With all this frightfulness flying at your ship, you'd want some kind of defense, besides just hoping they'll miss. Go to The Tough Guide to the Known Galaxy and read the entry "SHIELD".

As mentioned before, advances in effectiveness of weapon lethality and defensive protection are mainly focused on the targeting problem. That is, the weapons are generally already powerful enough for a one-hit kill. So the room for improvement lies in increasing the probability that the weapon actually hits the target.

And the room for improvement on the defensive side is to decrease the probability of a hit.

Weapons can be improved two ways: increase the precision of each shot (precision of fire), or keep the same precision but increase the number of shots fired (volume of fire).

Precision of fire is governed by

  1. the location of the target when the weapons fire arrives
  2. the flight path of the weapons fire given characteristic of the shot and the environment though which the shot passes
  3. the weapon's aiming precision

Volume of fire is governed by

  1. the weapon's rate of fire
  2. the lethality of a given shot

A defense can interfere with the [a] location of the target by evasive maneuvers.

There isn't really a way to interfere with [b] the characteristics of a shot, short of inserting a saboteur into the crew of the firing ship (in science fiction there are sometimes technobabble "nuclear damper fields" that prevent nuclear warheads from exploding). A defense can interfere with the environment through which the shot passes by such things as jamming the weapon's homing frequencies or clouds of anti-laser sand (which may work in the Traveller universe, but not in reality).

There isn't really a way to directly interfere with [c] the weapon's aiming precision (again short of a saboteur), though one can indirectly do so by decreasing the target's signature, increasing the range or jamming the firing ship's targeting sensors and degrade their targeting solution (in science fiction is the infamous "cloaking device").

Finally, while one cannot do much about the [d] weapon's rate of fire, the [e] lethality of a given shot can be effectively reduced by rendering harmless shots that actually hit. This is done by armor and point defense (in science fiction there are "force screens" and "deflector shields").

Defensive Systems

The innermost of a starship’s defensive systems is its armor. The primary armor is a multilayer (“honeycomb”) system over the core hull, composed of multiple vacuum-separated layers of refractory cerametals, sapphiroids, and artificially dense metal nanocomposites, strapped together via flexible, shock-absorbing forms. Atop this, a thick sprayed-on layer of foamed-composite ablative armor (whose vaporized form is designed to scatter incoming energy weapon fire) provides additional protection.

To provide thermal protection, each of these layers is threaded through with a mesh of thermally superconducting material, preventing heat input from lasers or other energy weapons from creating localized “hot spots”. This mesh spreads out external heat inputs, and ultimately dumps them into tanks of “thermal goo”, an artificial substance of very high specific heat capacity. Under normal circumstances, this heat is disposed of via the ship’s radiative striping and external radiators, but if necessary, the thermal goo can be vented to space, taking its heat (and, unfortunately, its heat capacity) with it.

Outside the armor, starship defenses come in three more layers:

First and innermost, the kinetic barriers. These are not a single, all-encompassing bubble; rather, they are a grid of plates of gravitic force, instantiated as needed to intercept incoming material objects. (They cannot shield against massless radiation.) They don’t attempt to directly retard incoming projectiles; rather, their job is to “slap them aside”, imposing enough sideways vector on them to generate a miss.

(ed note: kinetic barriers are science fictional)

Outside that, the defense drones: a military starship at general quarters surrounds itself with a “cloud” of small defense drones, serving multiple purposes: as electronic warfare platforms to obscure its signature; as participants in the kinetic-barrier generation and point-defense grid; as additional sensors; and ultimately, as sacrificial platforms capable of physically intercepting incoming projectiles or autonomous kill vehicles (AKVs) before they reach the ship itself.

Outermost is the point-defense zone guarded by the point-defense laser grid, extending substantially outward from the ship itself. Composed of phased-array plasma lasers which can be generated across large regions of the starship’s hull, the point-defense grid is used to vaporize incoming projectiles (or to use partial vaporization to decelerate incoming projectiles for the kinetic barriers and armor to deal with more effectively) and to force AKVs operating nearby – which have relatively little heat-dissipation capacity – into thermal shutdown.

The point-defense laser grid can also be used as an offensive weapon against any other starships unwise enough to stray into its range, but few captains are stupid enough to bring their starship into another ship’s point-defense zone.

The final defensive system that any starship has is drunkwalking: when at any alert state higher than peacetime cruising, every military starship engages in a pseudo-random “drunk walk” of vector changes around its station-keeping point or base course. This ensures that the starship is almost impossible to achieve a firing solution upon from a distance, since its movement since your most current observation of the target is unknown, and further increases the difficulty of achieving a solid firing solution in close.

(Of course, this depends greatly upon the quality of your drunkwalk algorithms and that they have been kept secure from the opposing force, which again underscores the importance of information warfare in the modern battlespace. A starship whose base course is identifiable and whose drunkwalk algorithms are known is a sitting duck even in the outer engagement envelope!)


Unlike starship armor, neither the point-defense laser grid nor the kinetic barriers are subject to direct attrition; if subjected to low-volume or low-power incoming fire, either or both could continue to destroy or repel it essentially forever.

In order to defeat these defensive systems, it is necessary to swamp them; to concentrate incoming fire to the point at which the defensive systems are unable to handle it all simultaneously. At this point, attrition may take effect as kinetic effectors and laser emitters are destroyed, but more importantly, it generates heat.

Heat is the primary limitation on combat endurance. Maneuvering burns, the use of high-energy equipment such as the point-defense grid, the kinetic barriers, and so forth, as well as the ship’s normal operation, all produce heat. In combat – when the ability to radiate heat is limited, usually to radiative striping and small (and exhaustable, if the starship is forced to maneuver) droplet radiators alone – military starships generate heat more rapidly than they can radiate it to space. As heat increases beyond the critical point, the efficiency of onboard equipment begins to fall (processor error rates rise, for example, and tactical officers must conserve their remaining heat capacity), some equipment goes into thermal shutdown, and the crew spaces become increasingly uninhabitable.

While some starships in any major space battle are destroyed physically, reduced to hulks, the majority of starships are defeated by either heat-induced equipment failure, or by being forced to surrender and deploy radiators lest their crew literally cook.

Evasive Maneuvers

The first rule of fighting is: Don't get hit.

If you can complicate your opponent's firing solution enough (i.e., dodge enough so all the shots miss), you do not need all that heavy bulky armor. Of course, if a shot does hit, you are up doo-doo pulsar without a gravity generator.

With fighter aircraft: weapon speeds, aircraft speeds, and target ranges are such that the main targeting problem is the large angular changes the undertaken by the target (you cannot slew the gun around quick enough). With spacecraft, however, the problem is light-speed lag and weapon lag. Light-speed lag means if your target is at a range of one light-second, you are seeing it where it was located one second ago, not where it is now. Weapon lag means you have to lead your target so that your plodding weapon shot will intercept it (the technical term is "deflection").

The attacker can try to minimize this by reducing the range, or using homing weapons that guide themselves to the target. The defender will try to open the range, and use various counter-measures to confuse the weapon's guidance system.

Naturally, if the target moves at a constant velocity with no course changes, light-speed lag and weapon lag cease to be a problem. Attackers love a target with a perfectly predictable course. Therefore, a target that wishes to live had better dodge and jink as much as possible.

Light speed lag and weapon lag will put an upper limit on the maximum probablity that an unguided weapon will strike the target. If you dare, you can calculate it with an equation I cobbled together all by myself (which means you had better double-check it first as I have been known to make childish mistakes in algebra).

H = Cm / (0.7854 * a2 * ((Dm / 299,792,458) + (Dm / Wv))4)


  • H = maximum percent chance to hit target given light-speed lag (0.0 - 1.0 with 1.0 = 100%)
  • Cm = target ship's mean cross section (m2, for a purely convex object this is approximately 1/4 of the surface area)
  • a = target's acceleration (m/s, where 9.81 = 1 g)
  • Dm = range to target (m)
  • Wv = weapon velocity (m/s)
  • 299,792,458 = meters in one light-second
  • 0.7854 = π / 4

Cm is the average target cross section. The target will be trying to orient itself so it presents the minimum possible cross section to the attacker, but the requirements of its propulsion system and other factors will interfere.

Since laser weapons travel at lightspeed (Wv = 299,792,458), for them the formula simplifies to:

H = Cm / (0.7854 * a2 * ((Dm + Dm) / 299,792,458)4)

Please note that this equation does not work if the target's acceleration is zero (since dividing by zero is mathematically undefined). In that case the target's official status is Sitting Duck and H = 1.0 or 100%. Neither does the equation work if the range is zero, in which the target's official status is At Point Blank Range or Eating The Gun Muzzle, and again H = 1.0 (Thanks to Eric Henry for pointing this out). Just remember that H cannot go over 1.0 and you'll be fine.

How was this equation derived? (just wait until you get a load of my assumptions...) Well, if H is chance to hit, a is acceleration in m/s, Dm is range in meters, and Cm is target's mean cross section:

Scircle = π * Rcircle2


  • Scircle = surface area of a circle
  • Rcircle = radius of the circle
  • π = Pi = 3.14159...

H = Cm / (π * displacement2)


  • displacement = maximum distance perpendicular to line of fire that the target can move in time between a shot being fired and the shot arriving at target

In other words, take the cross section surface area of the target, divide it by the surface area of the circle the target can move to, and you have your maximum hit chance. e.g., if the target has a surface area of 1, and it can displace anywhere into a circle of surface area 3, then the maximum hit chance is 1/3.

d = 0.5 * a * t2


  • d = distance (m)
  • a = acceleration (m/s2)
  • t = duration of acceleration (s)

which is the classic acceleration equation, assuming a starting velocity of zero. We can assume zero because all we care about is the change in the target's current velocity, that is, the jinking

Now, to use acceleration equation to calculate displacement:

t = (Dm / 299,792,458) + (Dm / Wv)


  • t = time it takes light from target to travel to targeting sensors plus time it takes weapon to travel to target (s)
  • t = time target has to jink before weapon arrives
  • Dm = range to target (m)
  • Wv = weapon velocity (m/s)
  • 299,792,458 = meters in one light-second
  • Dm / 299,792,458 = time it takes light from target to travel to targeting sensors (s)
  • Dm / Wv = time it takes weapon to travel to target (s)

d = 0.5 * a * t2

replace t with jink time

displacement = 0.5 * a * ((Dm / 299,792,458) + (Dm / Wv))2

Inserting displacement equation into hit chance equation and simplifying:

H = Cm / (π * displacement2)

H = Cm / (π * (0.5 * a * ((Dm / 299,792,458) + (Dm / Wv))2)2)

H = Cm / (π * 0.25 * a2 * ((Dm / 299,792,458) + (Dm / Wv))4)

H = Cm / (0.7854 * a2 * ((Dm / 299,792,458) + (Dm / Wv))4)

Note this equation only calculates the percentage chance of missing due to light-speed lag. There are many other factors that can contribute to a miss. However, most of these are not under control of the target.


Deep recon and forward scouting is the job of the recon destroyers. And contrary to “Running Cold”, it’s not a particularly stealthy job. For all the boffins dream about stealth starships and talk airily about basement universes and domain walls and dimensional transcendence, I’ve never seen one. And no, that’s not a joke.

The aim is not to avoid being seen. They can see you, bright, hot, and clear. (It’s not all bad — this also means that you can see them.) The aim is to be seen sailing through the target area fast and high and out of the way — beyond intercept range and outside their engagement envelope – so they can’t touch you. Except for exchanging the usual bluster.

Not that that stops people from taking a pot-shot or two at you anyway on general principles. So you jink, jink, jink and trust to light-lag! But all that drunkwalking cuts deep into your delta-v reserve for evading and running, which is why they give you a whole library of variable-power drunkwalk algorithms, from a pro-forma wobble on the reaction wheels up through the affectionately named Torpedo Tango, Missile Minuet, Warhead Waltz, Firing Solution Foxtrot, and so forth, right on up to the good old Hellfire Hop. Choose carefully, ‘cause you might need whatever you burn now later. And if you’re really worried, you can fire up the kinetic barriers to military power — if you wouldn’t rather keep that energy to go into thrust, and if you don’t mind being provocative by shining the EM signature of a battle-ready warship all over the system. Any misjudgment at this point may result in a salvo or two of unanticipated k-slugs ripping big holes in your ‘can.

This is why every recon captain I ever served under had an ulcer and the temperament of a grouchy bear.

- Senior Chief Viviré Galicios, Imperial Navy (unpublished memoir)

Eric Tolle

I think some ideas in this article are applicable to hard-SF space tactics. The goal that is, of not trying to keep from being detected, but of having a high enough delta-V that one cannot be intercepted. It would depend on the offensive and defensive weaponry of course, but if weapons are comparatively short range it might be a viable tactic.

Of course I think scouting would depend more on long-range observation and masquerading as civilian traffic, ut still, I wonder if a edge case can be made for scout vessels?

Alistair Young

Well, that's more or less the intent (granted, the kinetic barriers are more sort of "firmish SF", but...), so thank you kindly.

In-setting, effective weapons range isn't limited by the weapons, but by the ability of your fire control to generate a hit, which light-lag usually keeps down to no more than about a light-second against any target that isn't obligingly moving under constant single-vector acceleration. The outer engagement envelope is larger, one or two light-minutes, but firing in that is mostly to force your opponent to expend point-defense resources (including delta-v) and generate heat, rather than in serious hope of generating a hit.

(Hence the drunkwalking, since I doubt very much that any practical ship design could let you avoid interception by a k-slug — which can have a millionth or less of your mass and still mission-kill your ship. If it hits.)

On the other note, light-lag tends to be a problem with extracting tactical intelligence from long-range observation, as the situation tends to have changed unrecognizably by the time the information hits long (by space standards) range. I do agree, though, that a lot of recon is going to be done by masquerading as civilian traffic and other presumptively harmless things (comets, say, comets are always good)...

...but around secure bases and in times of, ah, interstellar tension, civilian traffic isn't going to be allowed near those places you really want to take a look at. And that's when these chaps go in. :)

Moment of Inertia

Roger M. Wilcox (creator of the indispensable Internet Stellar Database) spotted a major flaw with the above equation. In it, I assume that the target can instantly dodge in any desired direction. Bad assumption. Most rockets can only accelerate in the opposite direction of the main engine's thrust plume. True, a rocket can turn using its attitude control system so the plume is aimed in any desired direction, but this takes time. And it takes longer if the rocket is long and skinny like a pencil, instead of short and fat like an orange.

I was perusing your page wherein you calculate the probability for missing an accelerating target with a light-speed weapon at a distance of 1 light-second.

You carefully prefaced your equations with "Just wait'll you get a load of my assumptions" — but there's one additional assumption you were making that I think you might not be aware of. You assumed that the target's acceleration (a) could be applied in ANY randomly-chosen direction with equal ease.

This implies that the target is able to point its engine in any direction instantly, or nearly instantly. I did some calculations and discovered that it's much harder for a sizable spacecraft to rotate along its pitch or yaw axis than I thought.

Consider a modestly-sized 100 meter long spacecraft with a mass of 1000 tonnes, with a great big torch engine at the back capable of producing 20 million Newtons of thrust (enough for 2g of acceleration) and small attitude thrusters pointing sideways at its nose and tail. These attitude thrusters are what the spacecraft uses to rotate. We'll assume that the spacecraft is roughly rod shaped with its mass uniformly distributed along its length, so that its center of mass is at the 50 meter mark.

Let's say this spacecraft wants to start rotating. We want to apply a VERY MODEST angular acceleration of 1 radian per second squared — that is, after firing its attitude thrusters for 1 second continuously, its angular velocity will be 1 radian per second (it'll take 3.14 seconds to face the opposite direction at this angular speed). We fire the attitude thruster on one side of the nose, and simultaneously fire the attitude thruster on the opposite side of the tail.

How hard will each those attitude thrusters have to push?

For a rod-shaped object, the Moment of Inertia. (I) is 1/12*M*L2. Here, L = 100 meters, so I is 833 * 1,000,000 kg = 833 million. Our angular acceleration is 1 rad/s2. Thus, the total amount of TORQUE we need to apply to the spacecraft is 833 million meter-Newtons. Each of the 2 attitude thrusters will have to provide half this torque, or 416 mega-meter-Newtons each. Since each thruster is situated 50 meters from the center of mass, each will have to push with a FORCE of 416/50 = 8.33 million Newtons.

In other words, each of the ATTITUDE THRUSTERS has to produce enough thrust to accelerate the ENTIRE SPACECRAFT at 0.83 g !! The thrusters themselves would have to be torch drives!

And this is JUST to produce a very modest 1 radian/sec2 angular acceleration.

If you want to be able to point your nose in any direction in only, say, half a second, you'd need at least 12 radians/sec2 of acceleration — 24 rad/s2 if you wanted to angularly accelerate through half this angle and then angularly decelerate through the other half.

Oh, and the amount of attitude thrust force required works out to being proportional to your spacecraft's length as well as its mass. A 200 meter long 1000-tonne spacecraft would require 16.6 million Newtons from each thruster for 1 rad/s2 of angular acceleration. Note that I haven't increased the spacecraft's MASS there, JUST its length. A 200 meter long 2000-tonne spacecraft would require 33 million Newtons from each thruster.

As a side note, if a 100-meter long spacecraft WERE rotating at 1 radian/sec, everything in its nose and tail section would be pinned to the outer wall by a centripetal acceleration of 5g.

And if you keep making your spacecraft longer from nose-to-engines, there'll come a point where you can actually jink more rapidly by thrusting SIDEWAYS with your attitude thrusters than you will by rotating and using your main engine. (In my example, a 100 meter long spacecraft requires 0.83 g from its nose thruster and another 0.83g from its tail thruster to get 1 rad/s2 of angular acceleration. If you point both of those thrusters in the same direction, though, you'd get 1.66 g of acceleration sideways, which is almost as much as the 2g that its main engine can provide! You'd still have to ROLL the spacecraft to position those side thrusters onto the correct side, but rolling a rod-shaped spacecraft requires much less torque than pitching or yawing.)

Roger M. Wilcox

I too had no idea such huge amounts of torque were required (though I did know about the moment of inertia problem, having read Sir Arthur C. Clarke's short story "Hide and Seek"). Since torch drive attitude jets are highly unlikely, this drastically reduces the angle the ship's nose can be changed by, and thus drastically reduces the area the ship can dodge into. Instead of a sphere, it will be reduced to a cone sharing its long axis with the ship. The angular width of the cone will depend upon the target's moment of inertia, the torque produced by the target's attitude system, and the time it takes the weapon to fly to the target.

If your rocket exhaust is cool enough that material objects can actually survive being inserted into the exhaust plume, a possible solution to the dilemma is Cascade Vanes. So instead of making your attitude jets as big as your main engine, you actually use your main engine as an attitude jet. However, an exhaust that cool is probably way below torch rocket levels.


(ed note: Secret agent K-15 is being chased by enemy ship Doradus. If K-15 can survive a few hours, friendly spacecraft will come to his rescue. So to buy time, K-15 bails out of his ship in a spacesuit, and goes to ground on the tiny Martian moon of Phobos. The commander of the Doradus is most displeased.)

To the layman, knowing nothing of the finer details of astronautics, the plan would have seemed quite suicidal. The Doradus was armed with the latest in ultra-scientific weapons: moreover, the twenty kilometers which separated her from her prey represented less than a second’s flight at maximum speed. But Commander Smith knew better, and was already feeling rather unhappy. He realized, only too well, that of all the machines of transport man has ever invented, a cruiser of space is far and away the least maneuverable. It was a simple fact that K-15 could make half a dozen circuits of his little world while her commander was persuading the Doradus to make even one.

There is no need to go into technical details, but those who are still unconvinced might like to consider these elementary facts. A rocket-driven spaceship can, obviously, only accelerate along its major axis-that is, "forward." Any deviation from a straight course demands a physical turning of the ship, so that the motors can blast in another direction. Everyone knows that this is done by internal gyros or tangential steering jets, but very few people know just how long this simple maneuver takes. The average cruiser, fully fueled, has a mass of two or three thousand tons, which does not make for rapid footwork. But things are even worse than this, for it isn’t the mass, but the moment of inertia that matters here — and since a cruiser is a long, thin object, its moment of inertia is slightly colossal. The sad fact remains (though it is seldom mentioned by astronautical engineers) that it takes a good ten minutes to rotate a spaceship through 180 degrees, with gyros of any reasonable size. Control jets aren’t much quicker, and in any case their use is restricted because the rotation they produce is permanent and they are liable to leave the ship spinning like a slow-motion pinwheel, to the annoyance of all inside.

In the ordinary way, these disadvantages are not very grave. One has millions of kilometers and hundreds of hours in which to deal with such minor matters as a change in the ship’s orientation. It is definitely against the rules to move in ten-kilometer radius circles, and the commander of the Doradus felt distinctly aggrieved, K-15 wasn’t playing fair.

HIDE AND SEEK by Arthur C. Clarke (1949)

      Re. "Hide and Seek": Actually the commander of the Doradus used a very inefficient method of circling Phobos. As I recall he would fire his "jets", travel in a straight line for some minutes while making a 180-degree turn, fire his jets again to brake to a halt, reorient, then repeat.

     A much more efficient method of circumnavigating Phobos would be to align the rotation and thrust so that the Doradus was thrusting in towards Phobos while spinning at the same rate as it took to complete the circumnavigation. In other words, substitute the ship's drive for the nearly non-existent gravity needed to make an orbit. The maximum rate would be dictated by the circumference of the circle, the drive's thrust and the maximum allowable spin rate.

     I was trying to calculate hard figures for the Doradus, how fast it would have to go to circumnavigate Phobos and how much acceleration would be required. If my math is correct here's what I've got:

     The story mentions a spin rate of 10 minutes to flip 180 degrees, so 20 minutes for 360 degrees. Let's take that as the working spin rate and circumnavigation time. The story then mentions a 10 km radius. That would mean a circumference of 2Pi x 10, which rounds up to 62,832 meters.

     If Doradus travels that distance in 1200 seconds the tangential velocity is (rounded up again) 52.36 m/s. If the formula for centripetal acceleration is A=V2/R then Doradus has to maintain a centripetal acceleration of 0.27415 m/s2 to artificially orbit Phobos at maximum speed, or about 1/36 of a G.

     If Doradus attempts to maintain this for (iirc) six hours, the delta-V budget needed would be a little over 5.9 km per sec. Of course a slower speed can probably be used since that's faster than a man in a suit can move around Phobos.

by Michael Hutson (2019)

"Pursuit fighters," I told the Ship. "Easily fast enough to catch one of our boats, if they can do it within their limited range. It's limited because they're the only kind of craft designed for dogfight tactics.

They're just enormous multidirectional motors in a spheroid hull with one pilot in the centre and a few missile tubes scattered between the motor vents. Fast maneuvering in space means killing momentum one way as well as building it up in another, so there's murderous acceleration and deceleration every few seconds, with the motor blasting in all directions, eating up hydrogen and putting incredible stress on the pilots. Even with all the aids — liquid suspension cocoons, special suits, body reinforcement, field-shields, the lot — it takes years of training to stand it for more than a few minutes at a time.

The American call fighter pilots Globetrotters, for some old game where you had to bounce a ball all the time. I've been in a fighter simulator once — I came out black and blue, and they say the real thing's worse.

And that's our hope — that Liang can hold them off, make them maneuver so much they'll have to give up, or just outrun them. That's what he's trying to do now, but he's got to be careful. They mustn't box him in and stop him maneuvering, that'd let them swarm over him like hornets, killing the boat or crippling it till the gunships catch up —"

From RUN TO THE STARS by Michael Scott Rohan (1982)


Conventional military camouflage or ship camouflage intended to allow armored fighting vehicles to blend in with the jungle background obviously won't work in the inky depths of space. No jungle, for one thing.

In any event, "blending in" is synonymous with "invisibility", which means stealth in space. And we all know what RocketCat thinks about that.

But camouflage means more that invisibility. It is also disguise. Much like a decoy. However, while a tiny decoy missile is not cost-effective, it may be quite feasible for a full-sized spacecraft to alter its sensor signature enough so it looks like a different spacecraft or even a different kind of spacecraft. Ask any pirate how an unexpected q-ship can be a fatal surprise.

For instance, the ship's exhaust plume color can suggest the thrust level. Given the observed acceleration, the ship's mass can be calculated. But is that a defenseless 10,000 metric ton ore freighter loaded with nickle-iron asteroid ore or a 10,000 metric ton death-battleship loaded with nuclear missiles and x-ray lasers? An enemy ship is going to need different sensor data to narrow down the target ship's identity.

Similarly if only warships have engine thrust power rated in gigawatts, a warship might travel with the drive throttled down to megawatt levels hoping to be mistaken for a civilian craft. If you were an evil convoy raider preying on a rival nation's logistics tail, watching your target's thrust power suddenly climb from a few megawatts to a few gigawatts may be the last thing you will ever see.

I hate to be a killjoy, but using actual paint as camouflage is not very scientific. Most space combat will be at ranges such that the enemy ships will not be visible to the naked eye, you'll need telescopes to see your opponents. And any laser of medium power will instantly fry a telescope, since the scope helpfully directs the ravening destructive energy right into the most critical scope components. Visual camouflage is pointless. It is more likely that ships will use radar or something.

But as George Lucas demostrates, it is easy to laugh all the way to the bank at such questions about scientific accuracy. For your self respect, don't totally jettison every scrap of science and plummet down to "Lost in Space" levels. Just jettison enough science to be cinematic, and stop your plummet at an intermediate level.

What I'm saying is if you want your ships to have painted camouflage to make scifi eye-candy for your audience, at least make the camo semi-scientific. Make it dazzle camouflage.


(ed note: this is from a leaflet insert that came with each package of metal starship miniatures used for table-top war gaming)

      The question of color for interstellar warships is an interesting one to contemplate, particularly when one considers which aspects of modern naval and aeronautic practice in this regard are likely to be retained in the far-distant future.

     First, one must bear in mind that the color schemes and markings for armed vessels are determined primarily tactical necessity, not aesthetics. Color Serves two tactical purposes — concealment and identification — so that to a large extent the colors you finally decide upon will be determined by the rules you are using.

     If visual sighting plays any role at all, camouflage schemes would seem to be a logical development. Bear in mind that the usual purpose of camouflage is not so much to conceal an object as it is to break up the outline and make it difficult to identify. This might be done in space by using a dark blue or black, possibly mottled with white. A dazzle-type camouflage similar to that used in the Atlantic in World War II works very well and is shown in the drawing above (possible colors: black, blue, gray, lavender or white).

     Another possibility is the use of special paints that would foil electronic detection; just what color or colors this might entail is anybody's guess.

     If visual sighting has no tactical significance, the only purpose of color would be for identification. In this case the colors would probably be rather bright for high visibility and easy recognition, possibly involving different colors for different federations or empires. Colors we have found to work well include natural metal (just polish the castings with fine steel wool and give them a wash of thinned black paint or India ink). yellow, dark blue, light blue, lavender, red, and of course white.

     Ship numbers, squadron markings and federation or empire symbols are nil likely to be used as well, National markings should he simple and easy to identify at a distance. Squadron markings have always varied from rather spartan to outrageously gaudy, depending on the tastes of the individuals concerned, and would probably continue to be so; to get ideas for these. look at some World War II and modern aircraft — variations of any of these themes are liable to be used in space, possibly even by a descendant of the original unit!


Dazzle camouflage, also known as razzle dazzle (in the U.S.) or dazzle painting, was a family of ship camouflage used extensively in World War I, and to a lesser extent in World War II and afterwards. Credited to the British marine artist Norman Wilkinson, though with a rejected prior claim by the zoologist John Graham Kerr, it consisted of complex patterns of geometric shapes in contrasting colours, interrupting and intersecting each other.

Unlike other forms of camouflage, the intention of dazzle is not to conceal but to make it difficult to estimate a target's range, speed, and heading. Norman Wilkinson explained in 1919 that he had intended dazzle primarily to mislead the enemy about a ship's course and so cause them to take up a poor firing position.

Dazzle was adopted by the Admiralty in the UK, and then by the United States Navy. Each ship's dazzle pattern was unique to avoid making classes of ships instantly recognisable to the enemy. The result was that a profusion of dazzle schemes was tried, and the evidence for their success was at best mixed. So many factors were involved that it was impossible to determine which were important, and whether any of the colour schemes were effective. Experiments were carried out on aircraft in both World Wars with little success.

Dazzle attracted the notice of artists such as Picasso, who claimed that Cubists like himself had invented it. Edward Wadsworth, who supervised the camouflaging of over 2,000 ships during the First World War, painted a series of canvases of dazzle ships after the war, based on his wartime work. Arthur Lismer similarly painted a series of dazzle ship canvases.

Intended purposes

At first glance, dazzle seems an unlikely form of camouflage, drawing attention to the ship rather than hiding it. The approach was developed after Allied navies were unable to develop effective means to hide ships in all weather conditions. The British zoologist John Graham Kerr proposed the application of camouflage to British warships in the First World War, outlining what he believed to be the applicable principle, disruptive camouflage, in a letter to Winston Churchill in 1914 explaining the goal was to confuse, not to conceal, by disrupting a ship's outline. Kerr compared the effect to that created by the patterns on a series of land animals, the giraffe, zebra and jaguar.

Taking up the zebra example, Kerr proposed that the vertical lines of ships' masts be disrupted with irregular white bands. Hiding these would make ships less conspicuous, and would "greatly increase the difficulty of accurate range finding". However, in the same letter, Kerr also called for countershading, the use of paint to obliterate self-shading and thus to flatten out the appearance of solid, recognisable shapes. For example, he proposed painting ships' guns grey on top, grading to white below, so the guns would disappear against a grey background. Similarly, he advised painting shaded parts of the ship white, and brightly lit parts in grey, again with smooth grading between them, making shapes and structures invisible. Kerr was thus hoping to achieve both a measure of invisibility and a degree of confusion for the enemy using a rangefinder. Whether through this mixing of goals, or the Admiralty's skepticism about "any theory based upon the analogy of animals", the Admiralty claimed in July 1915 to have conducted "various trials" and decided to paint its ships in monotone grey, not adopting any of Kerr's suggestions. It had made up its mind, and all Kerr's subsequent letters achieved nothing.

