Can you catch a spaceship with a train on the Moon?
In Kim Stanley Robinson's new novel Red Moon the first few pages describe a method of Earth-to-Moon transportation that I had not encountered before. The idea is to use a magnetically levitated and accelerated train on the surface of the Moon to catch a spaceship from Earth flying by the Moon at thousands of kilometers per hour. The advantage of this system is that the spaceship does not need to bring fuel to decelerate itself and instead is decelerated by the train.
In more detail, a ship is launched from Earth and is put on a course tangential to the surface of the Moon such that it would just brush past the surface at 8300 kilometers per hour (according to the novel). As it approaches the Moon a maglev train on a 200-kilometer long track is accelerated to match speeds with the incoming spaceship. As the ship comes closest to the surface of the Moon the train is there to catch it and hold on to it. The train then gradually decelerates with the ship using the long track. Because the train is magnetically levitated and there is practically no air resistance on the Moon the train can easily reach such fast speeds. Because the ship doesn't have to bring its own decelerating fuel much more weight can be dedicated to cargo.
This system is very economically attractive and if practical would appear to cut costs of sending people and supplies to the moon significantly. However, I have never encountered this idea before and a cursory search doesn't find any other references for the system. Will this scheme work or are there practical difficulties that make it unfeasible?
P.S. I'm also curious whether this is a novel idea of Kim Stanley Robinson's or if someone else has proposed this before?
This post was sourced from https://worldbuilding.stackexchange.com/q/128601. It is licensed under CC BY-SA 4.0.
1 answer
Your massive maglev train is, obviously, levitated and accelerated by (electro) magnets, and must be many times the mass of the spaceship.
The energy to power those magnets must come from somewhere on the planet, like a nuclear power plant, or perhaps by solar energy. There won't be any fossil fuels on Mars, and although nuclear fuel might be shipped from Earth, other forms of fuel would not be; that would defeat the purpose.
The obvious solution, to me, is to put both the nuclear power plant and the electro magnets into space, a few million kilometers from Mars. The same magnetic energy used to lift and accelerate this giant train can be applied to the ship itself over a very long distance in space, and you won't have to deal with the problem of tolerances on an uneven and possibly shifting (and definitely rotating) planetary surface.
In space, you don't have to deal with confounding force factors like the moving Martian atmosphere (it is thin but there is weather), terrain, Martian dust storms and compensating vectors for planetary rotation or curvature. It is a cleaner and simpler environment and for engineers, this allows much greater accuracy and closer approaches, tiny nudges from steering rockets can change course by a hundredth of a centimeter.
So your ship can navigate to the same distance from the rails, the rails are the same length as those on Mars (and could be shorter, because in space you could have six rails circle the ship at the points of a hexagon), the power applied is the same. But the rails can be perfectly straight, the path of the spaceship perfectly straight and centered. The rails themselves can be linked into a ring, to keep them aligned. They can be as massive (or much more massive) than the train; iron is very cheap in space (asteroids). With the nuclear power plant, any shift of the deceleration rig can be corrected by nuclear powered magnetic propulsion (accelerating atoms at near-light speed in the opposite direction of desired travel).
After slowing the ship enough, it lands by parachute, just like our probes have, or you could even guide it into orbit and (robotically) deploy only the supplies by parachute to the planetary surface. Then the ship could be turned around, and the exact same rail guns in space could accelerate it away from Mars back to Earth.
It may be empty, or could carry crew and products back to Earth. However those return goods get to Mars orbit, it would certainly be less energy intensive to send just them into orbit, than it would be to send them AND the ship.
By this scheme (which I invented here on the fly) the ship never leaves space, so the space ship can be just a space ship, it does not have to be engineered to work both on the ground (Earth or Mars), withstand launch stresses, have landing gear, or even be oriented for gravity, it can be, for example, a permanently rotating cylinder with centrifugal 0.25G gravity, more comfortable for human passengers (washing, sleeping, cooking, eliminating, exercising, working, etc) and more convenient for packing and storage (you don't have to net everything or tie it in place).
Of course, this cylinder ship may have a centrifugal section and a non-rotating zero-G section, if zero-G is desired for storage or is useful for some scientific or technical operations.
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