Moon Guns

You will find several essays in the Artemis Data Book about using electromagnetic launchers, or "mass drivers," to send payloads from the lunar surface into orbit. Quite often, while we are discussing these electric guns, the intriging idea of building a chemical gun to launch payloads will come up. This great Moon Gun would shoot payloads into space much the same as a large cannon on Earth throws its projectiles.

If we wanted to build a chemical gun, we would be faced with a whole new set of interesting technology challenges. We might need all the industries required to support an electromagnetic launcher, plus a lot more.

Propellants

The first issue is making the propellants from lunar material. Many of the likely chemical propellants we use on Earth contain nitrogen. However, nitrogen is very scarce on the moon, and it is one of the four elements essential to life as we know it. (The four essential ingredients of life are carbon, hydrogen, oxygen, and nitrogen -- CHON, for short.) Some nitrogen is mixed in with the regolith, but exporting scarce, life-critical elements from the moon would be a self-defeating business.

The same goes for using hydrogen in our propellants. Sometimes a suggestion come up that we could hot hydrogen as a working fluid for the Moon Gun, but if we start a business that wastes hydrogen, our gun will last just as long as it takes other residents of the moon 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 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 we will have to develop at the cost of delaying the development of whatever else the lunar commkunity would like to have.

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.

Gun Structure

Now that we have some propellants, we need a gun. This is where 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. Manufacturing to the tolerances of a canon barrel in an absolutely straight cylinder many miles long poses a daunting technological challenge.

We could design the Moon Gun so that the propellant moves along the barrel with the payload, but here we run into even more problems. 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 it might not be the best approach.

That leaves us with the metallurgical challenge of making the material for the barrel, and then machining it. Now we're into really heavy industry on the moon. We haven't estimated the initial capital required to get that much heavy machinery to the moon, but just the thought of it is mind-boggling. The alternative is building up heavy industry on the moon one step at a time, which adds a few decades to the program schedule.

Conclusions

Based on the technological 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 quite 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 moon dirt, separating the chemicals, and making metal; and both need a good power supply. For electromagnetic propulsion we're ready to go as soon as 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.

Nevertheless

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, some day, 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 -- pristine, quiet, and unfailingly accurate compared to our giant blunderbuss. 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.