The catch is, you have to arrange for the protons to impact with 300 keV of energy, and even then the reaction cross section is fairly small. Shoot a 300 keV proton beam through a cloud of boron plasma, and most of the protons will just shoot right through. 300 keV proton beam against solid boron, and most will be stopped by successive collisions without reacting. Either way, you won't likely get enough energy from the few which fuse to pay for accelerating all the ones which didn't.

Now, a dense p-B plasma at a temperature of 300 keV is another matter. With everything bouncing around at about the right energy, sooner or later everything will fuse. But containing such a dense, hot plasma for any reasonable length of time, is well beyond the current state of the art. We're still working on 25 keV plasmas for D-T fusion.

If you could make it work with reasonable efficiency, you'd get on the order of ten gigawatt-hours of usable power per kilogram of fuel.

H-Li6 Fusor
Exhaust Velocity	19,620 m/s
Specific Impulse	2,000 s
Thrust	67,100 N
Thrust Power	0.7 GW
Mass Flow	3 kg/s
Total Engine Mass	54,000 kg
T/W	0.13
Frozen Flow eff.	92%
Thermal eff.	90%
Total eff.	83%
Fuel	Hydrogen-Lithium6 Fusion
Reactor	Electrostatic Confinement
Remass	Reaction Products
Remass Accel	Electrostatic Acceleration
Thrust Director	Magnetic Nozzle
Specific Power	82 kg/MW

A Farnsworth-Bussard fusor is little more than two charged concentric spheres dangling in a vacuum chamber, producing fusion through inertial electrostatic confinement. Electrons are emitted from an outer shell (the cathode), and directed towards a central anode called the grid. The grid is a hollow sphere of wire mesh, with the elements magnetically-shielded so that the electrons do not strike them. Instead, they zip right on through, oscillating back and forth about the center, creating a deep electrostatic well to trap the ions of lithium 6 and hydrogen that form the fusion fuel. With a one meter diameter grid and a fuel consumption rate of 7 mg/sec, the fusion power produced is 360 MW_th.

Half of this energy is bremsstrahlung X-rays, which must be captured in a lithium heat engine. The other half are isotopes of helium (³He and ⁴He), each at about 8 MeV. (Overall efficiency is 36%). Since both products are doubly charged, a 4 MeV electric field will decelerate them and produce two electrons from each, producing an 18 amp current at extremely high voltage.

An electron gun using this 4 million volt energy would emit electrons at relativistic speeds. This beam dissipates quickly in space, unless neutralized by positrons or converted into a free electron laser beam.

“Inertia-Electrostatic-Fusion Propulsion Spectrum: Air-Breathing to Interstellar Flight,” R W. Bussard and L. W. Jameson, Journal of Propulsion and Power, v. 11, no. 2, pp. 365-372.

(Philo Farnsworth, the farm boy who invented the television, spent his last years in a lonely quest to attain break-even fusion in his ultra-cheap fusor devices. His ideas are enjoying a renaissance, thanks to Dr. Bussard, and working fusion reactors are making an appearance in high school science fairs. On the theory that the fusor is power-limited, I have scaled down Bussard’s 10 GW design to 360 MW.)

• 

D-T fusion has a starting mass of 5.029053 and a mass defect of 0.018882. Divide 0.018882 by 5.029053 to get E_p of 0.00375.

Plugging that into our equation V_e = sqrt(2 * 0.00375) = 0.0866 = 8.7% c. In meters per second 0.0866 * 299,792,458 = 25,962,027 m/s.

Abstract

Excluding speculations about future breakthrough discoveries in physics, it is shown that with what is at present known, and also what is technically feasible, manned space flight to the limits of the solar system and beyond deep into the Oort cloud is quite well possible. Using deuterium as the rocket fuel of choice, abundantly available on the comets of the Oort cloud, rockets driven by deuterium fusion, can there be refueled. To obtain a high thrust with a high specific impulse, favors the propulsion by deuterium micro-bombs, and it is shown that the ignition of deuterium micro-bombs is possible by intense GeV proton beams, generated in space by using the entire spacecraft as a magnetically insulated billion volt capacitor. The cost to develop this kind of propulsion system in space would be very high, but it can also be developed on earth by a magnetically insulated Super Marx Generator. Since the ignition of deuterium is theoretically possible with the Super Marx Generator, rather than deuterium-tritium with a laser where 80% of the energy goes into neutrons, would also mean a breakthrough in fusion research, and therefore would justify the large development costs.

2. On deuterium, argon ion lasers and keV super explosives

     The goal is a space craft which can be refueled while landing on a planetary body, which can be a planet, an asteroid or a comet. With heavy water in water abundantly available on many planetary bodies, but in particular on comets, suggests to use deuterium as the thermonuclear rocket fuel. The ignition of deuterium though is more difficult than the ignition of the deuterium-tritium (DT) reaction, or the deuterium-helium3 (DHe³) reaction.

     The DT reaction is the easiest to ignite, but there 80% of the energy goes into neutrons, which can not be deflected by a magnetic mirror. In the DHe³ reaction all the energy goes into charged fusion products, but in a mixture of D with He³ there are still some neutron producing DD reactions. More important is the fact that unlike deuterium, He³ is largely unavailable. There is some indication for He³ on the surface of moon. In the Daedalus starship study of the British Interplanetary Society, it was proposed to “mine” He³ from the atmosphere of Jupiter. In either case the cost to recover appreciable amounts of He³ would be very high.

     For the DD reaction the situation is quite different. Because there the instantaneous burn with deuterium of the T- and He³- reaction products of deuterium, makes possible a detonation wave in dense deuterium. In this detonation wave only 38% of the energy released goes into neutrons, unlike the 80% for the DT reaction.

     Deuterium can be extracted from water with relative ease in three steps:

1. Water is electrolytically split into hydrogen and oxygen.
2. The hydrogen gas composed of H₂ and HD is cooled down until it liquifies, whereby the heavier HD is separated by the force of gravity from the lighter H₂.
3. The newly produced HD is heated up and passed through a catalyst, splitting HD into H₂ and D₂.

     Since the gravitational field on the surface of a comet or small planet, from where the D₂ shall be extracted, is small, the apparatus separating the liquid HD from H₂ must be set into rapid rotation.

     The comparatively small amount of energy needed for the separation can ideally be drawn from a ferroelectric capacitor (for example a barium-titanate capacitor with a dielectric constant ε ≈ 5000), to be charged up to many kilovolts by a small fraction of the electric energy drawn from the deuterium fusion explosions through a magneto hydrodynamic loop. One can also draw this energy from a small on-board nuclear reactor requiring only a small radiator, slowly charging the capacitor. Alternatively, one may store the needed energy in the magnetic field of a superconductor.

     For the launching of the spacecraft into earth orbit a very different scheme is proposed. It requires special materials are readily available on earth, but not on extraterrestrial bodies serving as landing points to refuel the spacecraft. There primarily water as a source of deuterium is needed.

     In the Orion bomb propulsion project a large number of fission bombs, or fission-triggered fusion bombs, were proposed to lift the spacecraft into space. Since this would release a large amount of highly radioactive fission products into the atmosphere, it was one of the causes which killed Orion. Even though large payloads can be brought into earth orbit by chemical rockets, this remains very expensive and an acceptable less expensive nuclear alternative is highly desirable.

     There seem to be two possibilities:

1. A laser driven by a high explosive, powerful enough to ignite a DT micro-explosion, which in turn can launch a thermonuclear detonation in deuterium.
2. The second possibility is more speculative: It is the conjectured existence of chemical keV superexplosives. These are chemical compounds formed under high pressure, resulting in keV bridges between inner electron shells, able to release intense bursts of keV X-rays, capable of igniting a DT thermonuclear reaction, which in turn could by propagating burn ignite a larger deuterium detonation.

For the realization of the first possibility, one may consider pumping a solid argon rod with a convergent cylindrical shock wave driven by a high explosive. If the argon rod is placed in the center of convergence to reach a temperature of 90,000° K, this will populate in the argon the upper ultraviolet laser level, remaining frozen in the argon during its following rapid radial expansion. The energy thusly stored in the upper laser level can then be removed by a small Q-switched laser from the rod in one run into a powerful laser pulse, to be optically focused onto a thermonuclear target.

     For the realization of the second possibility, one would have to subject suitable materials to very high pressure. These energetic states can only be reached if during their compression the materials are not appreciably heated, because such heating would prevent the electrons from forming the bridges between the inner electron shells. Details of the second possibility will be given in the appendix.

3. On magnetic insulation and inductive charging

     There are two concepts which are of great importance for the envisioned realization of a deuterium fusion driven starship:

1. The concepts of magnetic insulation. It permits the attainment of ultrahigh voltages in high vacuum.
2. The concept of inductive charging, by which a magnetically insulated conductor can be charged up to very high electric potentials.

To 1.

     In a greatly simplified way magnetic insulation can be understood as follows: If the electric field on the surface of a negatively charged conductor reaches a critical field of the order E_c ~ 10⁷ V/cm, the conductor becomes the source of electrons emitted by field emission. The critical electric field for the emission of ions from a positively charged conductor is ~ 10⁸ V/cm. Therefore, if in a high voltage diode the electric field reaches ~ 10⁷ V/cm, breakdown will occur by electric field emission from the cathode to the anode. But if a magnetic field of strength B measured in Gauss, is applied in a direction parallel to the negatively charged surface, and if B > E, where E (like B) is measured in electrostatic cgs units, the field emitted electrons make a drift motion parallel to the surface of the conductor with the velocity

     To keep ν_d/c < 1 then requires that E < H. Let us assume that H ≈ 2 × 10⁴ G, which can be reached with ordinary electromagnets, mean that E ≤ 2 × 10⁴ esu = 6 × 10⁶ V/cm. For a conductor with a radius of ℓ ~ 10m = 10³ cm, an example for a small starship configuration, one can reach a voltage of the order Eℓ ≤ 6 ×10⁹ Volts.

To 2.

     To charge the spacecraft to the required gigavolt potentials we choose for its architecture a large, but hollow cylinder, which at the same time serves to act as a large magnetic field coil. If on the inside of this coils thermionic electron emitters are placed, and if the magnetic field of the coil rises in time, Maxwell’s equation curl E = -(1/c)∂B/∂t induces inside the coil an azimuthal electric field:

where B = B_z is directed along the z-axis, with ν the radial distance from the axis of the coil. In combination with the axial magnetic field, the electrons from the thermionic emitters make a radial inward directed motion

By Maxwell’s equation div E = 4πner, this leads to the buildup of an electron cloud inside the cylinder resulting in the radial electric field

This radial electric field leads to an additional azimuthal drift motion

superimposed on the radially directed inward drift motion v_r.

     For the newly formed electron cloud to be stable, its maximum electron number density must be below the Brillouin limit:

where mc² = 8.2×10^-7 erg is the electron rest mass energy. For B = 2×10⁴G one finds that n_max ≈ 4×10¹³ cm^-3. To reach with a cylindrical electron cloud of radius R a potential equal to V, requires an electron number density n ≈ V / πeR². For V = 10⁹ volts ≈ 3×10⁶ esu and R = 10³cm, one finds that n ~ 2×10⁹ cm^-3, well below n_max.

