Originally, I thought the propellant tanks of Taxis and Moon Shuttles would have to be pressurized with oxygen because there wasn't enough helium 4 on the Moon. Surprisingly, or pleasantly, I was wrong. I found varying figures on helium concetrations in lunar regolith but the most reliable figure comes from a NASA study. They give 28.5 ppm He and 2.75 ppm Ne but next to no other noble gases.
MOON SHUTTLE HELIUM REQUIREMENTS
The Moon Shuttle tanks had a total of about 6000 cubic meters of volume (5995 actually). If we pressurize them to 25 atmospheres we get 367.5 pounds per square inch. That should be enough to drive out the monopropellant. Since helium has a density of 0.1785 kg. per cubic meter at 0 deg. C and one atmosphere of pressure we can say that we need 6000*0.1785*25= 26,775 kg. or about 27 tons to do the job of pressurizing fuel tanks. The helium will contract but we might warm it up to develop more pressure and this won't cause any chemical reactions because helium is inert. In any case, if we have one thousand rockets at work in 2100 or beyond, we need 27,000 tons of helium. We will shut off the valves before burnout to prevent helium loss and pump it out, separate residual oxygen, and recompress the helium for reuse after each flight. We can use plain oxygen to blow out any metallic dust that remains in the tanks if necessary.
MINING THE MOON FOR HELIUM
In the course of mining for one ton of helium three we will harvest 2850 tons of helium four from 100 million tons of moondust. This is just a pit 10 kilometers square to a depth of half a meter. We will mine over 100 tons of helium 3 per year so we will get 285,000 tons of 4He. We will mine for decades before building giant Moon Shuttles for hauling millions of tourists. We will build smaller Shuttles in the meantime for work crews and the richest travelers who always get things before everyone else does. If 50 years of 3He mining goes on before we build the monster rockets, we will have a stock of 14,250,000 tons of 4He! We will have enough for pressure fed rockets and gas cooled nuclear fission reactors and some party balloons also! Unless I've made some big errors in my arithmetic, we've got plenty especially if we don't waste any by losing it to the vacuum. There will be losses since no system is perfect, but we will have enough to replenish those losses.
PRESSURE TANKS
Also, I have been conservative by stating 100 tons of 3He a year. We might mine 300 tons a year or more. We might run the rocket fuel tanks at higher pressure. Also, if we have 25 atm. in 6000 cubic meters of tanks we could put 300 atm. or 4410 psi in 500 cubic meters of gaseous storage tanks or ten tanks 4.6 meters wide or 15 feet apeice. Since I am being conservative once again, we could raise the pressure to 6000 psi easily enough and have ten 4.13 meter wide tanks or 13.55 feet. We could probably go to 10,000 psi, but this is all a matter of aesthetics and gaseous storage tank mass. The two big monopropellant tanks are 13m and 21m wide. I do know they put 2250 psi in quarter inch thick steel scuba tanks and 3000 psi in half inch thick aluminum tanks. I know they have scuba tanks with even higher pressures. Titanium might be a better helium gas storage tank material than aluminum/lithium even. If we really want to be radical we could make the whole thing out of titanium alloyed with some lunar aluminum and manganese, but some will argue that welding titanium is a bear. So I am only a writer and speculator. I just hope some budding engineers and physicists take up where I leave off and build the real thing. I look forward to the surprises they have in store for me.
