Oh, Werner von Braun had it all figured out. In six issues of Collier's magazine he laid out a plan to send men to Luna and Mars. First you build a space ferry as a surface to orbit cargo transport (which was the great-grandfather of the Space Shuttle). Then you use it to make a space station.

And it was going to be a beauty of a space station, too. Three decks, 250 feet in diameter, and a crew of fifty. Makes the ISS look like a tin can. This outpost in space was where the Lunar expedition fleet would be constructed.

It would pay for itself as well. Meteorologists could plot the path of storms and predict the weather with unprecedented accuracy. Radio and TV signals could be transmitted all over the globe. Not to mention observing the military activities of hostile nations.

Why was this marvel never constructed? Because some clown invented the printed circuit. Freed from the tyranny of fragile and short-lived vacuum tubes, technologists could make unmanned satellites for Meteorologists, radio and TV signals, and watching hostile militaries. Such satellites could be assembled and launched at a fraction the cost of a manned station. They also did not require constant resupply missions to keep the crew alive.

If we had followed von Braun's plan, we would have ended up with a fleet of space ferries, a titanic manned space station, a large lunar base, and men on Mars. Instead, we have four overly complicated space shuttles near the end of their operational life, a four man space station due to be de-orbited and destroyed in 2016, and a few bits of space trash on the Lunar surface. And we haven't been back to Luna since 1972. So it goes.

Why doesn't a space station fall down? A station is in "orbit," which is a clever way to constantly fall but never hit the ground. The ground curves away just enough so that the station never strikes it.

Actually, the International Space Station is in a low enough orbit that atmospheric drag decays its orbit. Periodically, Russian resupply rockets have to boost it higher. Otherwise it would de-orbit and burn up in re-entry. Many readers of this website are too young to remember when NASA's Skylab unexpectedly fell.

Many station designs are wheel shaped, and large wheels at that. They are wheel shaped so you can spin them for artificial gravity. They are large wheels so the rotation rate can be kept low enough so the crew does not experience nausea. This is conservatively 3 RPM, though studies suggest some can acclimate themselves to tolerate up to 10 RPM. The Collier space wheel is 250 feet in diameter and spins at 3 RPM, providing about one-third gee of artificial gravity at the rim.

Many early SF stories fret about the military advantage an armed space station confer upon the owning nation. Heinlein says trying to fight a space station (or orbiting spacecraft) from the ground is akin to a man at the bottom of a well conducting a rock-throwning fight with somebody at the top. One power-crazed dictator with a nuclear bomb armed station could rule the world! Space faring nations would need space scouts for defense.

But most experts nowadays say that turns out not to be the case. A nation can threaten another with nuclear annihilation far more cheaply with a few ICBMs, no station is required. And while ground launching sites can hide in rugged terrain, a space station can hide nowhere. Pretty much the entire facing hemisphere can attack the station with missiles, laser weapons, and propaganda.

This clever design solves the problem of how to quickly assemble a wheel space station, with one tiny little drawback. You see, there is a reason that wheel space stations are shaped like, well, wheels and not like hexagons.

The amount of centrifugal gravity experienced is determined by the distance from the axis of rotation (the greater the distance, the stronger the gravity). So if you want the amount of gravity to be the same, the station has to be a circle.

Now, look at the image above. The segment labeled "SPACE STATION RIGID MODULE" is one of the hexagonal sides. The green lines lead to the axis of rotation (i.e., that is the direction of "up". Note the little dark men figures, they feel like they are standing upright). And the red lines are lines of equal gravity. You will note that they do not align with the module.

In the module, centrifugal gravity will be weakest at the center of the module, and strongest at the ends where it joins with the neighbor modules. Even though the module is straight, the gravity will feel like it is a hill. If you place a marble on the deck in the center, it will roll "downhill" to one of the edges.

As you see, the designers tried to compensate for this by angling the decks, but it really doesn't work very well.