4. On deuterium as the preferred nuclear rocket fuel

     To appreciate the importance of deuterium as the preferred and abundantly available nuclear rocket fuel, one must consider the secondary reactions of the He³ and T D-D fusion reaction products with D. Taking these reactions into account one obtains from 6 deuterium nuclei an energy of 26.8 MeV in charged fusion products, made up from He³ and H, and an energy of 16.55 MeV in neutrons. This means that 62% of the energy is released into charged fusion products and 38% into neutrons. This is a substantial improvement over the DT reaction where only 20% of the energy goes into He⁴.

     Of interest is also the average velocity, averaged over the momentum of the charged fusion products, because it is a measure of the maximum specific impulse, respectively the maximum exhaust velocity:

     For the six charged fusion reaction products (given in table 1) one obtains ν = 1.5×10⁹ cm/s.

     Because the 6 charged fusion products are accompanied by 9 electrons, they have to share their kinetic energy with 9 electrons. This reduces the maximum specific impulse by the factor

     A reduction of the specific impulse does not occur if the electrons have enough time to escape the burning plasma behind the detonation front that is in a time shorter than the time for them to be heated up by the charged fusion products.

     The time needed for the electrons to be heated by the charged fusion reaction products can be computed from the range λ0 of the charged fusion products and their velocity ν, in plasma of the temperature T. For the He⁴ fusion products the range is given by

This time then is

It has to be compared with the time the electron can escape the burning plasma behind the detonation front, given by

where r₀ is the radius of the burning deuterium cylinder, and v_e the electron velocity. One thus has,

Putting n = 10²³ cm^-3, T ≈ 10⁸ K with v_e ~ 10⁹ cm/s, v ~ 10⁹ cm/s one finds t_esc/τ ~ 2.5r₀. Therefore, for t_esc < τ requires that r₀ < 0.4cm.

     To reach the highest specific impossible, one should make the deuterium cylinder as thin as possible.

5. Magnetic entrapment of the charged fusion products and the stopping of the proton beam in dense deuterium

     If a fission bomb is used to trigger a thermonuclear detonation, there is so much energy available that almost any radiation implosion configuration is likely going to work. As an example, one may place a fission bomb and a sphere of solid deuterium in a shell of gold, in the two foci of an ellipsoidal cavity. The radiation released by the exploding fission bomb, by ablating the gold, launches a convergent shock wave into the liquid deuterium. With the temperature in the shock wave approximately rising as 1/r, where r is the distance of the shock wave from the center of the deuterium sphere, the ignition temperature is reached at some distance from the center. But only if this distance is larger than the stopping length of the DD fusion reaction products, typically a few cm, is a radially outward moving detonation wave ignited. This configuration is essentially the same kind of “hohlraum” (cavity) configuration, used in the indirect drive mode of laser fusion for a small DT sphere.

     A configuration of this kind can still be used to burn deuterium, if the DT micro-explosion is used to trigger a larger deuterium explosion. For a starship which shall depend on deuterium as its only rocket fuel, and nothing else, this possibility is excluded.

     But there is another possibility. It arises if the ignition is done with a 10⁷ Ampere-GeV proton (or deuterium) beam. If focused onto the one end of a slender cylindrical deuterium rod, the beam not only can be made powerful enough to ignite the deuterium, but its strong azimuthal magnetic field entraps the charged DD reaction fusion products within the deuterium cylinder, launching a deuterium detonation wave propagating with supersonic speed down the cylinder. There the fusion gain and yield can in principle be made arbitrarily large, only depending on the length of the deuterium rod.

     The range of the charged fusion products is determined by their Larmor radius

where,

     In (13) c, e are the velocity of light and the electron charge, M the hydrogen mass, A the atomic weight and Z the atomic number. E is the kinetic energy of the fusion products. If the magnetic field is produced by the proton beam current I, one has at the surface of the deuterium cylinder the azimuthal magnetic field

Combining (12) with (14) and requesting that r_l < r, one finds that

where I_c = 5α. In table 2 the values for α and I_c for all the charged fusion products of the DD reaction are compiled. For all of them the critical current is below I_c = 3.84 ×10⁶ A. Therefore, with the choice I ~ 10⁷ A, all the charged fusion products are entrapped inside the deuterium cylinder.

     For the argon ion laser configuration proposed for the launch into earth orbit, where a small amount of DT serves as a trigger for the ignition of a larger amount of deuterium, the ignition of a magnetic field supported detonation wave in deuterium is there possible with an auxiliary high explosive driven megampere current generator, setting up an axial magnetic field, by an azimuthal current around the rod. The charged fusion products are there spiraling down the rod. The current needed to entrap the charged fusion products are there of the same order of magnitude, that is of the order of ~ 10⁷ A.

     For the deuterium-tritium thermonuclear reaction the condition for propagating burn in a sphere of radius r and density ρ, heated to a temperature of 10⁸ K, is given by ρr ≥ 1 g/cm². This requires energy of about 1 MJ. For the deuterium reaction this condition is ρr ≥ 10 g/cm², with an ignition temperature about 10 times larger. That a thermonuclear detonation in deuterium is possible at all is due to the secondary combustion of the T and He³ DD fusion reaction products. The energy required there would be about 10⁴ times larger or about 10⁴ MJ, for all practical purposes out of reach for non-fission ignition. However, if the ignition and burn is along a deuterium cylinder, where the charged fusion products are entrapped by a magnetic field within the cylinder, the condition ρr ≥ 10 g/cm² is replaced by

where z is the length of the cylinder.

     If the charged fusion products are entrapped within the deuterium cylinder, and if the condition ρr ≥ 10 g/cm² is satisfied, and finally, if the beam energy is large enough that a length z > (10/ρ) cm of the cylinder is heated to a temperature of 10⁹ K, a thermonuclear detonation wave can propagate down the cylinder. This then leads to large fusion gains.

     The stopping length of single GeV protons in dense deuterium is much too large to fulfill inequality (16). But this is different for an intense beam of protons, where the stopping length is determined by the electrostatic proton-deuteron two-stream instability. In the presence of a strong azimuthal magnetic field the beam dissipation is enhanced by the formation of a collision less shock. With the thickness of the shock by order of magnitude equal to the Larmor radius of the deuterium ions at a temperature of 10⁹ K, which for a magnetic field of the order 10⁷ G is of the order of 10^-2 cm. For the two-stream instability alone, the stopping length is given by

     Where c is the velocity of light, and ω_i the proton ion plasma frequency, furthermore ε = n_b / n, with n the deuterium target number density and n_b = 2×10¹⁶ cm^-3 the proton number density in the beam. For a 100-fold compressed deuterium rod one has n = 5×10²⁴ cm^-3 with ω_i = 2×10¹⁵ s^-1. One there finds that ε = 4×10^-9 and, λ ≅ 1.2×10^-2 cm. This short length, together with the formation of the collision-less magneto-hydrodynamic shock, ensures the dissipation of the beam energy into a small volume at the end of the deuterium rod. For a deuterium number density n = 5×10²⁴ cm^-3 one has ρ = 17 g/cm³, and to have ρz > 10 g/cm², then requires that z ≥ 0.6 cm. With λ < z, the condition for the ignition of a thermonuclear detonation wave is satisfied. The ignition energy is given by

where T ≈ 10⁹ K.

For 100-fold compressed deuterium, one has πr² = 10^-3 cm², when initially it was πr² = 10^-1 cm². With πr² = 10^-3 cm², z = 0.6 cm one finds that E_ign ≤ 10¹⁶ erg or ≤ 1GJ. This energy is provided by the 10⁷ Ampere-GeV proton beam lasting 10^-7s. The time is short enough to ensure the cold compression of deuterium to high densities. For a 10³-fold compression, found feasible in laser fusion experiments, the ignition energy is ten times less.

     In hitting the target, a fraction of the proton beam energy is dissipated into X-rays by entering and bombarding the high Z material cone, focusing the proton beam onto the deuterium cylinder. The X-rays released fill the hohlraum surrounding the deuterium cylinder, compressing it to high densities, while the bulk of the proton beam energy heats and ignites the deuterium cylinder at its end, launching in it a detonation wave.

     If the GeV -10⁷ Ampere proton beam passes through background hydrogen plasma with a particle number density n, it induces in the plasma a return current carried by its electrons, where the electrons move in the same direction as the protons. But because the current of the proton beam and the return current of the plasma electrons are in opposite directions, they repel each other. Since the stagnation pressure of the GeV -10⁷ Ampere proton beam is much larger than the stagnation pressure of the electron return current, the return current electrons will be repelled from the proton beam towards the surface of the proton beam.

     The stagnation pressure of a GeV proton beam is (M_H proton mass)

For n_b ≅ 2×10¹⁶ cm^-3 one obtains p_i ≅ 3×10¹³ dyn/cm³. For the electron return current one has (m electron mass)

With the return current condition n_eev_e = n_iev_i, where for GeV protons v_i ≅ c, one has

Taking the value n_e = 5×10²² cm^-3, valid for uncompressed solid deuterium, one obtains v_e ≅ 10⁴ cm/s and hence p_e ≅ 5×10³ dyn/cm². This is negligible against p_i, even if n_e is 10³ times larger, as in highly compressed deuterium. The assumption, that the magnetic field of the proton beam is sufficiently strong to entrap the charged fusion products within the deuterium cylinder, is therefore well justified.

6. Solution in between two extremes

     With chemical propulsion manned space flight to the moon is barely possible and only with massive multistage rockets. For manned space flight beyond the moon, nuclear propulsion is indispensible. Nuclear thermal propulsion is really not much better than advanced chemical propulsion. Ion propulsion, using a nuclear reactor driving an electric generator has a much higher specific impulse, but not enough thrust for short interplanetary transit times, as they are needed for manned missions. This leaves the propulsion by a chain of fission bombs (or fission triggered fusion bombs) as the only credible option. There the thrust and specific impulse are huge in comparison. But a comparatively small explosive yield is there be desirable. Making the yield too small, the bombs become extravagant in the sense that only a small fraction of the fission explosive is consumed. The way to overcome this problem is the non-fission ignition of small fusion explosions. A first step in this direction is the non-fission ignition of deuterium-tritium (DT) thermonuclear micro-explosions; the easiest one to be ignited, expected to be realized in the near future. Because of it, I had chosen this reaction for the first proposed thermonuclear micro-explosion propulsion concept, with the ignition done by an intense relativistic electron beam. But because in the DT reaction 80% of the energy is released into neutrons which cannot be reflected from the spacecraft by a magnetic mirror, it was proposed to surround the micro-explosion with a neutron-absorbing hydrogen propellant, increasing the thrust on the expense of the specific impulse. It was for this reason that in the “Daedalus” interstellar probe study of the British Interplanetary Society, the neutron-less helium3-deuterium (He³-D) reaction was proposed, because for such a mission the specific impulse should be as high as possible. But even in a He³-D plasma, there are a some neutron producing DD reactions. There is no large source of He³ on the Earth, even though it might exist on the surface of the moon, and in the atmosphere of Jupiter. In the DD reaction much less energy goes into neutrons, but it is more difficult to ignite. The situation is illustrated in Fig 1.