The American artist Abbott Handerson Thayer had developed a theory of camouflage based on countershading and disruptive coloration, which he had published in the controversial 1909 book Concealing-Coloration in the Animal Kingdom. Seeing the opportunity to put his theory into service, Thayer wrote to Churchill in February 1915, proposing to camouflage submarines by countershading them like fish such as mackerel, and advocating painting ships white to make them invisible. His ideas were considered by the Admiralty, but rejected along with Kerr's proposals as being "freak methods of painting ships ... of academic interest but not of practical advantage". The Admiralty noted that the required camouflage would vary depending on the light, the changing colours of sea and sky, the time of day, and the angle of the sun. Thayer made repeated and desperate efforts to persuade the authorities, and in November 1915 travelled to England where he gave demonstrations of his theory around the country. He had a warm welcome from Kerr in Glasgow, and was so enthused by this show of support that he avoided meeting the War Office, who he had been intending to win over, and instead sailed home, continuing to write ineffective letters to the British and American authorities.

The marine artist and Royal Naval Volunteer Reserve officer Norman Wilkinson, agreed with Kerr that dazzle's aim was confusion rather than concealment, but disagreed about the type of confusion to be sown in the enemy's mind. What Wilkinson wanted to do was to make it difficult for an enemy to estimate a ship's type, size, speed, and heading, and thereby confuse enemy ship commanders into taking mistaken or poor firing positions. An observer would find it difficult to know exactly whether the stern or the bow was in view; and it would be correspondingly difficult to estimate whether the observed vessel was moving towards or away from the observer's position.

Wilkinson advocated "masses of strongly contrasted colour" to confuse the enemy about a ship's heading. Thus, while dazzle, in some lighting conditions or at close ranges, might actually increase a ship's visibility, the conspicuous patterns would obscure the outlines of the ship's hull (though admittedly not the superstructure), disguising the ship's correct heading and making it harder to hit.

Dazzle was created in response to an extreme need, and hosted by an organisation, the Admiralty, which had already rejected an approach supported by scientific theory: Kerr's proposal to use "parti-colouring" based on the known camouflage methods of disruptive coloration and countershading. This was dropped in favour of an admittedly non-scientific approach, led by the socially well-connected Wilkinson. Kerr's explanations of the principles were clear, logical, and based on years of study, while Wilkinson's were simple and inspirational, based on an artist's perception. The decision was likely because the Admiralty felt comfortable with Wilkinson, in sharp contrast to their awkward relationship with the stubborn and pedantic Kerr.

Wilkinson claimed not to have known of the zoological theories of camouflage of Kerr and Thayer, admitting only to having heard of the "old invisibility-idea" from Roman times.

Possible mechanisms

Disrupting rangefinding

In 1973, the naval museum curator Robert F. Sumrall (following Kerr) suggested a mechanism by which dazzle camouflage may have sown the kind of confusion that Wilkinson had intended for it. Coincidence rangefinders used for naval artillery had an optical mechanism, operated by a human to compute the range. The operator adjusted the mechanism until the two half-images of the target lined up in a complete picture. Dazzle, Sumrall argued, was intended to make that hard, as clashing patterns looked abnormal even when the two halves were aligned, something that became more important when submarine periscopes included such rangefinders. Patterns sometimes also included a false bow wave to make it difficult for an enemy to estimate the ship's speed.

Disguising heading and speed

The historian Sam Willis argued that since Wilkinson knew it was impossible to make a ship invisible with paint, the "extreme opposite" was the answer, using conspicuous shapes and violent colour contrasts to confuse enemy submarine commanders. Willis pointed out, using the HMT Olympic dazzle scheme as an example, that different mechanisms could have been at work. The contradictory patterns on the ship's funnels could imply the ship was on a different heading (as Wilkinson had said). The curve on the hull below the front funnel could seem to be a false bow wave, creating a misleading impression of the ship's speed. And the striped patterns at bow and stern could create confusion about which end of the ship was which.

That dazzle did indeed work along these lines is suggested by the testimony of a U-boat captain:

It was not until she was within half a mile that I could make out she was one ship [not several] steering a course at right angles, crossing from starboard to port. The dark painted stripes on her after part made her stern appear her bow, and a broad cut of green paint amidships looks like a patch of water. The weather was bright and visibility good; this was the best camouflage I have ever seen.

Motion dazzle

In 2011, the scientist Nicholas E. Scott-Samuel and colleagues presented evidence using moving patterns on a computer that human perception of speed is distorted by dazzle patterns. However, the speeds required for motion dazzle are much larger than were available to First World War ships: Scott-Samuel notes that the targets in the experiment would correspond to a dazzle-patterned Land Rover vehicle at a range of 70 m (77 yd), travelling at 90 km/h (56 mph). If such a dazzling target causes a 7% confusion in the observed speed, a rocket propelled grenade travelling that distance in half a second would strike 90 cm (35 in) from the intended aiming point, or 7% of the distance moved by the target. This might be enough to save lives in the dazzle-patterned vehicle, and perhaps to cause the missile to miss entirely.

From the Wikipedia entry for DAZZLE CAMOUFLAGE


If the pressurized habitable section of your warship was one single area, a hull breech would depressurize the entire ship (I was going to recount the ancient joke about "why is a virgin like a balloon", but luckily good sense intervened). A prudent warship design would use air tight bulkheads to divide the interior of the pressurized section into separate areas. This comes under the heading of "not keeping all your eggs in one basket". The keyword is redundancy.

For the same reason, you'd want back-up life-support systems, power plants, control rooms, and other vital components. And these duplicate systems should be located in widely separated parts of the ship. Otherwise a single lucky enemy weapon shot could take both of them out.

Even in the non-pressurized section, bulkheads can help contain destructive effects of hostile weapons fire. So an explosive warhead, with any luck, will merely damage the interior of one compartment, instead of gutting the entire interior of the ship.

Mark Temple says:

Recently at one of the RPG boards I visit, a discussion about "armor belts" and the durability of space warships has cropped up. This got me thinking about compartments and how they'd be an integral part of a ships survival.

Modern naval vessels are divided up into compartments to make them more survivable. Compare a naval frigate to a main battle tank. A tank is basically one compartment. Breach its (very thick) armor and you wreck the tank, since the hit will usually kill the entire crew and/or destroy the internal systems.

A frigate however has multiple compartments. Breach the hull of the frigate and while you might wreck one compartment, the entire ship will still float and will often still be able to fight. You have to wreck many compartments, or very specific compartments, in order to mission-kill the ship.

It seems to me this vital part of naval design would not be overlooked in space warship design. Beyond the obvious benefits of making it easier to control atmospheric leaks, a space warship built with many compartments that can be isolated would gains a structural benefit in combat.

Now, compartments would be worthless if one hit could completely disable vital systems like life support or command-and-control. Thus all these systems would be distributed all across the ship, with multiple redundancies. Thus if you lose a compartment with life support systems, you have others to fall back on. Having the main CiC compartment destroyed will not totally eliminate your ability to control the ship. This is standard for real world navy ships. Engine systems, command rooms (bridges, CiC's, etc.) would have secondary locations kept manned in battle in case the main compartments for them are destroyed.

This is also why those compartments would be buried as deep inside the ship as possible. No sense in making things easy for your enemy. True, on modern wet navey warships bridges are still mainly at the highest point of the ship, but that's mainly to facilitate visual tracking and identification. In space, you cannot see the enemy with the naked eye anyway, so you might as well put your command centers where the enemy has to destroy the entire ship to get at it.

In science fiction movies and television, we have never really seen all of these features at once. Ironically Star Trek managed to get the distributed systems part correct, we eventually even saw that Federation starships had "battle bridges" to provide emergency control should the main bridge be damaged. But Star Trek has utterly failed to put the bridge in defended positions, or show proper compartments in their designs (As David Gerrold noted, that silly bridge perched on the saucer top of the Starship Enterprise would have been shot off a long time ago). Apparently they rely on their handwavium deflector shields to do the job, which is great until you run out of power.

(New) Battlestar Galactica came pretty close, though. The ships systems are distributed, the ship itself compartmentalized, and it has a bridge buried deep in the hull. We just never see redundant engine rooms or command centers, which is probably more of a failing of the script writers than of design.

In novels we see this idea used properly, though. The Honorverse novels showcase the benefits of compartmentalization in a very obvious and graphic form, in nearly every novel.

Mark Temple

"There are six main classes of fighting machines. The great battleships are first, weighing in the neighborhood of one million five hundred thousand tons. A battleship is almost indestructible. Even when blown completely in two, it Is exceedingly dangerous, as it maintains maneuverability and fighting power... "

...Thirty great battleships formed the front, against twenty-nine whole Tefflan battleships, but there were no less than eleven half ships in action, and each of these was fully half as deadly as a full battleship...

...A battle between battleships of space is not like a sea battle, for the battleship of space never sinks, and every portion is capable of fighting until every man within is killed; a battle between space battleships is to the death of every individual...

From THE MIGHTIEST MACHINE by John W. Campbell jr. (1934)


Armor is a shell of strong material encasing and protecting your tinfoil spacecraft. Unfortunately as a general rule, armor is quite massive, so it really cuts into your payload allowance.

Basically, the energy requirement to damage a surface is measured in joules/cm2. If you exceed that value, you do damage, otherwise you fail. Keep in mind that a Joule is the same thing as a watt-second.

There are three ways that weapon energy damages a surface: thermal kill, impulse kill, and drilling.

Thermal kill destroys a surface by superheating it. Impulse kill destroys a surface by thermal shock. In the calculations for the SDI, the amount to thermal kill a flimsy Soviet missile is about 1 to 10 kilojoules/cm2 (100 MJ/m2) deposited over a period of a second. The same energy deposited over a millionth of a second is required for an impulse kill. Since the laser beam tends to be meters wide, the beam energy is in the hundreds of megaJoules.

However, neither thermal kill nor impulse kill works very well with armor. So we use the third method: drilling. The amount of energy required to drill through an object is within a factor of 2 or so of the heat of vaporization of that object. There are also two other limits: the maximum aspect ratio of the hole is usually less than 50:1, and the actual drilling speed, for efficient drilling, is limited to about 1 meter per second (depending on the material).

Therefore, the best anti-laser armor will be that material with the highest vaporization energy for its mass. The best candidate is some form of carbon, at 29.6 kilojoules/gram. You do not want a form that is soft or easily powdered, or the vapor action under laser impact will blow out flakes of armor, allowing the laser to penetrate much faster. Steel has a higher vaporization energy, but it masses more as well.

Under laboratory conditions, if an armor layer was 5 g/cm2 of carbon, burning through a 1 cm2 (1.12 cm diameter) spot of armor would take about 148 kilojoules and 20 milliseconds. An AV:T laser cannon with 50 megaJoules could burn through 330 such armor layers in a few seconds, under laboratory conditions (i.e., enough layers to burn through the entire ship the long way).

However, under combat conditions there is no way one could focus the laser down that tiny and keep it on the same spot on the target ship for multiple seconds.

It would be better to use a beam focused down to a larger 10 cm2 spot (11.2 cm diameter). Granted the beam power required to penetrate jumps from 148 kilojoules to 15 megaJoules, but now if we have an uncertainty in the target's velocity of up to 5 meters per second it doesn't matter.

Of course, if price is no object, you can do better than carbon. Boron has a vaporization energy of 45.3 kilojoules/gram and is only slightly denser than carbon. Expensive, though.

In a 1984 paper on strategic missile defense, it suggested that your average ICBM would require about 10 kilojoules/cm2 to kill it. This would rise to 20 to 30 kilojoules/cm2 with ablative armor, and it would be tripled if the ICBM was spinning on its long axis since the laser couldn't dwell on the same spot 100% of the time.

As a side note, a Whipple shield is very effective at stopping hypervelocity weapons. With kinetic weapons at closing velocities in excess of 10 km/sec, you're getting into the realm where armor is less important than blow-through. For armor, you want something that will resist being turned into a plasma for as long as is possible, followed by gaps made of vacuum to make it a Whipple shield.

Anti-radiation armor is discussed here.

Abraham Wald and Armor Placement

This is a rather famous story from World War 2, but I'm going to re-tell it anyway because it is so cool.

During World War 2, the United States had several top-secret programs where mathematicians and statisticians helped with the war effort by fighting with math. Their analysis and optimizations were aimed at giving the Allies the edge in combat, their work saved lives. One of the more important programs was the Statistical Research Group, based in Manhattan.

And at the SRG, the smartest guy in the room was Abraham Wald.

Wald was a genius, Wald was brilliant, and Wald was yet another example of the Nazi racist policy towards those of the Hebraic persuasion turning around and savagely biting off both Nazi Gluteus Maximus cheeks (another example being Albert Einstein). Wald had a nice little job in Austria, but after the Nazi conquest it didn't take the genius of a Wald (or an Einstein) to see if you didn't get the heck out of Austria you'd soon be in a concentration camp. Wald moved to the United States where he was quickly offered a professorship of statistics at Columbia. From there he was recruited into the SRG.

At the time the Allies were sending near-constant bombing missions to blow up German factories and otherwise destroy the German ability to wage war. The German response was a near-saturation level of anti-aircraft barrage that made Allied bombing missions into suicide missions. The bombers had to fly way high in the sky where everybody could see you, and linger for hours while every ack-ack gun in Germany did its darnedest to kill you. The bomber crews figured they had the same chance of surviving a given mission as winning a coin toss, you were quite likely to die long before you got your fifty-mission crush.

The Army Air Force knew that any improvement of the odds no matter how small would make a big difference over multiple missions. They came to the SRG. They already knew that additional armor would be a huge help. But you couldn't just armor plate the entire bomber or it couldn't get off the ground. The question was applying armor to which spots would get the maximum survival benefit for their armor poundage dollar.

The Army Air Force had already done some preliminary work. They did an analysis of bullet-holes in the bomber aircraft and made Chart 1 (see above) which they proudly displayed to Wald. They told Wald that the idea was to put the armor plate where the bullet holes where thickest, since obviously that was the places that needed it the most.

But lucky for them, Wald was a genius.

Wald shook his head and told them they were wrong, the place to put the armor was the spots with the least holes. Yes, this seems illogical, but as it turns out the Army Air Force had fallen into the trap of failing to consider data bias. Specifically Survivorship bias.

Wald pointed out that the data was not that of bullet holes in bomber aircraft. It was data of bullet holes in bomber aircraft that had survived long enough to make it back to base.

Since bullet holes are more or less randomly spread over the body of a bomber, the fact that there are bullet-free places on the surviving bombers implies that the chart for the bombers that died would be the exact opposite. It would look something like Chart 2 (see above). Chart 2 is the chart for bombers that did not return to base. Therefore a bullet in the bullet-zones of Chart 2 makes the bomber go down in flames. That's where you need to put the armor.

The bullet zones in Chart 1 were merely the spots you could shoot a hole in a bomber and not kill it. Those were the spots the bomber was strongest.

History doesn't record the reaction of the Army Air Force, but I'd hazard a guess they turned pale at the magnitude of the blunder that Wald had just rescued them from.

Wald refined the data and actually developed equations showing the relative vulnerability of each bomber part, and the probability of being shot in a given part depending upon the intensity of the anti-aircraft fire. Equations that are still in use to this day.

The Gods Hate Kansas

In this science fiction story, author Joseph Millard does what I have advocated: find some real scientific fact that is really weird, and use it as a springboard for your story plot. It gives you the author some inspiration. And if your readers check your facts, they will stub their toe on the reality of the matter and be duly impressed by your research.

Mr. Millard started with the fact that apparently Kansas is a meteorite magnet.

As of July 2009, there are 1,530 verified meteorites that have been discovered in the continental United States. There are 49 continental states so you'd figure each state would have 1/49 = 2% share of the 1,530 meteorites. But the state of Kansas has a whopping 9% share (137). Indeed it does seem to be a meteor-magnet.

Starting from that, Mr. Millard created a lurid tale of invaders from outer space. These initial meteorites were just advanced scouts. Then one fine day nine of the suckers landed simultaneously. A meteorite investigating team arrived to investigate the meteorites, and were immediately turned into alien-controlled automatons. They started building an infernal device for the invasion. About this time the deadly Crimson Plague struck, yet another fiendish part of the alien master plan.

As you can see, Mr. Millard sure got a lot of springboard action out of one little fact. So much action, in fact, that the novel was made into a movie called They Came From Beyond Space. Granted the movie was a big flop but I'm sure Mr. Millard was happy his little novel got that far.

On a tangent, I noticed that when the people in charge of the Superman franchise actually mentioned the location of Superman's home town of Smallville, a common choice was in Kansas.

As you impatiently tap your feet and pointedly look at your smart phone for the time, I'm sure you want to know why is Kansas a meteorite magnet. Well, actually it isn't. As mentioned above, the spectre of data bias raises its ugly head; this time in the form of Sampling Bias.

You see, there is an unspoken assumption that meteorites are equally easy to find in all of the continental states. But as it turns out, that just ain't so. For geological reasons Kansas has very few rocks of any kind close to the surface. In other states people spotting a meteorite would react "Oh, just another of the zillions of rocks lying around, how boring". In Kansas however, the reaction is "What the heck is that??!?". So Kansas meteors are immediately identified as something weird to take to your local geologist and find out if it is worth any money.

In addition most of the land in Kansas is heavily cultivated with crops. This means there is a distinct lack of [a] trees, [b] buildings, and [c] paved roads. All of which hinder the spotting of meteorites lying on the ground. Kansas is also relatively arid, so meteorites will disintegrate more slowly than in more damp states.

Therefore sampling bias does reveal that there is nothing special about Kansas, meteorite-wise. But unlike Abraham Wald's bullet-hole maps, it does not diminish Kansas' value as a springboard for authors. Many readers will only find the initial reports and still be impressed. And if the author wants to cover all the bases they only have to suggest that the sampling bias stories are part of a sinister government cover-up.


Armor is not a common topic of discussion in space warfare circles, but it deserves at least a cursory look.  There are numerous options available, ranging from those introduced decades ago to advanced materials that only exist in the lab.  Armoring a spacecraft is made more difficult by the fact that it is expected to be faced with a variety of attacks, each of which has a radically different damage mechanism.  

Various metallic armors are likely to be the cheapest option available, but suffer from a very high mass to strength ratio and are not terribly effective against lasers.  They are only likely to be used in situations where nothing else is available, probably when the combat vessel in question is being improvised.  In more advanced armor packages, metallic materials most likely to be used only in the Whipple shield, as they break up the projectile more effectively than do various other materials.  This reduces the risk of penetration of the main armor, but increases the risk of spalling.  This effect has been observed in hypervelocity tests on earth, but the effect of metallic bumpers at velocities above 10 km/s is unknown.

The next step up is some form of ceramic armor.  The most likely candidate for this is silicon carbide.  The high refractory properties of this material make it ideal for anti-laser use, and the general toughness and low density make it useful against kinetics.  Luke Campbell has suggested layering SiC with tungsten to gain a sort of composite armor which would be effective both against lasers and kinetics thanks to high refractory properties and the alternating high density-low density layers give a form of stand-off (Whipple) armor.  Alloying the tungsten with steel would prevent it from cracking after the first hit, and adding iron and boron-doped polyethylene would protect against neutrons.  This material is probably the limit in space armor available today.

The next step up is some form of advanced carbon material, ranging from diamondoid armor to nanotubes.  These materials are even more refractory then silicon carbide, and are also stronger.  The biggest problem is that we are unable to produce industrial quantities of these materials today, and the exact properties achievable are not known with any degree of accuracy.  Carbon armors could benefit from the use of a liquid film coating, as described in Section 7.

The shape of the armor is just as important as the material itself.  As mentioned in the section on kinetics, a Whipple shield greatly enhances the effectiveness of armor in protecting against kinetics.  (In fact, at typical velocities for orbital debris impacts, Whipple shields provide protection equal to a monolithic barrier 10 to 20 times as massive (Orbital Debris: A Technical Assessment p. 124). How effective a Whipple shield is against lasers depends on the type of laser.  For x-ray lasers or particle beams, the stand-off will not really help, but they will not hurt either. For high powered continuous lasers in the IR/Visible/UV, it will either not help much but not hurt (when the primary damage mechanism is by melting or evaporation) or it may actually be less effective than a single monolithic slab (for impulsive momentum transfer blasting out a section of armor, where thicker armor provides more support to resist deformation of the slab). For high energy pulsed lasers in the near IR/Visible/UV, the Whipple shield will be as good or better than a monolithic slab - as good for a rapid burst of pulses used to drill deeply, better for a single very high energy pulse that blasts a crater from the surface. (The preceding material was posted by Luke Campbell on Rocketpunk Manifesto (Space Warfare VI), with minor editing by the author).

The standoff required to allow disruption of the projectile will depend on the velocity and mass of the projectile.  For example, a 1 gram projectile at 32 km/s (conveniently giving it 1 MJ of energy) will probably require standoffs on the order of 10 to 20 centimeters to ensure minimum damage.  More detail on the specific mechanics of kinetic impacts can be found in Section 8.  

Inside the main armor is likely to be an anti-spall liner, made of a material such as Kevlar.  This serves to catch fragments thrown off by the armor, limiting the damage they can do.  This is particularly important in spacecraft, as spalling is relatively more likely at higher impact velocities.

Damage and survivability are by far the most difficult aspects of modeling space warfare.  Unlike maneuver or weapons fire, there are no simple formulas to implement.  What formulas there are are complicated and difficult to find.  For armor penetration by kinetic weapons, Luke Campbell, in the Rocketpunk Manifesto thread Home Away From Home, recommended a version of the Tate formula for crater depth:

where d is the depth of penetration, L is the length of the projectile, V is the initial velocity of the projectile, ρp is the density of the projectile, ρt is the density of the target, and Yp is the compressive yield strength of the projectile (ex is e raised to power of x or natural antilog of x, B and μ are calculated by second and third equations). This result isn't too bad at moderate velocities and approaches the correct hydrodynamic limit at high velocities.

This analysis does not take into account additional penetration which might come from the kinetic explosion of the projectile. The size of the resulting crater can be estimated if you know a value Rt, which is the energy per volume to excavate a crater in the target (note that this has units of pressure). This value is approximately three times the compressive yield of the target. So if you know the kinetic energy of the impactor you can determine the volume of the crater, and the depth of the crater will be somewhere close to the cube root of the volume. If this is larger than the penetration depth, use the crater penetration. Otherwise, use the penetration depth.

Later in the same thread, Luke described the mechanism of evaluating laser penetration:

For CW (continuous wave, as opposed to pulsed) lasers at high enough intensity, the primary means of penetration is melt-erosion of the solid (the laser impinges on the solid, melts a thin layer of the surface, and evaporates enough of the melt to produce a hypersonic jet of escaping vapor), in which nearly all of the energy of the laser goes into melting the solid and only a small amount goes into evaporating and accelerating the jet. This doesn't work on carbon (like the sooper carbon nano-stuff armor) because carbon sublimates rather than melts, so you will need to input the heat of evaporation rather than the heat of fusion. This method starts to break down once you don't have enough intensity to develop a high pressure vapor jet, in which case you need to input enough energy to vaporize a hole rather than just melt it.

For pulsed lasers, the energy per volume to excavate a crater is about three times the compressive yield strength of the material (there's a formula involving not just the compressive yield strength but the shear modulus as well, but the result depends only logarithmically on the ratio of the shear modulus to the compressive yield, so it can mostly be ignored and replaced by three times the compressive yield). The diameter of a crater will thus be about the cube root of the pulse energy. Once you have figured the number of pulses and the energy per pulse, it becomes a simple matter to determine both the depth and radius of the hole that is excavated. This method breaks down when the spot size of the laser pulse is larger than the crater it would produce if focused to a spot. When this threshold is passed, it quickly reduces to the CW laser values.

He also elaborated on the effects of penetration, and the energy consumed therein:

The calculations for laser penetration immediately give the amount of energy lost from the beam. For CW beams, it takes an amount of energy equal to the energy it takes to melt all the volume of material in the beam path. Any energy remaining is delivered to what's on the other side of the barrier.

For pulsed beams, you know how deep each pulse excavates. So once you have delivered enough pulses to chew through the entire thickness, the rest of the pulses get through.

For long rod impactors, it is a bit more complicated. For the depth penetrated when the velocity of the end of the rod is v, my previous formula (valid for v=0) becomes

where d is the depth of penetration, L is the length of the projectile, V is the initial velocity of the projectile, ρp is the density of the projectile, ρt is the density of the target, and Yp is the compressive yield strength of the projectile (ex is e raised to power of x or natural antilog of x, B and μ are calculated by second and third equations). This can be solved for v if you know d

However, it is also important to take into account that the front end of the rod has eroded as it passed through the barrier. The amount of rod remaining at velocity v is

(exp[x] is the same as ex) With this, you can find how much bang the rod does to stuff behind the barrier using the original formula.

For impactors that penetrate the barrier simply by virtue of exploding to gouge a crater, there is no penetration of the impactor through the barrier. You will still get fragments of the barrier forming high velocity projectiles, a blast wave and overpressure, and possibly a jet of hypervelocity vapor and re-condensed grit.

For simpler analysis, documents from Los Alamos National Laboratories about ASAT and spacecraft defense indicate that a good first approximation is that an effective shield needs to have the same areal density as the projectile it is going to stop.  In other words, the mass per unit area of the shield for spherical projectiles needs to be equal to the diameter of the projectile times its density.  This is in line with the Newtonian impact approximation, and it is likely that at orbital velocities, this is a reasonable assumption.  Unfortunately, the paper that is cited to justify this has proven inaccessible, so it has been impossible to determine the limits of this approximation.

Once an attack has penetrated the armor, it must still damage the target to be effective.  There are numerous different damage mechanisms that could be involved, depending on the weapon and the target.  For kinetic weapons, the primary damage mechanism is likely to be simple physical impact, with shock close behind.  As discussed in Section 7, kinetic impacts at the velocities expected in space warfare will tend to produce clouds of fragments or plasma, endangering a greater area around the impact, but also limiting the depth of penetration.  Spalling from the inside of unpenetrated bulkheads is another potential source of fragments.  A warship designer might place compartments that are not combat-critical next to the skin of the vessel, with a reinforced bulkhead inboard of them to protect against just such fragments.  However, the only such compartments that are likely to exist are the crew quarters, which are obviously absent from drones.  Another way to reduce spalling is to laminate the outer plates, bonding several plates of different materials together.  A ductile inner material will help to keep the fragments of the outer material together, and might also reflect some of the shock due to impedance mismatch between the two materials.  

The National Academy of Sciences report Orbital Debris: A Technical Assessment describes the effects of an impact at kinetic weapon speeds (p. 93):

“The effects of the impact of a 1-cm-diameter aluminum sphere on a 0.5-cm-thick aluminum spacecraft wall at 10 km/s are illustrative of the damage that can be caused by debris impact. Such a collision would fully melt and partially vaporize the impactor and would create a perforation in the spacecraft wall with an outer diameter of approximately 3.3 cm and an inner hole diameter of approximately 2.7 cm. The peak impact stress caused by the expanding liquid projectile and wall material on a component located 2.5 cm behind the perforated wall would be approximately 450 kbar, well above the yield strength for most typical spacecraft structural materials. The peak impact stress decreases with the cube of the distance from the wall, so that the loading on a component 15 cm behind the wall would only be about 2 kbar (still close to the yield strength of commonly used aluminum alloys).

Even small impactors can cause component failures. For example, a particle as small as 0.75-mm diameter impacting 0.5-cm-thick aluminum housing covering a component such as a solar array pointing/steering motor could result in the spallation of the internal housing wall, potentially damaging or jamming the motor. At collision velocities of 10 km/s, particles as small as 1 mm in diameter can perforate a radiator with thin-walled heat pipes (such as those used for space nuclear reactor cooling).”

Another source on the effects of kinetic impacts on spacecraft is NASA SP-8042, Space Vehicle Design Criteria – Meteoroid Damage Assessment.  Besides describing the use of laminated materials to reduce spalling mentioned above, it also mentions the development of a self-sealing system built for double-walled manned habitats.  The outer wall could be satisfactorily sealed, but the inner wall could not, and the added mass amounted to 4.88 kg/m2 to 7.32 kg/m2.  It was concluded that manned patching was a better alternative to the system.  It also notes that titanium tanks containing gaseous or liquid oxygen almost always begin to burn when subject to hypervelocity impact, and sometimes explode.  This does not happen with other oxidizers tested (which probably means things like N2O4 and Nitric Acid, not ClF5), and most other tank materials (besides magnesium and low-alloy steel) do not exhibit this property.

The paper Designing Dual-Plate Meteoroid Shields, originally published as part of the analysis done for the Giotto probe, which flew by Halley’s Comet and was subjected to some of the highest-velocity debris impacts of any spacecraft, as high as 70 km/s, making it very useful for space warfare purposes.  It suggests (p. 12), that the shield is most effective when the sectional density of the barrier is 0.25 times the sectional density of the impacting object.  This indicates that smaller, unitary projectiles can be protected against, but long-rod penetrators will be prohibitive to armor against.

