Lunar Aluminum Applications

David L. Burkhead

Aluminum is an abundant material on the moon, and can be used in a variety of applications. Rocket propellant and export to LEO may be two major applications.

Hydrogen/Oxygen engines are very efficient. However, if you're on the moon, you may very well not want hydrogen at all, not until you have an extraterrestrial source. The penalty in lifting that hydrogen from Earth's surface (including lifting very large tanks) may be higher than the penalty of using a lower Isp rocket that uses indigenous fuels. Aluminum/oxygen is one possibility. The figures usually published give it an Isp of about 270. That's on the low side, but it's a dense fuel, so you can get pretty high mass ratios. Also the delta-v requirements of most missions are much lower than that of launch from Earth's surface to orbit.

On the production side of the equation, thanks to the US Bureau of Mines we already have techniques to extract aluminum from feldspars (which are plentiful on the moon). Perhaps the most adaptable to lunar conditions is the melt-quench-crush-leach process. Melt the rock (big solar furnace, only runs during the lunar day), and quench it to glass, then crush it. This breaks up the crystalline structure so that acids can leach out the aluminum oxide. Precipitate the aluminum oxide and the rest is identical to Earth-based processes.

In terms of exports, I did some economic type numbers on just that kind of operation for my article "The Economics of Lunar Mining" which appeared in the August/September 1993 issue of High Technology Careers. The upshot is that lunar aluminum, delivered to Earth orbit, and using current launch prices, will cost about $52,000 a ton. Compare that with terrestrial materials also delivered to Earth orbit at more than $4 million a ton. I didn't include the use of lunar aluminum as a propellant in the figures, or low thrust/high efficiency engines such as ion drives, but it's pretty clear that either would drive down the cost of the lunar aluminum, as would closing the life support system (I assumed a completely open cycle life support system, with everything but oxygen lifted from Earth). Likewise, lower launch costs would drive down the cost of both lunar and terrestrial aluminum.