• 

On the left side it shows the experimentally verified ignition of a DT pellet with the X-rays drawn in an underground test from a fission bomb (Centurion Halite experiment at the Nevada Test Site). For the ignition of the DT reaction with propagation thermonuclear burn (i.e. detonation) requires a few megajoule, with a density×radius target product, ρr > 1g/cm². And on the right side is the 15 Megaton “Mike” test, where with the Teller-Ulam configuration a large amount of liquid deuterium is ignited with a fission bomb. For the DD reaction propagating thermonuclear burn (i.e., detonation) requires that ρr ≥10g/cm². In between is the proposed hypothetical deuterium target, where a detonation wave in a thin cylindrical deuterium rod is ignited by a pulsed 10⁷ Ampere-GeV proton beam, utilizing the strong magnetic field of the beam current.

     To estimate the order of magnitude what is needed, we consider a spacecraft with a mass of M₀ = 10³ ton = 10⁹ g, to be accelerated by one g ≅10³ cm/s², with a thrust T = M₀g ≅ 10¹² dyn. To establish the magnitude and number of fusion explosions needed to propel the spacecraft to a velocity of v = 100 km/s = 10⁷ cm/s, we use the equation

where we set c ≅ 10⁸ cm/s, equal to the expansion velocity of the fusion bomb plasma. We thus have

The propulsion power is given by

in our example it is P = 5×10¹⁹ erg/s.

     With E = 4×10¹⁹ erg equivalent to the explosive energy of one kiloton of TNT, P is equivalent to about one nuclear kiloton bomb per second. From the rocket equation

where M₀ is the initial, and M₁ the final mass, and setting M₁ = M₀ - ΔM, where ΔM ≪ M₀ is the mass of all used up bombs one has,

     where v is the velocity reached (delta V) by the spacecraft after having used up all the bombs of mass ΔM. If one bomb explodes per second, its mass according to (23) is m₀ = 10⁴ g.

     Assuming that the spacecraft reaches a velocity of v = 100 km/s = 10⁷ cm/s (egads!), the velocity needed for fast interplanetary travel, one has ΔM = 10⁸ g, requiring N = ΔM / m₀ = 10⁴ one kiloton fusion bombs, releasing the energy E_b = 5×10¹⁹ × 10⁴ = 5×10²³ erg. By comparison, the kinetic energy of the spacecraft E_s = (1/2)M₀v² = 5×10²² erg, is 10 times less. In reality it is still smaller, because a large fraction of the energy released by the bomb explosions is dissipated into space.

     One can summarize these estimates by concluding that a very large number of nuclear explosions is needed, which for fission explosions, but also for deuterium-tritium explosions, would become very expensive. This strongly favors deuterium, more difficult to ignite in comparison to a mixture of deuterium with tritium, but abundantly available. Here I will try to show how bomb propulsion solely with deuterium might be possible.

7. The non-fission ignition of small deuterium nuclear explosives

     With no deuterium-tritium (DT) micro-explosions yet ignited, the non-fission ignition of pure deuterium (DD) fusion explosions seems to be a tall order. An indirect way to reach this goal is by staging a smaller DT explosion with a larger DD explosion. There the driver energy, but not the driver may be rather small. A direct way requires a driver with order of magnitude larger energies.

     I claim that the generation of GeV potential wells, made possible with magnetic insulation of conductors levitated in ultrahigh vacuum (in a laboratory on Earth), has the potential to lead to order of magnitude larger driver energies. It is the ultrahigh vacuum of space by which this can be achieved without levitation. Therefore, the spacecraft acting as a capacitor can be charged up to GeV potentials.

     If charged to a positive GeV potential, a gigajoule intense relativistic ion beam below the Alfvén current limit can be released from the spacecraft and directed to the deuterium explosive for its ignition. If the current needed for ignition is below the Alfvén limit for ions, the beam is “stiff”. The critical Alfvén current for protons is I_A = 3.1×10⁷βγ, where β = v / c, γ = (1-β²)^-½, with v the proton velocity and c the velocity of light. For GeV protons I_A is well in excess of the critical current (15) to entrap the DD fusion reaction products, the condition for detonation.

     In a possible bomb configuration shown in Fig.2, the liquid (or solid) D explosive has the shape of a long cylinder, placed inside a cylindrical “hohlraum” h. A GeV proton beam I coming from the left, in entering the hohlraum dissipates part of its energy into a burst of X-rays compressing and igniting the D bomb-cylinder. With its gigajoule energy lasting less than 10^-7 s, the beam power is greater than 10¹⁶ Watt, sufficiently large to ignite the D explosive. The main portion of the beam energy is focused by the cone onto the deuterium rod, igniting at its end a detonation wave.

• 

     Because the condition for thermonuclear burn and detonation depends only on the critical current (15), but not on the radius of the deuterium cylinder, one may wish to make the diameter of the deuterium cylinder as small as possible, because as it was shown above, the specific impulse can become largest, with the electrons not taken away kinetic energy from the charged fusion products. There then, the yield of the deuterium fusion explosions can be made much smaller, eliminating the need for Orion-type shock absorbers, but the thrust there is also much smaller. For very deep space missions that would not be a disadvantage.

     A problem in either case is that 38% of the energy is released as neutrons. In hitting the space craft they lead to its heating, requiring a presumably large radiator. For a long and thin deuterium rod this problem can likely be reduced by a boron diaphragm on the deuterium rod, as shown in Fig 3, because boron is a good neutron absorber. It can be enhanced by a hydrogen moderator, because the absorption cross section for neutrons is greatly increased with a reduced neutron kinetic energy. Both the boron and the hydrogen there simply become part of the propellant, reducing the specific impulse but increasing the thrust.

     We know that in comets there is large amount of deuterium, readily available for mining. And we know that comets have also nitrogen and carbon. From this knowledge it is very likely, that other light elements like boron should be in relative high concentrations.

• 

     The problem of the waste heat radiator remains high, but it favors large explosions because there most of the waste heat goes with the propellant into space. Droplet radiators, with the droplets slowly evaporating, are unlikely to work. Placing the neutron absorbing radiators near the shock absorber, permitting them to get red-hot, and thermally insulating the rest of the space craft from the radiators, may solve the problem.

8. Delivery of a Gev proton beam onto the deuterium fusion explosive

     The spacecraft is inductively charged against an electron cloud surrounding the craft, and, with a magnetic field of the order 10⁴ G, easily reached by superconducting currents flowing in an azimuthal direction around the craft, is magnetically insulated against the electron cloud up to GeV potentials. The spacecraft and its surrounding electron cloud form a virtual diode with a GeV potential difference. To generate a proton beam, it is proposed to attach a miniature hydrogen filled rocket chamber R to the deuterium bomb target, at the position where the proton beam hits the fusion explosive (see Fig. 2). A pulsed laser beam from the spacecraft is shot into the rocket chamber, vaporizing the hydrogen, which is emitted through the Laval nozzle as a supersonic plasma jet. If the nozzle is directed towards the spacecraft, a conducting bridge is established, rich in protons between the spacecraft and the fusion explosive. Protons in this bridge are then accelerated to GeV energies, hitting the deuterium explosive. Because of the large dimension of the spacecraft, the jet doesn’t have to be aimed at the spacecraft very accurately.

     The original idea for the electrostatic energy storage on a magnetically insulated conductor was to charge up a levitated superconducting ring to GeV potentials, with the ring magnetically insulated against breakdown by the magnetic field of a large toroidal current flowing through the ring. It is here proposed to give the spacecraft a topologically equivalent shape, using the entire spacecraft for the electrostatic energy storage (see Fig. 3). There, toroidal currents flowing azimuthally around the outer shell of the spacecraft, not only magnetically insulate the spacecraft against the surrounding electron cloud, but the currents also generate a magnetic mirror field which can reflect the plasma of the exploding fusion bomb. In addition, the expanding bomb plasma can induce large currents, and if these currents are directed to flow through magnetic field coils positioned on the upper side of the spacecraft, electrons from there can be emitted into space surrounding the spacecraft by thermionic emitters placed on the inner side of these coils, inductively charging the spacecraft for subsequent proton beam ignition pulses. A small high voltage generator driven by a small onboard fission reactor can make the initial charging, ejecting from the spacecraft negatively charged pellets.

     With the magnetic insulation criterion, E < B (E, B, in electrostatic units, esu), where B is the magnetic field surrounding the spacecraft measured in Gauss, then for B ≅ 10⁴G, E = 3×10³ esu = 9×10⁵ V/cm, one has E ~ (⅓)B hence E < B. A spacecraft with the dimension l ~ 3×10³ cm, can then be charged to a potential El ~ 3×10⁹ Volts, with the stored electrostatic energy is of the order ε ~ (E² / 8π) l³.

     For E = 3×10³ esu, and l = 3×10⁹ cm, ε is of the order of one gigajoule. The discharge time is of the order τ ~ l / c where c = 3×10¹⁰ cm/s is the velocity of light. In our example we have τ ~ 10⁷ sec. For a proton energy pulse of one gigajoule, the beam power is 3×10¹⁶ erg/s = 30 petawatt, large enough to ignite a pure deuterium explosion.

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9. Lifting of large payloads into earth orbit

     To lift large payloads into earth orbit remains the most difficult task. For a launch from the surface of the earth, magnetic insulation inside the earth atmosphere fails, and with it the proposed pure deuterium bomb configuration. A different technique is here suggested, which I had first proposed in a classified report, dated January of 1970, declassified July 2007, and thereafter published. A similar idea was proposed in a classified Los Alamos report, dated November 1970, declassified July 1979. In both cases the idea is to use a replaceable laser for the ignition of each nuclear explosion, with the laser material thereafter becoming part of the propellant. The Los Alamos scientists had proposed to use an infrared carbon dioxide (CO₂) or chemical laser for this purpose, but this idea does not work, because the wavelength is too long, and therefore unsuitable for inertial confinement fusion. I had suggested an ultraviolet argon ion laser instead. However, since argon ion lasers driven by an electric discharge have a small efficiency, I had suggested a quite different way for its pumping, illustrated in Fig. 5. There the efficiency can be expected to be quite high. It was proposed to use a cylinder of solid argon, surrounding it by a thick cylindrical shell of high explosive. If simultaneously detonated from outside, a convergent cylindrical shockwave is launched into the argon. For the high explosive one may choose hexogen with a detonation velocity of 8 km/s. In a convergent cylindrical shockwave the temperature rises as r^-0.4, where r is the distance from axis of the cylindrical argon rod. If the shock is launched from a distance of ~1 m onto an argon rod with a radius equal to 10 cm, the temperature reaches 90,000 K, just right to excite the upper laser level of argon. Following its heating to 90,000 K the argon cylinder radially expands and cools, with the upper laser level frozen into the argon. This is similar as in a gas dynamic laser, where the upper laser level is frozen in the gas during its isentropic expansion in a Laval nozzle. To reduce depopulation of the upper laser level during the expansion by super-radiance, one may dope to the argon with a saturable absorber, acting as an “antiknock” additive. In this way megajoule laser pulses can be released within 10 nanoseconds. A laser pulse from a small Q-switched argon ion laser placed in the spacecraft can then launch a photon avalanche in the argon rod, igniting a DT micro-explosion.

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     Employing the Teller-Ulam configuration, by replacing the fission explosive with a DT micro-explosion, one can then ignite a much larger DD explosion.