Another interesting fact from this paper is how the shockwave-cancellation works.  If the shield is not thick enough, the shockwave reaches the back of the shield, and is reflected back.  The resulting releasewave overtakes the shockwave in the projectile, and dramatically reduces its strength.  A similar situation occurs when the shield is too thin.  In that case, the shockwave reflects off the back of the projectile, and the releasewave cancels out, ensuring that parts of the shield remain mostly unshocked.  This may result in fragments of the shield being thrown inward as liquid drops or solid fragments.  These fragments could pose a serious threat to the ship being protected.  In an aluminum barrier, the fastest possible solid fragments will be limited to no more than 5 km/s, as physics prevents particle velocities above twice the average particle velocity downstream of the shockwave, and a shockwave with a particle velocity of 2.5 km/s will just about melt aluminum.  Other materials have higher potential solid fragment velocities.  Iron should be around 5.6 km/s.  If the shockwave is fast enough to melt the armor, then the impact of the fragments is somewhat dispersed.  In an ideal case, the armor vaporizes entirely, dispersing and significantly reducing the impact on the main armor.  Carbon is particularly bad about this, as it usually goes straight from solid to gaseous states.  In theory, graphite particles could be ejected as fast as 12 km/s, while nanotube particles might reach 16 km/s.   However, vaporization starts at much lower energy levels, as far down as 6 km/s ejection velocity for graphite and a calculated value of 1.7 km/s for nanotubes.  The numbers for nanotubes are rather suspect, as the author is not an expert on shockwave physics.

The paper also includes some data on analysis of these impacts based on the theory that they behave essentially as explosions, creating a bubble of debris from both the barrier and the projectile.  This does not hold true if large portions of the projectile survive, or if the projectile largely fails to penetrate, but does hold true regardless of the state of the shocked material (solid, liquid, gas, or even plasma).  In this case, the velocity of the CG (center of gravity) of the debris cloud is described as

where Vc is the velocity of the CG, Vp is the velocity of the projectile, K is the ratio of projectile mass per unit area to barrier mass per unit area, and G is the ratio of the diameter on the barrier that produces material for the debris cloud to the projectile diameter.  G is usually close to 1.  This equation is obviously derived from conservation of momentum, with the assumption that there is no momentum transfer into the main mass of the barrier.  This transfer would be accomplished through shear stress on the material, the magnitude of which is limited by material strength.

As an aside, there are some who believe that the primary kill mechanism for kinetics will be jellying of the crew due to momentum transfer from the projectiles.  This is incorrect for several reasons.  First, unrealistically large projectiles are generally assumed.  Second, the collision is assumed to be perfectly elastic, which requires the projectile to transfer its entire momentum to the target, while it is obvious that a spacecraft structure stressed for maybe a few to a few tens of Gs will not be able to stop a projectile that would require an acceleration of thousands of Gs.

The paper continues, giving an equation for energy loss Er during the impact

where Ep is the kinetic energy of the projectile before impact and K and G are as above.  This energy is what powers all of the phenomena associated with the impact that are not part of the debris cloud.  This includes (and is in fact equal to) the energy contained in debris that is thrown out of the front of the armor, thermal and kinetic energy transferred to the armor, energy radiated by the shocked material, heating of the debris and kinetic energy of the debris cloud associated with expansion away from the CG.  The paper suggests that the last will dominate the situation, with all other factors proving minor relative to it, and introduces a value, Q, that is the ratio of expansion KE (kinetic energy) to total Er.  It should be near, but slightly below, 1.  This value allows the velocity of the expansion, Ve, to be calculated

from which we can calculate the angle θ that the impact will subtend on the next layer of armor

(sin-1(x) is arc sine of x ) as well as the maximum and minimum velocities of the fragment cloud Vmax and Vmin

Note that Vmax can be greater than Vm.  In fact, Vmax reaches its greatest value of 1.2066 Vm when K = 0.191, and G and Q are 1.

All of this data allows us to work out the mass and momentum per unit area at impact (mc and ρc) for a given standoff distance X and a given projectile mass Mm.

ρc is called the standard impact parameter.

From these equations, we can learn about the physics of the impact on the rear barrier.  The peak kinetic energy of the cloud per unit surface area when it strikes the rear barrier is

and if the debris cloud is assumed to have thickness B times its radius (B<1), the time required for impingement will be

and the kinetic power associated with stopping the cloud fragments is then ec/tc.  This is important because data from the Halley’s shield paper (the only study the author has found of very high velocity impacts) suggests that the ‘stagnation plate’ (their term for the rear barrier) will be present in about equal mass with the debris when it first comes to rest.  This allows calculation of the temperature that the mix of debris and barrier will initially reach (or, more accurately, the temperature change caused by the instant when the impacting mass stagnates)

where cp is the average specific heat per unit mass (at constant pressure, if relevant) over the range of interest.  More accurate values may require use of tables, but basic thermodynamics are not within the scope of this paper.  The bottom line is that the peak stagnation temperatures the Halley mission calculated (for 70 km/s with all-aluminum components) was on the order of 500,000 K.

The above equations (from Designing dual-plate meteoroid shields) are valid for ranges of K between 0.1 and 5-10.

The paper also suggests that for impactor velocities around 70 km/s, the ejecta begins to behave in some ways like a particle beam instead of a simple cloud of debris.  The debris from a metallic plate at these velocities will be capable of penetrating distances on the order of 250 nm into metallic targets.  

The obvious solution to the fragment problem is to create a multi-layered armor package, intended to break up the projectile in stages and catch any resulting fragments.  The layers might be made of different, alternating materials, probably some form of metal (either aluminum or steel) and carbon or some other non-metallic armor.  This system would be quite effective, but also require a large amount of volume, quite a lot like the torpedo defense systems of battleships.  This would limit use to the front of spacecraft, instead of use of all-around armor.  The parallels between likely armor designs for spacecraft and torpedo defense systems are interesting, but issues of hydraulic ram make it unlikely that liquid layers are a worthwhile addition to anti-kinetic armor packages.

Shocks produced by kinetic impact or pulsed lasers can propagate through the vessel’s structure and cause damage to systems a significant distance away from the impact.  Spalling is the most obvious effect of shocks, but the damage caused by shock to critical systems could be even more damaging than the direct impact of a projectile.  This is particularly true given the number of high-precision systems on a space warcraft.  Optical trains are likely to be particularly vulnerable, but even the most basic of systems, like circuit breakers, could be affected.  Nearby nuclear detonations are also likely to produce shock effects due to ablation of the spacecraft’s outer skin and armor.

A related phenomena important to spacecraft is the transmission of shock through liquids, known as hydraulic ram.  This occurs when a projectile enters a tank of fluid and creates hydraulic loads on the walls of the tank.  The initial shock loads are often enough to rupture the walls of the tank, and the oscillation of the fluid after the impact pumps more fluid out of the holes.  Lasers might cause a similar effect, depending on the nature of the laser, although information on this is lacking.  This is obviously a major threat to spacecraft with large remass tanks, and the pressure could propagate through piping as well.

Blast effects in pressurized spacecraft cannot be totally ignored.  The introduction of large amounts of energy at a single point will obviously produce an effect similar to an explosion in air.  This is true for both laser and kinetic impacts, and is often a significant damage mechanism for modern fragmentation warheads.  The potential for this type of damage would be an obvious reason to depressurize the spacecraft before entering combat, leaving the crew to fight in space suits.  Fire is another reason to do so, as air escapes fairly slowly through a small hole, and a fire could do significant damage before it ran out of oxygen.  A third reason is another damage mechanism from modern fragmentation warheads, ignition of the fragments after impact with the target.  This is most notable when dealing with fragments (either of the projectile or of the target) made of aluminum, although titanium would probably exhibit similar behavior.  Zirconium and hafnium are used to enhance this effect, and there has been research into coruscatives, which contain their own oxidizer and function in a similar manner.  Only very limited information on this concept exists.  However, the ease with which this effect can be countered by depressurizing the compartment makes use of those materials dubious.

The damage mechanisms produced by lasers will vary based on the intensity of the beam.  For a low-intensity laser, melting is the primary means of damage, and the products involved are fairly slow-moving and not terribly dangerous to those who are not directly in its path.  A higher-intensity laser would instead convert the target to plasma, which would either expand back out of the hole or be pushed through on penetration.  Quite obviously, this is unhealthy to be around.  Radiant heating from the area of impact is a minor concern, compared to all of the other effects.  Even higher beam intensities would add shock damage to this, as described above.  Even if the laser does not deposit enough energy to melt the target, it could still weaken the structure, causing it to deform or buckle.

by Byron Coffey (2016)

We have seen designs for long ranged particle beams and powerful lasers. Could they be the end-all, be-all of space warfare? Not if we fend off their destructive power with actively cooled armor.

Let's have a look at the different cooling solutions, from high pressure gas to liquid metal, and evaluate their relative effectiveness.


The traditional solution for defeating directed energy weapons such as particle beams and lasers is to use solid plates of armor. The armor material would ideally have a high heat capacity so that it doesn't heat up too quickly, and an excellent melting or boiling energy.

Graphite excels in both. It is clearly superior when compared to steel or aluminium. 

We have written about the effectiveness of graphite when facing laser beams, and how we can use different techniques such as sloping, rotation and reflective surfaces to further increase the energy required to remove armor.

However, certain techniques described in Lasers, Mirrors and Star Pyramids have limitations. Reflectivity in particular cannot be counted upon in all situations. While we might have broadband dielectric mirrors that effectively reflect a vast range of wavelengths, an enemy will eventually field lasers specifically meant to defeat them.

They might select polarizations against which the mirror is less effective, they might use beams of wavelengths too short to reflect (usually below 200nm), or even replace lasers with particle beam weapons that ignore surface features entirely.

What can we do against such mirror-defeating techniques? Can we increase the effectiveness of armor even further than what was calculated in Lasers, Mirrors and Star pyramids?

Maximizing that energy value means you can get by with less armor and have more mass dedicated to winning tools such as propulsion or ammunition.


We can start with a reference to compare everything else to.

Passive armor is simple in design and construction. It is made to handle as much heat as possible and prevent it from leaking into the spaceship.

A good example, as mentioned above, is graphite. It first needs to be heated to about 3,500K before it starts being degraded. At 4,000K, it turns into a gas. Between its heat capacity and vaporization energy, it takes roughly 60 MJ/kg to vaporize. A stronger carbon-based material would take an equivalent amount of energy while also being physically strong.

We can therefore expect graphite to handle a laser intensity of 8.5 MegaWatts per square meter for extended periods of time. This is a value calculated from the Stefan Boltzmann blackbody radiation equation, as it is the intensity required for a black surface to sit at an equilibrium temperature of 3,500K.

For the laser beam to start digging into the carbon at an appreciable rate, it must first overcome a 14.5 MW/m^2 threshold so that the temperature rises to 4,000K, then it must add 60 MJ for each kilogram of carbon to be vaporized.

We mentioned active cooling. Being able to remove 11 MW/m^2 using a coolant looping through the armor material significantly raises the damage threshold. In this case, it is increased to 19.5 and 25.5 MW/m^2 respectively.

Two other techniques covered in Lasers, Mirrors and Star Pyramids are valid in all situations: sloping and rotating.

We can add a good compound slope to the carbon surface: 80 degrees vertical and 67.5 degrees horizontal. This spreads the laser beam over a surface area about 15 times greater. Consequently, the laser now needs to reach an intensity of at least 292.5 MW/m^2.

Rotation spreads the beam further. We could have a situation where the beam diameter is 12.56 times smaller than the armor’s circumference, and the average beam intensity is reduced by the same factor.

All in all, carbon materials can survive 3.67 GW/m^2 for long periods of time or 4.8 GW/m^2 while ablating.

If we have a 100 MW laser with a wavelength of 450nm, being focused by a 4 meter wide mirror, we would be able to damage a simple layer of carbon at a distance of 27,025 km. Using all the techniques just mentioned, the range is shortened to 1,300 km.


At very long ranges, space battles are boring with little room for tactical decisions.

We cover this issue in The Laser Problem: any moderately powerful laser can render maneuvers pointless. Warships cannot escape beam weapons. Even at distances where light lag becomes significant, the beam can be divided to cover a wider area while maintaining its destructive intensity.

Shorter ranges allow for acceleration to matter more. Other weapon systems that do not have the supreme range of lasers and particle beams can come into play, such as missiles and kinetics. Angular separation starts to matter, allowing for flanking attacks against warships forced to be more well-rounded to fend off multiple weapons from different directions.

It is also important from a narrative and visual perspective. Actions taken have a more immediate effect. Events happen quicker and the danger or relief is greater. It is easier to depict battles and the faster pace will be more agreeable to viewers.


The 1,300 km figure given in the previous example is situational.

It relies on the beam coming in from an ideal angle, which is straight down the edges of the star pyramid shape. If instead it came from a flanking angle, perhaps 80 degrees from the front, then the vertical slope of 80 degrees is completely negated. The benefit from sloping falls from a factor 15 to just 2.6, which makes the laser effective from a distance (15/2.6)^0.5: 2.4 times greater.

In other words, a warship that sustain laser fire from the front at 1,300 km is vulnerable to flanking attacks out to 3,122 km.

The benefit from rotation also varies with distance, as it changes the size of the beam’s spot relative to the target’s circumference.

     Intensity reduction by rotation = Beam spot radius / (3.142 * Armor radius)

The reduction is a dimensionless number
Beam spot radius and armor radius are in meters

The beam spot radius decreases linearly as the distance decreases.

If the warship is being shot at a distance two times closer, then the benefit from rotation is twice as great. This inverse relationship means significant gains from rotation at close distances and small gains at long distances.

Distances, mirror sizes, beam wavelength, warship radii and even spot shapes are all variable, so we are unlikely to ever have one single number to describe the effect of rotation.

What technique instead can we rely upon in all situations? Can we increase armor effectiveness without the enemy having to sit in certain positions and relative angles?

Active cooling is the solution.

We looked at figures of 11 MW/m^2 being removed by flowing water over hot surfaces. Further research shows fusion diverters and rocket engine chambers surviving 100 MW/m^2.

We can go further, to absorb much more power.

For the sake of comparison, we will be using a standardized model where a flat plate of armor sits under the glare of a laser beam. It conducts heat through a heat exchanging surface to a coolant flow underneath. The armor is 1cm thick and the coolant flow channel 1cm wide. We will calculate the power required to pump coolant to a certain velocity, and won’t allow velocities that cause turbulence in liquid or supersonic compression in gases. The heat is radiated away using simple 1mm thick carbon fiber radiators.

There are four factors to consider for a proper comparison. The first is the heat that can be removed per square meter. The second is the pumping power requirement. The third is battle damage resistance, which is not as straightforward. The last is the radiator surface area required to handle the heat.

We won’t be working out the effect of thermal conductivity as in most cases it is not the limiting factor.


Gases are an interesting coolant as they have no upper temperature limit. If the armor material is carbon and it can withstand a 3500K temperature, then we can select any gas and heat it up to that temperature in the heat exchanger.

We are looking for the gas with the highest heat capacity. High heat capacity means less gas needs to flow through the heat exchanger to pick up the heat and carry it away. Less gas means reduced pumping power. We also want a gas that doesn’t condense at lower temperatures.

Hydrogen is the best. However, it is reactive. It will chemically reduce the carbon and degrade it. Helium is a second-best alternative that is chemically inert. We want it entering the armor at 500K, heating up to 3,500K and exiting to be cooled in a radiator back down to 500K. If it does not reach the desired temperature in one pass through the heat exchanger, it can be recirculated through at the cost of doubling the pumping power.

The speed of sound in helium at 500K is 1,315m/s. At 3,500K, it is 3,480m/s. We are limited to pumping below the smaller figure, so let’s give it Mach 0.9 (1183m/). This means we can push 1.13 kg/s under each square meter of armor at 1 bar of pressure, and 28.25 kg/s at 25 bar.

Helium absorbs 5.19 kJ/kg/K. Over a temperature rise of 500 to 3,500K, 28.25 kg/s will absorb 439.7 MW/m^2.

Pumping power requirement depends on the pressure drop across the heat exchanger. It will be the highest of all designs considered in this post.

The radiators happily handle the heat using 4,994 m^2 and 5 tons of mass.

Gaseous cooling has the advantage that it can increase its performance with increased pumping pressure, and can maintain some degree of functionality while sustaining battle damage. A sudden increase in temperature is not so dangerous as pressure build-up can be vented into space.

However, they have the highest pumping power requirements. While we are ignoring thermal conductivity in our calculations, gaseous cooling using helium has the lowest thermal conductivity of all the fluids we will be considering. This imposes certain design restrictions on the heat exchanger interface with the gas, which is trouble when we want the gas to be flowing through it at near Mach speeds.


Helium has high heat capacity but low density. We need a lot of pumping power to push enough volume through the heat exchanger to draw a decent amount of heat away.

The gases with the highest densities are metal vapours. The same volume brings a lot more mass throughput and therefore cooling capacity.

We want a metal that is dense but boils easily. Mercury is ideal. It boils at 630K, so we’ll set the minimum temperature to 750K to prevent it condensing back into a liquid. As before, we heat it up to 3,500K.

The average density of a mercury vapor at 25 bar, between 750 and 3,500K, is 48.7 kg/m^3. It would have a heat capacity of 104 J/kg/K and a speed of sound of 227 m/s at 750K. Putting all this together, we expect to push 99.5 kg/s through the 1cm wide heat exchanger and extract 28.5 MW/m^2. This is a much lower performance than with helium.

Pumping requirements are a significantly lower. It is estimated that mercury vapours remove 5 kW of heat from the armor for every watt of pumping power, which is about 20% better than for helium. Only 1,563 m^2 of radiators weighing 1.6 tons are needed to handle the heat.

The reduced pumping requirements means that you can use many smaller pumps to push the mercury gas through heat exchangers, which helps with redundancy. However, a sudden pressure drop from holes created in the armor are likely to cause the mercury to suddenly expand, cool and revert to its liquid form. The droplets would quickly block gas flow through narrow channels in the heat exchanger.

Another difficulty is that mercury can solidify completely behind unheated or damaged armor. Re-establishing a coolant flow is impossible unless the mercury is boiled again to clear the heat exchanger’s channels. Armor would have to ‘go hot’ before battles and prevented from cooling down too much when not under beam attack.


The traditional cooling liquid is water. It is much denser than a gas, has good thermal conductivity and very high heat capacity.

Water’s temperature range is its main limit. If we use it as a liquid, we impose that we use very low temperatures to reject heat from the radiators. Consequently, huge radiating areas would be needed. If we use it as a gas at the same temperatures as helium or metal vapours, it will corrode the heat exchanger and chemically attack everything it touches.

Instead, we use a phase change design. High pressures allow water to stay liquid beyond the standard 373K boiling point. It is then heated into steam, up to the maximum temperature the heat exchanger can handle without corrosion. After passing through the radiators, it condenses back into water.

At 25 bar, we can retain liquid water at 480K. That will be our minimum temperature. It has a density of 1,197 kg/m^3. We find steam turbine coatings such as chromium steel able to resist 873K steam for thousands of hours, or chromium-niobium alloys at 923K. That will be our maximum temperature. Steam has an average heat capacity of 2.56 kJ/kg/K between 480 and 920K.

The phase change from liquid to gas also absorbs energy. For water, this is a whopping 1,840 kJ/kg when starting from 480K.

Adding the heat absorbed by the phase change and then rise in temperature, we obtain a total of 2,966 kJ/kg.

We cannot allow turbulent flow through the heat exchanger, as this drastically decreases the heat transfer rate. Based on a fluid’s Reynolds number and viscosity, we can estimate the maximum velocity before the start of turbulent flow. In this case, it can only be 24 m/s when passing through a 1cm gap.

With that flow rate, we get as much as 286.8 kg/s passing through the heat exchanger removing 850 MW/m^2.

This is impressive performance. The pumping power requirements are drastically lower than any gas (on the order of 2 kW of heat removed per watt of pumping power). Water has the advantage of gaining most of its heat removing capacity through its phase change when cooling armor from as low as 373K. Increasing the temperature of the armor and therefore of the heat exchanger only improves performance.

Another advantage is that it is likely to serve a second role as propellant on spacecraft. Electric, nuclear and solar rockets can all use water. The consequences are that the coolant needed for the armor’s active cooling does not have to be dead weight, and that after being heated into steam, it can be pushed through a nozzle instead of passed into radiators. During battle, if radiators are hidden or destroyed, the armor can still be kept cool by using water as an open-cycle coolant.

There are downsides though. Water increases in volume over a thousand-fold between liquid and gaseous states. Designing a phase change heat exchanger where liquid enters one side and gas exits the other is tricky to do, and is unlikely to work after receiving battle damage. In fact, creating a hole in the heat exchanger would release pressure and allow the water entering as a liquid to suddenly boil and practically explode in the pipes.

Just like mercury, water can freeze into ice when not heated. Damaged pipes can see themselves blocked by this ice, cutting further cooling. Thankfully, the phase change from liquid into solid also takes a lot of energy, so there is usually plenty of time to re-heat the water and get it flowing again. 

The biggest disadvantage is the maximum temperature restriction on the armor and heat exchanger. Above 920K, the thin layer of water or steam in contact with the heat exchanger starts corroding the protective layer quickly.

If the armor is at 3,000K, it will be superheating a small quantity of steam to a vigorous oxygen-hydrogen plasma, even if the average temperatures within the heat exchanger as within bounds. Therefore, if the laser intensity overwhelms the cooling capacity and the armor starts heating up to higher temperatures, we will start to see degraded heat exchangers and a decrease in cooling capacity. This is a self-reinforcing cycle that destroys the armor.


Liquid coolants have much reduced pumping power requirements. Instead of water with its restrictive temperature limitations, we might select a coolant that can handle much temperatures.

Eutetics are mixtures of two or more elements that have a lower melting point than either pure element. A prime example is sodium and potassium (NaK) used as ‘molten salt’ coolant in nuclear reactors or solar energy storage facilities. Sodium and potassium melt at 371K and 337K respectively, but their eutectic mixture melts at just 260K.

We will be using Galinstan. It is a mixture of gallium, indium and tin. It melts at 254K and boils at 1,573K. With a density of 6,440 kg/m^3, a heat capacity of 296 J/kg/K and a laminar flow velocity of 85m/s, we find that we can remove 1,738MW/m^2 while radiating away heat at 500K.

This incredibly better performance is possible due to the fluid’s high density and high viscosity. Pumping requirements will be significant, and you’d need 490,606 m^2 or 491 tons of radiators for every square meter of armor receiving this intensity, but it is worthwhile when it can reduce ranges by so much.

A note on the radiator requirements: this number is not to be used simply as it is presented. 490,606m^2 are only needed if the enemy beam covers an entire square meter with 1,738 MW of power. It is much more likely that a much less powerful beam, for example 100 MW, is focusing its power onto a small spot, perhaps 27cm wide. This gives the same intensity (1,738 MW/m^2) but the total heat that must be handled is only 100 MW. The radiator area needed to handle the heat is just 28,218 m^2.

One advantage of Galinstan is that it remains liquid at very low temperatures, so there is a much reduced risk of it solidifying and blocking cooling channels. Another is that as an electrically conductive mix of metals, we can use electromagnetic pumping that can end up being more efficient and more damage resistant than conventional pumps.

The main disadvantage is that lasers or particle beams can strike multiple spots along an armor surface without warning, so the coolant flow much be able to compensate for any heating across its entire surface. In other words, the pumps must consume large amounts of power to keep Galinstan flowing across the entire armor surface!

Another challenge is when beam intensity overwhelms cooling capacity. 1,573K is a decently high boiling point, but it is still lower than the 3,500K that carbon materials can handle. A hot spot can create vapor bubbles in the Galinstan flow that could cause destructive cavitation or blocked flow in small channels of a hat exchanger.


There are liquids that can handle much higher temperatures without boiling. Liquid metals have the highest boiling points.

To be a good coolant, we could use a metal with a fairly low melting point, a very high boiling point and the best heat capacity possible. There are many good choices, including plutonium, but we will look at these four in particular: tin, indium, aluminium and cerium.

Indium has a melting point of 430K and a boiling point of 2,345K. It has a density of 7,020 kg/m^3 and a heat capacity of 233 J/kg/K. We work out that 2,944 MW/m^2 can be removed between 500K and 2,300K.

Tin has a melting point of 505K and a boiling point of 2,875K. It has a density of 6,990 kg/m^3 and a heat capacity of 228 J/kg/K. We work out that 3,666MW/m^2 can be removed between 550 and 2,850K.

Aluminium has a melting point of 934K and a boiling point of 2,792K. It has a density of 2,375 kg/m^3 and a heat capacity of 896 J/kg/K. We work out that 3,767 MW/m^2 can be removed between 980 and 2,750K.

Cerium has a melting point of 1,071K and a boiling point of 3,697K. It has a density of 6,550 kg/m^3 and a heat capacity of 192.4 J/kg/K. We work out that 2,999 MW/m^2 can be removed between 1,120 and 3,500K.

For all of the calculations, we limited the flow velocity to 100m/s, despite maximum laminar flow velocities reaching double and more. This gives a more plausible 1m^3 per second volumetric flow rate.

The performance of liquid metal cooled armor far exceeds that of other cooling solutions.

Indium is at the lowest risk of solidifying in the pipes, but has the highest pumping requirement and imposes the lowest temperature limit on the armor.

Aluminium provides the best performance and the lowest pumping power requirement, but it is the most reactive of the metals and so needs a protective layer in between it and the carbon armor.

Cerium, with its very high boiling point, is unlikely to ever create vapour bubbles in the heat exchanger and has the smallest radiators, but it is also at the greatest risk of solidifying inadvertently.

Tin is the overall best choice.

The danger of course is that a battle starts with the tin in its solid state. Directed energy weapons could add heat to an armor plate too quickly for the tin to melt and start flowing to draw it away. Ideally, the tin is constantly flowing at the minimum temperature, which is 550K in this case. For efficiency’s sake, it could be kept molten using waste heat from a nuclear reactor. However, pumping the metal would consume electrical power that has to be taken away from other systems.

Spaceship designers could make use of the armor layer as another radiator. It would be durable and usable even in battle. Other than the sections under laser attack, it could be rejecting up to 3.5 kW of waste heat for each square meter sitting on liquid tin. This feature could compensate for the extra heat from a nuclear reactor that needs to operate at a higher power level to feed the pumps with electricity.


Let’s work out the effective range of a beam weapon facing a carbon armor layer using all the tricks available to us: rotation, compound sloping and active cooling.

As before, we set the weapon to be a 100 MW laser of 450nm wavelength, being focused by a 4m wide mirror.

The target will be an eight-pointed star (octagram) pyramid 6m wide at the base and 17.3 meters long. This gives it a vertical slope of 80 degrees and a horizontal slope of 67.5 degrees. Each face of the octagram is 1.76m long, which means a total circumference of 28.1m.

Before combat, the armor maintains a flow of liquid tin through it. It serves as a radiator which handles about 1.26 MW of heat on its own.

During combat, 3.6 GW/m^2 can be absorbed when the liquid tin gets really hot. The maximum temperature we’ll allow is 2,800K to prevent any hotspots from boiling the tin. The heated carbon also radiates another 3.5MW/m^2, but this is a tiny contribution.

An enemy attacking the pointy end of the star pyramid would face a compound slope that spreads their beam over a surface area 15 times greater. This is a 15-fold reduction in intensity. Working it out, the armor can handle 54.2 GW/m^2 with ideal sloping. If the enemy attacked from the side, they would face only the horizontal slope that reduces intensity by 2.6-fold. The armor can only handle 9.4 GW/m^2 in that case.

We can see already that the intensities the armor can handle are much greater than in previous examples.

The laser we are considering can only produce an intensity of 9.4 GW/m^2 at distance of 812.6 km. If our warship is outnumbered, it would want to stay at least this distance away from its closest opponent.

54.2 GW/m^2 is only possible at a distance of 338.4 km. Our warship can get this close if it is confident that it can always face its pointy end to the enemy.

What about rotation?

In these scenarios, the benefits could be massive. At 338.4km, the spot radius of the laser is 2.4cm, and at 812.6km, it is 5.8cm. These are 1,170.8x and 484.5x smaller than the circumference of our armor. In ideal conditions, it is a reduction in intensity of 1,170.8x and 484.5x. A supremely confident commander could bring our warship to single-digit kilometres in front of the beam and expect to spread its power enough to never overwhelm its cooling capacity!

There are important consequences for this sort of cooling capacity and reduction beam ranges. Many depend on the specifics of the setting where space warfare takes place.

In general though, we can reasonably expect that battles will revolve around achieving a flanking position, suppressing the cooling capability of the armor or preventing electrical generation that powers the pumps. The latter two objectives can be achieved by pulsed lasers, penetrating particle beams, kinetic strikes and other weaponry that are not continuous beams.

At the very least, the threat of giant laser-equipped warships and their dampening effect on any sort of eventful space combat can be reduced or eliminated in science fiction.


A Comprehensive Theory of Composite Armor for Space Battles

The most effective armor is "composite" armor, made out of multiple layers of different materials. What should these materials be? How should they be arranged? No one really knows, including professionals who actually design real-world composite armors. These are essentially educated guesses, and owing to limitations of volume, are typically not very interesting (usually just a ceramic between two steel plates).

However, it is possible to come up with a far more elaborate theory to employ when armoring spaceships, which I have done here. This is based on some rudimentary ballistics theory, some guesswork, and community experience with the excellent simulation game "Children of a Dead Earth" ("CoaDE").

(CoaDE places a high value on physical accuracy, but do note that it is still a simplified, approximated version of the real thing, and in-particular some important effects seem to be lacking. Besides those mentioned below, it seems hard to get shattering effects from brittle materials, and aerogel inexplicably works better on the outside of Whipple shields. Both may be bugs, and may have been fixed in recent patches to the game.)

I'll start with some general tips and threat analysis, which are inseparable from actually choosing the armor type desired. Then, I'll elaborate on the armoring strategy to be used for each threat profile, along with some example armor compositions fitting it. I would be interested if CoaDE users would help me in confirming that these example armors work well in-practice by doing more-rigorous testing, as well as tweaking the layers' relative thicknesses to find some kind of optimum.

