Figure 1. Full moon, as photographed in 2010. (c)2010 Wikimedia
Comments & Submissions ____________
Could a Lunar Space Elevator Become a Solar Powered Pipeline?
A space elevator is a very dangerous object, if it breaks.
I think the first space elevator should be built on / to the Moon.3 The second should be built on or to Mars. Both are much more within the capabilities of known materials, than an Earth space elevator. The Mars elevator is particularly attractive, since it is the shortest, and landing on Mars is quite difficult. If it could be made from materials on Phobos or Deimos, by robots, well, that's elegant. However, the Moon is much closer, and building a Moon elevator is feasible at this time.
How do you go about building a space elevator to the Moon? Start with the International Space Station.
You could disassemble the ISS, when that project comes to an end, and ship the pieces to Lunar L1, using ion drives. Even part of the ISS would provide an acceptable anchor for the Lunar space elevator.
Kevlar works out to just about 1kg per km of cable, so let's assume that as our tether. Here comes the hard part. The tether must be 57,000 km long, longer than the Earth space elevator cable. So, if you loaded 1000 kg of cable into each cargo shipment to the ISS, you would need 57 shipments to build a minimal space elevator cable. You would probably want 10 to 20 times as much material, for factors of safety. An alternative would be to ship the cable with 1 to 10 flights of Falcon Heavy rockets.
Hauling up the cable, using solar power, would be very slow, probably for cargo only, but going down, you could go fast, and hit the brakes at 1g, or even 2gs, near the end. If you went down the cable at 2000 km/hr, it would still take ~28 hours to make the trip. If there is a way to transfer that falling energy to the rising payloads, that would be really good. (28 hours is roughly as fast as Apollo made the trip from L1 to Lunar orbit.)
The Lunar Pipeline
There has been talk that a Moon elevator is useless, since the thrust needed to land and take off from the Moon is so small, and climbing the elevator takes so long. I've argued that it's great for cargo, especially rocket fuel. I have an improvement on that idea, that I'd like to share.
Let's say you have a Moon space elevator, and you want to move water and rocket fuel up the elevator, using Solar power. What's the easiest way to do it?
The answer is to make the cable a gas sealed tube. Now, gaseous diffusion will do all the work for you. Kinetics of the Ideal Gas law gives:1,2 As the molecules climb in the tube, gravity will slow them down, cooling the gas. This will have to be compensated for, by sunlight falling on the tube, heating the gas. So, even a tube only 1 or 2 cm in diameter can produce quite a large flow. 10 cm diameter would be even better, because you could operate at lower pressures, with less leakage.
If there is a problem with reverse pressure building in the tube due to gravity, it is easily solved. One simply places a valve in the tube every 100 km or so, in the lower parts of the tube, and less frequently in the higher parts of the tube. By opening the valve above and shutting the valve below, assuming the top of the system is in near vacuum, thermal velocity will pump the gas up hill. Only about 1/2 the gas will move to the next segment each time a valve is opened, but, starting with say, 100 atmospheres of pressure in the tube, there will be plenty of pressure to drive the flow in upper parts of the system.
The reason I listed so many different gasses is because I'm worried about Hydrogen leaks, over such a long distance. I'd be even more worried if you tried to ship Helium 3 through the pipeline, though that would be the most valuable product of all, some say. If Hydrogen does not ship well because of leaks, it might be necessary to ship the Hydrogen as water steam, or as Methane. If shipped as steam, the Solar cells to make H2 and O2 would have to be in orbit, and that's a pain. Gravity really helps separate the gas.
With water (steam) there is danger of freezing, expansion, and then pipe rupture. Whenever part of the tube is not heated by the Sun (2 weeks out of each month) it would be necessary to flush the tube with some other gas before the tube freezes. (Orbiting mirrors could warm the tube, but that's complicated.)
If enough carbon is available, it would probably be best to ship Methane and Oxygen in the tube, but not at the same time. In between, shipments of Nitrogen or Helium should flush the tube, to prevent fire danger.
ConclusionsSo there you have it. A Solar powered space elevator, for shipping rocket fuel to Lunar L1 and beyond.