     As an alternative one may generate a high current linear pinch discharge with a high explosive driven magnetic flux compression generator. If the current I is of the order I = 10⁷A, the laser can ignite a DT thermonuclear detonation wave propagating down the high current discharge channel, which in turn can ignite a much larger pure DD explosion.

     If launched from the surface of the earth, one has to take into account the mass of the air entrained in the fireball. The situation resembles a hot gas driven gun, albeit one of rather poor efficiency. There the velocity gained by the craft with N explosions, each setting off the energy E_b is given by

For E_b = 5×10¹⁹ erg, M₀ = 10⁹g, and setting for v = 10 km/s = 10⁶ cm/s the escape velocity from the Earth, one finds that N ≥ 10. Assuming an efficiency of 10%, about 100 kiloton explosions would there be needed.

10. Neutron entrapment in an autocatalytic thermonuclear detonation wave – a means to increase the specific impulse and to solve the large radiator problem

     The principal reason why neutrons released by thermonuclear reactions pose such a serious problem is that they cannot be repelled from the spacecraft by a magnetic field. Choosing a neutron absorbing target as shown in Fig. 3 one can though reduce the flux of neutrons hitting the spacecraft. Besides the material damage the neutrons can inflict on the spacecraft, it is that they release heat which must be removed by a radiator, and this radiator will be very large.

     The idea of the autocatalytic thermonuclear detonation wave presents a solution, which if feasible would very much reduce the magnitude of this problem. For its implementation it requires very large bremsstrahlungs flux densities in the burn zone behind the thermonuclear detonation front. Such large bremsstrahlungs flux densities will occur in deuterium detonation burn, at the highest temperature for all the thermonuclear reactions.

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     In an Autocatalytic thermonuclear detonation, explained in Fig. 6, soft X-rays generated through the burn of the thermonuclear plasma behind the detonation front, compresses the still unburned thermonuclear fuel ahead of the front. The increase in the fuel density, both in the Teller-Ulam configuration and the autocatalytic thermonuclear detonation wave, is of crucial importance, with the reaction rate going in proportion to the square of the density.

     From the burning plasma behind the detonation front, energy flows into all spatial directions. Part is by bremsstrahlung and part by electronic heat conduction. Roughly half of the energy flows into the liner, and a quarter into the still unburned fuel ahead of the wave and one quarter into opposite direction. The bremsstrahlung emission rate is given by

     For T = 10⁹ K one has ε_r ≈ 3.2×10^-23 n² [erg/cm³s]. The flux of the bremsstrahlung which goes into the liner is ε_r r /2, where r is the radius of the deuterium rod just behind the detonation front. About ½ of this radiation runs ahead of the detonation front where it pre-compresses the deuterium. Its intensity is:

with nr ≈ 3×10²⁴cm^-2, (corresponding ρr ≈ 10 g/cm²), one has

We are aiming at a density where the neutrons are absorbed in the burning plasma cylinder of radius r. If the neutron-deuteron collision cross section is σ , then the neutron path length λ_n must be smaller than r:

or

One typically has σ ≈ 10^-24 cm² , hence

If r = 0.01 cm then n ≥ 10²⁶ cm^-3 = 5×10³ n_o, where n_o = 5×10²² cm^-3, the particle number density of liquid deuterium. With n=10²⁶ cm^-3, one finds that ε_r r /4 ≈ 3×10²⁷ erg/cm²s. The flux on a deuterium tube of radius r and length z, where for the burn zone r ≈ z, one obtains onto its surface (ε_r /4)2πrz ≈ 10²⁴ erg/s = 10¹⁷ watt = 100 petawatt, certainly powerful enough to compress the deuterium to more than 1000 fold density.

     Through the entrapment of the neutrons goes an increase of the specific impulse. With 38% of the energy going into the kinetic energy of the neutrons, and 62% into charged fusion products, the specific impulse increased by the factor √1 + 38 / 62 = √10 / 6.2 = 1.275 This increases the maximum exhaust velocity from v=1.5×10⁹ cm/s to v=1.9×10⁹ cm/s = 0.063c.

     In the course of their thermalization in the supercompressed plasma, the neutron absorption cross section is greatly increased, both in the liner and tamp, with the liner also compressed to high densities. If the liner and tamp are made from boron which has a large neutron absorption cross section, the space craft is only heated by a greatly reduced thermal neutron flux, and only this much smaller amount of heat must be removed by a radiator.

12. Conclusion

     If large scale manned spaceflight has any future, a high specified impulse – high thrust propulsion system is needed. The only known propulsion concept with this property is the nuclear bomb propulsion concept. However, since large yield nuclear explosions are for obvious reason undesirable, the nuclear explosions should by comparison be small. But because of the “tyranny of the critical mass” (quote by F. Dyson), small fission bombs or fission triggered fusion bombs, become extravagant, with only a fraction of the nuclear material consumed (The same is true for nuclear fission gas core rocket reactors, where much of the unburnt fission fuel is lost in the exhaust).

     In the original Orion bomb propulsion concept, the propulsive power was through the ablation of a pusher plate. There the energy is delivered to the pusher plate by the black body radiation of the exploding bomb. The propulsion by non-fission triggered fusion bombs, not only has the advantage that it is not subject to the “tyranny of the critical mass”, but the propulsive power is there delivered by the kinetic energy of expanding hot plasma fire ball repelled from the spacecraft by a magnetic mirror. This is in particular true for a pure deuterium bomb, where in comparison to DT more energy is released into charged fusion products. In a DT bomb, 80% of the energy goes into neutrons.

     While in a fission explosion most of the energy is lost into space by the undirected blackbody radiation, much more propulsive energy can be drawn from the plasma of a pure deuterium fusion bomb explosion, in conjunction with a magnetic mirror.

     Manned space flight requires lifting large masses into earth orbit, where they are assembled into a large spacecraft. While this can be done with chemical rockets, it would be much more economical if it could be done with a chain of small nuclear explosions. Without radioactive fallout this can be done with a chain of laser ignited fusion bombs, with one laser for each bomb, where the lasers become part of the exhaust. Ignition cannot be done by infrared chemical or CO₂ lasers as it was suggested by the Los Alamos team, but rather by the kind of an ultraviolet laser driven by high explosives (see appendix in original paper).

     Looking ahead into the future with deuterium as the nuclear rocket fuel, widely available on most planets of the solar system and in the Oort cloud outside the solar system, this would make manned space flight to the Oort cloud possible, at a distance at about one tenth of one light year.

2. Ignition by a convergent shock wave

(see details here)

4. The mini-fusion bomb configuration

     As shown in Fig. 4, the deuterium-tritium (DT) fusion explosive positioned in the center is surrounded by a cm-size spherical shell made up of a super-explosive, surrounded by a metersize sphere of liquid hydrogen. The surface of the hydrogen sphere is covered with many high explosive lenses, preferably of a high explosive made up of a boron compound, to increase the absorption of the neutrons making up 80% of the energy released in the DT fusion reaction. Each explosive has an igniter, and to produce a spherical convergent shock wave in the hydrogen the ignition must happen simultaneously, which can be done by just one laser beam, split up in as many beamlets as there are ignitors.

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5. The propulsion unit

     The propulsion unit is very similar to the one in a previous publication, where the fusion bomb assembly is placed in the focus of a 10 meter-size large metallic reflector, positioned around the focus of a magnetic mirror. The expanding fire ball compressing the magnetic field will there generate surface currents in the metallic reflector, making a magnetized plasma layer protecting the reflector from the hot plasma. The meter-size hydrogen sphere of the mini-fusion bomb is transformed into a fireball with a temperature of ~ 10⁵ K, or somewhat higher, with an exhaust velocity of ~ 30 km/s (Fig. 5). Cooling the metallic reflector can be done with liquid hydrogen becoming part of the exhaust, as in chemical liquid fuel rocket technology. This is unlikely to amount to more than 10% of the liquid hydrogen heated by the neutrons of the fusion explosion.

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     A meter-size ball of liquid hydrogen heated to 10⁵ K, has a thermal energy of 10¹⁸ erg, equivalent to 25 tons of TNT. At this temperature the pressure is ~10¹¹ dyn/cm². If the fireball expands from an initial radius of R₀ ~1 m to R₁ ~10 m, the pressure goes down to 10⁹ dyn/cm² ~10³ atm, which is about 2 orders of magnitude smaller, and less than the tensile strength of steel. At this pressure, the magnetic field strength at the surface of the steel will be of the order 10⁵ Gauss. The energy released by the eddy currents in the reflector can hardly be more than 10% of the energy released in the fusion explosion. The mass of a meter-size ball of liquid hydrogen is of the order 0.1 tons, such that 0.01 tons of liquid hydrogen would be available for the cooling of the reflector.

     The pressure of ~10⁹ dyn/cm² acting on the metallic mirror is transmitted to the space-lift to produce thrust. Because the thickness of the parabolic mirror is small in comparison to its radius, this would lead to a large circumferential hoop stress on the mirror, larger by a factor equal to the ratio of radius of the mirror to its thickness. This requires that the mirror be supported by external forces. These forces could be realized making the mirror’s thickness comparable to its radius, which for a mirror made from steel would make it very heavy.

     Adopting an idea by P. Schmidt and B. Pfau, who had shown that the wall thickness of cylindrical and spherical pressure vessels can be greatly reduced by surrounding them with a thick compact layer of a disperse medium composed of high tensile strength micro-particles. As shown in Fig. 5, to utilize this effect, the metallic mirror is placed inside a box filled with a compactified disperse medium, such as SiC (carborundum) or AlO₃.

     If the disperse medium consists of mono-crystal particles (“whiskers”), it has a compressive strength of the order ~10¹¹ dyn/cm². Because of the friction between the particles of the disperse medium, a shear stress is set up in the medium which makes possible the radial reduction of the stress in a spherical configuration.

     Setting ρ as the friction angle between the particles of the disperse medium, one has for the maximum shear stress

Where σ_n is the normal component of the stress tensor. If plotted in a Mohr stress diagram as shown in Fig. 6, the maximum possible shear stress cannot exceed the line τ < σ_n tan ρ.

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     The maximum shear stress

which in the Mohr stress diagram is given by

and hence

The friction angle ρ can be visualized by the slope of an imaginary “sandhill” made from particles of the disperse medium. For a “real” sandhill seen in nature, we estimate that ρ = 45°. Inserting this value into (19) one finds that σ_min / σ_max ≈ 0.1.

     The pressure distribution in the disperse medium surrounding the metallic reflector is determined by the static equilibrium equation, which in Cartesian coordinates is given by

and in curvilinear coordinates by

where the colon stands for the covariant derivative. For (21) one can also write

with the line element squared ( ds² = g_ikdxⁱdx^k ) defining the metric tensor and g = det g_ik . The Γ^l_ik are the Christoffel symbols of the 2^nd kind. For simplicity we may approximate the metallic reflector by a spherical shell. Then, introducing in the dispersive medium spherical coordinates r, θ ,φ , where the metric tensor is determined by the line element

one has Γ¹₁₁ = 0, Γ²₁₂ = Γ³₁₃ = 1 / r and √g = r² sin θ, and therefore from (22)

where σ_r and σ_θ are the components of the stress tensor in the radial and transverse direction.