General Armor Tactics

  • Sloped armor: deflects projectiles instead of absorbing them—an incredibly important and obvious tactic. Also, increases the effective thickness by the secant of the angle. Space warships should have long, thin nosecones. The primary battle tactic is flanking, to get a better attack angle. This is best accomplished by spreading out groups during attacks, since it is impossible for defenders to face in multiple directions at once.

  • All or nothing / citadel armor: it's very heavy to armor an entire spaceship. Therefore, we can do what ocean battleships did and put almost all the armor around the most-vital systems. In space, this would be the crew compartment and reactors. Losing fuel tanks, engines, and radiators is bad, but potentially survivable.

  • Radiator armor: just don't. It makes them less-efficient (i.e., you need them to be even larger) and it tends not to work anyway. You're better off making smaller and hotter radiators, adding redundant radiators, and retracting them before battle and dumping heat into a heat-sink.

  • Empty spaces: close to weapon mounts and radiators (both being weak points in the hull and primary targets), it might be nice to put empty spaces in your ship (with thin or absent armor), under the assumption that the region will take a lot of damage no matter what, and there oughtn't to be anything else to be damaged there.

  • Spaced armor: putting small (or in some cases large) gaps between layers of armor mitigates many related ballistics effects discussed in-detail below. As a general tactic, it should be employed regularly (ubiquitously?).

Threat Enumeration

In space, we're going up against several types of attacks.

  • On the kinetic/ballistics side, we have projectiles ranging from low-mass/high-speed (a few grams at many km/s) to high-mass/low-speed (many kg at at-best a km/s) projectiles. And we have stuff in the middle (tens or hundreds of g at maybe a few km/s).

  • There are also lasers, which generally work by melting (possibly, ablative heating, especially as-applied to pulse lasers).

  • Finally, we have missiles. Standard missiles tend to either act like extremes of the kinetics (a lot of localized low-mass/high-speed for flak bombs or a single/few high-mass/low-speed for kinetic-kill vehicles). For nuclear missiles, the main damage effect is radiation flux, which is essentially an intense but brief flash. (One can also make casaba-howitzer type missiles, which are so ridiculously effective at close range, I won't discuss armoring against them, because you can't.)

Therefore, we have low-mass/high-speed to high-mass/low-speed projectiles and radiation from lasers and nukes. To effectively counter all of these, we'll build armor out of layers of materials to withstand each in turn.

There's also something of an order to which threats must be dealt with first, which we will consider:

  • When space battle begins, the combatants start at long range. Here, lasers and low-mass/high-speed projectiles are the most viable (because high-mass/low-speed projectiles can't close the distance).

  • As the battle wears on, countermeasures for deflecting these are gradually eroded, and if the conflict isn't ended by this, the ships have probably approached each other somewhat. In this intermediate range, intermediate-mass/speed projectiles and guided missiles (including nukes) become viable. Lasers and low-mass/high-speed projectiles remain viable.

  • If the combatants survive to close range, high-mass/low-speed projectiles become practical. Low-mass/high-speed projectiles and lasers become great (however, conflicts are usually ended before this, so getting here usually means both sides lacked long-range weaponry at the outset). Missiles tend to become impractical here because their low launching speeds make them especially vulnerable to point defense. Also, the danger of friendly collateral damage, especially for nukes, becomes significant.

Defense Against Lasers

Basically, you want a high specific heat and a high melting point (so that lasers take longer to heat up your armor). It's complicated whether you want low or high thermal conductivity. High is good because it spreads out the laser's energy. Low is good because the damage is localized, and the damage is limited by ablation rate. However, empirically and in-general, it seems that high conductivity just can't help enough (or it helps too much and melts the whole layer).

In CoaDE, the best material for this task (low thermal conductivity) by far is silica aerogel (i.e., the usual kind of aerogel) (although a patch may have nerfed it). In real life, its effectiveness is strongly dependent on the laser wavelength (it's transparent at most visible wavelengths), and I suspect this is not modeled by CoaDE. Happily, for higher (less-diverging and hence more-weaponizable and -probable) frequencies, aerogel seems (data is hard to find) to be opaque. CoaDE users also report that aramid fiber, basalt fiber, or alpha-titanium-aluminide are also good, while graphite aerogel and especially reinforced carbon-carbon ceramic are unexpectedly weak.

The theory is that keeping a laser trained on one spot is difficult, especially if you, for example, rotate your ship. Any long distance of filled volume (as is practical with low-density materials like aerogel) may be advantageous as well; as the ship moves, laser beams must start all over in ablating through it, since the angle of the path through the volume changes.

One might also like to consider Fresnel losses, which increase as the index of refraction (IOR) increases and angle of attack (sloped armor) increases. Aerogel seems to have an IOR close to 1, making the losses slight. Again, I suspect Fresnel losses are not modeled by CoaDE.

Defense Against High-Mass/Low-Speed and Intermediate Mass/Speed Projectiles

Heavier projectiles damage armor in a variety of ways, essentially boiling down to fracturing, deforming, plugging, and spalling. You can see pictures here:

It's unclear what material properties are desirable in resisting these, although an approximative answer would be easily sourced from a materials scientist or a thorough literature review.

Since I'm too lazy to dive deeply, I'll substitute an educated guess. It seems that higher shear modulus resists plugging, but increases fracturing. Deformation is probably countered by yield strength. Resistance to spalling seems to be modeled scientifically as ∝ √((bulk modulus)*(density)), so denser, less-compressible materials spall less. I think a higher speed of sound might help too. Therefore, a high tensile and yield strength, combined with a moderate shear modulus seems reasonable to recommend.

Such materials aren't especially rare, and the obvious approach is just a big slab of it. It's fairly easy to make a thick slab, but spalling remains a concern no matter what. The usual approach is to put a spall liner behind the slab (i.e., another layer that catches the spallation), a Whipple shield (see next section), or another slab of armor (which reduces the problem, but may itself spall). In any case, it is vital to leave a small amount of space, so that the next layer doesn't itself participate in the spalling (or other contact damage, even if the liner is of a different material).

Spall liners are designed to capture small spallation fragments of intermediate velocity. The usual approach is a thin layer of high-tensile but flexible (low Young's modulus) material. High yield/ultimate strength is also desirable. CoaDE users recommend spider silk. Reinforced carbon-carbon ceramic is another choice (with amorphous carbon an acceptable substitute in a pinch). Some also report great results with nickel-phosphorus microlattice and ultra-high-molecular-weight polyethylene (UHMWPE).

Defense Against Low-Mass/High-Speed Projectiles

For kinetics, low-mass/high-speed projectiles can sometimes simply be deflected entirely, if the armor is at a shallow-enough angle. However, the angle must be very shallow and the projectiles very light, so this cannot be wholly depended upon.

In general, low-mass/high-speed projectiles explode when they hit something. This is because at high speeds, the ratio of kinetic energy to momentum is higher; objects are more like bombs than penetrators. The standard defense is a "Whipple shield" (name comes from an astronomer), which is a thin layer of sacrificial armor that, in failing locally, triggers this explosion. The resulting material then diffuses through an empty space, eventually hitting another armor layer that, due to the spreading out, can handle the incident flux better.

The Whipple shield should be hard enough to deflect some projectiles, but also lightweight (as the Whipple shield's mass adds to the debris formed). NASA research suggests that ceramic fiber works better than aluminum, by mass, and cloth has also been used. CoaDE users recommend diamond (though this seems impractical in real life due to the possibly of shattering) and more-recently boron.

The empty space can be filled with some lightweight material to further retard the debris (a "stuffed Whipple shield"). CoaDE users report some improvement with graphite aerogel. As mentioned above, filling the large distance might also help in repelling laser attacks. In the real-world, Kevlar and Nextel fibers have been deployed.

The absorption layer behind the Whipple shield should have a high melting point, specific heat capacity, and thermal conductivity, as the debris will cause significant heating (high-energy, remember?). Thicker layers offer more mass to this cause. A higher speed of sound has also been said to reduce heating. It should also be hard and strong to block any larger fragments (of armor or a heavier projectile) that survive. CoaDE users suggest elemental boron. Any spalling occurring (from heavier projectiles, or particularly ambitious debris) on this layer can be dangerous, so amorphous carbon may be preferred (weaker, but owing to the higher speed of sound has less spalling; its decent tensile and yield strengths also make it better for hybrid armor requirements).

The usual approach puts a spall liner (catches spalling) behind this, and possibly bulk armor. However, this is increasingly wasteful of mass, and possibly unnecessary (see long-range armoring section for rationale).

Whipple shields start to become worthwhile at about 3km/s, when projectiles begin to melt on impact. At 7km/s, Whipple shields become maximally efficient. Faster, and projectiles begin to vaporize, making Whipple shields less-effective (but still far better than an equivalent mass of solid plate).

Having successive layers of Whipple shields (a "multi-shock shield") is desirable in real-life since each shock expands the cloud of debris, reduces the speed of the projectile, and heats the debris (this latter effect increases the efficiency, somehow). For equivalent mass, multi-shock shields can be up to twice as efficient, and they reduce spalling in real-world tests. In CoaDE, though, this doesn't seem to be modeled, and multi-shock shields are at best unhelpful.

You can read about real-world Whipple shield designs here:

Armor Strategy

The exact armor strategy chosen depends immensely on what your combat assumption is.

If your ship has lots of long-range lasers, low-mass/high-speed kinetics, or (to a lesser extent) missiles, this implicitly means that you expect to engage at long range. Hence, armoring against high-mass/low-speed kinetics is sortof pointless (if they try to attack with those, just dodge instead). You should instead have only strong anti-laser and hypervelocity armor.

Conversely, if your ship has high-mass/low-speed kinetics or slower missiles, you are expecting to battle at close range. The armor types that work versus these shorter-range weapons tend to fail against long-range weapons, so it is inadvisable to attempt a battle versus a long-range ship.

One can, as it turns out, make a hybrid armor which functions adequately over all engagement ranges, but this is inefficient. It only sortof makes sense on larger ships. In such a case, volumetric efficiency due to the square-cube law makes it less wasteful. Also, larger ships are inherently more-flexible, able to field a wider variety of weaponry, but also unable to be economically specialized for particular roles; as capital investments, a polity would require them to be adaptable and more-general-purpose.

We will discuss all three scenarios specifically.

Long-Range Armor

The main threats, again, are lasers and low-mass/high-speed kinetics.

Here, the main strategy will be a combined Whipple shield / laser armor approach. Note that, since a long-range ship ought never to encounter truly massive projectiles (because it ought to only fight at long range; see the hybrid armor section if this presumption doesn't apply because you're an all-ranges battleship or paranoid), we'll forgo the heavier armor. This also allows the ship to have more Δv and acceleration (useful for dodging any slower kinetics they might hurl at you just on-principle, or for running away if they try to close the distance).

Some slightly-heavier (though still only several-gram- or at worst hundred-gram-range) projectiles may come your way too from heavier weaponry. This is extremely dangerous, because it counters your Whipple shield and attacks your armor below. The solution is thicker Whipple shields or layers of Whipple shield / absorption layer pairs. A spall liner may also be applicable.

Thin example armor (innermost to outermost):

2mm spider silk
3cm empty space
2.5cm boron or amorphous carbon
1m empty space or graphite aerogel
10cm silica aerogel
1mm diamond or vanadium-chromium steel

CoaDE endurance test (boron, aerogel, VCS variant) vs. 1g (5mm) @ 100km/s * 10/s (50 MJ of kinetic energy / s) railgun: 2 minutes, 24 seconds.

For heavier armor, you can mostly just double-stack it and tweak the numbers to make the outer layer defend against lighter attacks and the inner layer against heavier ones.

Heavy example armor (innermost to outermost):

5cm vanadium chromium steel or 5mm osmium
3cm empty space
5mm spider silk
3cm empty space
3cm boron or amorphous carbon
25cm empty space or graphite aerogel
5cm silica aerogel
3mm diamond or vanadium-chromium steel
2cm empty space
2mm spider silk
2cm empty space
1.5cm boron or amorphous carbon
1m empty space or graphite aerogel
1cm silica aerogel
0.5mm diamond or vanadium-chromium steel

In real life, adding a few more layers of Whipple shields in the gaps will dramatically improve performance.

Short-Range Armor

The main threats, again, are high-mass/low-speed kinetics. Some short-range lasers, but probably not missiles, may also be in-play. Low-mass/high-speed kinetics are assumed to not be in-play (because long-range-fighting ships prefer longer-range), but there's no reason they won't work. If they might be a threat, intermediate-range / all-range hybrid armor may be desired instead.

Here, the main strategy will be a conventional composite armor. The main layer is a monolithic plate, with possibly a few thin additions to improve it against any lasers, etc. This layer should have a high yield strength to minimize spalling (CoaDE users recommend osmium, diamond, boron, ceramic oxide fiber, or vanadium-chromium steel). Nevertheless, heavy projectiles can still cause spalling, so we should also back it with a similarly hefty spall liner.

If you're expecting lasers, as a second layer you can put in a combined ballistic / anti-laser armor, such as (CoaDE again) titanium, para-aramid fiber, liquid-crystal-polymer fiber, or s-glass composite. Aerogel should also work.

For heavier armor you can back this with more monolithic plates and spall liners; you have more materials options since we can assume lasers won't get to this point (if lasers are in-play and they get through both the heavy outer layer and the anti-laser armor, you're probably dead no matter what you try, so it's not worth worrying about). CoaDE users recommend UHMWPE as a good combined layer here. It has an unbeatable tensile-strength to mass ratio, good flexibility, and high speed of sound (all meaning it's hard to spall), while (in real life) it's also fairly cheap. It's not good against high-speed projectiles (owing to a lackluster shear modulus) or lasers (easy to melt), but that's why it's a backing layer.

It should be noted that preliminary tests in CoaDE aren't encouraging. The best way to stop heavy penetrators unfortunately remains lots of thick, heavy materials. Splitting and spacing the main layer may help, since although both are weakened, subsequent penetrations through the first layer are unlikely to hit second in the same place. In any case, clearly, more research is required. It is also partly due to CoaDE being incorrect; adding a 0.5mm vanadium-chromium steel Whipple shield approximately tripled the armor's endurance (given that Whipple shields only begin to help at 3+ km/s, this is clearly a bug).

Thin example armor (innermost to outermost):

2cm spider silk
2cm empty space, silica aerogel, or graphite aerogel
10cm vanadium-chromium steel
1–10cm silica aerogel

CoaDE endurance test (boron, aerogel, VCS variant) vs. 1000g (30mm) @ 2km/s * 10/s (20 MJ of kinetic energy / s) railgun: 10–30 seconds.

The outer layer of aerogel is to shrug off any lasers. This is important because lasers will destroy kinetic shields fairly quickly. The inner void or aerogel pairs with the spider silk to handle spalling. Aerogel would double as a last-ditch effort against lasers.

Heavy example armor (innermost to outermost):

5–10cm UHMWPE or vanadium-chromium steel
1cm empty space
5cm silica aerogel, 5cm graphite aerogel, or 1cm titanium (optional laser layer)
1cm empty space (optional laser layer)
2cm spider silk
3cm empty space
15cm vanadium-chromium steel
5–20cm silica aerogel
1mm amorphous carbon

The amorphous carbon's high-melting point and specific heat is a good counter to surprise nukes and, along with the aerogel, helps versus lasers.

Intermediate- and All-Ranges Armor

All threats are assumed to be in-play, including nuke flash.

The hybrid armor will need to guard against a variety of attacks, and therefore has less effectiveness against all. Hence, while most CoaDE users have implicitly tried to construct this sort of hybrid armor, as-above it is better to specialize your ships for expected engagement range. If you're attempting hybrid armor anyway, it more-or-less implies that your ship is a large capital investment, and your armor can be heavier regardless.

The thin Whipple shields of long-range armor get shredded by high-mass/low-speed projectiles (at least in real life), while the thick plates of short-range armor is quickly eroded by lasers and high-speed kinetics—that is, long-range armor is bad versus short-range weapons and vice-versa. Therefore, the basic approach is to layer the long-range armor over the short-range armor, with the presumption that combatants start at long range and gradually close the distance. By the time the range is short, the long-range armor will have been heavily damaged anyway. However, it is wise to blend these layers somewhat; in-particular having a small amount of low-mass/high-speed protection in the bulk armor layers is helpful, as is nuke flash protection throughout.

Here's just a simple layering, with laser and nuke flash protection added. The nuke flash layers tend to go on the outside of each section (they're thin, and everything else does badly if unprotected):

Thin example armor (innermost to outermost):

5mm spider silk
2cm empty space, silica aerogel, or graphite aerogel
4cm vanadium-chromium steel
2cm silica aerogel
2mm amorphous carbon
2cm empty space
2mm spider silk
2cm empty space
1.5cm boron or amorphous carbon
1m empty space or graphite aerogel
2cm silica aerogel
1mm diamond or vanadium-chromium steel
0.5mm amorphous carbon

The following is more complex; the absorption laser in each of two layers of Whipple shields doubles as the bulk plate for short-range. The outer bulk plate is still relatively thin, owing to the greater chance it will be chewed up by lasers while still at range. The second Whipple shield is also heavier to provide better performance against higher-mass projectiles:

Heavy example armor (innermost to outermost):

5–10cm UHMWPE or vanadium-chromium steel
1mm amorphous carbon
2cm empty space
2cm spider silk
4cm empty space
10cm vanadium-chromium steel
3cm boron or amorphous carbon
30cm empty space or graphite aerogel
2mm diamond or vanadium-chromium steel
1cm silica aerogel
2mm amorphous carbon
2cm empty space
2mm spider silk
2cm empty space
2cm vanadium-chromium steel
1cm boron or amorphous carbon
1m empty space or graphite aerogel
1cm silica aerogel
0.5mm diamond or vanadium-chromium steel
0.5mm amorphous carbon

As before, in real life, adding a few more Whipple shields in the otherwise empty space will dramatically improve performance.


I hope this is a helpful guide to creating plausible composite armors for all your space-battling needs! As before, more actual experimentation would be extremely useful in validating the effectiveness of these armor examples. They work in theory, but not necessarily in practice! It's at-least essentially certain there's room for improvement.

Troy Campbell

     Interesting that of near-term material, steel comes out tops for bulk armour, but I also came to the same conclusion for skyhooks and tethers. Specifically, the same steel currently used for logging cables. Since temperature and radiation are involved along with alternating loads, the boring stuff seems to be the best. No pun intended.
     Plus it's a heck of a lot easier to manufacture with local resources than something like Zylon with its complex chain of dependencies (something I learned about recently)

Michael Earl (author of CoaDE)

A couple of the CoaDE-derived material analysis points here are outdated based on current patches and player studies:

  • The original game had a typo in the definition of bulk/crystalline boron, using gigapascals where megapascals was correct - this lead to it being omnipresent as a structural/armor material (it was a lighter, cheaper diamond). With the current patch it's a poor substitute for aluminum, although boron fiber is still worth consideration.
  • CoaDE impact testing now seems to indicate that soft, dense materials make the best Whipple shields - tin, in particular, is excellent and cheap. Unclear if this is physically accurate.
  • Again, testing in CoaDE indicates certain polymers are weirdly resistant to laser ablation - PTFE and nitrile rubber are currently heavily used to that purpose.

All in all a great summary, though. I would also add that what constitutes long/short range depends on your cross-section, so very small targets (especially missiles) will like consider armoring almost exclusively against long-range weapons, even if expecting short-range engagements.

Wouter Debois

     Michael Earl, about laser ablation in CoaDE: recently the heat capacity of polyethylene got increased, making it hands down the best anti-laser armour. But In CoaDE, heat capacity is overrated, and melting point is underrated as the game doesn't take into account that armour will radiate heat away when it heats up. And since radiation of heat scales with the 4th power of temperature, high melting point materials should be able to tolerate low to moderately intense lasers indefinitely without armour ablation. This is currently not modeled in the game, though I hope a future update corrects this.

Troy Campbell

     Melting modelling is another issue. In zero g, the molten material will form a dome or sphere, which would limit heat conduction to the surrounding armour. When the armour boils, there'd be a molten donut around it.
     Molten armour layers beneath the laser strike would drift and stick to the outr or inner armour layers. A Luke Campbell level of laser power would ablate it explosively but I think in that case there would still be chunks near the beam site.
     Of course, the second acceleration is applied this goes out the window so its not that big of a deal.

Ian Mallett

     Troy, from discussion on the ToughSF, another related point is that hot materials radiate heat, so high-melting-point materials can potentially just shrug off a laser—even a pulsed laser—indefinitely. This does not seem to be modeled in CoaDE.

Luke Campbell

     Troy, I don't think you get true boiling from opaque laser-heated material. You get boiling when heating from the bottom and the material is subject to acceleration (including gravity as acceleration here, a la Einstein). This naturally leads to convective motion. If you heat the material from the top, you make an inversion layer and you just get surface evaporation. In microgravity, there is no driver for convective motion and again you just get evaporation from the surface.
     Molten material suffering no additional forces will change its shape according to the surface tension forces. If the melt is a layer on top of a solid slab, surface tension will tend to pull the surface taught, preventing the melt from escaping. If large areas of molten material are unsupported by solid material underneath, it will be unstable to perturbations causing the melt to break up into globs.
     Under intense laser irradiation, the recoil of evaporating material can also exert forces that push down on the melt layer, expelling it out of the material (this is one of the reasons you get lots of sparks from laser cutting — the sparks are bits of molten goo ejected by the evaporate jet. Of course, laser cutters also get to use gas-assisted cutting where a jet of oxygen assists the removal of melt, and incidentally also chemically attacks the laser-heated regions — a method not really available to laser weapons).

from a post by Ian Mallett (2018)

(ed note: the context is how to armor your laser cannon optics so that the enemy cannot destroy them with pin-point laser strikes)

Isaac Kuo

It is possible to armor laser mirrors, and it's also possible to use optics which are inherently difficult to damage. We've had extensive discussions about this (with Rick Robinson and others) on sfconsim-l.

Armor is based on protecting an otherwise delicate mirror with grids of armor. This assumes the use of a pulsed laser. Each armor grid is a bundle of parallel sheets. When the grid is rotated, it briefly lines up with the target in passing—that's when a pulse laser can fire. With two or more grids, the window of vulnerability can be made arbitrarily short. And the duty cycle can be made unpredictable.

So, for example, a pulsed laser that could only pulse 1/10000 of the time. Incoming laser fire would only hit the mirror 1/1000 of the time. The other 99.99% of the time, it hits the grid armor.

If you want to get even fancier, you can space apart the grids by, say, 1/1000 light seconds (300km). This requires the use of an armor drone, or a pair of warships. This lets you have a duty cycle of almost 50% and still have armor protection 100% of the time. The time delay is sufficient that your photons can pass through to the target, while photons going the other way will get blocked by either one grid or the other.

Still, this grid armor is very bulky. Assuming the grids block 10% of the outgoing photons, it takes 100cm thick grid armor to provide the equivalent protection of only 10cm of solid armor. And it's possible that damage to these grids may significantly diminish their efficiency.

Another interesting possibility is to use damage resistant optics. If you use diffraction rather than reflection or refraction, you can make your focusing element arbitrarily thick. Your focusing element is a zone plate drone some distance away from the beam generator ship. The zone plate is a sturdy thick set of concentric cylinders. It can be arbitrarily thick...if you want, it can be 1m thick. All that really matters is the pattern of concentric circles. Enemy lasers could blast away at this thing all day, and it still functions perfectly so long as there's enough left over to block the concentric circles.

Such a zone plate is not the most efficient focusing element—it only focuses about 25% of the source beam's energy on target. But if you want the ultimate in damage resistance, it can't be beat.

The bottom line is...don't bother shooting at the laser optics. It can be HEAVILY armored.

Ray McVay:


This way, when one side gets it's ass kicked, rotating armored grids can be part of the next generation of warship!

Isaac Kuo

Yes, perhaps, but anti-laser armor protection in general seems like a pretty obvious idea once lasers become powerful enough to require armor protection. I wouldn't think it would be something that requires an ass kicking to realize should be done.

However, if you want to justify laser warship with no anti-laser armor...

You might posit that up until the time of your setting, missiles have been overwhelmingly dominant over weapons lasers. As such, warships are designed purely with missile combat in mind. It's not that weapons lasers don't exist, it's just that no one bothers to put them on spacecraft because missiles are superior weapons.

In this situation, it might make sense to design a laser armed spacecraft designed to fight only missiles because there are lots of enemy missiles and no enemy lasers. It's like how the first tanks weren't designed to fight enemy tanks because enemy tanks didn't exist yet.

Then, when the enemy sees them and reacts by slapping some laser on their own spacecraft, the first guys are like, "Derp, we didn't think that would happen."

(No really, people can be that stupid.)

Aaarggh...this scenario really hurts my head. It just doesn't seem to make sense from the start. Weapons lasers seem to be inevitably useful thing to have around. If the POTUS wants to zap terrorists from orbital drones, a near future laser will be able to do it with precision but a missile would take minutes to cross the distance required. Orbital lasers could zap hostage taking pirates like a team of snipers, while orbital missile launchers just...can't. There's just no way missiles will ever be better than lasers for this, and it's hard to imagine a future where that isn't one of the first things mankind does with space weaponry.

Isaac Kuo

Getting back to an earlier question — the level of armor protection on the laser turrets vs other systems is a matter of design priorities and the level of the threat. If the armor required for all around armor protection is reasonably low mass, than the obvious thing is to provide all around armor protection.

But if offensive weaponry gets strong enough to make that impossible, you have to prioritize things. Historically, the nature of the fuses on increasingly powerful armor piercing explosive shells led to the concept of "all or nothing" armor. It wasn't possible to provide all around protection and still float, so the idea was to concentrate armor around the most critical systems while hopefully providing so little armor on the other systems that shells would pass fully before detonating (causing far less damage than if it detonated within the ship).

The nature of tank armor vs weaponry led to directional concentration of armor--concentrating armor on the front and sides, to protect in a frontal arc. The engine would provide some protection against attacks from the rear (albeit it would still be a mission kill, but the crew would stand a decent chance of survival).

With hypothetical space warships, I see two very different sorts of threats--beam weaponry and missiles. You plainly don't want excessively powerful beam weaponry, or it would dominate utterly (never runs out of ammo, reaches the target more quickly, can be used for things like zapping terrorists). That's okay, because near term practical beam weaponry might be limited to 1MW or less anyway...good enough to take out missiles with some dwell time, but perhaps with minimal armor penetration capability. You can tweak things upward from there to provide whatever level of armor penetration is appropriate to balance against other stuff.

The precision of lasers is such that the enemy can likely specifically target any spot on the target vehicle. This strongly suggests that all around protection is called for. You can dramatically improve the level of armor protection with the use of rotating armor...this prevents concentrating on a single spot until it penetrates. The laser basically can't ever penetrate unless it can slice through all at once.

You probably don't want to redesign all your artwork and ships and design to take rotating armor into consideration. A less radical solution is to assume some sort of multi-layer spaced armor filled with some sort of open mesh foam. As bits of foam get melted/vaporized, nearby foam expands to fill in the gap. Ideally, most of the melted/vaporized material gets absorbed by the mesh, so armor loss is minimal. I think you're already assuming a foam armor layer, so that works out.

The bottom line is that any particular enemy laser have a pretty sharp transition from being entirely ineffective at a particular range to being extremely effective at a slightly lower range. If you do anything other than even protection for all components, then you'll end up with a situation where the less protected components are practically guaranteed to be wiped out as soon as the range closes to that transition range. It might make sense to completely write off living quarters, drop tanks, and missile racks. But lasers, power, radiators, and propulsion probably need the same level of protection all around.

(Yes, radiators can be armored, and should be. Schemes to fold away radiators are a false economy.)

Ray McVay:

I totally agree about the armored radiators — assuming quartz for anti-debris armor on civilian craft, and diamonoid for military.

I LOVE the idea of rotating armor, or other active measures. The virtue of the setting I'm developing is that it pre-supposes technological innovation once an interplanetary war begins. This means that I can design different generations of spacecraft as the arms race heats up.

Isaac Kuo

I'm not sure anti-debris armor makes sense. I mean, I like making military radiators out of quartz or diamond because they're basically invulnerable to visible wavelength lasers (lasers mostly harmlessly pass through them, causing ignorable local heating of coolant). But for civilian radiators, debris impacts should be rare enough that it generally makes more sense to simply patch leaks than prevent them. Current space radiators are based on a combination of conduction and circulation, being mostly solid conductive panels with small armored coolant pipes. The small armored coolant pipes try to present a small target to random debris--trying to prevent any leak in the first place. But once you have robotic drones to autonomously detect and patch leaks, then it's more efficient to simply have transparent radiator pipes so the coolant directly radiates heat to space (no conduction step necessary). The repair drones might be shaped like dumb-bells. The two spherical ends isolate a small higher pressure section. If the drone detects a drop in pressure, it knows it's passing by a leak, so it sprays some transparent patch glue.

Defense against missiles is a bit different than defending against random space debris, since it's going to be incomparably nastier in damage potential, and you can see it coming. Because of the momentary nature of the threat, it makes sense to use some sort of "shutters"...simply turtle up entirely for the moment the missiles arrive. Who cares if you're momentarily unable to fire lasers or radiate any heat?

Personally, I prefer these "shutters" to be low mass defensive anti-missile drones a small distance from the ship, rather than armor integrated with the hull. These form a temporary wall between the incoming missiles and the ship. It's not necessary to stop the missiles entirely, it's just necessary to break them up enough so the remaining debris will mostly miss the ship. (Another layer or two of these drones will stop what small fraction of the debris is headed for the ship.)

Hull armor, in contrast, has to stop the incoming missile dead in its tracks--redirecting the explosive force back into the hemisphere from whence it came.

Isaac Kuo in a Google+ thread

Composite metal foams (CMFs) are tough enough to turn an armor-piercing bullet into dust on impact. Given that these foams are also lighter than metal plating, the material has obvious implications for creating new types of body and vehicle armor – and that’s just the beginning of its potential uses.

