#92 February 1996
Section 22.214.171.124.092.of the Artemis Data Book
by Gregory R. Bennett,
Chief Architect of the Artemis Project
About Moon guns... As it happens, I even have some thoughts about the subject at hand.
The idea of building a chemical gun to launch propellants from the moon is intriguing, but I'd shy away from comparing it to an electric gun. We're faced with its own interesting technology challenges. As best I can tell, to make a chemical gun work we'd need all the industries required to support an electromagnetic launcher, plus a lot more. Here's what I'm thinking. Correct my concept of this thing if I got it wrong.
The first issue is making the propellants from lunar material. You mentioned the lack of nitrogen on the moon. Some nitrogen is mixed in with the regolith, but exporting scarce, life-critical elements from the moon would be a self-defeating business.
Ditto for the idea of exporting hydrogen. You mention using hot hydrogen as a working fluid in one of your posts, but I'd advise against it. If you start a business that wastes hydrogen on the moon, it will last just as long as it takes other loonies to wreck it. The stuff is too precious.
The good news is that there are lots of ways to make propellants from lunar material. You need a fuel, oxydizer, and perhaps some other elements to control the burning rate. Any mixture that gives you gasses which will support a supersonic burning wave front (a Chapman-Jouget wave) will work.
The hard part is getting that mixture. We'll have to disassemble moon rock to do this. To get the oxygen we're exporting, we'll need essentially the same processes. Chemically the function of the gun is to reassemble the compounds we took apart. But making propellants for a chemical gun means additional chemical processes, all new technology that will have to be developed at some non-zero cost.
Once we make the propellants, we'll need containers, for storage, logistics, and getting the propellants into the gun. That means more manufacturing processes, more machining, more technology to develop.
Here it gets really complicated. We need a precisely machined barrel, able to contain some pretty high chamber pressures. It'll be miles long unless you're willing to build your payload to withstand thousands of g's.
A fuel tank robust enough to take that kind of acceleration is a whole new engineering challenge. It wouldn't be useful as a container for rocket propellants and in this scenario won't be recoverable, so I assume you don't want to do this.
That leaves us with the metallurgical challenge of making the material for the barrell, and then machining it. Now we're into really heavy industry on the moon. I don't have time to estimate the initial capital required to get that much heavy machinery to the moon, but just the thought of it boggles my mind. The alternative is building up heavy industry on the moon one step at a time, which adds a few decades to the program schedule.
Based on the technology challenges and capital required to develop that big chemical gun on the moon, my best guess is that electromagnetic propulsion would be considerably betterfastercheaper to implement. Linear induction motors are fairly straightforward so the technology isn't scary. The important structural components -- supports for the magnets and payloads -- are massive but don't need to be as precise as a chemical gun's barrel; we're not building a giant pressure-containing cylinder.
The basic industries supporting either operation involve mining moondirt, separating the chemicals, and making metal; and both need a good power supply. For electromagnetic propulsion we're ready to go when we can make aluminum wire and machine the fiddly bits that keep the parts together. We'll even need wire to support control of the chemical gun, too. The chemical gun needs much lower power levels for its operation, but the fundamental industry is the same; we're still smelting aluminum and extruding it through a die to make wire.
All that said, I can still think of an economic argument for the gun: it looks neat. The muzzle flash against a dark sky would be awesome; you'd know something is happening when that behemoth fires! It might even sound neat if the ground shock gets transmitted to a nearby habitat. So even if it takes more industry development to do it, a big chemical gun might be economically Worth Doing because its entertainment value might add enough to make up for the cost of developing the additional machining capability. In comparison, an electromagnetic launcher would be boring. Think of it as an attraction at the Great Theme Park In The Sky.
One more argument in favor of doing this: These days we flinch at the word "spinoff", but the additional industrial capability we'd get from being able to do all that heavy machining should spark your imagination. If we can make a cannon barrel on the moon, we'll be able to make just about anything.
Contents of this issue of Moon Miners' Manifesto