Because σ_r = σ_max and σ_min ≈ 0.1σ_r one has

hence

where r₀ is the radius of the reflector with r₁ > r₀. Assuming, for example, that r₁ ≈ 3r₀, approximately shown in Fig. 5, one finds that σ_r⁽¹⁾ = 0.14σ_r⁽⁰⁾. For σ_r⁽⁰⁾ = p₀ = 10⁹ dyn/cm², one has σ_r⁽¹⁾ = p₁ = 1.4×10⁸ dyn/cm² = 140 atm. As in the Orion concept, this pressure is transmitted through shock absorbers to the spacecraft.

     Because more than one propulsion unit is needed, a cluster of propulsion units are put together forming a disk as shown in Fig. 7. There, the pressure in the radial horizontal direction is computed in cylindrical coordinates r, φ . With ds² = dr² + r²dφ², and ∂ / ∂φ = 0, one finds that Γ¹₁₁ = 0, Γ²₁₂ = 1/r and √g = r. From (22) one finds that here

or with σ_φ ≈ 0.1σ_r that

and hence

where r₂ > r₁ is the radius of the disc.

For r₂ ≈ 3r₁, as shown in Fig. 7, one has σ_r⁽²⁾ ≈ 0.37σ_r⁽¹⁾. For σ_r⁽¹⁾ ≈ 1.4×10⁸ dyn/cm², one has σ_r⁽²⁾ ≈ 5.2×10⁷ dyn/cm² = p₂.

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     Under these conditions the static equilibrium condition for a disc of radius r₂ and thickness t is given by

where t is the thickness of the hoop put around the disc at its radius r₂, and σ_∥ the hoop stress. From (30) one obtains for the hoop stress

or

Assuming that the material of the hoop has a tensile strength of about σ_∥ = 10¹⁰ dyn/cm², then with a disc radius r₂ ≈ 10 m = 10⁴ cm, and for p₂ = 5×10⁷ dyn/cm², one obtains t ≈ 50cm. Therefore, a hoop with such a thickness would hold the disc together.

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Research is stopped on a system of space propulsion which broke all the rules of the political game.

      In January 1965, unnoticed and unmourned by the general public, Project Orion died. The men who began the project in 1958 and worked on it through 7 strenuous years believe that if offers the best hope, in the long run, of a reasonable program for exploring space. By "a reasonable program" they mean a program comparable in cost with our existing space program and enormously superior in promise. They aimed to create a propulsion system commensurate with the real size of the task of exploring the solar system, at a cost which would be politically acceptable, and they believe they have demonstrated the way to do it. Now the decision has been taken to follow their road no further. The purpose of this article is neither to bury Orion nor to praise it. It is only to tell the public for the first time the facts of Orion's life and death, and to explain as fairly as possible the political and philosophical issues which are involved in its fate.

Vehicle Design and Capabilities

First, a brief technical summary. Orion is a project to design a vehicle which would be propelled through space by repeated nuclear explosions occurring at a distance behind it. The vehicle may be either manned or unmanned; it carries a large supply of bombs, and machinery for throwing them out at the right place and time for efficient propulsion; it carries shock absorbers to protect the machinery and the crew from destructive jolts, and sufficient shielding to protect against heat and radiation. The vehicle has, of course, never been built. The project in its 7 years of existence was confined to physics experiments, engineering tests of components, design studies, and theory. The total cost of the project was $10 million, spread over 7 years, and the end result was a rather firm technical basis for believing that vehicles of this type could be developed, tested, and flown. The technical findings of the project have not been seriously challenged by anybody. Its major troubles have been, from the beginning, political. The level of scientific and engineering talent devoted to it was, for a classified project, unusually high.

     The fundamental issue raised by such a project is: Why should one not be content with alternative means of propulsion which are free from the obvious biological and political disadvantages of nuclear explosions? The answer to this question is that, on the purely technical level, an Orion vehicle has capabilities which no other system can approach. All alternative propulsion systems which we know how to build are either temperature-limited or power-limited. Conventional rocket systems, whether chemical or nuclear, are temperature-limited in that they eject gas at a velocity V limited by the temperature of chemical reactions or of solid structures. The upper limit for V appears to be about 4 kilometers per second for chemical rockets, 8 kilometers per second for nuclear rockets. For missions involving velocity changes many times V, multiple-staged rockets are required, and the initial vehicle size needed in order to carry a modest payload soon becomes preposterous. The initial weight is multiplied by about a factor of 3 whenever an amount V is added to the velocity change of a mission. It is for this reason that programs based on conventional propulsion run into a law of heavily diminishing returns as soon as missions beyond the moon are contemplated.

     The other class of propulsion systems at present under development is the so-called nuclear-electric class. These systems use a nuclear reactor to generate electricity, which then accelerates a jet of ions or plasma by means of electric or magnetic forces. The velocity of the jet is no longer limited by considerations of temperature, but the available thrust is limited to very low values by the power of the electric generator. Vehicles using nuclear-electric propulsion necessarily accelerate very slowly and require long times to achieve useful velocities. They have undoubtedly an important role to play in long-range missions, but they offer no hope of transporting men or machines rapidly around the solar system.

     The Orion propulsion system is neither temperature-limited nor powerlimited. It escapes temperature limitations because the contact between the vehicle and the hot debris from the explosions is so brief that the debris does no more than superficial damage. It escapes power limitations because the nuclear engine (bomb) is outside the vehicle and does not depend on coolants and radiators for its functioning. An Orion vehicle is unique in being able to take full advantage of the enormous energy content of nuclear fuel in order to achieve, simultaneously, high exhaust velocity and high thrust.

     Let me give an example of the specific performance that would be achieved by first-generation Orion vehicles. Designs were worked out in detail for vehicles that could carry eight men and a payload of 100 tons on fast trips to Mars and back. The vehicles were small enough to be lifted into space by Saturn chemical rockets, and the cost of the Saturn boosters turned out to be more than half the estimated cost of the whole enterprise. These designs do not, of course, prove that a manned expedition to Mars is a worthwhile undertaking; they indicate only that if you wish to go to Mars, then Orion will take you there more rapidly and cheaply than other vehicles that are now being developed.

     So much for the technical background of Orion. Next comes the political history. The idea of a bombpropelled vehicle was first described by Ulam and Everett in Los Alamos in 1955. It was transformed into a serious and practical proposal by a group of physicists and engineers at General Atomic Division of General Dynamics Corporation in ·san Diego, under the leadership of Theodore Taylor. Work at General Atomic started in the spring of 1958, as a direct response to the first Sputniks. The initial group at General Atomic, including Taylor, were old weaponeers from Los Alamos, and they seized happily upon this opportunity to make their knowledge of nuclear explosions serve a loftier purpose than weaponry. Within a few months they had worked out the basic theory of the Orion system, and found that it worked even better than they had supposed.

Government Sponsorship

     The problem then arose of obtaining government sponsorship and money for the project. The National Aeronautics and Space Administration (NASA) did not yet exist. There was only one government agency which could logically take responsibility and fund the project -namely, the Advanced Research Projects Agency (ARPA) of the Defense Department. It was a thoroughly anomalous situation to have a group of weapons experts in a private company working on a space project, and it took many months of negotiation to obtain the first contract from ARP A. At that early date in its history ARPA did not insist that anything which it supported must have a military justification. The terms of the first contract permitted designation of peaceful interplanetary exploration as the major goal of the project. Nevertheless the project was administered through Defense Department channels, and military influences were inevitably at work upon it.

     Quite soon after Orion officially began, NASA was established, with legal responsibility for all nonmilitary space activities. NASA quickly began to annex parts of ARPA's nonmilitary functions, and the Air Force responded by annexing ARP A's military space projects, so that the situation of ARP A was reminiscent of the partition of Poland between Prussia and Russia in the 18th century. In the end, Orion was left as the only space project in the hands of ARP A, largely because neither NASA nor the Air Force considered it a valuable asset. Taylor's efforts to interest NASA in Orion during this period met with no success. In 1960 ARP A decided to drop Orion, and Taylor was compelled to go to the Air Force for sponsorship. According to the law, the Air Force may handle only military projects, and must apply a rigid definition of the word military. A project is defined as military only if a direct military requirement for it exists. There is no military requirement for interplanetary exploration. Thus Taylor paid a high price for his Air Force contract. Although the technical substance of the work was not changed, the project became in name a military enterprise directed toward real or imagined military requirements. This arrangement continued in force until the end of the project in 1965.

     The effect of the military sponsorship of Orion was, in the end, disastrous. The Air Force officials administering the project were sympathetic to the long-range and nonmilitary aspects of the work, but they were compelled by their own rules to disguise their sympathies. Each year when they applied to the high authorities in the Defense Department, Harold Brown and McNamara, for more money to expand the project, they had to argue in terms of immediate military requirements. Men as wise and critical as Harold Brown and McNamara could easily see that the military applications of Orion are either spurious or positively undesirable. So the requests for expansion were turned down. The Air Force was told that if it wished to continue the project for nonmilitary reasons it should enlist the cooperation of NASA.

     In 1963 NASA finally showed some official interest in Orion. Jim Nance, acting first as assistant director of the project under Taylor and later as director in his own right, established friendly relations with the Marshall Space Flight Center in Huntsville, Alabama. Within NASA, Orion's possibilities appealed particularly to the Office of Manned Space Flight, where people are beginning to worry about what they should do after the Apollo mission is over. NASA awarded Orion a small study contract, and from this resulted the design of ships for specific interplanetary missions. Also in 1963 the test-ban treaty was signed, and nuclear explosions became more than ever politically questionable.

     In 1964 the shadows began to close in. The Air Force grew tired of supporting a project which McNamara would not allow to grow, and announced that further support would be forthcoming only if NASA would make a serious contribution. At the eleventh hour, in October 1964, Nance succeeded in getting the basic technical facts concerning Orion declassified, so that it became possible for the first time to discuss the issue publicly. A certain interest in Orion belatedly developed within the engineering community but did not extend to the scientific community. In December 1964 the question of the support of Orion came to a final decision within NASA, with the result which was announced in January 1965.

Concerning the Verdict

     As is proper in conducting an inquest, we have first assembled the historical evidence, and now we come to the question of a verdict. Who killed Orion, and why? And was the murder justifiable?

     Four groups of people were directly responsible for the death of Orion. These are the Defense Department, the heads of NASA, the promoters of the test-ban treaty, and the scientific community as a whole. Each group encountered Orion within the context of a larger struggle in which Orion appeared to them as a relatively minor issue. In each group a negative attitude toward Orion was dictated by general principles which, in the wider context, were wise and enlightened. In each group the men who killed Orion acted from high and responsible motives. And yet their motives were strangely irrelevant to the real issues at stake in this highly individual case. I will examine the four groups in turn and describe how the problem of Orion presented itself to them.

     The Defense Department chiefs have been waging for many years a successful battle to stop the Air Force from embarking upon a great variety of technically interesting projects whose military importance is questionable. The nuclear-propelled airplane was one such project, which was stopped only after large sums of money had been wasted on it. More recently, as in the cases of the B-70 bomber and the Dynasoar orbital airplane, McNamara has been strong enough to call a halt before the big money was spent. There is little doubt that, when the Air Force asked for more money for Orion, the authorities in the Defense Department mostly thought of it as one more in the long series of Air Force extravaganzas which it was their duty to suppress. The way in which the money was requested made it difficult for them to view it otherwise. And within this context their decision was unquestionably right.