Afsaneh Rabiei, a professor of mechanical and aerospace engineering at NC State, has spent years developing CMFs and investigating their unusual properties. The video seen here shows a composite armor made out of her composite metal foams. The bullet in the video is a 7.62 x 63 millimeter M2 armor piercing projectile, which was fired according to the standard testing procedures established by the National Institute of Justice (NIJ). And the results were dramatic. (see video above).

“We could stop the bullet at a total thickness of less than an inch, while the indentation on the back was less than 8 millimeters,” Rabiei says. “To put that in context, the NIJ standard allows up to 44 millimeters indentation in the back of an armor.” The results of that study were published in 2015 (Ballistic performance of composite metal foams).

But there are many applications that require a material to be more than just incredibly light and strong. For example, applications from space exploration to shipping nuclear waste require a material to be not only light and strong, but also capable of withstanding extremely high temperatures and blocking radiation.

Last year, with support from the Department of Energy’s Office of Nuclear Energy, Rabiei showed that CMFs are very effective at shielding X-rays, gamma rays and neutron radiation. And earlier this year, Rabiei published work demonstrating that these metal foams handle fire and heat twice as well as the plain metals they are made of.

Now that these CMFs are becoming well understood, there could be a wide array of technologies that make use of this light, tough material. Armor, if you’ll forgive the pun, barely scratches the surface.

(ed note: the stuff has a Boron carbide ceramic {B4C} strike plate.

Troy Cambell said: Boron carbide is some serious shizzle. It has a stupidly high heat capacity, and can eat more joules than carbon does, even though it it melts at a lower temperature vs. carbon sublimation.

The core is Steel-steel composite metal foam (S-S CMF) using hollow spheres embedded in a stainless steel powder matrix. Hollow spheres have a 2 mm outer diameter and a 200 μm sphere wall thickness.

The backing plates are either Kevlar or aluminum 7075. Total armor thickness is 25 mm.)


And you can forget about laser defenses like Traveller style Sandcasters. These fire clouds of magic "prismatic" dust that provide protection from hostile laser fire. In reality they would not work. There is no way that they can project a cloud dense enough to do any good.

In Frank Chadwick's starship combat game Star Cruiser, there are anti-laser fields.

Screens are not mysterious force fields that prevent enemy weapons from penetrating. Instead they are electromagnetic fields which hold reflective particles in suspension. When a laser hits the screen, the particles reflect a portion of the laser light and then vaporize, absorbing the rest of the laser's energy. Although some energy will penetrate the screen, often the screen absorbs or reflects enough energy that the remainder is insufficient to damage the ship.

From STAR CRUISER by Frank Chadwick

However, the gang at are skeptical:

I don't remember that thread but the idea intrigues me. Why would a levitating cloud of metallic particles be any better at protecting a ship than the same metal used to make ordinary hull plating?

It sounds ike you are just wasting energy on maintaining armor with more holes in it than conventional armor. On the other hand there may be heat dissipation issues with conventional armor. On the third hand if you have a magnetically shaped armor you could concentrate the cloud on the side you are being attacked from so you don't have to create thick armor on all sides. This could cut the weight in half or more - but levitating plates instead of a cloud would seem better suited for the task.

Michael Grosberg

Seems to me it might even be worse. If you're talking about insanely powerful laser beams, when they hit the particles they'll just turn them into projectiles that will hit the ship. It doesn't seem to me like you could plausibly get a "shield" of magnetically-levitated particles in such a way that would give you any kind of real coverage -- especially if you're positing it being used in defense against superpowerful laser beams. The beams just knock the particles out of the way and fire straight through.

I would think it's because every little metallic particle would be exposed to the beam only a short time. Then more would fill in. Like having your hull plates jump in front of any hole. Sort of. That presumes particles circulating around in this levitating cloud.

: Seems to me it might even be worse.

Well... yes, there is that. Much easier to vaporize each particle, though it might be quite hard to get a particle to actually recoil and hit the ship. Hmmm. Anyways... yes, I suspect it wouldn't really work well, and you'd have to levitation a large, large: mass of particles.

Seems like most of the particles would hit the ship. To serve their purpose, after all, they are have to be between the beam and the ship. Whether that would be really dangerous to the ship depends on how thick the "shield" is and how big each particle is.

If we're granting superpowerful laser beams, it seems to me that the energy required to displace or even vaporize these particles will be much smaller than the amount of energy in the beam, which suggests, as you say, that such a "shield" won't be of much use unless it's very thick. At some point, it seems to me you're just better off having armor; you have to carry around the extra mass anyway. But without attaching numbers it's hard to be sure.

Put simply, a layer of sand is no more effective at stopping a laser beam than a similar areal density of monolithic armor (in fact, it's a bit less effective due to structural issues); you can simply shoot holes in a cloud of sand, just like you can shoot holes in armor. As such, why spend X tons of your mass budget on temporary armor when you can just spend the same X tons on permanent armor?

In addition, a cloud of sand

  1. needs to be somewhat larger than the ship it shields (reducing areal density, and thus armor value)
  2. cannot maneuver if the parent ship maneuvers (so if you deploy sand, you're stuck in your current position)
  3. without some form of containment will simply disperse in a time frame that's comparable to the deployment time (if the cloud can cover the entire ship in 10 seconds, after 20 seconds it will have expanded to twice the size of the ship, reducing protection by a factor of 4. You can improve this time somewhat by using multiple projectors)
Anthony Jackson

Creative Measuring Units

In his novel The Wellstone, Wil McCarthy proposes a unit called the TW or "train wreck". It is measure of impulsive acceleration (i.e., from a crash or explosion) equal to an inertial acceleration of 40 g. A human being can survive a 1 TW impulse lasting no more than a couple of seconds, while a 2 TW impulse of longer than a second is typically fatal. In The Hitch-Hiker's Guide To The Galaxy, Douglas Adams creates a tongue-in-cheek unit called the "hurt", with spacecraft weapons rated in "mega-hurts". Har-har.

Force Fields

Force shields, deflectors, energy screens, they are all handwavium science fictional armor composed of some form of energy instead of dull boring matter.

Names include force fields, force shields, force screens, energy screens, energy shields, deflectors, deflector shields/screens, ray-screens, repulsor screens, and many more. Understand, you young whipper-snappers, Star Trek did not invent the term "deflector screen." John W. Campbell, Jr. used it in 1937, for a science essay called "Interplanetary Dividends", and there are probably earlier uses.

Since they are imaginary their abilities and properties are only limited by the imagination of the science fiction author. Some stop bullets but let laser beams through, some stop lasers but are useless against bullets, some stop both. They can be invisible, mirrored like chrome, or glowing in various colors and levels of brilliance. They are occasionally used in a creative fashion, e.g., "structural integrity fields" are force fields interpenetrating a spaceship's metal frame in order to make it stronger than is possible with mere matter. Or gravity shields that allow one to ignore the gravity of a nearby planet.

In other words the author has to be real careful in order to avoid unintended consequences.


[first use unknown; also "force field"]

General SFnal term for a hypothetical technology which can interact with any material object in the ways that magnetic fields interact with magnetized ones. A forcefield may be deployed as a shield around a spaceship or other object (a force shield). Other applications include the tractor beam, the pressor beam and antigrav.

force shield

[dates to early space opera, c. 1930]

The concept goes back to E.E. "Doc" Smith's Skylark stories, a classic space opera series originally launched in 1928; those books referred to force screens. By the time of Asimov's Foundation in the 1940s the idea had been sufficiently naturalized that his world featured personal force shields as a defense against the blaster.

In "Doc" Smith's and most later versions, a force shield has no thickness and has resistence proportional to the power of the generator. It is possible to overload a shield by throwing more energy at it than the generator can handle, or to wait for the exhaustion of the generator's power sources.

From An SF Glossary by Eric S. Raymond (2006)

In sf Terminology — unlike Physics, where it has a different meaning — a force field (sometimes a force shield or energy screen) is usually an invisible protective sphere or wall of force. It first seems to have been used in this sf sense in E E "Doc" Smith's Spacehounds of IPC (July-September 1931 Amazing; 1947). Throughout the 1930s and 1940s the force field performed sterling service, notably in Smith's Skylark and Lensman series, where force fields under attack routinely glow red and orange and then all the way up through the spectrum until they reach violet and black and break down. Isaac Asimov's "Not Final!" (October 1941 Astounding) uses force-field research as a vehicle for the message that absolute statements about scientific possibility tend to be unwise.

(ed note: Spoiler Alert - in Not Final, force fields were proved to be impossible because they only last a fraction of a second. Until some joker figured you can do an end run around the problem by activating the field stroboscopically.)

Force fields are also a sovereign remedy against Rays, Death Rays and usually bullets as well — though not against "space-axes" in E E Smith's First Lensman (1950) or against swords in Charles L Harness's Flight into Yesterday (May 1949 Startling; exp 1953; vt The Paradox Men 1955 dos; rev 1984). In these books the efficacy of the shield is directly proportional to the cube of the velocity (Smith) or to the momentum (Harness) of the object it resists. This property of force fields gives Harness a good excuse to introduce swordplay, where the momentum involved is relatively small, into a technologically advanced society — an example of Medieval Futurism that other writers were not slow to follow, most notably Frank Herbert with his personal "shields" and knife-fighting in Dune (fixup 1965).

The eponymous device in Poul Anderson's Shield (June-July 1962 Fantastic Stories of the Imagination; 1963) can recharge its batteries by soaking up the kinetic energy of the bullets it stops. But these are comparatively late examples, when the concept was sufficiently familiar in sf to allow Parody and sophisticated variations. Still later force fields are often simply accepted as a given, as in Star Trek with its shields or in Iain M Banks's Culture stories.

It is the essence of the traditional sf force field that by a kind of judo it converts the energy of an attacking force and repels it back on itself. Few writers, however, were able to give — or concerned to try to give — a convincing rationale for forces being conveniently able to curve themselves around an object and to take on some of the properties of hard, resistant matter. A well-ground mirror might more plausibly carry out the same function, at least against death rays. Indeed, Colin Kapp's "The Pen and the Dark" (in New Writings in SF 8, anth 1966, ed John Carnell) features an Alien defensive wall which is essentially a 3D mirror, opposing each physical assault with a precisely similar reflection. James Blish nevertheless made an interesting attempt, using analogies from radar technology, to justify a kind of standing-wave force field placed around New York by some unspecified hostile power in "The Box" (April 1949 Thrilling Wonder). The true rationale for the force field and for its close relations, the Tractor Beam (which pulls objects towards the beam projector) and the Pressor Beam (which pushes them away), is that — like Faster Than Light travel — they help tell stories.


Also called "Force Fields", these are invisible (or, if the budget allows it, barely-visible) energy barriers placed around objects to protect them from harm.

Most common around space ships, but also seen around bases and — very occasionally — individual people.

How much protection they offer is usually proportional to their power. This also makes for yet another reason you are boned if the energy runs out. They may also be subject to Phlebotinum Overload if they get hit by too many Energy Weapons.

An energy barrier can zap or repel anyone who touches it, or can simply behave like an impenetrable wall. It's often represented in the form of a Beehive Barrier or another Hard Light construct. In practice, this is somewhat less scientifically feasible than Energy Weapons, but not by much, at least in the way it is usually depicted — a strong electromagnetic field really can be used to deflect particle beam weapons or railgun/coilgun slugs. A better example is the Earth's magnetic field which safeguards us from charged particles in the solar wind. NASA and ESA are actually researching how to reproduce the effect on future interplanetary spaceships, to protect Mars travellers from cosmic radiation.

Shields may be handled as a single egg-shell or as several independent barriers covering different areas of the ship. The latter encourages certain maneuvers, such as making sure your shielded side is always facing the enemy.

Shields are popular in fiction because it allows the Cool Ship to participate in battles without the inconvenience of having to spend the rest of the episode making repairs to physical armour and systems. In older or lower-budget movies and TV, they also eliminate the need to show battle damage on the ship; e.g., having your Ensign call out "Shields down to twenty percent!" can be a lot cheaper than showing a gash blasted in your ship's armor. The downside is that creators often have to resort to Explosive Instrumentation to provide combat casualties on a shielded ship. In somewhat-harder science fiction, shields are useful to overcome the seemingly overwhelming attacker's advantage—cities on planets can't dodge, so if there isn't some way to defend against space-launched missiles and kinetic projectiles, wars are going to be short and boring.

In Video Games, deflector shields have a special use. They are effectively a way to justify regenerating hit points, but only for a part of a unit's health. Shields get to regenerate, but if there is something beneath them, like armor, the armor doesn't regenerate. Also, for many games where a target can take Subsystem Damage, that won't start until you penetrate the deflector shields.

Compare and contrast Containment Field and Reinforce Field. See also Some Kind of Force Field.

(ed note: see link below for lots of examples from TV, movies, and novels)

From the DEFLECTOR SHIELDS entry at TV Tropes

In the real world, defensive force fields do not exist. But if they did it would make things so much easier.

There are a couple of remotely possible real-world "force fields". Researchers have been experimenting with using magnetic and electrostatic fields to ward off particle radiation. More on the fringe are plasma windows, which could defend against microwaves and particle radiation. But they have a long way to go before they can stop weapon-grade particle beam weapons.

Plasma windows can separate pressurized areas from unpressurized areas with a sort of force-shield "door". Air cannot pass through the plasma window into the unpressurized region, even under a pressure differential of up to nine atmospheres. So if a radiation bean generator requires vacuum but the item being irradiated is in an atmosphere, the plasma window works nicely.

It is currently used for electron beam welding. The beam generator is inside a vacuum chamber, the electron beam passes through the plasma window and welds the metal sheets on the workbench in the shirt-sleeve environment. In theory one could use this as a quick-pass airlock or hangar-bay door on a spacecraft.

Plasma windows have a bright glow, the color depends upon what gas the plasma is using. Argon is violet, nitrogen is orange.

The drawback of plasma windows is that they are power hogs. For a round window they need 8 kilowatts per centimeter of window diameter. Other than that there is no limit on diameter.

But there isn't anything in the real world like E.E."Doc" Smith's electromagnetic radiation stopping "ray-screens", nor his matter stopping "repellor screens."


      Light particles normally do not «feel» each other because there is no interaction acting between them. Researchers at ETH have now succeeded in manipulating photons inside a semiconductor material in such a way as to make them repel each other nevertheless.

     Two light beams crossing each other do not deflect one another. That is because, according to the laws of quantum physics, there is no interaction between light particles or photons. Therefore, in a collision two photons simply pass through each other instead of bouncing off one another — unless one helps them along in some way. In fact, researchers have tried for quite some time now to find techniques for making photons “feel” each other. The hope is that this will result in many new possibilities for research as well as for practical applications. Ataç Imamoğlu, professor at the Institute for Quantum Electronics at ETH in Zurich, and his collaborators have now taken a further important step towards the realization of strongly interacting photons. Their research results were recently published in the scientific journal Nature Materials.

Transformation into polaritons

     “Strongly interacting photons are something of a Holy Grail in our field of research, photonics”, explains Aymeric Delteil, who works as a post-doc in Imamoğlu’s laboratory. To make light particles repel each other, he and his colleagues have to go to some length, though. Using an optical fibre, they send short laser pulses into an optical resonator, inside of which the light is strongly focused and finally hits a semiconductor material. That material (produced by Imamoğlu’s colleagues in Würzburg and St. Andrew’s in Scotland) is cooled inside a cryostat — a kind of extremely powerful refrigerator — down to minus 269 degrees centigrade (about 4 degrees above absolute zero). At those low temperatures the photons can combine with electronic excitations of the material. That combination results in so called polaritons. At the opposite end of the material the polaritons become photons again, which can then exit the resonator.

     As there are electromagnetic forces acting between the electronic excitations, an interaction arises also between the polaritons. “We were able to detect that phenomenon already a while ago”, says Imamoğlu. “However, at the time the effect was so weak that only the interactions between a large number of polaritons played a role, but not the pairwise repulsion between individual polaritons.”

Correlations signal interactions

     In their new experiment, the researchers were now able to demonstrate that single polaritons — and hence, indirectly, the photons contained in them — can, indeed, interact with each other. This can be inferred from the way in which the photons leaving the resonator correlate with each other. To reveal those so called quantum correlations, one measures the probability of a second photon leaving the resonator shortly after another one. If the photons get in each other’s way through their polaritons inside the semiconductor, that probability will be smaller than one would expect from non-interacting photons.

     In the extreme case there should even be a “photon blockade”, an effect which Imamoğlu already postulated 20 years ago. A photon in the semiconductor that has created a polariton then completely prevents a second photon from entering the material and turning into a polariton itself. “We are quite some way from realizing this”, Imamoğlu admits, “but in the meantime we have improved further on our result that has just been published. This means that we are on the right track.” Imamoğlu’s long-term objective is to make photons interact so strongly with each other that they start behaving like fermions — like quantum particles, in other words, that can never be found at the same place.

Interest in strongly interacting polaritons

     In the first instance, Imamoğlu is not interested in applications. “That’s really basic research,” he says. “But we do hope to be able, one day, to create polaritons that interact so strongly that we can use them to study new effects in quantum physics which are difficult to observe otherwise.” The physicist is particularly interested in situations in which the polaritons are also in contact with their environment and exchange energy with it. That energy exchange, combined with the interactions between the polaritons, should, according to calculations by theoretical physicists. lead to phenomena for which there are only rudimentary explanations so far. Experiments such as those carried out by Imamoğlu could, therefore, help to understand the theoretical models better.


Delteil A, Fink T, Schade A, Höfling S, Schneider C, Imamoğlu A: Towards polariton blockade of confined exciton–polaritons. Nature Materials 2019, 18: 219, doi: 10.1038/s41563-019-0282-y

{ed note: so if I am reading this correctly, theoretically if you use special semiconductor armor and cool it to four degrees above absolute zero, it is possible to make it prevent the entry of a hostile laser weapon bolt (since laser beams are composed of photons). Maybe. Sounds like great technobabble at any rate. Will probably either glow white or look like a mirror under photon bombardment.

Kind of like Scotchgard, for your warship. “Got a life? Gotta ask for Lasergard.”

For more technobabble, wave your hands and say you don't have to cool it to 4 K if you magically use a Polariton superfluid, since those can theoretically be made at room temperature.

See also: Polariton, Polaritonics, Polariton laser, and Polariton superfluid

I see a more serious objection (I am not sure, because Quantum Mechanics makes my brain hurt).

Say the incoming hostile laser beam has a power density of 10 kiloJoules per square centimeter (10 kJ/cm2) and a spot size of 1 cm2. Say the defending spacecraft has the same average cross section of an Oscar submarine or 1.51×107 cm2.

This means the spacecraft would have to charge up all the armor facing the enemy to 150 gigaJoules to ensure protection against one beam strike (10 kJ * 1.51×107). In other words the armor needs fifteen million times the energy of the laser bolt. Granted, once the armor is charged up it probably only needs replacement energy to keep up with energy lost in dissipating hostile weapon strikes.

Or maybe not, because Quantum is involved. Perhaps one polariton can photon-block an infinite number of incoming photons, so weapon strikes do not dissipate polariton energy. The question is above my pay grade.}

From REPULSIVE PHOTONS by Oliver Morsch
Eidgenössische Technische Hochschule (ETH) Zürich (2019)

(ed note: this was written before the invention of the laser. The fortress is called Project Thor, no relation to Rods From God, and is located on the Moon near Pico crater. It is about to be attacked by three spacecraft. Dr. Steffanson has invented a technobabble "ray screen" that reflects electromagnetic radiation. Jamieson and Wheeler are hiding in a crevasse several miles away near their crippled tractor. The description is so cinematic that it is just begging for an SF artist to depict it.)

EVEN TODAY, little has ever been revealed concerning the weapons used in the Battle of Pico. It is known that missiles played only a minor part in the engagement. In space warfare, anything short of a direct hit is almost useless, since there is nothing to transmit the energy of a shock wave. An atom bomb exploding a few hundred meters away can cause no blast damage and even its radiation can do little harm to well-protected structures. Moreover, both Earth and the Federation had effective means of diverting ordinary projectiles.

Purely non-material weapons would have to play the greatest role. The simplest of these were the ion-beams, developed directly from the drive-units of spaceships. Since the invention of the first radio tubes, almost three centuries before, men had been learning how to produce and focus ever more concentrated streams of charged particles. The climax had been reached in spaceship propulsion with the so-called "ion rocket," generating its thrust from the emission of intense beams of electrically charged particles. The deadliness of these beams had caused many accidents in space, even though they were deliberately defocused to limit their effective range.

There was, of course, an obvious answer to such weapons. The electric and magnetic fields which produced them could also be used for their dispersion, converting them from annihilating beams into a harmless, scattered spray.

More effective, but more difficult to build, were the weapons using pure radiation. Yet even here, both Earth and the Federation had succeeded. It remained to be seen which had done the better job—the superior science of the Federation, or the greater productive capacity of Earth.

Less than a million kilometers away, Carl Steffanson sat at a control desk and watched the image of the sun, picked up by one of the many cameras that were the eyes of Project Thor. The group of tired technicians standing around him had almost completed the equipment before his arrival; now the discriminating units he had brought from Earth in such desperate haste had been wired into the circuit.

Steffanson turned a knob, and the sun went out. He flicked from one camera position to another, but all the eyes of the fortress were equally blind. The coverage was complete.

Too weary to feel any exhilaration, he leaned back in his seat and gestured toward the controls.

"It's up to you now. Set it to pass enough light for vision, but to give total rejection from the ultra-violet upward. We're sure none of their beams carry any effective power much beyond a thousand Ångström. They'll be very surprised when all their stuff bounces off. I only wish we could send it back the way it came."

"Wonder what we look like from outside when the screen's on?" said one of the engineers.

"Just like a perfectly reflecting mirror. As long as it keeps reflecting, we're safe against pure radiation. That's all I can promise you."

(ed note: 1000 Ångströms = 100 nanometers = extreme ultraviolet)

Then he saw that something was happening to the dome. It was no longer a gleaming spherical mirror reflecting only the single image of the sun. Light was splashing from it in all directions, and its brilliance was increasing second by second. From somewhere out in space, power was being poured into the fortress. That could only mean that the ships of the Federation were floating up there against the stars, beaming countless millions of kilowatts down upon the Moon. But there was still no sign of them, for there was nothing to reveal the track of the river of energy pouring invisibly through space.

The dome was now far too bright to look upon directly, and Wheeler readjusted his filters. He wondered when it was going to reply to the attack, or indeed if it could do so while it was under this bombardment. Then he saw that the hemisphere was surrounded by a wavering corona, like some kind of brush discharge. Almost at the same moment, Jamieson's voice rang in his ears.

"Look, Con—right overhead!"

He glanced away from the mirror and looked directly into the sky. For the first time, he saw one of the Federation ships. Though he did not know it, he was seeing the Acheron, the only spaceship ever to be built specifically for war. It was clearly visible, and seemed remarkably close. Between it and the fortress, like an impalpable shield, flared a disk of light which as he watched turned cherry-red, then blue-white, then the deadly searing violet seen only in the hottest of the stars. The shield wavered back and forth, giving the impression of being balanced by tremendous and opposing energies. As Wheeler stared, oblivious to his peril, he saw that the whole ship was surrounded by a faint halo of light, brought to incandescence only where the weapons of the fortress tore against it.

It was some time before he realized that there were two other ships in the sky, each shielded by its own flaming nimbus. Now the battle was beginning to take shape; each side had cautiously tested its defenses and its weapons, and only now had the real trial of strength begun.

In utter silence, the battle was rising to its climax. Millions of years ago the molten rock had frozen to form the Sea of Rains, and now the weapons of the ships were turning it once more to lava. Out by the fortress, clouds of incandescent vapor were being blasted into the sky as the beams of the attackers spent their fury against the unprotected rocks. It was impossible to tell which side was inflicting the greater damage. Now and again a screen would flare up, as a flicker of heat passes over white-hot steel. When that happened to one of the battleships, it would move away with that incredible acceleration, and it would be several seconds before the focusing devices of the fort had located it again.

Both Wheeler and Jamieson were surprised that the battle was being fought at such short ranges. There was probably never more than a hundred kilometers between the antagonists, and usually it was much less than this. When one fought with weapons that traveled at the speed of light—indeed, when one fought with light itself—such distances were trivial.

The explanation did not occur to them until the end of the engagement. All radiation weapons have one limitation: they must obey the law of inverse squares. Only explosive missiles are equally effective from whatever range they have been projected: if one is hit by an atomic bomb, it makes no difference whether it has traveled ten kilometers or a thousand.

But double the distance of any kind of radiation weapon, and you divide its power by four owing to the spreading of the beam. No wonder, therefore, that the Federal commander was coming as close to his objective as he dared.

The three ships appeared to be moving in some complex tactical pattern, so that they could maintain the maximum bombardment of the fort while reducing its opportunity of striking back. Several times one of the ships passed vertically overhead, and Wheeler retreated as far into the crack as he could in case any of the radiation scattered from the screens splashed down upon them.

And now a strange thing was beginning to happen. The rays with which the battle was being fought were no longer quite invisible, for the fortress was no longer in a vacuum. Around it the boiling rock was releasing enormous volumes of gas, through which the paths of the rays were as clearly visible as searchlights in a misty night on Earth.

The tenuous and temporary atmosphere round the dome was producing another unexpected effect. Occasional flashes of lightning darted between ground and sky, draining off the enormous static charges that must be accumulating around the fort. Some those flashes would have been spectacular by themselves—but they were scarcely visible against the incandescent clouds that generated them.

From EARTHLIGHT by Arthur C. Clarke (1955)

      Maybe I should explain a little about Malibu, about how it all came about. They always said Io was the least hospitable place in the solar system. I can think of worse, but it is true that the inner Jovians were uninhabitable until some Earthbound tecky came up with the MP screen. Mesoproteic. Something about mesons and positron moods and the strong nuclear force. I don’t know, it's fifty-seven years since my doctorate. A six-year-old could probably explain it better than me now. What it does is it cuts out everything you want, solids, radiation, whatever. MPs let us live on Io, a big one encases all of Malibu. It keeps our air in, keeps the charged particles and the stink of sulphur out where they belong.
     Tin Man was one of the first people to set up on Io. Employed by one of the old lunar corporations, he and some others discovered just how useful an MP screen was. Malibu was little more than a small prospectors’ dome when a keyboard man called Berg Ruttgers came down and saw Tin Man and his friends skidding around on MP-screened buggies, skimming down a slow-moving channel of molten sulphur, riding the golden surf. Ruttgers had gone back to Callisto to make a deal with Ruby Gerome and raise finance and then, as a newly formed leisure agency, they bought into Malibu and started building.
     There were other hotriding stars but Tin Man was always the favourite. With all his alterations and implants he could jack directly into his buggy, link directly with the sim-recorder. The trip technicians could access the raw data from his prosthetic eye, they could trace his tensions through the body-machine interfaces. Others followed, other agencies competed, but Tin Man was the first and best. Tin Man and Malibu had made the fortunes of Ruttgers and Gerome and now they were dropping him.

     I realized something was wrong so I made for a panel, suddenly scared for Tin Man. I knew he had been angry and I didn't like to think what he might have tried.
     I looked out and there were the two black chase-buggies, circling slowly, no sign of Tin Man. Even a magniview only showed whirlpool turbulence where my friend had last been seen. No buggy. I picked up what had happened from the tourists. Tin Man had gone down. The chasers had closed on him, drifting in behind a three metre crest of sulphur. Tin Man had spotted them, hesitated too long and the wave had folded over him, carrying the chasers on past. When the surface had levelled his buggy had vanished. In over sixty years off-Earth I had lost a few friends, some very close, but there was something too terrible to comprehend about going the way I believed then that Tin Man had gone. The MP shielding would protect him from the heat and the pressure, but there would be no way to dissipate his own heat from the buggy or, failing that, he would eventually run out of power or air. I couldn't bear to think about it, but then I couldn't fix my mind on anything else. Maybe he would sink so far that even the MPs would fail, maybe that would be best.
     Eventually, the chasers came back in. I saw their faces as they left their buggies. They were creased, shaken. Then a tecky came out and—
     “Mega, mega!” said a whining riche voice and I looked up and saw Tin Man's buggy skimming back up the drag towards Malibu and, pretty soon, MP screens merging and he was in and home and alive.
     He must have been under the brimstone sea for near to forty-five minutes yet he came out of the crowd smiling. People were yelling at him, asking what had happened and all he would do was grin and tap his bulbous plastic eye and say, “It’s all in here. Property of R & C. Sorry, you’ll have to buy the sim.”

From HOTRIDER by Keith Brooke (1991)


As always when dealing with rubber science, the smart move is to nail down the ground rules for the item in question, think out all the logical consequences and implications, and stick to them.

If the force field blocks incoming laser fire, will it block your outgoing fire as well? In Isaac Asimov's "Black Friar of the Flame", a ship has to drop its field entirely in order to fire its weapons. This lead to chain reactions, ship A drops and fires, then it is hit by ship B who drops and fires, who is hit by ship C who drops and fires... In Larry Niven and Jerry Pournelle's The Mote in God's Eye, the Langston Field can have temporary holes opened to allow egress of your laser fire. In other novels, the field is on stroboscopically, that is, it flickers. It will be on, say, 80% of the time, and off for 20%. If your lasers flicker in synch with your field, 100% of their energy will penetrate. But since your opponent's lasers will probably not be in synch, only 20% of their energy will penetrate. However, if your opponent manages to match your synch rate, you'll be clobbered.

Does the force field block matter only (e.g., kinetic weapons), energy only (e.g., lasers), or both? Doc Smith had separate types of force fields for each ("repellors" and "ray-screens"), while the Langston Field would absorb both the kinetic energy of projectiles as well as the electromagnetic energy of lasers. The fields in "Black Friar of the Flame" only block energy, so the good guys get a bright idea from the Battle of Salamis.

