MMM Classics Year 10: MMM #s 91-100 - Moon Society

Moon Guns & Other Mysteries
by Gregory R. Bennett
CEO: The Lunar Resources Company,
“Chief Responsible Party”: “The Artemis Project”
Jan. 20th, 1996 - from: grb@asi.org
To: ederd@bcstec.ca.boeing.com (Dani Eder)
CC: kokhMMM@aol.com (Peter Kokh, for MMM)
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 is intriguing, but I'd shy away from comparing it
to an electric gun. We're faced with it's 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.
Propellants . . .
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 selfdefeating
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 Lunans 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, oxidizer, 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.
Gun Structure . . .
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 barrel, 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. (And you know my mind doesn't boggle easily.) 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 technology challenges and capital
required to develop that big chemical gun on the Moon, my
best guess is that electromagnetic propulsion would be considerably
better-faster-cheaper 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.
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 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.
GB
cartoon by Neil Durst, ASI Pittsburgh
Moon Miners’ Manifesto Classics - Year 10 - Republished January 2006 - Page 21