     The heads of NASA were not interested in Orion at the time NASA began for the simple reason that it was a classified project supported by the Defense Department and therefore outside their terms of reference. They were explicitly enjoined by Congress not to trespass upon military ground, and they had no wish to become gratuitously involved with a project encumbered by all the bureaucratic nuisances of secrecy. The established policy of NASA is to conduct as many as possible of its operations openly and without requiring all its employees to be cleared for security. Few will question that this policy is wise as a general rule, and indeed essential to the maintenance of a healthy scientific atmosphere within NASA.

     When the heads of NASA came to their final decision concerning Orion, in 1964, the jurisdictional issue was no longer central. The Air Force had officially appealed to NASA for a declaration of support, and participation in a future development of Orion would not have compromised the nonmilitary status of NASA. In 1964 the dominating concern at the top levels of NASA was the search for political stability.

     The heads of NASA have learned that their first duty to the space program is to keep it politically popular. Without consistent support from the public and from Congress, there would be no possibility of an effective program. H is therefore wise to sacrifice technical improvements if technical improvements carry risks of failure which may be politically upsetting to the entire program. Above all, spectacular and public failures are to be avoided. When a responsible public official thinks of Orion he inevitably envisions a shipload of atomic bombs all detonating simultaneously and wiping out half of Florida. Though it is technically easy to make such an accident impossible, it is not possible to exorcise the fear of it. The heads of NASA know that fear is the most potent force in politics, and they have no wish to be feared.

     The promoters of the test-ban treaty are a heterogeneous group of people, including the Arms Control and Disarmament Agency, the State Department, a large segment of Congress, the White House staff, and the President's Science Advisory Committee (PSAC). About the only thing that all the people working for the treaty had in common was a total unconcern for the welfare of Project Orion. Most of them had never heard of Orion, and most of those who had heard of it (for example, some influential members of PSAC) had met it only in a context in which they were committed to oppose it. They had met it within the context of a continuing battle to stop the military arm of the U.S. Government from gratuitously expanding the arms race into arenas where no arms race yet existed. The PSAC had been successful in opposing a race to build bigger bombs than the U.S.S.R. was building, and had also successfully opposed the idea of placing offensive nuclear weapons in orbit. The members of PSAC have developed a deep commitment to the policy of military restraint, of deploying new weapons systems only when a military need exists and not just for the sake of technological novelty. Their commitment to this goal has served their country well, and has borne fruit in many other wise decisions besides the decision to negotiate the test-ban treaty. Seeing Orion from this viewpoint, as an Air Force project ostensibly aimed at large-scale military operations in space, they felt no qualms in crushing it.

     Lastly, the scientific community as a whole is responsible, in a negative sense, for the death of Orion. The vast majority of scientists have consistently refused to become interested in the technical problems of propulsion, believing that this was a job for engineers. A clear illustration of their point of view is provided by the report on national goals in space for the years 1971-85, recently published by the Space Science Board of the National Academy of Sciences. This report describes in detail a recommended program of space activities which is based on the assumption that the propulsion systems available until 1985 will be those now under development. The Space Science Board does not concern itself with the question of whether a scientific effort might bring radical improvements in the art of propulsion before 1985. To somebody familiar with the potentialities of Orion, the Space Science Board program seems both pitifully modest and absurdly expensive.

     Here again, the disinterest of scientists in problems of propulsion arises from attitudes which in a wider context are wise and healthy. In their dealings with NASA and with the public, scientists have constantly preached that the payload is more important than the rocket, that what you do there is more important than how you get there. They have argued repeatedly, and usually without success, that ten dollars spent on unmanned vehicles are scientifically more useful than a hundred spent on manned vehicles, and that often one dollar spent on ground-based observations is scientifically more useful still. They have been alienated from the field of propulsion by the spectacle of NASA officials claiming a scientific justification for space-propulsion developments which have little or nothing to do with science. They have, after long years of listening to the pseudoscientific propaganda of the manned space program, learned to confine their attention to that small part of the NASA empire within which they have some real influence-namely, the Office of Space Science and Applications (OSSA). Within OSSA they have created an atmosphere of scientific sanity which has allowed excellent and manysided programs of unmanned scientific exploration to be carried out with the eighth of the NASA budget which is allotted to this purpose.

     The Space Science Board of the National Academy, in its consideration of future activities, was mainly concerned with preserving the quality and the scientific integrity of these existing unmanned programs. The board rightly sees as its primary task the definition of the ends, rather than the means, of the space science enterprise.

     What then is the attitude of a scientist who is actively engaged in scientific space activities toward a project such as Orion? He has perhaps just been denied by NASA a half-million-dollar ground-based telescope with which to observe planets. Or he has designed an experiment which was excluded, because of space limitations, from the next orbiting solar observatory. And then he hears that a wonderful new propulsion system has been invented which might allow him, 15 years later, to make high-quality nearby observations of Jupiter and Saturn. The price of the new system is quoted as only a few billion dollars. He is understandably not enthusiastic.

     This brief summary of Orion's history has shown that every one of the four murderers had good and laudable motives for killing the project, or, in the case of the scientific community, for not lifting a finger to save it. Orion had a unique ability to antagonize simultaneously the four most powerful sections of the Washington establishment. The remarkable thing is that, against such odds, with its future never assured for more than a few months at a time, the project survived as long as it did. It held together for 7 long years a band of talented and devoted men, and produced in that time a volume of scientific and engineering work which in breadth and thoroughness has rarely been equaled.

     The story of Orion is significant, because this is the first time in modern history that a major expansion of human technology has been suppressed for political reasons. Many will feel that the precedent is a good one to have established. It is perhaps wise that radical advances in technology, which may be used both for good and for evil purposes, be delayed until the human species is better organized to cope with them. But those who have worked on Project Orion cannot share this view. They must continue to hope that they may see their work bear fruit in their own lifetimes. They cannot lose sight of the dream which fired their imaginations in 1958 and sustained them through the years of struggle afterward—the dream that the bombs which killed and maimed at Hiroshima and Nagasaki may one day open the skies to mankind.

      The race to the moon, in the forms of Project Apollo and the still-shadowy Soviet lunarprogram, dominated manned space flight during the decade of the 1960's. In the United States, the project sequence Mercury-Gemini-Apollo succeeded in putting roughly sixty people into space, twelve of them on the moon. Yet, during the late 1950's and early 1960's, the U.S. government sponsored a project that could possibly have placed 150 people, most of them professional scientists, on the moon, and could even have sent expeditions to Mars and Saturn. This feat could conceivably have been accomplished during the same period of time as Apollo, and possibly for about the same amount of money. The code name of the project was Orion, and the concepts developed during its seven-year life are so good that they deserve serious consideration today.

     Project Orion was a space vehicle propulsion system that depended on exploding atomic bombs roughly two hundred feet behind the vehicle (1). The seeming absurdity of this idea is one of the reasons why Orion failed; yet, many prominent physicists worked on the concept and were convinced that it could be made practical. Since atomic bombs are discrete entities, the system had to operate in a pulsed rather than a continuous mode. It is similar in this respect to an automobile engine, in which the peak combustion temperatures far exceed the melting points of the cylinders and pistons. The engine remains intact because the period of peak temperature is brief compared to the combustion cycle period.

     The idea of an "atomic drive" was a science-fiction cliche by the 1930's, but it appears that Stanislaw Ulam and Frederick de Hoffman conducted the first serious investigation of atomic propulsion for space flight in 1944, while they were working on the Manhattan Project (2). During the quarter-century following World War II, the U.S. Atomic Energy Commission (replaced by the Department of Energy in 1974) worked with various federal agencies on a series of nuclear engine projects with names like Dumbo, Kiwi, and Pluto, culminating in NERVA (Nuclear Engine for Rocket Vehicle Application) (3). Close to producing a flight prototype, NERVA was cancelled in 1972 (4). The basic idea behind all these engines was to heat a working fluid by pumping it through a nuclear reactor, then allowing it to expand through a nozzle to develop thrust. Although this sounds simple the engineering problems were horrendous. How good were these designs? A useful figure for comparing rocket engines is specific impulse (Isp), defined as pounds of thrust produced per pound of propellant consumed per second. The units of Isp are thus seconds. The best chemical rocket in service, the cryogenic hydrogen-oxygen engine, has an Isp of about 450 seconds (5). NERVA had an Isp roughly twice as great (6), a surprisingly small figure considering that nuclear fission fuel contains more than a million times as much energy per unit mass as chemical fuel. A major problem is that the reactor operates at a constant temperature, and this temperature must be less than the melting point of its structural materials, about 3000 K (7).

     A number of designs were proposed in the late 1940's and 1950's to get around the temperature limitation and to exploit the enormous power of the atomic bomb, estimated to be on the order of 10 billion horsepower for a moderate-sized device (8). The Martin Company designed a nuclear pulse rocket engine with a "combustion chamber" 130 feet in diameter. Small atomic bombs with yields under 0.1 kiloton (a kiloton is the energy equivalent of 1000 tons of the high explosive TNT) would have been dropped into this chamber at a rate of about one per second (9); water would have been injected to serve as propellant. This design produced the relatively small Isp of 1150 seconds, and could have yielded a maximum velocity change for the vehicle of 26,000 feet/second. The vehicle would have been boosted to an altitude of 150 miles by chemical rockets, and the extra 8000 ft/sec or so thus provided would have allowed it to escape the Earth's gravity (10). The Lawrence Livermore Laboratory produced a similar although much smaller design called Helios at about the same time (11).

     In a classified 1955 paper (12), Stanislaw Ulam and Cornelius Everett eliminated the combustion chamber entirely. Instead, bombs would be ejected backwards from the vehicle, followed by solid-propellant disks. The explosions would vaporize the disks, and the resulting plasma would impinge upon a pusher plate. The advantage of this system is that no attempt is made to confine the explosions, implying that relatively high-yield (hence high-power) bombs may be used. Such a system is neither temperature- nor power-limited. Ulam may have been influenced by experiments conducted at the Eniwetok proving grounds, where graphite-covered steel spheres were suspended thirty feet from the center of an atomic explosion. The spheres were later found intact; a thin layer of graphite had been ablated from their surfaces (13).

     Project Orion was born in 1958 at General Atomics in San Diego. The company, now a subsidiary of defense giant General Dynamics, was founded by Frederick de Hoffman to develop commercial nuclear reactors. The driving force behind Orion was Theodore Taylor, a veteran of the Los Alamos weapons programs. De Hoffman persuaded Freeman Dyson, a theoretical physicist then at the Institute for Advanced Study in Princeton, New Jersey, to come to San Diego to work on Orion during the 1958-1959 academic year. Dyson says that Taylor adopted a specific management model for the project: the Verein fur Raumschiffahrt (VfR), the German rocket society of the 1920's and 1930's which numbered among its members Werner von Braun. The VfR had little structure: no bureaucracy and essentially no division of labor between its members; it accomplished much before it was taken over by the German army. Orion at first was similar: scientists did practical engineering and engineers built working scale models, all on a shoestring budget (14).