Is the field a bubble around the ship, or flat planes that can be positioned? There was that throwaway line in the movie Star Wars, where Red Leader tells the Red Squadron X-Wing pilots to angle their deflector shields "double-front". Presumably this means rotating the rear shields to face forwards, so there is double the protection forwards and zero protection aft.

How fast can the field be charged up? The usual model is that energy is fed into the field, and each incoming shot reduces the energy in the field ("Deflector shields are down to 40%, Captain"). When the field energy reaches zero, the field goes down and the incoming weapons fire impacts directly on the ship. For dramatic reasons, it is desirable to have the rate of shield charging to be a fraction of the rate of shield reduction. Otherwise ship's shields will never go down.

Does the field obey the law of Conservation of Momentum? Say your force field generator is located in the Engineering deck. You put the force field around the ship, then quite by accident the ship crashes into an asteroid. One would expect that as the field hit the asteroid, the shock of impact would be transmitted to the field generator. You might wind up with the generator plowing through the hull and out the rear of the ship.

In Poul Anderson's novel Shield, the field has a sharp gradient on the outside, and a more gradual one on the inside. This means if you were running and collided with the shield it would feel like hitting a brick wall. But if you were inside the shield it would feel like hitting a mound of feather pillows.


(ed note: Isaac Asimov took his inspiration from the Battle of Salamis)

      In two hours, the Lhasinuic demand for surrender had been scornfully rejected and the hundred ships of the Human squadron spread outwards on the expanding surface of an imaginary sphere—the standard defense formation of a surrounded fleet—and the Battle for Earth was on.

     A space-battle between approximately equal forces resembles in almost every detail a gigantic fencing match in which controlled shafts of deadly radiation are the rapiers and impermeable walls of etheric inertia are the shields. The two forces advance to battle and maneuver for position. Then the pale purple of a Tonite beam lashes out in a blaze of fury against the screen of an enemy ship, and in so doing, its own screen is forced to blink out. For that one instant it is vulnerable and is a perfect target for an enemy ray, which, when loosed, renders its ship open to attack for the moment. In widening circles, it spreads. Each unit of the fleet, combining speed of mechanism with speed of human reaction, attempts to slip through at the crucial moment and yet maintain its own safety.

(ed note: the important point being that the etheric inertia shields will stop energy weapon fire, but will do nothing to stop material objects like bullets and shells.)

     Loara Filip Sanat knew all this and more. Since his encounter with the battle cruiser on the way out from Earth, he had studied space war, and now, as the battle fleets fell into line, he felt his very fingers twitch for action.
     He turned and said to Smitt, “I’m going down to the big guns.”
     Smitt’s eye was on the grand ‘visor, his hand on the ether-wave sender, “Go ahead, if you wish, but don’t get in the way.”
     Sanat smiled. The captain’s private elevator carried him to the gun levels, and from there it was five hundred feet through an orderly mob of gunners and engineers to Tonite One. Space is at a premium in a battleship. Sanat could feel the crampedness of the room in which individual Humans dove-tailed their work smoothly to create the gigantic machine that was a giant dreadnaught.
     He mounted the six steep steps to Tonite One and motioned the gunner away. The gunner hesitated; his eye fell upon the purple tunic, and then he saluted and backed reluctantly down the steps.
     Sanat turned to the co-ordinator at the gun’s visiplate, “Do you mind working with me? My speed of reaction has been tested and grouped 1-A. I have my rating card, if you’d care to see it.”
     The co-ordinator flushed and stammered, “No, sir! It’s an honor to work with you, sir.”
     The amplifying system thundered, “To your stations!” and a deep silence fell, in which the cold purr of machinery sounded its ominous note.
     Sanat spoke to the co-ordinator in a whisper, “This gun covers a full quadrant of space, doesn’t it?”
     “Yes, sir.”
     “Good, see if you can locate a dreadnaught with the sign of a double sun in partial eclipse.”
     There was a long silence. The co-ordinator’s sensitive hands were on the Wheel, delicate pressure turning it this way and that, so that the field in view on the visiplate shifted. Keen eyes scanned the ordered array of enemy ships.
     “There it is,” he said. “Why, it’s the flagship.”
     “Exactly! Center that ship!”
     As the Wheel turned, the space-field reeled, and the enemy flagship wobbled toward the point where the hairlines crossed. The pressure of the co-ordinator’s fingers became lighter and more expert.
     “Centered!” he said. Where the hairlines crossed the tiny oval globe remained impaled.
     “Keep it that way!” ordered Sanat, grimly. “Don’t lose it for a second as long as it stays in our quadrant. The enemy admiral is on that ship and we’re going to get him, you and I.”
     The ships were getting within range of each other and Sanat felt tense. He knew it was going to be close—very close. The Humans had the edge in speed, but the Lhasinu were two to one in numbers.
     A flickering beam shot out, another, ten more.
     There was a sudden blinding flash of purple intensity!
     “First hit,” breathed Sanat. He relaxed. One of the enemy ships drifted off helplessly, its stem a mass of fused and glowing metal.
     The opposing ships were not at close grips. Shots were being exchanged at blinding speed. Twice, a purple beam showed at the extreme limits of the visiplate and Sanat realized with a queer sort of shiver down his spine that it was one of the adjacent Tonites of their own ship that was firing.
     The fencing match was approaching a climax. Two flashes blazed into being, almost simultaneously, and Sanat groaned. One of the two had been a Human ship. And three times there came that disquieting hum as Atomo-engines in the lower level shot into high gear—and that meant that an enemy beam directed at their own ship had been stopped by the screen.
     And always, the co-ordinator kept the enemy flagship centered. An hour passed; an hour in which six Lhasinu and four Human ships had been whiffed to destruction; an hour in which the Wheel turned fractions of a degree this way, that way; in which it swivelled on its universal socket mere hair-lines in half a dozen directions.
     Sweat matted the co-ordinator’s hair and got into his eyes; his fingers half-lost all sensation, but that flagship never left the ominous spot where the hairlines crossed.
     And Sanat watched; finger on trigger—watched—and waited.

     Twice the flagship had glowed into purple luminosity, its guns blazing and its defensive screen down; and twice Sanat’s finger had quivered on the trigger and refrained. He hadn’t been quick enough.
     And then Sanat rammed it home and rose to his feet tensely. The co-ordinator yelled and dropped the Wheel.
     In a gigantic funeral pyre of purple-hued energy, the flagship with the Lhasinulo Admiral inside had ceased to exist. Sanat laughed. His hand went out. and the co-ordinator’s came to meet it in a firm grasp of triumph.

     But the triumph did not last long enough for the co-ordinator to speak the first jubilant words that were welling up in his throat, for the visiplate burst into a purple bombshell as five Human ships detonated simultaneously at the touch of deadly energy shafts.
     The amplifiers thundered, “Up screens! Cease firing! Ease into Needle formation!”
     Sanat felt the deadly pall of uncertainty squeeze his throat. He knew what had happened. The Lhasinu had finally managed to set up their big guns on Lunar Base; big guns with three times the range of even the largest ship guns—big guns that could pick off Human ships with no fear of reprisal.
     And so the fencing match was over, and the real battle was to start. But it was to be a real battle of a type never before fought, and Sanat knew that that was the thought in every man’s mind. He could see it in their grim expressions and feel it in their silence.

     It might work! And it might not!

     The Earth squadron had resumed its spherical formation and drifted slowly outwards, its offensive batteries silent. The Lhasinu swept in for the kill. Cut off from power supply as the Earthmen were, and unable to retaliate with the gigantic guns of the Lunar batteries commanding near-by space, it seemed only a matter of time before either surrender or annihilation.
     The enemy Tonite beams lashed out in continuous blasts of energy and tortured screens on Human ships sparked and fluoresced under the harsh whips of radiation.
     Sanat could hear the buzz of the Atomo-engines rise to a protesting squeal. Against his will, his eye flicked to the energy gauge, and the quivering needle sank as he watched, moving dowm the dial at perceptible speed.

     The co-ordinator licked dry lips, “Do you think we’ll make it, sir?”
     “Certainly!” Sanat was far from feeling his expressed confidence. “We need hold out for an hour—provided they don’t fall back.”
     And the Lhasinu weren’t. To have fallen back would have meant a thinning of the lines with a possible break-through and escape on the part of the Humans.
     The Human ships were down to crawling speed—scarcely above a hundred miles an hour. Idling along, they crept up the purple beams of energy; the imaginary sphere increasing in size ; the distance between the opposing forces ever narrowing.
     But inside the ship, the gauge-needle was dropping rapidly, and Sanat’s heart dropped with it. He crossed the gunlevel to where hard-bitten soldiers waited at a gigantic and gleaming lever, in anticipation of an order that had to come soon—or never.
     The distance between opponents was now only a matter of one or two miles—almost contact from the viewpoint of space warfare—and then that order shot over the shielded etheric beams from ship to ship.
     It reverberated through the gun level:

     “Out needles!”

     A score of hands reached for the lever, Sanat’s among them, and jerked downwards. Majestically, the lever bent in a curving arc to the floor and as it did so, there was a vast scraping noise and a sharp thud that shook the ship.
     The dreadnaught had become a “needle ship!”
     At the prow, a section of armor plate had slid aside and a glittering shaft of metal had lunged outward viciously. One hundred feet long, it narrowed gracefully from a base ten feet in diameter to a needle-sharp diamond point. In the sunlight, the chrome-steel of the shaft gleamed in flaming splendor.
     And every other ship of the Human squadron was likewise equipped. Each had become ten, fifteen, twenty, fifty thousand tons of driving rapier.

     Swordfish of space!

     Somewhere in the Lhasinuic fleet, frantic orders must have been issued. Against this oldest of all naval tactics—old even in the dim dawn of history when rival triremes had maneuvered and rammed each other to destruction with pointed prows—the super-modem equipment of a space-fleet has no defense.
     Sanat forced his way to the visiplate and strapped himself into an anti-acceleration seat, and he felt the springs absorb the backward jerk as the ship sprang into sudden acceleration.
     He didn’t bother with that, though. He wanted to watch the battle! There wasn’t one here, nor anywhere in the Galaxy, that risked what he did. They risked only their lives; and he risked a dream that he had, almost single-handed, created out of nothing.
     He had taken an apathetic Galaxy and driven it into revolt against the reptile. He had taken an Earth on the point of destruction and dragged it from the brink almost unaided. A Human victory would be a victory for Loara Filip Sanat and no one else.
     He, and Earth, and the Galaxy were now lumped into one and thrown into the scale. And against it was weighed the outcome of this last battle, a battle hopelessly lost by his own purposeful treachery, unless the needles won.
     And if they lost, the gigantic defeat—the ruin of Humanity—was also his.
     The Lhasinuic ships were jumping aside but not fast enough. While they were slowly gathering momentum and drifting away, the Human ships had cut the distance by three-quarters. On the screen, a Lhasinuic ship had grown to colossal proportions. Its purple whip of energy had gone out as every ounce of power had gone into a man-killing attempt at rapid acceleration.
     And nevertheless its image grew and the shining point that could be seen at the lower end of the screen aimed like a glittering javelin at its heart.
     Sanat felt he could not bear the tension. Five minutes and he would take his place as the Galaxy’s greatest hero—or its greatest traitor! There was a horrible, unbearable pounding of blood in his temples.
     Then it came.


     The screen went wild in a chaotic fury of twisted metal. The anti-acceleration seats shrieked as springs absorbed the shock. Things cleared slowly. The screenview veered wildly as the ship slowly steadied. The ship’s needle had broken, the jagged stump twisted awry, but the enemy vessel it had pierced was a gutted wreck.
     Sanat held his breath as he scanned space. It was a vast sea of wrecked ships, and on the outskirts tattered remnants of the enemy were in flight, with Human ships in pursuit.

From BLACK FRIAR OF THE FLAME by Isaac Asimov (1942)

      The attackers closed in. The Outsider raised screen. Word went back: The screen was Guardship quality.
     The Outsider could not have been in a poorer position. It could not deploy riders. A more powerful enemy lay between it and access to the Web. And it was deeper in the gravity well.
     Attackers englobed the Outsider. They floated just meters off the screen. XXVIII Fretensis rotated to present its broadest face, closed to three hundred meters. At that range even the most inept Twist Master could not miss.
     A hundred pulsating green eyes bumed on the Guardship’s face.
     WarAvocat XXVIII Fretensis ordered his Hellspinners loosed. Those balls of mad energy drifted onto the Outsider’s screen like the slow fall of a fine oil mist onto the surface of a summer-warmed pond. Rainbow points spread and faded slowly. Fighters darted to the impact points like fish to motes of food. They pounded those spots, probing for an opening or weakness.
     The screen withstood the salvo. But the Twist Masters had permission to loose at will. No screen could absorb Hellspinners long.
     The Outsider finally grasped the gravity of its situation. It began to move.
     Its assailants moved with it.
     Here, there, soft spots in the screen yielded. A one-meter gap opened and persisted for seven seconds. An interceptor put one hundred rounds of 40mm contraterrene (old term for antimatter) shot through the hole. The Outsider’s skin blossomed, a garden of small fires.
     Other gaps opened. Some attack craft chose marksmanship, gunning for specific installations. Others just blazed away. None tried running the gaps. A screen shielded both ways. A fighter inside would become the target of every Outsider weapon otherwise unable to fire.
     The gaps grew larger and lasted longer. The Twist Masters began pairing, hoping to get a second Hellspinner through a gap cut by a first.
     A Tregesser ship, crawling the outer surface of the screen, laying down continuous fire, exploded.
     “How did they do that?” Tregesser shrieked.
     “He got too close, running with his own screen down. They fired a CT burst and opened a port just long enough for the shells to pass through. Our ship shaded the port.”
     “Its screens are holding, Simon."
     “For how long? Eh? What’re they doing?”
     Slivers had begun sliding over the surface of the Guardship, behind its screen, roilsome as maggots in a carcass.
     “Launching fighters. Holding them inside the screen.”
     “Why? They can’t get them out.”
     Tregesser said, “Those fighters are like bugs on the inside of a light globe.” Then, “Hey! They’re launching.”
     It was called a bubble-through launch though neither Provik nor Tregesser had heard of the tactic. It was used only by Guardships with little or no concern for living crew: I Primagenia, III Victrix, IV Trajana, XII Fulminata, others gone extremely strange. Losses in a bubble-through were heavy.
     Fighters came out with their own screens maxed, osmotically. The gaps they exited never opened bigger than fighter and screen. The Guardship risked little. But fighter screens were of a lesser grade, and the ships they protected were easy targets for a moment. If they did not get through fast and start dodging, they were dead.
     A lot got dead this launch.
     But then the survivors were everywhere, making life miserable for the attackers, forcing them behind their own screens.
     “They’re as crazy as your damned suicide squadrons,” Lupo said.

From THE DRAGON NEVER SLEEPS by Glen Cook (1988)

   Force-fields - or shields, screens, deflectors, etc. - have been a recurrent trope in SF since the creation of the genre; scarcely has there been a mainstream novel or movie that does not feature them to some extent.  Like many other SF magitech devices force-fields often shape the story and 'Verse in which it is set; without planetary defence shields the Rebel Alliance on Hoth would have had little chance against the might of the empire.  Star Wars is not alone, the universe of Dune, Star Trek, and countless others use the technology in a unique way.  Usually the factors and applications considered revolve around the primary usage of forcefields in SF, defensive measures for military vessels.  In this article I will look at a few, most of which have admittedly little impact overall on the 'Verse in which they are used, but which help to anchor the story and the readers attention firmly in the future world.

   For the sake of simplicity the forcefields in this blogpost are assumed to have the following characteristics; can be projected in a variety of geometrical shapes, have mechanical strength, repel kinetic and radiant energy.  It is also assumed that either the forcefield is invisible when in matter-repelling mode, or it can be tuned to block or admit certain frequencies of light.
  • Airlocks:  this has been seen in both Star Trek and Star Wars, and might be one of the least stupid of the non-defensive ways that forcefields have been used.  The entrance to the hangar bay has physical doors, a forcefield that allows slow moving shuttles to push through is activated when required.  As air molecules move quite fast, they cannot penetrate the field, and so pressurisation is maintained.  However, everyone had better remain in p-suits; a blown fuse could otherwise result in the inconvenience of explosive decompression.  Note that a real world device called a Plasma Window can achieve much the same result. 
  • Landing Gear:  this example comes from the SF comedy Galaxina, and is employed by the spaceship Infinity to overcome the problems with landing on uneven ground.  Like the forcefield airlock this is well and good, until the power fails and several hundred or thousand tonnes of spaceship crashes to the ground.  While it would be somewhat foolhardy to equip a normal spaceship with these, they could serve for special landings where the ground is unhardened, say for military or rescue missions, with the forcefield acting like a futuristic giant snowshoe.
  • Structural Reinforcement: the ships of Star Trek's Federation are much maligned among engineers and those of scientific bent for their structurally stupid design.  The narrow struts connecting the hulls and warp nacelles are under incredible stress in any manoeuvre, and to cope with this the ship uses Structural Integrity Fields.  Again, good until the power goes out, which is probably when you don't want the ship snapping into three pieces. 
  • Emergency Containment:  although - yes, power (requirements) again - physical barriers are probably preferable for containing anything from prisoners to poisonous gasses, forcefields might prove indispensable in an emergency.  If they can power up in instants they could seal off an area far sooner than ponderous blast or containment doors, failing to seal off of dangerous situation in moments. 
  • Life Support:  the vacuum of space makes simple jobs a nightmare, pushing the costs or orbital construction sky high.  If a starship under construction in orbit could be enclosed with a forcefield just strong enough to contain an atmosphere, matters would be much simpler.  Even though for safety p-suits would still be worn, the 'air' could equally easily be there to enable plasma cutting as (well as) breathing.  A forcefield on a planet could be used as an emergency shelter from natural attacks such as volcanos, tsunamis, and hurricanes.
  • Power & Drive Reactors: the lower limit to the size and weight of a nuclear fission power supply is the critical mass needed for a sustained reaction.  However, if the forcefield is a perfect neutron reflector it is easy to see how it could cause even a few grams of uranium or plutonium to fission.  The resulting plasma could be confined by the forcefield and piped through a magnetohydrodynamic generator, the whole package limited in size only by the forcefield generators.  Fusion could be treated in the same way, opening up the way to abundant clean energy.  The most obtuse of the implications that result from this is the development of torch drive spacecraft, which I will discuss in a future post.
  • Energy Storage: antimatter is often proposed as the ultimate starship fuel; a misleading statement.  Like the use of hydrogen as car fuel, antimatter acts only as an energy storage device, a battery.  Starships need astronomical amounts of energy, so antimatter is used as giving the ultimate power-to-weight.  Create a hollow container from a forcefield, fill it with a vapour that absorbs a frequency of light allowed to pass through the field.  Energy is added to the 'battery' with lasers, and thanks to the phenomenon of electron shells the light released by the vapour is unable to pass the field, most of it at a different than original frequency.  A device such as this could have no limit to the energy contained, making 'nuclear hand-grenades' look like damp firecrackers.
  • Directed Energy Weapons: as anyone familiar with the Kzinti Lesson knows, the exhaust plume of a starship or torch drive spaceship is as deadly a weapon as can be found.  The Fission Fragment Rocket, possible to build in the real world, has an exhaust velocity of a few % of c, making it a deadly beam weapon at close range.  By piping the plasma from a fusion reaction out through a forcefield nozzle a devastating weapon could be created; it disadvantage a fairly short range due to the dispersion of the beam.
  • Airships: if a forcefield has no mass in and of itself, it makes the perfect airship hull.  Possibilities include vacuum airships, high temperature thermal, airships that change shape to attain supersonic speed, etc.  Nor would an airship built in this way suffer from the fragility of conventional designs, making it much more practical than any physical airship. 
  • Re-entry Shielding: for a large or fast spaceship a re-entry shield is much too heavy, despite the advantage in fuel reduction that the Leonov demonstrated.  A forcefield, especially if already fitted for military defence, is a perfect substitute, and if power fails, you're probably screwed anyway.  By allowing aerobraking the available delta Vee for a mission could be doubled, or the fuel load halved; a significant improvement.  Also, as it could be much larger than the actual ship, and of a more aerodynamic shape, g-load could be altered to be less taxing.
  • Cassions & Civil Engineering: a cassion is a temporary dry-dock of sorts, built to enable the construction of underwater structures.  A forcefield by its virtue of easy deployment - on/off switch - is ideal for rapid or emergency construction.  
  • Tools: this is seldom seen, the only example I have personally come across being in Asimov's extensive 'Verse, where forcefield tools are far more capable than mechanical counterparts.  The advantages of such theoretical tools are many, and are discussed on Atomic Rockets.
  • Ramscoop:  the greatest challenge of constructing a Bussard Ramjet, one of the most advanced and powerful starship designs, is the construction of a magnetic field capable of collecting the interstellar hydrogen needed by the design.  A forcefield could be just the solution, especially if it is massless and frictionless.
  • Solar Sail: the solar sail is an interesting design that cannot ever come to its full potential due to material constraints.  In essence the solar sail uses the momentum of reflected sunlight or solar wind, having theoretically infinite delta Vee.  However, the mass of the sail, combined with the inability of find a material that will withstand solar heat at the closer, and thus more effective, distances means it is not likely ever to be widely used.  A forcefield, however, might be massless and perfectly reflective, making it the ideal method of producing a solar sail, and thus providing a reliable method of interstellar transport.  In fact, this is such an effective interstellar design that I have decided to do a post on Interstellar Transport, featuring such a design and explaining its implications. 

  • (ed note: and as he points out in a later post, force fields allow Torch Drives)

From FORCE-FIELDS; NOT JUST DEFENCE by William Moran (2014)

What do they do?

Before embarking on any speculation of mechanism, we should first determine what a shield does. Shields in sci-fi generally serve 2 purposes:

  1. Stop "energy weapons" (lasers, phasers, blasters, etc).

  2. Stop physical objects (bullets, knives, people, vehicles).

Fair enough, but what does it mean to "stop" energy weapons and physical objects? Physical objects tend to stop abruptly when they hit a shield, but not always: sometimes the "slow blade passes" concept from Dune seems to be in effect, and objects can slip through (see the Gungan dome shields in the Battle of Naboo). Sometimes incoming objects bounce, and sometimes they explode on impact. And what about "energy weapons"? Some ships (such as the Trade Federation battleship in the end of TPM) look as if they're taking every hit right on the hull, but their shields are said to remain up, as if they are coincident with the hull surface. Others project their shields out into space, and these shields light up in a multi-coloured display when hit.

Are shields forcefields?

The most popular candidate for shields is forcefields. In fact, shields and forcefields are often treated as interchangeable terms in the literature and dialogue. This is encouraging, because the term "forcefield" comes from real-world science, not science fiction. Unfortunately, the resemblance between real forcefields and sci-fi forcefields ends at the name.

The two types of forcefield you are most familiar with are electromagnetic and gravitational. Sure enough, those are the forces routinely mentioned in sci-fi. In the 1950's classic "War of the Worlds", the Martian spacecraft were said to be using an "electromagnetic blister", which easily warded off artillery shells and all other methods of attack. In Star Trek, the writers gleefully steal terminology from particle physicists and say that they're based on "gravitons" (the theoretical carrier particle for gravitational forces).

But electromagnetic and gravitational forcefields share an interesting characteristic: they are both long-ranged, and their effects weaken with the square of distance. So if you double your distance from the centre of the Earth, the force of gravity drops to one quarter of its original value. Simple enough, correct? Unfortunately, this creates a problem for our shields: you see, they typically have no effect whatsoever until you reach some invisible point. When a man runs into a forcefield on Star Trek, he feels nothing until he touches the invisible wall, which produces a sparkly effect.

Now, if this were a gravitational forcefield, he should have felt its effects from anywhere in the room, gradually increasing in strength as he approaches the window. A forcefield has a volume effect, hence the name "force field", not "force wall". But this is obviously not what we saw. Have you ever tried to force the positive poles of two magnets together? You can't do it, can you? And you will notice that the forcefield effect is gradual, not abrupt. It gradually increases as they approach, until it eventually becomes so large that you cannot force them any closer together.

And what about the fact that they wear down? We've all seen the displays: DEFLECTOR POWER 70%

In Star Trek, rather than simply being up or down, shields have a strength property. It wears down after multiple hits, and when it goes to zero, the shields are nonexistent. But why would a forcefield weaken after use? Does the magnetic field of an electromagnet get weaker each time you use it to pick something up? No, so why would shields get weaker? Is there a "fatigue" property? None of this is consistent with a forcefield.

Are shields made of energy?

Rather than imagining shields as forcefields, some people imagine them as a "wall of energy". That seems like an improvement (after all, there are no real walls of energy which we can use for comparison, so it's not as easy to say that it's wrong), but even if we disregard the question of how you would go about constructing this beast, some obvious questions leap to mind:

  1. What holds this energy in place? Pure energy is light, and moves at c. It does not sit in a particular spot, nor does it form walls of arbitrary shape. If you had some kind of mechanism which could control the energy and force it to move in a contour around the ship, why bother with the energy component? This mysterious energy-manipulating mechanism would obviously be capable of deflecting incoming energy weapons by itself if it can already manipulate a wall of energy to hold an arbitrary shape.

  2. Why aren't incoming objects destroyed by this energy? Not only can Picard touch one of these shields with his hand, but incoming objects such as Roga Danar's flimsy escape pod in "The Hunted" have been observed to bounce off a starship shield with no ill effects.

  3. Where does the energy go when they turn off the shields? If it's released, should it not be quite violent?

  4. Why would the energy necessarily interact with other energy?

The problems with the "wall of energy" idea are extremely difficult to resolve for many reasons, not least of which is the fact that the mechanism for manipulating the energy into a shield would perform the function of a shield all by itself.

What about frequency?

Star Trek shields have a "frequency" characteristic, which implies that they oscillate. It should be noted that this behaviour is unusual to Star Trek, and there is no reason to assume it is universal to all shield concepts. Many natural phenomena are frequency-based, but even a device based on a frequency-based principle need not be phase-coherent, so it would not exhibit an aggregate "frequency" characteristic. There are some general advantages and disadvantages of frequency coherence in shields:



Against a frequency-based attack, a phase-coherent shield could be theoretically optimized to give greater protection than a flat shield with the same average amplitude, by synchronizing with the attack.

Against a non-coherent attack, a phase-coherent shield would allow partial penetration even if it's working perfectly.

It should be possible for a ship to fire outgoing weapons out through its own phase-coherent shield by matching frequencies but being 180 degrees out of phase. You would have to open a small hole in a flat shield in order to fire through it, which requires fine control over shield geometry.

The knife cuts both ways. An attacker could penetrate a phase-coherent shield by matching frequencies and being 180 degrees out of phase.

Note that it's possible to oscillate with respect to amplitude or vector, although we would expect a vector oscillation would cause significant scattering effects with outgoing beams (turning a tight beam into more of a spray), and we generally don't see that with Trek weapon/shield interactions, or those of any other sci-fi series for that matter. A square-wave (as opposed to sinusoidal wave) would allow perfect penetration with a synchronized weapon, thus eliminating the scattering effect of a vector oscillation, but it would also allow 50% penetration from an incoherent weapon.

There are interesting theoretical possibilities for a shield which oscillates, but the disadvantages outweigh the advantages, depending on what kinds of weapons the enemy is using, what kind of control you have over shield geometry, etc. Worse yet, if the enemy has sensors which can detect the activity of your shields, it should be trivially easy for him to match frequencies, synchronize phase, and shoot through your defenses. Ultimately, the idea of a phase-coherent frequency-based shield seems more attractive for its script-writing flexibility than its tactical attributes.


Sci-fi is diverse, and not all shield systems do the same thing. But there are a few questions you can ask to narrow down what basic phenomenon a shield represents (note that this is different from trying to invent some technobabble explanation for how it works).

  1. Do energy bolts or beams "bounce off" the shields, still intact? If so, you are looking at reflection (for examples, see the trash compactor scene in ANH, or the battle droid shot which ricocheted off Anakin's Naboo starfighter in TPM).

  2. Do energy bolts or beams splinter, or break apart into a shower of smaller bolts? If so, you are looking at scattering (for an example, see the ISD turbolaser bolt that struck the blockade runner's shields in the opening scene of ANH).

  3. Do energy bolts or beams make a large area of the shield glow? If so, you are looking at absorption, conduction, and subsequent retransmission (for examples, see most TNG-era Trek shield incidents, as well as the incident in TESB where the Falcon was knocked about its longitudinal axis by a turbolaser hit).

  4. Does shield geometry affect the shield/energy interaction? If so, it may be a vector effect, deflecting the bolt in specific directions based on geometry (phrases like "angle the deflector shield" in SW or "continuously vary the shield geometry" in ST hint at this possibility).

  5. Does the shield completely block incoming energy, or does it allow a portion through even if it is still functional?

Ultimately, there are far more questions than answers when it comes to shielding, and one must be careful not to leap to facile conclusions.


Shielding is more complex than it may appear on first glance. There are more issues to consider, and more difficulties involved in evaluating strength than one may initially realize. However, this hardly means that the exercise is futile. On the contrary, a thorough and systematic examination of observed events can be used to determine realistic limits, given certain caveats:

  1. The fact that they often call it a "forcefield&quoquot; does not mean it actually conforms to the description of one.

  2. There is no intrinsic need for a great volume of energy in a shield, so it is wrong to assume that shields must consume large amounts of energy. Do not become attached to a particular model of shields simply because everyone else seems to accept it. A solid case can even be made for the idea that shields have mass.

  3. Always remember to consider the weakest link in the chain, not the strongest link. This is an important lesson from real-life engineering which is often lost on sci-fi debaters, who tend to conceptualize sci-fi in a purely abstract theoretical sense in which they pick one particular phenomenon and concentrate on that phenomenon to the exclusion of all others (ie- focus on the non-physical shields and ignore physical constraints).

  4. Do not assume that all energy shields employ the same mechanisms. We can view the behaviour of energy shields in action and see that there is significant diversity in their operation, and this must be considered when attempting to synthesize a consistent model of their operation.