     Taylor's specialty at Los Alamos had been the effects of atomic weapons. He was an expert at making small bombs at a time when the drive was toward ever-bigger superweapons. He was also aware of techniques for shaping explosions, for making bomb debris squirt in one particular direction. Taylor adopted Ulam's pusher-plate idea but instead of the propellant disks he combined propellant and bomb into a single pulse unit. The propellant material of choice was plastic, probably polyethylene (15). Plastic is good at absorbing the neutrons emitted by an atomic explosion (i.e. it couples well with the prompt radiation energy) and in addition it breaks down into low-weight atoms such as hydrogen and carbon which move at high speeds when agitated. There are indications that a plastic similar to Styrofoam is used inside hydrogen bombs to "channel" the energy from the atomic trigger into the fusible material (16). The advantage of the pusher plate design, as Taylor and Dyson saw it, was that it could simultaneously produce high thrust with high exhaust velocity. No other known propulsion system combined these two highly desirable features. The effective Isp could theoretically be as high as 10,000 to one million seconds (17). The calculated force exerted on the pusher plate was immense; it would have created intolerable acceleration for a manned vehicle. Therefore, a shock absorber system was placed between the plate and the vehicle itself. The impulse energy delivered to the plate was stored in the shock absorbers and released gradually to the vehicle.

     The Orion workers built a series of models, called Put-Puts or Hot Rods, to test whether or not pusher plates made of aluminum could survive the momentary intense temperatures and pressures created by chemical explosives. Several models were destroyed, but a 100-meter flight in November 1959, propelled by six charges, was successful and demonstrated that impulsive flight could be stable (18). These experiments also proved that the plate should be thick in the middle and taper toward its edges for maximum strength with minimum weight (19).

     The durability of the plate was a major issue. The expanding plasma bubble of each explosion could have a temperature of several tens of thousands of Kelvins even when the explosion occurred hundreds of feet from the plate. Following the lead of the Eniwetok tests, a scheme was devised to spray grease (probably graphite-based) onto the plate between blasts (20). It is not known if this scheme was retained in later versions of the Orion design. Extensive work was done on plate erosion using an explosive-driven helium plasma generator. The experimenters found that the plate would be exposed to extreme temperatures for only about one millisecond during each explosion, and that the ablation would occur only within a thin surface layer of the plate (21). The duration of high temperatures was so short that very little heat flowed into the plate; active cooling was apparently considered unnecessary. The experimenters concluded that either aluminum or steel would be durable enough to act as plate material.

     The Orion workers realized early that the U.S. government had to become involved if the project was to have any chance of progressing beyond the tinkering stage. Accordingly, the Advanced Research Projects Agency (ARPA - later DARPA with "D" standing for "Defense") was approached in April1958. In July, it agreed to sponsor the project at an initial funding level of $1 million per year; it was at this time that the code name of Orion was assigned (22). Work proceeded under ARPA order 6, task 3, entitled "Study of Nuclear-Pulse-Propelled Space Vehicles" (23).

     Taylor and Dyson were convinced that the approach to space flight being pursued by NASA (which had just been created in January 1958) was the wrong one. Von Braun's chemical rockets in their opinion were very expensive, had very limited payloads, and were essentially useless for flights beyond the moon (24). The Orion workers wanted a spaceship that was simple, rugged, capacious, and above all affordable. Taylor originally called for a ground launch, probably from the U.S. nuclear test site at Jackass Flats, Nevada (25). The vehicle has been described as looking like a bishop's miter or the tip of a bullet, sixteen stories high and with a pusher plate 135 feet in diameter (26). Intuitively it seems that the bigger the pusher plate, the more efficiently the system would perform. For a derivation of a formula for the effective specific impulse of a nuclear-pulse rocket and for the relations between pusher plate diameter, pulse energy, and Isp, the reader should consult Reynolds (27). The launch pad would have been composed of eight towers, each 250 feet high. The mass of the vehicle on takeoff would have been on the order of 10,000 TONS (28); most of this mass would have gone into orbit. The bomb units ejected on takeoff would have yielded 0.1 kiloton; initially the ejection rate would have been one per second. As the vehicle accelerated the rate would slow down and the yield would increase until 20-kiloton bombs would have been going off every ten seconds (29). The idea seems to have been for the vehicle to fly straight up until it cleared the atmosphere so as to minimize radioactive contamination.

     At a time when the U.S. was struggling to put a single man into orbit aboard a modified military rocket, Taylor and Dyson were developing plans for a manned voyage of exploration through much of the solar system. The original Orion design called for 2000 pulse units, far more than enough to attain Earth escape velocity. "Our motto was 'Mars by 1965, Saturn by 1970'", recalls Dyson (30). Orion would have been more akin to the rocket ships of science fiction than to the cramped capsules of Gagarin and Glenn. One hundred and fifty people could have lived aboard in relative comfort; the useful payload would have been measured in thousands of tons (31). Orion would have been built like a battleship, with no need for the excruciating weight-saving measures adopted by chemically-propelled spacecraft. It is unclear how the vehicle would have landed; it is reasonable to assume that specialized chemically-powered craft would have been used for exploration. Taylor may have anticipated that a conventional Space Shuttle-type vehicle would have been available to transport people to and from orbit. Dyson gives the astounding figure of $100 million per year as the cost of the proposed twelve-year program (32); surely this does not include development costs for the thousands of items from spacesuits to scientific instruments that such a program would require. The Orion program would have most likely "piggybacked" on the military weapons programs and the existing civilian space projects. Still, even if Dyson underestimated the cost by a factor of 20, the revised total would have been only $24 billion, roughly the same as the accepted cost for the Apollo program.

     The times were changing, however. The fledgling space administration began to acquire all civil-oriented space projects run by the federal government; the Air Force got all projects with military applications. ARPA was left with Orion as its only space project, for two reasons. The Air Force felt that Orion had no value as a weapon, and NASA had made a strategic decision in 1959 that the civilian space program would be non-nuclear for the near future (33). NASA was and is a very publicity-conscious organization, and it is hard to overcome the negative perception of atomic devices of any kind on the part of the public. In addition, NASA was filled with engineers who had spent their careers building ever-larger chemical rockets and either did not understand or were openly opposed to nuclear flight. In this situation the Orion workers were truly outsiders.

     A crisis came in late 1959, when ARPA decided it could no longer support Orion on national-security grounds. Taylor had no choice but to approach the Air Force for funds. It was a hard sell. A common reaction from both military and civilian officials is displayed by the quote: "...you set off one big bomb and the whole shebang blows up."(34) The Air Force finally decided to take on Orion, but only on the condition that a military use be found for it. Dyson says that his Air Force contacts, although sympathetic to the goal of space exploration, felt that their hands were tied (35). One immediate result of the change of management was that all model flight testing was stopped (36). The freewheeling era was over; Taylor's dream of a company of "men of goodwill" exploring the solar system had died.

     One can imagine that Orion could be used as a weapon platform, in a polar orbit so that it would eventually pass over every point on the Earth's surface. It could also protect itself easily, at least against attacks by small numbers of missiles. However, this idea has the same disadvantages as the early bomb-carrying satellite proposals. Terminal guidance would have been a problem (assuming that hardened, high-value installations were the intended targets), since the technology for steering missile warheads accurately had not yet been developed. Both the U.S. and the Soviet Union were deploying missiles that were capable of reaching their targets in fifteen minutes with multi-megaton warheads, making orbiting bomb platforms irrelevant.

     Robert McNamara, Defense Secretary under the Kennedy Administration, realized that Orion was not a military asset. His department consistently rejected any increase in funding for the project, effectively limiting it to a feasibility study (37). Taylor and Dyson knew that another money source had to be found if a flyable vehicle was to be built. NASA was the only remaining option. Accordingly, Taylor and James Nance, a General Atomics employee and later director of the project, made at least two trips to Marshall Space Flight Center (MSFC) in Huntsville, Alabama (38). MSFC was von Braun's domain and it was where most of NASA's space propulsion research and development took place. Von Braun was hard at work on the Saturn project, which NASA had inherited from the old Army Ballistic Missile Agency. The Saturn V would eventually transport men to the moon. The Orion workers had produced a new, "first generation" design that abandoned ground launch and instead would have been boosted into orbit as a Saturn V upper stage. The core of the vehicle was a 200,000-pound "propulsion module" with a pusher-plate diameter of 33 feet, limited by the diameter of the Saturn. This design limitation also restricted Isp to from 1800 to 2500 seconds (39). While disappointingly low by nuclear- pulse standards, this figure still far exceeded those of other nuclear rocket designs. The shock absorber system had two sections: a primary unit made up of toroidal pneumatic bags located directly behind the pusher plate, and a secondary unit of four telescoping shocks (like those on a car) connecting the pusher plate assembly to the rest of the spacecraft (40).

     How many Saturn V's would have been required to put this vehicle into orbit? Dyson says one or two (41); a simple inspection of published drawings indicates at least two, possibly three if the crew module (with crew aboard) was intended to be flown separately (42). In this case, some assembly would have been done on-orbit. Several mission profiles were contemplated; the one developed in greatest detail appears to have been a Mars flight. Eight astronauts, with around 100 tons of equipment and supplies, could have made a round trip to Mars in 125 days (43); most modern plans call for one-way times of at least nine months. Another impressive figure is that as much as 45% of the gross vehicle weight in Earth orbit could have been payload (44). Presumably the flight would have been made when Mars was nearest to the Earth; still, so much energy was available that almost the fastest-possible path between the planets could have been chosen. Inspection of the drawings indicates that a lander may also have been carried.

     What about the cost? Pedersen's 1964 estimate of $1.5 billion for the project (45) suggests the superior economics of nuclear pulse spaceships. Dyson felt that Orion's appeal was greatly diluted by the chemical booster restriction: the Saturns would have represented over 50% of the total cost (46).

     Von Braun became an enthusiastic Orion supporter, but he was able to make little headway among higher-level administration officials. In addition to the general injunction against nuclear power, very practical objections were raised: what if a Saturn bearing a propulsion module with hundreds of bombs aboard should explode? Was it possible to guarantee that not a single bomb would explode or even rupture? NASA's understandable fear of a public-relations disaster contributed to its reluctance to provide money (47); however, its Office of Manned Spaceflight was sufficiently interested to fund another study (48).

     A hammer blow was delivered in August 1963 with the signing of the nuclear test-ban treaty by the U.S., U.K., and U.S.S.R. Orion was now illegal under international law. Yet the project did not die immediately. It was still possible that an exemption could be granted for programs that were demonstrably peaceful. Surely the treaty reduced Orion's political capital even further, though. Yet another problem was that, because Orion was a classified project, very few people in the engineering and scientific communities were aware of its existence. In an attempt to rectify this, Nance (now managing the project) lobbied the Air Force to declassify at least the broad outline of the work that had been done. Eventually it agreed, and Nance published a brief description of the "first generation" vehicle in October 1964 (49).

     The Air Force, meanwhile, had become impatient with NASA's temporizing. It was willing to be a partner but only if NASA would contribute significant funds. Hard-pressed by the demands of Apollo, NASA made its decision in December 1964 and announced it publicly the following month: no money would be forthcoming (50). The Air Force then anounced the termination of all funding, and Orion quietly died. Some $11 million had been spent over nearly seven years (51).

     Overshadowed by the moon race, Orion was forgotten by almost everybody except Freeman Dyson and Theodore Taylor. Dyson in particular seems to have been deeply affected by his experience. The story of Orion is important, he says, "...because this is the first time in modern history that a major expansion of human technology has been suppressed for political reasons"(52). His 1968 paper (53) gives more physical details of nuclear pulse drives, and even suggests extremely large starships powered by fusion explosions. Ultimately he became disillusioned with the concept, primarily because of the radiation hazard associated with the early ground-launch idea. Yet he says that the most extensive flight program envisaged by Taylor and himself would have added no more than 1% to the atmospheric contamination then (circa 1960) being created by the weapons-testing of the major powers (54).

     Does it make any sense to even think of reviving the nuclear-pulse concept? Economically the answer is yes. Pedersen (55) says that 10,000-ton spaceships with 10,000-ton payloads are feasible. Spaceships like this could be relatively cheap compared to Shuttle-like vehicles due to their heavyweight construction. One tends to think of shipyards with heavy plates being lowered into place by cranes. How much would the pulse units cost? Pedersen gives the amazingly low figure of $10,000 to $40,000 per unit for the early Martin design (56); there is reason to think that $1 million is an upper limit (57). Primarily from strength of materials considerations, Dyson (58) argues that 30 meters/second (about 100 feet/second) is the maximum velocity increment that could be obtained from a single pulse. Given that low-altitude orbital velocity is about 26,000 feet/second, around 350 pulses would be required (59). Using $500,000 as a reasonable pulse-unit cost, this implies a "fuel cost" of $175 million, cheaper than a Shuttle launch. Whereas the Shuttle might carry thirty tons of payload, the pulse vehicle would carry thousands. If one uses the extreme example of spending $5 billion to build a vehicle to lift 10,000 tons (or 20 million pounds) to orbit, the cost if spread over a single flight is $250 per pound, far cheaper than the accepted figure of $5,000 to $6,000 per pound for a Shuttle flight.

     Efficiency improvements could be made by improving the design of the pulse units. Considerable progress has been made in nuclear bomb design over the past thirty years. Neutron bombs, for instance, demonstrate that it is possible to change the form of the energy emitted by the explosion. Recent work on X-ray lasers bears on the important problem of shaping the explosion into a beam. Yet it is impossible to prevent the formation of radioactive fission fragments. For a ground launch, choosing a very remote site such as a floating platform in the extreme southern Atlantic or Pacific would minimize the radiation hazard to humans. The chemical-rocket imperative to launch as close to the equator as possible disappears when such an abundance of energy is available. Even this might be judged environmentally unacceptable, though; perhaps ANY release of radiation into the atmosphere is wrong. In this case the option of a space launch remains open. Even this has been criticized on the grounds that it would leave a radioactive debris trail in space. However, interplanetary space is a very dangerous environment to begin with, being periodically saturated with fast charged particles from solar flares and with extremely energetic cosmic rays occasionally blasting through. The notion that the bomb debris would form a trail is challenged by the fact that the velocity of most of the debris would exceed solar escape velocity (60).

     Although the Saturn V no longer exists, U.S. engineers are currently studying several heavy-lift systems. Given the recent reduction in world tensions, even the Russian Energia could be considered. Russian nuclear scientists, unemployed after the Cold War, might collaborate with Americans on nuclear-pulse space projects. Fast flights to the planets might be made in ten years or less, at reasonable expense, instead of thirty to fifty years.

     Unfortunately, the Orion concept is inherently "dirty" because it uses fission fuel. It is also inefficient; this is acceptable only because of the vast amounts of energy available. A much better alternative is fusion, since a fusion rocket would not leave a wake of heavy radioactive ions. The British Interplanetary Society's Daedalus project (61) was a study of an unmanned interstellar probe. It would have been driven by fusion "microexplosions" caused by irradiating fuel pellets with electron beams at pulse rates up to 250 Hz, in a magnetic "combustion chamber". Confinement and shaping of the plasma with a magnetic field would make Daedalus vastly more efficient than Orion. Daedalus would work just as well in the solar system as between the stars, and one can imagine that in 75 to 100 years fusion freighters will be sailing regularly between the planets. An important point is that no one has yet produced controlled fusion energy with electron beams or anything else, while the technology required to build an Orion-type spaceship has existed for over thirty years. Nuclear propulsion will get into space eventually. Orion might be the device that makes possible human occupation and economic exploitation of the solar system.

Notes

1.		Erik S. Pedersen, Nuclear Propulsion in Space (Englewood Cliffs, NJ: 
		Prentice-Hall Inc.,1964), p. 275.

2.		William R. Corliss, Nuclear Propulsion for Space (U.S. Atomic Energy
		Commission: Division of Technical Information, 1967), p. 11.

3.		Corliss, pp. 1-16.

4.		James A. Dewar, "Project Rover: The United States Nuclear Rocket Program",
		in History of Rocketry and Astronautics (John L. Sloop ed. - San Diego:
		American Astronautical Society Publications Office, 1991), p. 123.

5.		"Specific impulse", article in McGraw-Hill Encyclopedia of Science and 
		Technology, vol. 17, p. 204.

6.		"Specific Impulse", p. 204

7.		Corliss, pp. 13-14.

8.		Pedersen (p. 276) gives 4.2 x 1019 ergs per kiloton; exploding one such
		bomb per second yields 4.2 x 1012 joules / sec (i.e. watts) or roughly
		5 billion average horsepower.

9.		Pedersen, p. 276.

10.	Pedersen, p. 276.

11.	Eugene Mallove and Gregory Matloff, The Starflight Handbook (New York:
		John Wiley and Sons, 1989),  p. 60.

12.	Mallove and Matloff, p. 61.

13.	John McPhee, The Curve of Binding Energy (New York: Farrar, Straus and
		Giroux, 1974), pp. 167-168

14.	Freeman Dyson, Disturbing the Universe (New York: Harper and Row, 1979),
		pp. 109-110.

15.	Mallove and Matloff, p. 63.

16.	The Ground Zero Fund, Inc., Nuclear War: What's In It for You? (New York:
		Simon and Schuster Inc., 1977), p. 27.

17.	Mallove and Matloff, pp. 60-61.

18.	Dyson, Disturbing, p. 113

19.	McPhee, p. 175.

20.	McPhee, p. 175

21.	J.C. Nance, "Nuclear Pulse Propulsion", IEEE Transactions on Nuclear
		Science (Feb. 1965), p. 177.

22.	McPhee, p. 170.

23.	DARPA letter to the author dated October 7th, 1992.

24.	Dyson, Disturbing, pp. 109-110.

25.	McPhee, pp. 173-174.

26.	McPhee, pp. 173-174.

27.	T.W. Reynolds, "Effective Specific Impulse of External Nuclear Pulse
		Propulsion Systems", Journal of Spacecraft and Rockets 10 (Oct. 1973),
		pp. 629-630

28.	The volume of a cone 200 feet high with a base diameter of 135 feet (the 
		approximate dimensions of the proposed Orion vehicle) is about 1.5 million 
		cubic feet.  If the average density is 10 pounds per cubic foot (about 1/6
		that of water) the weight is 15 million pounds or 7500 tons.

29.	McPhee, pp. 173-174.

30.	McPhee, pp. 180-181.

31.	McPhee, p. 158.

32.	Dyson, Disturbing, p. 111.

33.	Dyson, Disturbing, p. 113.

34.	Nance, p. 177.

35.	Freeman Dyson, "Death of a Project", Science (9 July 1965), pp. 141-144.

36.	Dyson, Disturbing, p. 113.

37.	Dyson, "Death", p. 142.

38.	McPhee (p. 183) says that Taylor traveled to MSFC in 1961; Dyson
		("Death", p. 142) says that Taylor and Nance established relations with
		MSFC management in 1963.

39.	Nance, pp. 181-182.

40.	Nance, p. 182.

41.	Dyson, Disturbing, p. 115.

42.	Nance, p. 182.

43.	Dyson, "Death", pp. 141-142.

44.	Nance, p. 179.

45.	Pedersen, p. 276.

46.	Dyson, "Death", pp. 141-142.

47.	Dyson, "Death", pp. 143-144.

48.	Dyson, "Death", pp. 143-144.

49.	Dyson, "Death", pp. 143-144.

50.	Dyson, "Death", p. 142.

51.	Dyson, "Death", p. 144.

52.	Mallove and Matloff, p. 61.

53.	Freeman Dyson, "Interstellar Transport", Physics Today (Oct. 1968),
		pp. 41-45.

54.	Dyson, Disturbing, p. 114.

55.	Pedersen, p. 275.

56.	Pedersen, p. 276.

57.	Kenneth A Bertsch and Linda S. Shaw, The Nuclear Weapons Industry
		(Washington D.C.: Investor Responsibility Research Center, 1984),
		on p. 55 state that warheads for 560 ground-launched cruise missiles 
		were expected to cost $630 million.  Not only were these military
		weapons but they were quite likely fusion devices as well and so would
		be significantly more expensive than simple fission bombs.

58.	The figure of 350 pulses was arrived at as follows: if the net 
		acceleration during the initial vertical phase is about 2 g's,
		about 100 pulses are required to reach an altitude of 60 miles (at
		an average of one pulse per second).  The velocity at this height is
		about 6400 ft/sec.   If the spaceship then performs an attitude
		correction and accelerates to orbital velocity at about 3 g's, roughly
		260 pulses are required, at which time the altitude is roughly 300 miles.
		This is a very crude estimate and the actual number of pulses might be
		much lower.

59.	Dyson, "Interstellar", p. 44.

60.	McPhee, pp. 167-168.

61.	British Interplanetary Society, Project Daedalus (London: British
		Interplanetary Society Ltd., 1981)

References

	Bertsch, Kenneth A. and Shaw, Linda S. The Nuclear Weapons Industry
	(Washington D.C.: Investor Responsibility Research Center, 1984)

	British Interplanetary Society, Dr. A.R. Martin ed. Project Daedalus
	(London: British Interplanetary Society Ltd., 1981)

	Corliss, William R. Nuclear Propulsion for Space (U.S. Atomic Energy
	Commission: Department of Technical Information, 1967)

	Dewar, James A. "Project Rover: The United States Nuclear Rocket Program",
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	Dyson, Freeman	"Death of a Project", Science 9 July 1965

	___.		Disturbing the Universe (New York: Harper and Row, 1979)

	___. 		"Interstellar Transport", Physics Today  October 1968

	Ground Zero Fund Inc., The Nuclear War: What's In It for You?
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	Letter from Defense Advanced Research Projects Agency to the author,
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	Mauldin, John Prospects for Interstellar Travel (San Diego: American
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	McPhee, John The Curve of Binding Energy (New York: Farrar, Straus and
	Giroux, 1974)

	Pedersen, Erik S. Nuclear Propulsion in Space (Englewood Cliffs, NJ:
	Prentice-Hall, Inc., 1964)

	Reynolds, T.W. "Effective Specific Impulse of External Nuclear Pulse
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	"Specific Impulse", article in The McGraw-Hill Encyclopedia of Science
	and Technology, 6th ed., vol. 17 (New York: McGraw-Hill Inc., 1987)