  5. At no point do any of these theories require that the shield must draw as much energy from the ship's systems as the incoming weapon carries with it. Yet I have often noted that virtually everyone assumes this energy equivalency to be the case, without making the slightest attempt to explain the logic. Why should a shield require energy equivalent to the weapon? Does a piece of armour on a tank consume energy when a shell bounces off its surface? Did you ever wonder why an air conditioner's rate of cooling can exceed its electrical power draw in watts?

Before making any leap in logic about how much power a shield must need or how it must work, just ask yourself whether your assumptions are coming from observation and logic, or from common practice. Because ultimately, common practice is a poor justification for anything.

From PHYSICAL NATURE OF SHIELDS at by Michael Wong (2002)


It is widely assumed that if a sci-fi shield can withstand X joules of energy from a laser, it must be able to withstand X joules of energy from a physical impactor. However, this is not necessarily the case. As attractive as the simplistic numbers game is, if we apply a little bit of physics knowledge to the situation, we can see that if anyone were to build such a beast, the situation would be more complex than that.

So what would make physical impactors more dangerous? The answer to that question comes down to damage mechanisms. To put it simply, a physical impactor inflicts damage upon its target in a variety of ways. While an energy weapon will generally attempt to heat the target, thus permitting specialized one-dimensional defensive strategies, a large, fast-moving physical impactor presents a more complex threat:

Threat type

Damage mechanisms

Energy weapon (laser, phaser, turbolaser bolt, etc.)

Heats the target surface.

Physical impactor (asteroid, high-velocity ramming attack, hyper-velocity railgun, etc.)

Subjects the target to severe structural stresses, usually resulting in penetration. If it fails to penetrate, it pulverizes and/or vapourizes at the point of contact due to internal stresses and work-heating, thus producing a large cloud of high-temperature material at the target surface. This cloud heats the target surface through convection and radiation.

That is why an effective defense strategy would use guns to destroy large physical impactors, forcefields to deflect small physical impactors, and shields to reflect, absorb and retransmit, or scatter energy weapons rather than the "one size fits all" approach that seems to be popular among fanboys.

Collision Physics

When a physical object strikes a shielded vessel, it must be decelerated by the vessel's defensive systems. Most people tend to assume that if the shield holds, the ship is undamaged. However, this is not necessarily the case. Consider the following image (and please, keep in mind that I am not a professional graphic artist):

Let's assume that the rectangular assembly at left is a shielded starship (yes, I know, it looks cheesy, but please bear with me). The big brown rock at right is hitting the ship's shields, and it is being decelerated (hence the rightward force F being applied to the rock by the forcefield). For every action, there is an equal and opposite reaction, so there must be a counter-balancing force for that forcefield. A forcefield must be coupled to something, and in this case, it would obviously be the shield generator. Therefore, there is a leftward force F being applied to the shield generator (the blue square) in the middle of the ship. But the shield generator cannot move relative to the ship or it would be torn loose from its moorings, so its mounting brackets (the four red blocks) must each apply a rightward force 0.25F in order to hold the shield generator in place. These four reaction forces, in turn, push the entire ship to the left with force F, so the net result is to stop the impactor while accelerating the ship.

Are we clear on that? Now here's where it gets interesting: what if the shield generator's projected forcefield is easily strong enough to decelerate the asteroid to zero before the moment of impact, but the four little red blocks aren't strong enough to hold the generator in place? Guess what: the shield generator will be torn from its moorings, and the rock will slam into the ship. This is where momentum can rule over energy; a low-momentum, high-energy weapon such as a laser might not be as dangerous to a shielded vessel as a high-momentum, low-energy physical impactor. In this scenario, the potential points of failure are the shield generator itself, the points where it is mounted to the vessel, and the structure of the vessel itself. In other words, the mounting brackets, bolts, welds, shield generator internal mechanisms, shield generator forcefield strength, and all other connecting bits are parts of a chain through which reaction forces must go in order to make the end-to-end connection between the ship and the impactor. It can be thought of as a chain, and as in any chain, it is the weakest link that will cause your downfall.

As you can see, even if it was possible to build a deflector shield generator of virtually infinite strength, the overall effectiveness of the system would still be limited by good old-fashioned structural limits. Ultimately, the survivability of a shielded spacecraft against physical impacts could (and would, given sufficient shield strength) conceivably come down to a set of bolts holding a shield generator onto the ship's spaceframe. This example highlights the severe problem with most attempts to rationalize sci-fi technologies, which is that people tend to look for the strongest link in the chain, not the weakest link in the chain.

Shield Collision Physics Summary

Physical impacts and energy weapons should not be treated as functionally identical, particularly in terms of the relationship of energy to structural stress in the target. Collision physics are still ruled by Newton, and all of the deflector shields and fancy tricks in sci-fi will not prevent reaction forces from acting upon the physical structure of a target spacecraft.

Ramming tactics are widely used in sci-fi (click here for an example analysis of a collision event). In Star Trek, Worf called for "ramming speed" in STFC, Jem'Hadar vessels rammed the USS Odyssey and destroyed it in DS9, and Commander Riker prepared to ram the Borg Cube in the TNG two-part episode "Best of Both Worlds". In Babylon 5, we saw a Starfury crash into and through a Minbari war cruiser's dorsal fin in the Battle of the Line as shown in the movie "In the Beginning", and Jeffrey Sinclair tried to ram another war cruiser later in that same battle. We also saw an Earth-force cruiser ramming a Minbari war cruiser in a brief flashback to the events leading up to that battle. Other examples include Battlestar Galactica, where Cylon raiders routinely crashed into the Galactica's flight decks (thus making the viewer wonder why there were no weapon emplacements near these flight deck entrances), Transformers (where the Autobots' stolen Quintesson corkscrew-ship rammed through Unicron's eye), and of course, ROTJ, where an A-wing crashed through the bridge windows of the Executor after its bridge shields were knocked out (which would imply that the Executor's unshielded bridge windows are similar in strength to the dorsal fin of a Minbari war cruiser).

The effectiveness of these popular ramming tactics has often been used as an excuse to downgrade shield estimates against energy weapons. But this implies an equivalency which does not exist. The "real-world" explanation for the effectiveness of ramming in sci-fi is that ramming is a very dramatic event, filled with imagery of martyrs and heroes. But the physics of collisions and reaction forces provide us with an "in-universe" explanation that works just as well.

From SHIELDS: PHYSICAL IMPACT ISSUES at by Michael Wong (2002)

For instance, the bridge set was one of the best designed science flction sets in motion picture history. There are few that can equal, let alone surpass, the bridge of the U.S.S. Enterprise. (Offhand, I can only think of two: the interiors of the spaceships used in Forbidden Planet and 2001; A Space Odyssey.)

As a design for the control of a giant starship, the bridge is a model of logic and efficiency. The Captain's eyes are before him in the form of a giant viewscreen. So is the pilot console, which is the equivalent of his hands. The Science Officer is to his right, ready to present whatever information is needed. Communications and Engineering are to the rear, right and left respectively, where they are out of the way, but convenient.

The upper walls are lined with information screens and the Captain's chair swivels so he can survey the whole bridge easily. The lights and controls on each panel are set in curved banks—curved to match the reach of the human arm. Whatever operator is seated at a console, he will find all of his controls in the most convenient possible position.

So workable is this design that the United States Navy sent a delegation to the studio to examine the bridge set in detail. They were considering a similar layout for a new aircraft-carrier bridge. (The new Enterprise maybe…?)

That's the brilliant part.

The clumsy part is that so many writers and directors continually misused this beautiful set.

Example: (A familiar scene.) The Enterprise is under attack. She's hit by a photon torpedo—kaboom!—everything tilts and everybody falls out of their chairs! They climb back into them and another torpedo comes zooming in—kaboom!—again, they're knocked to the floor! A third time—kaboom!—the camera tilts and they all fall down again!

And Scotty reports, “All defenses out, Captain. The next one will get us for sure.”

Several years ago, Bob Justman, associate producer of the show, was asked by a fan about this: “Why don't you put seat belts on the chairs?"

“Because,” he replied, “if we did, then the actors couldn't fall out of them."

—But the above scene—and Bob Justman's easy answer—are wrong. Both scientifically and dramatically. There has been little thought put into either.

From a scientific standpoint, the scene is fallacious. Each of those torpedoes would have had to have been a direct hit in order to shake the Enterprise. If they had been misses—even near misses—the ship wouldn't have been shaken at all. Shockwaves don't travel through the vacuum of space. Hence, in order to shake the ship, they must have been direct hits. If they were direct hits, the ship should have been destroyed three times over.

Or try it another way. Let's assume that a near miss does have the power to jolt the Enterprise. But the Enterprise has an artificial gravity—no, not just an artificial gravity, a whole force field to neutralize the effects of momentum, acceleration, and inertia. Assuming the speed and maneuverability already postulated for the vessel, a protective force field is a necessity to keep the ship's crew from being smeared into jelly every time she makes a rapid change of direction or speed. If this is so, a near miss with a photon torpedo would no more be able to rattle the crew of the Enterprise than it would be able to dislodge a fly trapped in amber.

(And assuming that such an artificial gravity/force field did exist, isn't it amazing that it was never knocked to hell and gone by one of those missiles?)

Actually, I could just as easily argue the other side of the question:

Well, you see, the Enterprise had her shields up. The torpedoes exploded against the ship's force screens and the shock of the explosion was transmitted to the ship via the shields. Or: the shields didn't stop all of the explosion, part of it leaked through. Just enough to shake everyone, but not kill anybody.


It doesn't really matter. Whichever side of the question you argue, it's only doubletalk. But when doubletalk is designed to justify a piece of bad writing, that's a reprehensible cheating of the viewer. Let's make it look exciting. We'll have all the actors fall out of their chairs. Three times."

The same scene could have been a lot more tense—and a lot more believable as science fiction—if it had been written with even the simplest awareness of the postulated capabilities of the Enterprise.

The purpose of the scene is to dramatize the menace of the attacking ship. Fine. Very simple. Instead of three missiles, just one. The Enterprise tracks it all the way in—and can't stop it. The officer at the weapon control console (let's put him at the upper left side of the bridge) reports all of his attempts to intercept the torpedo—phasers, anti-missile missiles, force screens, tractor beams, and various other doubletalk devices—and also reports the failure of each device to stop the approaching torpedo. The torpedo strikes the Enterprise's last set of screens and detonates, and in the process knocks out that line of defense. This is all reported on the bridge—at the moment of detonation, nobody falls out of their chairs and no sparks fly from any panels (somebody has invented fuses). The lights simply dim momentarily, then come back up again as (implied) the emergency power supply cuts in.

Kirk calls for a status report and Scotty replies: “All defenses out, Captain. The next one will get us for sure."

And that brings us to the exact same place that the first scene did.

The major difference is one of credibility. The scene with the people falling out of their chairs is visually exciting, but is not dramatically valid. We are abruptly and arbitrarily told that the Enterprise 's defenses have been knocked out.

The latter scene—admittedly a harder one to write—shows how those defenses are taken out, and in the process, it builds to a tense climax. The scene involves the viewer in the destruction of each line of defense and makes him more a participant in the action. The real point of the scene is to leave Kirk with no defenses at all—that's when he has to open his hailing frequencies and start talking. Fast. But it feels a lot better if the writer has brought us to this point honestly.

Science fiction is not a western with ray guns and spaceships. It is a genre so demanding that few of its practitioners are more than moderately competent at it. The responsibility to be logical and scientifically accurate, while at the same time telling a good dramatic story, will continually defeat any writer who approaches the field with less than total respect for its requirements.

Because so few screenwriters are well versed in science fiction, STAR TREK should have had a full-time science fiction writer on its staff, someone with a good background in science as well as science fiction. Such a staff advisor could have worked closely with the writers, the directors, the producers and the actors, as well as the designers and the decorators, to make everything as logical, believable, and accurate as possible.

There were too many instances of clumsiness in STAR TREK'S production that a science fiction advisor could have corrected. Doubletalk cannot disguise bad writing—and it will only hurt good writing. It's a little harder, yes, to be accurate, but the results are worth it.

From THE WORLD OF STAR TREK by by David Gerrold (1973)

Langston Field

In space combat it pretty much looks like the first to get a hit wins. This isn't really surprising; it's true of most combat these days (air combat, submarine combat, etc.). The weapons will be devastating enough that one hit will put a ship out of combat, if not vaporize it outright (i.e., they will have a very high Single Shot Kill Probability).

Larry Niven and Jerry Pournelle knew this, but wanted to write about dramatic extended space combat anyway. They contracted physicist Dr. Dan Alderson to design a self-consistent science-fictional gadget to allow this. He created the Langston Field.

In the SF trade, the Langston field is a "capacitor" or "tank" field. The field drinks up energy. It will absorb a laser beam, a nuclear blast or the kinetic energy in a coilgun shot. It then tries to radiate the energy away. However, the field cannot radiate away the energy as fast as the enemy can load the field with weapons fire. The field can only hold so much, and when the limit is reached, the field explodes, vaporizing the ship.

Also, the more destructive energy currently being held in the field, the more of the ship's own power that will be required to keep the field from exploding. If the field gets too full, the ship will not have energy to spare for movement or its own weapons.

Temporary "portals" or "holes" can be opened in the field to allow the ship's laser fire to hit enemy starships. Otherwise the laser beams will hit the underside of their own field. Of course the more energy being held in the field, the more difficult it is to open a hole.

Sensors are on booms so they can be extended outside of the field, otherwise the ship is blind. As the exposed sensors are blown away, the booms are retracted and fresh sensors are mounted. If the attack is ferocious enough, a ship can become blinded (i.e., all exposed sensors destroyed before any new ones can be deployed), and the enemy will quickly move out of the path of the ship's weapon fire while still pouring death and destruction into the blind ship's field. Then the blind ship frantically tries to deploy enough sensors so that at least one will last long enough to plot the position of the attacker.

Unfortunately, if the field becomes too full of energy, sensors or any other item being extended through the field will be fried or vaporized by the contained energy.

A hot field will also fry any object attempting to pass through the field en route to the ship inside (such as a shuttle containing a boarding party). Any object would also become embedded in the field, since the field also absorbs kinetic energy, unless is was moving really fast.

In a nod to E.E."Doc" Smith, when radiating, the field starts glowing red, then moves its way up the rainbow spectrum. In reality, the field would probably glow as a blackbody spectrum, which is not quite as cinematic.

The only thing a blinded ship can see is the color of the inside of its field. Which means any crew who can see the shield can estimate how close the ship is to being utterly destroyed, which can't be doing their level of morale any favors.

Note the implication. When a ship's field is ten seconds from detonation, the ship is near death. But nothing has been physically damaged. If the ship is left alone long enough the field will cool off and the ship is as good as new. This made surrender a tricky proposition. If you gave too much mercy to the surrendering ship, it would recover and you'd be right back where you started.

The solution was interesting. If a ship with hot fields surrendered to you, the captain asks for a volunteer from the midshipmen. If nobody volunteers, the captain shrugs and signals to destroy the enemy ship anyway. But if there is a volunteer, they get to strap on their chest a tactical nuclear weapon with a hand detonator (dead-man switch or other fail-deadly type). Under pain of destruction, the surrendering ship has to allow the midshipmen to board, and let the midshipmen go to the control room or other vulnerable spot. You can now allow the surrendering ship's field to cool off. If it doesn't do exactly what you say, the midshipmen will detonate the bomb (you hope).

For dramatic purposes, Dr. Alderson decreed that the Langston field was subject to "local burn-throughs". That is, a given weapon strike might be too intense to be absorbed all at once, so a fraction of the damage pokes through the field into the ship. This gives enough damage to the ship to be cinematically interesting, but not enough to vaporize the ship outright or something boring like that.

This had the intended side-effect of ensuring that the ship with the best damage control crew would win the battle.

As a bonus, the Langston field allows the creation of a photon drive with torch-ship performance.

The Langston field may be science fiction, but at least it is internally self-consistent. Niven and Pournelle used it in their novel "The Mote in God's Eye", which Heinlein said was "possibly the finest science fiction novel I have ever read." High praise indeed.


      The Langston Field (aka the Field) is a force field in the shape of a sphere, egg or more generally an ellipsoid. Its function is to absorb, store and release electromagnetic energy in a controlled manner. Electromagnetic energy includes photons, the kinetic energy of particles (both the random kinetic energy of heat and nonrandom kinetic energy) as well as magnetic and electrostatic fields. It is able to absorb energy from both real and virtual photons (hence its ability to absorb kinetic energy). It releases its stored energy as black-body radiation.

     The Field’s black-body radiation (frequency/color) depends on its temperature. In turn, the temperature depends on the amount of energy it has absorbed and is releasing and the Field’s surface area. At room temperature or below, the Field would appear black because the energy it absorbed would be released as infrared radiation (which we cannot see). As the temperature increases the frequency of the light (color) moves up from infrared, to red and then through the visible spectrum toward blue-white. If the Field’s temperature then decreases through the net release of energy, the color moves back down the visible spectrum toward infrared. The color green does not show up much in a black body spectrum and would rarely be seen. The Field has the appearance of a large balloon (as indicated above that doesn’t necessarily mean spherical). Around a city at room temperature, where half of the Field extends underground, it would appear like the top half of a buried black balloon. Around a spaceship, again at room temperature or below, it would appear to be a large black balloon.

     The Langston Field is analogous to a black hole but solely for electromagnetic phenomena. Think of it as an electromagnetic event horizon or optical singularity. The Field’s black-body radiation is similar to Hawking radiation. Real and virtual particles associated with the other four forces (i.e., strong force, weak force, gravity and Alderson force) in the CoDominium/Empire of Man universe are not affected by the Field. Although the Field’s energy release can be controlled the Field can still be damaged or destroyed. The integrity of the Field can be temporarily damaged (called a burn through) by a very quick and high level of localized energy input (like a big nuke). As a result a burst of energy can penetrate into the ship. The Field can experience a catastrophic failure (called an overload or collapse) if too much energy is pumped into it in too short a period of time, energy that it doesn’t have time to safely release. When the Field collapses, all the energy of the Field is released at once both inward and outward; everything inside vaporizes and damage is caused to anything nearby on the outside. Like a heat flash from a nuke, but much larger.

     The Langston Field is used in the First and Second Empire periods of the CoDominium/Empire of Man. It was discovered through a series of unlikely accidents to men in very different scientific fields. Pournelle wrote that the natural shape of a Field is a solid. If the Field absorbs real and virtual photons then the Field must generate an effect similar to the Casimir effect (the suppression of some virtual photon wavelengths between two plates.). Any virtual wavelengths inside the volume of space surrounded by the Field that are longer than the Field would be absorbed by the Field. This in turn would cause a small differential in the zero-point energy between the vacuum within the Field and the vacuum in the external universe. This would cause the Field to collapse inward (like two plates being pulled together) and would explain Pournelle’s statement.

     The Field is used primarily for protection and spaceship propulsion. In terms of protection there are two effects. First and most important, the Field absorbs incoming energy. Second, as it releases that energy it creates a wall of heat or firewall. The more energy the Field releases (and therefore the closer to blue-white its color) the more intense the radiation and the better protection it is against low-speed material objects penetrating the Field.

     The Field must conform to the physics laws of conservation of energy and conservation of momentum. It can absorb and store electromagnetic energy, but not momentum. Rather momentum from a laser, a nuke, particle beams, railguns or collisions is spread out to the Field as a whole, but simultaneously that momentum is transferred to the Field generator and ultimately to the ship or planet where it is mounted. This means that in a spaceship battle, there will be a lot of shaking. Not so much for a city. Therefore, the bigger the ship (in terms of mass) the better it will be able to absorb all the shaking.

     The Langston Field has size limits: an ellipsoid 30 meters along its longest axis is the smallest, 10,000 meters along its longest axis the largest. By 3000 A.D., after nine hundred years of development and manufacturing, the Empire’s Langston Field generators are as well designed and foolproof as they can be, absent battle damage. The Langston generators and accumulators are not found everywhere. They are controlled (licensed in the Second Empire period and likely in earlier periods) and expensive. They are modular in design so they can be removed or re-installed from a spaceship without it going into a shipyard’s dry-dock facilities.


A torpedo had penetrated her defensive fire to explode somewhere near the hull. The Langston Field, opaque to radiant energy, was able to absorb and redistribute the energy evenly throughout the field; but at cost. There had been been an overload at the place nearest the bomb: energy flaring inward.

All through Defiant nonessential systems died. It took power to maintain the Langston Field, and the more energy the Field had to contain the more internal power was needed to keep the Field from radiating inward. Local overloads produced burnthroughs, partial collapses sending bursts of energetic photons to punch holes in the hull. The Field moved toward full collapse, and when that happened, the energies it contained would vaporize Defiant. Total defeat in space is a clean death.

From REFLEX by Larry Niven and Jerry Pournelle
(the deleted first chapter of The Mote in God's Eye)

      A ship in Defiant's situation, her screens overloaded, bombarded by torpedoes and fired on by an enemy she cannot locate, is utterly helpless; but she has been damaged hardly at all. Given time she can radiate the screen energies to space. She can erect antennas to find her enemy. When the screens cool, she can move and she can shoot. Even when she has been damaged by partial collapses, her enemy cannot know that.
     Thus, surrender is difficult and requires a precise ritual. Weapons in the hand of a defeated enemy are still dangerous. Indeed, the Scottish skean dhu is said to be carried in the stocking so that it may be reached as its owner kneels in supplication.
     Defiant erected a simple antenna suitable only for radio signals. Any other form of sensor would have been a hostile act and would earn instant destruction. The Imperial captain observed and sent instructions. Meanwhile, torpedoes were being maneuvered alongside Defiant. (Captain) Colvin couldn't see them. He knew they must be in place when the next signal came through. The Imperial ship was sending an officer to take command. Colvin felt some of the tension go out of him. If no one had volunteered for the job, Defiant would have been destroyed.
     Something massive thumped against the hull. A port had already been opened for the Imperial. He entered carrying a bulky object: a bomb.
     "Midshipman Horst Staley, Imperial Battlecruiser MacArthur," the officer announced as he was conducted to the bridge. "I am to take command of this ship, sir."
     Captain Colvin nodded. "I give her to you. You'll want this," he added, handing the boy the microphone. "Thank you for coming."
     "Midshipman Staley reporting, sir. I am on the bridge and the enemy has surrendered." He listened for a few seconds, then turned to Colvin. "I am to ask you to leave me alone on the bridge except for yourself, sir. And to tell you that if anyone else comes on the bridge before our Marines have secured the ship, I will detonate the bomb I carry. Will you comply?"

From REFLEX by Larry Niven and Jerry Pournelle (the deleted first chapter of The Mote in God's Eye

In principle Defiant was a better ship than she'd been when she left New Chicago. The engineers had automated all routine spacekeeping tasks, and no United Republic spacer needed to do a job that a robot could perform. Like all of New Chicago's ships, and like few of the Imperial Navy's, Defiant was as automated as a merchantman.

Colvin wondered. Merchantmen do not fight battles. A merchant captain need not worry about random holes punched through his hull. He can ignore the risk that any given piece of equipment will be smashed at any instant. He will never have only minutes to keep his ship fighting or see her destroyed in an instant of blinding heat.

No robot could cope with the complexity of decisions damage control could generate, and if there were such a robot it might easily be the first item destroyed in battle. Colvin had been a merchant captain and had seen no reason to object to the Republic's naval policies, but now that he had experience in warship command, he understood why the Imperials automated as little as possible and kept the crew in working routine tasks: washing down corridors and changing air filters, scrubbing pots and inspecting the hull. Imperial crews might grumble about the work, but they were never idle. After six months, Defiant was a better ship, but...

From REFLEX by Larry Niven and Jerry Pournelle
(the deleted first chapter of The Mote in God's Eye)

Erickson's Model

And now for something totally different. Leonard Erickson came up with an interesting model for force fields: use the equation for gas pressure.

Best fit between real formulas and the desired behavior/model was one of the gas equations. The one that has Energy equaling pressure times volume times a constant. This works ok for a closed surface type field. And leads to some interesting performance issues.

P * V = k * E


  • P = pressure
  • V = volume
  • E energy
  • k = constant

Assuming k=1, you get something like 42 joules for a 1 meter radius sphere with 1 atmosphere of pressure inside. If you make k smaller, the energy requirements go up.

It "makes sense" that the bigger the enclosed volume, the more energy it'll take. And likewise for higher pressure (i.e. "stronger") fields taking more energy. One unexpected, but nice detail is that the field doesn't "use" energy. It takes energy to set it up, but that energy is "stored" in the field. So, aside from losses, you don't need to keep pumping energy in.

On the other hand, when you start considering the strength or "resistance to penetration" of a force field in terms of "pressure" (i.e. force per unit area), you suddenly realize that while holding in air is cheap, stopping bullets is gonna cost.

Another nice thing is that it would seem likely that "puncturing" the field doesn't hurt it. On the other hand, this is little comfort to the user when he finds that it wasn't turned up high enough to stop that bullet.

Leonard Erickson

Potential Barrier

Yet another possibility is the system described in Poul Anderson's novel Shield.

"So what is your invisible screen? A potential barrier?"

Surprised, he nodded. "How did you guess?"

"Seemed reasonable. A two-way potential barrier, I suppose, analogous to a mountain ridge between the user and the rest of the world. But I've determined myself, today, that it builds from zero to maximum within the space of a few centimeters. Nothing gets through that hasn't the needful energy, sort of like the escape velocity needed to get off a planet. So a bullet which hits the screen can't get through, and falls to the ground. But what happens to the kinetic energy?"

"The field absorbs it," he said, "and stores it in the power pack from which the field is generated in the first place. If a bullet did travel fast enough to penetrate, it'd get back its speed as it passed through the inner half of the barrier. The field would push it, so to speak, drawing energy from the pack to do so. But penetration velocity for the unit I've got, at its present adjustment, is about fifteen miles per second."

She whistled. "Is that the limit?"

"No. You can push the potential barrier as high as you like, until you even exclude electromagnetic radiation. That would take a much larger energy storage capacity, of course. For a given capacity, such as my unit has, you can expand the surface of the barrier at the price of lowering its height. For instance, you could enclose an entire house in a sphere centered on my unit, but penetration velocity would be correspondingly less-maybe only one mile a second, though I'd have to calculate it out to be certain."

From SHIELD, by Poul Anderson's (1963)

Paragravity Shield

Paragravity is synthetic gravity. Visualize it as a box that you put electricity into one end and gravity comes out the other.

There have been a few science fiction novels where the authors figured that synthetic gravity was as good a handwave as any to create a magic defensive force field. Note this is similar if not identical to the Potential Barrier.

The most famous example is the "impeller wedge" from the Honor Harrington series by David Weber. The gravidic propulsion of the starships is created by two planes of paragravity, one "above" and one "below" the starship. These are basically impenetrable to hostile weapons fire. On the port and starboard flanks are weaker planes of paragravity. They provide some protection but some weapons fire penetrates. There is can be one weaker protective plane on the fore or aft aspect of the ship, but only one aspect at a time.

The purpose of this weird invention on the part of the author is to force combat starships to maneuver and do battle much in the same way as in the age of tall ships. Ships are in formations aiming their broadside batteries at the enemy, trying to cross the T. The only difference is that wooden sailing ships are restricted to the two-dimensional plane of the ocean (in a "line of battle"), while Honververse starships can be stacked in three dimensions (a "wall of battle" as it were).


(ed note: the new-born Galactic Patrol is centered around The Hill, Terra's greatest fortress. They know that the unknown enemy, the Black Fleet is going to attack soon, and try and obliterate The Hill and Virgil Samms the First Lensman. The Black fleet does not know that the Galactic Patrol has a secret weapon up their sleeves: the Rodebush-Bergenholm Fields.)

      “Flagship Chicago to Grand Fleet Headquarters!” it blatted, sharply. “The Black Fleet has been detected. RA twelve hours, declination plus twenty degrees, distance about thirty light-years …”

     Kinnison started to say something; then, by main force, shut himself up. He wanted intensely to take over, to tell the boys out there exactly what to do, but he couldn’t. He was now a Big Shot—damn the luck! He could be and must be responsible for broad policy and for general strategy, but, once those vitally important decisions had been made, the actual work would have to be done by others. He didn’t like it—but there it was. Those flashing thoughts took only an instant of time.

     “… which is such extreme range that no estimate of strength or composition can be made at present. We will keep you informed.”
     “Acknowledge,” be ordered Randolph; who, wearing now the five silver bars of major, was his Chief Communications Officer. “No instructions.”

     He turned to his plate. Clayton hadn’t had to be told to pull in his light stuff; it was all pelting hell-for-leather for Sol and Tellus. Three general plans of battle had been mapped out by Staff. Each had its advantages—and its disadvantages. Operation Acorn—long distances—would be fought at, say, twelve light-years. It would keep everything, particularly the big stuff, away from the Hill, and would make automatics useless … unless some got past, or unless the automatics were coming in on a sneak course, or unless several other things—in any one of which cases what a Godawful shellacking the Hill would take!

     He grinned wryly at Samms, who had been following his thought, and quoted: “A vast hemisphere of lambent violet flame, through which neither material substance nor destructive ray can pass.”
     “Well, that dedicatory statement, while perhaps a bit florid, was strictly true at the time—before the days of allotropic iron and of polycyclic drills (a type of directed energy weapon that "drills" through force fields like an auger through a wooden plank). Now I’ll quote one: ‘Nothing is permanent except change’.”

     “Uh-huh,” and Kinnison returned to his thinking. Operation Adack. Middle distance. Uh-uh. He didn’t like it any better now than he had before, even though some of the Big Brains of Staff thought it the ideal solution. A compromise. All of the disadvantages of both of the others, and none of the advantages of either. It still stunk, and unless the Black fleet had an utterly fantastic composition Operation Adack was out.

     And Virgil Samms, quietly smoking a cigarette, smiled inwardly. Rod the Rock could scarcely be expected to be in favor of any sort of compromise.

     That left Operation Affick. Close up. It had three tremendous advantages. First, the Hill’s own offensive weapons—as long as they lasted. Second, the new Rodebush-Bergenholm fields. Third, no sneak attack could be made without detection and interception. It had one tremendous disadvantage; some stuff, and probably a lot of it, would get through. Automatics, robots, guided missiles equipped with superspeed drives, with polycyclic drills, and with atomic warheads strong enough to shake the whole world.

     But with those new fields, shaking the world wouldn’t be enough; in order to get deep enough to reach Virgil Samms they would damn near have to destroy the world. Could anybody build a bomb that powerful? He didn’t think so. Earth technology was supreme throughout all known space; of Earth technologists the North Americans were, and always had been, tops. Grant that the Black Fleet was, basically, North American. Grant further that they had a man as good as Adlington—or that they could spy-ray Adlington’s brain and laboratories and shops—a tall order. Adlington himself was several months away from a world-wrecker, unless he could put one a hundred miles down before detonation, which simply was not feasible. He turned to Samms.

     “It’ll be Affick, Virge, unless they’ve got a composition that is radically different from anything I ever saw put into space.”
     “So? I can’t say that I am very much surprised.”
     The calm statement and the equally calm reply were beautifully characteristic of the two men. Kinnison had not asked, nor had Samms offered, advice. Kinnison, after weighing the facts, made his decision. Samms, calmly certain that the decision was the best that could be made upon the data available, accepted it without question or criticism.
     “We’ve still got a minute or two,” Kinnison remarked. “Don’t quite know what to make of their line of approach. Coma Berenices: I don’t know of anything at all out that way, do you? They could have detoured, though.”
     “No, I don’t.” Samms frowned in thought. “Probably a detour.”
     “Check.” Kinnison turned to Randolph. “Tell them ‘to report whatever they know; we can’t wait any …” As he was speaking the report came in.
     The Black Fleet was of more or less normal make-up; considerably larger than the North American contingent, but decidedly inferior to the Patrol’s present Grand Fleet. Either three or four capital ships …
     “And we’ve got six!” Kinnison said, exultantly. “Our own two, Asia’s Himalaya, Africa’s Johannesburg, South America’s Bolivar, and Europe’s Europa.”
     … Battle cruisers and heavy cruisers, about in the usual proportions; but an unusually high ratio of scouts and light cruisers. There were either two or three large ships which could not be classified definitely at that distance; long-range observers were going out to study them.
     “Tell Clayton,” Kinnison instructed Randolph, “that it is to be Operation Affick, and for him to fly at it.”
     “Report continued,” the speaker came to life again. “There are three capital ships, apparently of approximately the Chicago class, but tear-drop-shaped instead of spherical …”
     “Ouch!” Kinnison flashed a thought at Samms. “I don’t like that. They can both fight and run.”
     “… The battle cruisers are also tear-drops. The small vessels are torpedo-shaped. There are three of the large ships, which we are still not able to classify definitely. They are spherical in shape, and very large, but do not seem to be either armed or screened, and are apparently carriers—possibly of automatics. We are now making contact—off!”

     Instead of looking at the plates before them, the two Lensmen went en rapport with Clayton, so that they could see everything be saw. The stupendous Cone of Battle had long since been formed; the word to fire was given in a measured two-second call.

(ed note: the Cone of Battle is a fleet formation. The base points at the enemy fleet, the point of the cone is furthest from the enemy, the walls of the cone are composed of warships sorted by weakest to strongest, the flagship is at the point, the cone is hollow. All the ship fire their weapons forwards, making a sort of combined energy beam. I frankly fail to see the advantage of a cone over a disk or other formation.)

     Every firing officer in every Patrol ship touched his stud in the same split second. And from the gargantuan mouth of the Cone there spewed a miles-thick column of energy so raw, so stark, so incomprehensibly violent that it must have been seen to be even dimly appreciated. It simply cannot be described.

     Its prototype, Triplanetary’s Cylinder of Annihilation, had been a highly effective weapon indeed. The offensive beams of the fish-shaped Nevian cruisers of the void were even more powerful. The Cleveland-Rodebush projectors, developed aboard the original Boise on the long Nevian way, were stronger still. The composite beam projected by this fleet of the Galactic Patrol, however, was the sublimation and quintessence of each of these, redesigned and redesigned by scientists and engineers of ever-increasing knowledge, rebuilt and rebuilt by technologists of ever-increasing skill.

     Capital ships and a few of the heaviest cruisers could mount (defensive) screen generators able to carry that frightful load; but every smaller ship caught in that semi-solid rod of indescribably incandescent fury simply flared into nothingness.

     But in the instant before the firing order was given—as though precisely timed, which in all probability was the case—the ever-watchful observers picked up two items of fact which made the new Admiral of the First Galactic Region cut his almost irresistible weapon and break up his Cone of Battle after only a few seconds of action: One: those three enigmatic cargo scows had fallen apart before the beam reached them, and hundreds—yes, thousands—of small objects had hurtled radially outward, out well beyond the field of action of the Patrol’s beam, at a speed many times that of light. (translation: the cargo scows were packed with thousands of FTL missiles with atomic warheads, all with The Hill's name on them) Two: Kinnison’s forebodings had been prophetic. A swarm of Blacks, all small—must have been hidden right on Earth somewhere!—were already darting at the Hill from the south.

     “Cease firing!” Clayton rapped into his microphone. The dreadful beam expired. “Break cone formation! Independent action—light cruisers and scouts, get those bombs! Heavy cruisers and battle cruisers, engage similar units of the Blacks, two to one if possible. Chicago and Boise, attack Black Number One. Bolivar and Himalaya, Number Two. Europa and Johannesburg, Number Three!”

(ed note: in the Lensman novels, defensive screens start radiating in color as they come under attack by hostile directed energy weapons. The color goes up the spectrum as the number of hosile beams increase. Once it reaches violet, the screen goes black and excess damage leaks through it. Warships typically have three "courses" of screens, arranged in concentric layers. All three have to be driven to black. There is a fourth screen, the "wall-shield" which is the most powerful. Once it fails, the ship takes damage. If before the wall shield fails the ship turns on its FTL drive and "goes free", it escapes the battle.)

     Space was full of darting, flashing, madly warring ships. The three Black super-dreadnaughts leaped forward as one. Their massed batteries of beams, precisely synchronized and aimed, lashed out as one at the nearest Patrol super heavy, the Boise. Under the vicious power of that beautifully-timed thrust that warship’s first, second, and third screens, her very wall-shield, flared through the spectrum and into the black. Her Chief Pilot, however, was fast—very fast—and he had a fraction of a second in which to work. Thus, practically in the instant of her wall-shield’s failure, she went free; and while she was holed badly and put out of action, she was not blown out of space. In fact, it was learned later that she lost only forty men.

     The Blacks were not as fortunate. The Chicago, now without a partner, joined beams with the Bolivar and the Himalaya against Number Two; then, a short half-second later, with her other two sister-ships against Number Three. And in that very short space of time two Black super-dreadnaughts ceased utterly to be.

     But also, in that scant second of time, Black Number One had all but disappeared! Her canny commander, with no stomach at all for odds of five to one against, had ordered flight at max; she was already one-sixtieth of a light-year—about one hundred thousand million miles—away from the Earth and was devoting her every energy to the accumulation of still more distance.

     “Bolivar! Himalaya!” Clayton barked savagely. “Get him!” He wanted intensely to join the chase, but he couldn’t. He had to stay here. And he didn’t have time even to swear. Instead, without a break, the words tripping over each other against his teeth: “Chicago! Johannesburg! Europa! Act at will against heaviest craft left. Blast ’em down!”

     He gritted his teeth. The scouts and light cruisers were doing their damndest, but they were outnumbered three to one—Christ, what a lot of stuff was getting through! The Blacks wouldn’t last long, between the Hill and the heavies, but maybe long enough, at that—the Patrol globe was leaking like a sieve! He voiced a couple of bursts of deep-space profanity and, although he was almost afraid to look, sneaked a quick peek. to see how much was left of the Hill. He looked—and stopped swearing in the middle of a four-letter Anglo-Saxon word.

     What he saw simply did not make sense. Those Black bombs should have peeled the armor off of that mountain like the skin off of a nectarine and scattered it from the Pacific to the Mississippi. By now there should be a hole a mile deep where the Hill had been. But there wasn’t. The Hill was still there! It might have shrunk a little—Clayton couldn’t see very well because of the worse-than-incandescent radiance of the practically continuous, sense-battering, world-shaking atomic detonations—but the Hill was still there! And as he stared, chilled and shaken, at that indescribably terrific spectacle, a Black cruiser, holed and helpless, fell toward that armored mountain with an acceleration starkly impossible to credit. And when it struck it did not penetrate, and splash, and crater, as it should have done. Instead, it simply spread out, in a thin layer, over an acre or so of the fortress’ steep and apparently still, armored surface! “You saw that, Alex? Good. Otherwise you could scarcely believe it,” came Kinnison’s silent voice. “Tell all our ships to stay away. There’s a force of over a hundred thousand G’s acting in a direction normal to every point of our surface (i.e., perpendicular to the surface of the Hill). The boys are giving it all the decrement they can—somewhere between distance cube and fourth power—but even so it’s pretty fierce stuff. How about the Bolivar and the Himalaya? Not having much luck catching Mr. Black, are they?”

     “Why, I don’t know. I’ll check … No, sir, they aren’t. They report that they are losing ground and will soon lose trace.”

     “I was afraid so, from that shape. Rodebush was about the only one who saw it coming … well, we’ll have to redesign and rebuild …”

     Port Admiral Kinnison, shortly after directing the foregoing thought, leaned back in his chair and smiled. The battle was practically over. The Hill had come through.

     TheRodebush-Bergenholm fields had held her together through the most God-awful session of saturation atomic bombing that any world had ever seen or that the mind of man had ever conceived. And the counter-forces had kept the interior rock from flowing like water. So far, so good.

     Her original armor was gone. Converted into …what? For hundreds of feet inward from the surface she was hotter than the reacting slugs of the Hanfords (the nuclear fuel rods of a major nuclear power plant).

     Delousing her would be a project, not an operation; millions of cubic yards of material would have to be hauled off into space with tractors and allowed to simmer for a few hundred years; but what of that? Bergenholm had said that the fields would tend to prevent the radioactives from spreading, as they otherwise would—and Virgil Samms was still safe!

     “Virge, my boy, come along.” He took the First Lensman by his good arm and lifted him out of his chair. “Old Doctor Kinnison’s peerless prescription for you and me is a big, thick, juicy, porterhouse steak.”

From FIRST LENSMAN by E. E. "Doc" Smith (1950)

      “Should we divert Raven toward the enemy?” Bazzoli asked.
     “Negative. Stay on course for now,” Henry ordered. “Let them come to us. Initiate evasive maneuvers once they’re clear of the gas giant.” Seconds continued to tick away. A gesture threw a timer of the hostiles’ estimated emergence time on the screen closest to Henry. Five minutes and counting.
     “Ihejirika, are the guns ready?” he asked calmly.
     “Solid shot in the main gun. Laser capacitors charged. Launchers loaded, conversion warheads.” The tactical officer nodded firmly, as much to himself as to his Captain. “All weapons are ready for action.”
     “Song? Is the shield ready?” Henry continued.
     “All shield systems are online and in the green. Sensors confirm a fifteen-thousand-gravity shear zone surrounding the ship.” Very little in existence could handle going from microgravity to fifteen thousand gravities. Even less could handle going back seventeen centimeters later. Any physical projectile was shredded by tidal forces. Even lasers were badly distorted, rarely hitting the target they were aimed at.
     With her shield up, it was difficult for anyone outside it to even locate Raven. They could easily detect the shield itself, though, and the Kenmiri, at least, had learned that the ship was always at the exact center of the spherical shield bubble.
     Their own projectiles suffered the same problems, but they could open “gunports” in the shear zone to let beams, missiles and grav-driver slugs through.
     The gravity shield hadn’t made the UPA’s ships invulnerable, but it had made them tough beyond any rational amount of firepower. They’d taken far fewer losses than their allies, which had allowed the UPA to maintain the war with only a moderate commitment from the member systems.

     “Starfang,” he told his XO. “The mind-concept they use for their lighter warships translates as starfang. Just the one of them?”
     “Isn’t that enough?” Iyotake replied. “I don’t suppose you know why I’ve got an intel warning here classifying the starfang as the biggest threat here?”
     “You’re not cleared for that,” Henry admitted. “If you need to know, I’ll brief you. Otherwise, let’s just not pick a fight with the ten-legged spiders, okay?”
     He had been briefed. One Terzan ship during Golden Lancelot had demonstrated a form of gravity shield when under pressure. The sensor data was unclear, but it had suggested that the Terzan not only shared the system the UPA had regarded as their main advantage over the rest of the galaxy…but that their version was significantly more sophisticated.
     And if they had a grav-shield, they almost certainly had grav-shield penetrators.

(ed note: The Kenmiri empire had enslaved thousands of planets, called the Vesheron. They tried to enslave Terra, but were defeated. During the war the Vesheron fought alongside the Terrans, but there are some hard feelings. Specifically about the Terrans keeping secret their grav-shield technology. The Gathering was called to try and get the various planets in the Vesheron to play nice with each other, now that the Kemiri are gone. But the situation is tense.)

     “Not counting our birds, there are a grand total of twelve starfighters flying combat patrol around the Gathering,” the XO said. “Twelve. Two are from Trintar—their corvettes carry four apiece, it looks like. Two are from the Londu. Two are from the Drifters. Four are Restan and two are from the Slant from the Bes Province.”
     “And ours are the only ones with a gravity shield,” Henry said slowly. “The Dragoons are getting eyed for capture, aren’t they?”
     “Exactly,” Iyotake confirmed. “I’m not sure who in this mess to even call allies, ser, but even the Restan wouldn’t hesitate if they thought they could manage to snap up one of our starfighters. Even if they had to give it back in a day or two, having unrestricted access to one of our birds to dissect for even twenty-four hours…”
     Henry nodded. They’d never based grav-shield starfighters off Vesheron ships, and while the occasional pilot had been picked up by the UPA’s allies, they’d always ditched and destroyed their fighters.
     It had taken a ruthless degree of paranoia to get through the war without the grav-shield technology ending up in Kenmiri hands. The price had been higher than Henry thought many of his superiors guessed. A lot of the Vesheron were bitter over it.

     Without maneuvering, Henry was wearing mag-boots to remain locked to the shuttle deck. The Londu shuttle had artificial gravity, but the UPA’s version of the tech was too finicky to easily install in that small of a ship.
     The systems aboard the shuttle could compensate thrust to allow the craft to accelerate at half a kilometer per second squared, but they couldn’t provide gravity while the shuttle wasn’t accelerating.
     The lack clearly surprised Kahlmor, and he nearly overbalanced and fell into the ship before Henry grabbed his arm.
     “Here.” He passed the other man a pair of mag-boots.
     “You are the masters of gravity technology among the Vesheron, yet your shuttle lacks artificial gravity?” the Lord of Ten Thousand Miles replied in Kem. He put on the boots regardless, and based on the ease with which he locked himself to the floor, the tall Londu man wasn’t unfamiliar with the concept.
     “Creating a basic gravity field with a device that can fit in the free space in a shuttle that has to match her mothership’s acceleration is a very different project than projecting a gravity shear with enough tidal force to deflect a laser,” Henry said. “Our gravity technology is rather…focused.”
     It also wasn’t true that the UPSF couldn’t put artificial gravity in their shuttles. They’d chosen to spend that mass and cubage elsewhere. Most of the time, after all, a shuttle was either in motion or in someone else’s gravity field.

Point Defense

Point Defense is a fancy name for all the short ranged weapons and anti-missile missiles used to shoot at incoming enemy missiles. They are analogous to anti-aircraft guns. Actually sometimes they are used to attack enemy aircraft but with current weapons it is rare for aircraft to get close enough to a ship to be within the point defence engagement envelope. The aircraft stay out of range and launch missiles.

A low powered weapon would do for defense against nuclear warheads. John Schilling says that nuclear weapons are rather complex and fragile devices, and it doesn't take much to put them out of action. And they do not undergo sympathetic detonation, i.e., they don't go boom just because you hit them real hard. So if your point-defense system can score a solid hit, the nuke is effectively useless.


Ken Burnside:

Acceleration and armor are two parts of the puzzle. The third part is something to shoot missiles down with.

In general, an object impacting at 3 km/sec (about 9,840 f/sec for those used to WWII shell velocities) will deliver KE equal to its mass in TNT. KE goes up at the square of the velocity; as delta V increases for the missile, damage goes up quadratically.

Picking the balance point between missile delta V, actual physics, ship delta v and acceleration is one of those places where it's real easy to paint yourself into a corner with a dull game.

If missile delta v is significant compared to ship delta V, missiles become very long range weapons, and maneuver doesn't matter, only point defense does. (For those who want the proof, in the archives from December and January, I showed the targeting solutions for Rick Robinson using the precepts of his universe with the tools I made for my game. The best targeting solution when you've got a finite number of missiles with the delta V constraints he has is to assume the target is always pointed away from the missile, and subtract the target's accel from the missile's accel — in essence, "target sits still" modus.)

My means of reducing missile delta V to "interesting to play" levels was to say that they're using solid fuel boosters with Isp's of about 306 (chosen because it came to a very convenient number and was within the acceptable range).

AMRAAM has an Isp of 295, and has serious maintenance issues because the oxidizer reduces its shelf life. Most air-to-air missiles and naval missiles seem to have Isps of 250 to 270.) Sure, you can get better Isps out of LOX/H2, or an HF rocket...but do you really want a 19 year old kid with a non technical education doing maintenance on cryogenic fuels or something that seeps fluorine or hydrofluoric acid?

Anthony Jackson

When dealing with missiles vs kirklin mines, there is a fairly easy way to analyse the probabilities of breaking through:

First of all, take it as given that without penaids (penetration aids), the mine always wins.

Obviously, we want to put penaids on the warhead; a cloud of offensive missiles to take out defending mines, for example. Equally obviously, the mine doesn't want to be taken out before it can kill the enemy missile, so we wind up putting penaids on the mine as well. So, basically, what we have are two objects (the missile and the mine) each with a 'cloud' of penaids in front of it.

Now, a point here: if we change reference frames, we just have two objects and clouds on a collision course; the clouds hit first, followed by the larger objects. There is only one difference between these two clouds: the mine wants to destroy the missile, the missile wants to bypass the mine.

Therefore, assume that the missile has an N% chance to pass through the mine's cloud. Since we've got a completely parallel situation, the mine in turn has an N% chance to pass through the missile's cloud, and if it gets through it destroys the mine. The chance for the missile to get through is thus N/100 * (100-N)/100.

The most efficient possible value, for missiles, is N = 50. This results in a maximum of 25% chance for a missile to get past one mine.

Now, put a second mine there. Assuming no degradation in the missile's cloud from the first mine, there's still no better than a 25% chance to get past the second mine either. If you have to pre-allocate all the mines, a 5:1 ratio is enough to stop 99.9% of the missiles. If you can reallocate mines after figuring out what was killed in the first pass, it still takes 5 waves of mines to get a 99.9% kill rate, but the second and subsequent waves require far fewer mines; a 2:1 ratio of defense to offense is plenty.

Now, the important thing for Attack Vector: Tactical (AV) is this: the delivery system for the kinetic warhead is very expensive. The delivery system for the kirklin mine is not. Therefore, 2:1 is not particularly difficult. Also, a 50% chance to pass through the 'cloud' is almost certainly optimistic.

As a separate issue in terms of planetary bombardment: the threshold for useful general bombardment, as opposed to specifically targeting cities, is very high; 10,000 megatons or so total, spread between a fairly large number of impactors. With a measly 100 km/sec impact velocity, that requires 8.4 million tons of impactors.

Citykillers are much easier, of course, but you still want multi-ton impactors to have any chance of getting through atmosphere and doing anything.

Rick Robinson

Yes. Kirklin mines pretty much nullify torch missiles — if they can be used. The key constraint is that kirklin mines contain the maneuvering of the deployer. The mine has a limited maneuver envelope, and the target has to stay "behind" it. Your only free direction of movement is (close to) directly away from the oncoming missile.

This is fine for a defender, but problematical for an attacker. If you are a task force closing on a planet, you can't readily protect yourself with kirklin mines. Hmm, well, maybe you can! You are decelerating as you approach the planet, so if torch missiles are fired toward you, kirklin mines you release will drift out ahead of you, pretty much right along your approach path. (This ignores planetary motion and the star's gravity well, but these are pretty "flat" on the scale even of a torch-missile attack.)

So, torch missiles look questionable in the face of kirklin mines.

But, this leaves another option. Kirklin mines can also be used as offensive weapons in a tactical sense. Instead of an using an expensive, expendable torch missile to deliver a high-speed kinetic punch, use an expensive but non-expendable ship.

This differs from my familiar "cavalry charge" attack in that the closing speed would be much faster, on the order of 100 km/s. (Each impactor would do correspondingly greater damage; in AV terms, a standard 50-kg slug would hit with 5000 damage points, if I'm figuring it correctly.)

The attacker can avoid the defender's mines if he has greater acceleration than the defender — there will be a zone in which he can veer enough to avoid defending mines, while the defender cannot avoid his. So, a more maneuverable attacker can use all his munitions for attack, not having to hold them back for point (or "zone") defense.

I don't think this would be precluded by the "real world" assumptions behind AV, but it would break the game if allowed. The scale is wrong, and it couldn't be played on a reasonable-size board, or maybe at all. 100 km/s is 80 hex/segment, IIRC, so you'd get a granularity issue.


Modern battles are fought at close range–a light-second or so–since lasers are big, heavy, slow, and radiate more than half their power into their parent ships as low-quality waste heat. That’s why every conflict since the 2110s has been fought with missiles and k-slugs.

The danger with missiles is that they’re fast and independently targeted. Try shooting them down with a gun of some type and you have a problem: the missile has traveled literally miles before your bullet gets halfway down the barrel.

Particle beam weaponry was once largely considered to be useless. About the best it can do is barf up some bremsstrahlung secondary radiation (radiation emitted by a charge particle when slowed down). Deliciously lethal, sure, but only in a localized area, and certainly not structurally damaging. The engineer-physicists eventually realized, however, that the particle beam is well-suited to defense. And so, the fan was invented.

The fan makes use of an otherwise annoying property of particle beams. When you deflect a stream of charged particles, you’re accelerating it, but the stream still goes basically the same speed afterward (just in a different direction). That extra energy gets dumped in the form of synchrotron radiation (radiation emitted by a charge particle when accelerated perpendicular to their velocity ), streaming out tangentially in a flood of hard x-rays. So you get a searing fan of radiation, spreading knifelike in a plane (radiation beam shaped like a hand fan).

Nowadays, when the call goes out for point defense, the ship fires up its spinal-mount linear accelerator. Huge flickering electromagnets in the bow deflect the beam semi-randomly, and a decollaminated blast of bit-flipping, electronics-frying radiation cooks the missiles as they reach the terminal guidance phase.

Small wonder the Jovian Trade Union’s radiation hardening expertise is widely-sought.

Kinetic PD

Eric Rozier has an on-line calculator here that does calculation of Kinetic point defense hit probabilities (i.e., a point defence using bullets).

Modeling kinetic point defense is no easy task, the simulation I've created is a discrete event simulator which simulates individual bullets fired from a Phalanx style weapons system. The initial parameters for the CIWS are equivalent to a Phalanx with a perfect targeting computer. You can increase the number of CIWS firing at an incoming missle by increasing the number of linked CIWS, it is not as simple as multiplying the probability.

Target parameters are set to those of an AIM-9 Sidewinder missle with "infinite fuel", i.e. it will accelerate continuously during the entire simulation, regardless of the distance.

The simulation begins firing at the given range to the target in meters, simulating each shot to the target, and calculating the percentage of a hit based on the apparent velocity of the missle (muzzle velocity + target velocity), and the acceleration capabilities of the target (much as in the laser calculations on your page, but with slower than light bullets).

During each time step (of length indicated by the intershot time), all of the linked CIWS simulate a firing and calculate a hit probability, the missle then accelerates to a new velocity, the distance is shortened and provided the missle hasn't closed to minimum targeting distance, the CIWS take another shot and the joint probability is recomputed.

Eric Rozier

Missile PD

The indefatigable Eric Rozier has an on-line calculator here that does calculation of Missile point defense hit probabilities (i.e., a point defence using anti-missile missiles).

Ok, I think I've got a pretty well justified CIWS missile system. It models anti-missile missile point defense, similar to the RAM system in development by the military.

Cm = (Rb + Rt)2 * π

  • Rb is the blast radius of the kill zone for the nuclear CIWS missile.
  • Rt is the radius of the missile we're attempting to kill.
  • Cm is the cross-sectional area we must hit to kill the missile.

Hp = Cm / (π * d2)

  • d is the displacement which can be achieved by the target missile
  • Hp is the hit probability

d = 0.5 * (9.8 * (At - (Ac * e)) * t2

  • At is the acceleration (in Gs) of the target missile
  • Ac is the acceleration (in Gs) of the CIWS missile
  • e is the effectiveness of the tracking system on the CIWS missile
  • t is the time to intercept

t is calculated in my model by approximating an integral which takes into account the increasing velocity due to acceleration of both the CIWS missile and the target missile.

The end model basically models the system by calculating when the two missiles will hit, and then calculating the possible displacement the missiles can achieve. Normally with a purely kinetic kill vehicle this is calculated by the acceleration potential of the target missile during the time it takes us to intercept. In this case since we can supply active thrust, we can cancel out some of this acceleration potential. Our ability to do so is modeled as our acceleration potential multiplied by an effectiveness of our tracking system. If we have a perfect tracking system, we match them move per move to the extent our acceleration allows (i.e. if At = Ac, we hit, if At > Ac we usually miss). If it is imperfect we only get a fraction of our acceleration, as a portion of the time we are correcting mistakes, (i.e. in general if At < Ac by a ratio proportional to effectiveness we hit, otherwise we usually miss).

Eric Rozier

Laser vs. Missile

When it comes to laser point defense vs incoming missiles, there is some controversy. This is the subject of a long-running "Purple/Green" debate on SFConSim-L.


The term "Purple/Green" comes from an episode of Babylon-5 called "The Geometry of Shadows". The episode involving the ritual Drazi civil war, where the sides are chosen by randomly choosing colored sashes from a barrel. It is a science-fictional version of Miller Lite partisans shouting "Tastes Great!" and "Less Filling!".

More specifcally, as Christopher Weuve explains:

"It's the SFConsim-L brevity phrase meaning 'an argument in which no actual agreement can be reached, usually (but not always) because it is dependent on going-in assumptions.'"

Anyway the argument is about what happens in the last hundred kilometers to the target ship.

For an in-depth look go to the Rocketpunk Manifesto and read Battle of Spherical War Cows: Purple vs Green and Further Battles of Spherical War Cows. For a brief summary, see below:

The laser gang asserts that they can zap a missile before it ever gets to kill range, even for a nuclear warhead. And do it every time, at least so much of the time that missiles aren't worth firing. Even if the missile fragments into 10,000 pieces of shrapnel (each with substantial killing power), tracking gear can determine the fragments that will hit, and zap them before they reach target.

The laser gang's theory is that lasers never miss. If you can paint the target with photons to see it, you can hit it with a laser. In addition: missiles, by definition, need to close on the target, which means there are some trigonometry tricks that will allow you to lock them up hard with lasers - they can't laterally juke in space without missiing the target, for example.

The missile gang contends that laser point defense can always be saturated. Fire a big enough missile, or a salvo of missiles, coming in fast enough, and there will just be more mosquitoes than the bug zappers can zap in the short time till impact.

The missile gang's theory is that you can derive the number of missiles needed to overwhelm a given number of lasers by inputting some variables, like amount of energy per square cm needed to guarentee a kill on a missile, the wattage of output of the lasers, and the cycle/recharge time of the lasers. Lasers do require some time to recharge, and need some time to cool off.

The laser gang reply that lasers have the advantage in that they are reusable, unlike missiles. If lasers are dominant, it's also an offensive weapon to zap enemy ships, not a purely defensive one.

The missile gang retorts that the missile can be fired outside of laser range, and if it does penetrate point defense and smoke your ship, your laser is no longer reusable, now is it?

There is the cost effectiveness argument. Can you afford to carry point-defense lasers that can stop my missiles? Can I afford to carry missiles that can penetrate your point defense? Which is cheaper?

Can there be any tactics in a long-range duel between two missile armed ships? It comes down to whether you can afford to fire a missile on anything but a certain intercept, this is also ultimately a matter of cost.

Can there be any tactics in a long-range duel between two laser armed ships? It can be argued that it is the equivalent of two crack marksmen at opposite ends of a football field, shooting at each other with scope-equipped, tripod-mounted sniper rifles.

Given equal quality lasers, if I can zap you, you can zap me. Given laser ranges of at least a few hundred km, maybe a few thousand how can ships maneuver? If they are slow, it will take minutes to change position, meanwhile zapping away with multimegajoule lasers. If they are fast, they'll hurtle past each other in a drive-by, then take hours to swing around for another pass, unless they have science-fictional levels of acceleration. Possible solutions include long recharge and/or cooling-off times between laser volleys, and restricted firing arcs on the laser turrets.

The argument rages on, which probably means you can just pick which side appeals to you and be able to justify it. By carefully selecting, say, the proper minimum laser recycle time one can decide whether missiles are a viable weapon or not.

The Attack Vector: Tactical wargame adds an additional wrinkle. The laser recycle time is set such that missiles are viable. However, laser cannons have a limited number of "flash cooler" loads which can drastically cut the recycle time. But once you've used up your flash cooler loads, the laser is stuck at the standard recycle time.

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