Newsgroups: sci.space.policy
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Lunar Fuels
Date: Tue, 9 Jan 1996 04:53:27 GMT

In article <30F0DFBE.5BF5@mail.GANet.NET> "William H. Mook" <wm0@mail.GANet.NET> writes:
>Hydrogen and the other volatiles are easily retrieved from the lunar soil 
>by simple heating to about 800 C.  So, a parabolic reflector pointed at 
>the sun heating a sample of lunar soil will give off water and CO2 and 
>such...

Unfortunately, only very slowly.  The lunar soil is too good a thermal
insulator for this sort of thing to work well in bulk.  People have looked
at ideas like microwave heating as alternatives.  Not an easy problem,
although it can probably be solved.

>What would the specific impulse of an Alumina or Silica engine exhaust
>be?  Has anyone worked that out?

Yes; see assorted papers in (for example) Resources of Near-Earth Space.

>Also, wouldn't the exhaust condense
>rather quickly?  That is, wouldn't the expansion factors be limited
>by exhaust cooling?

That's why schemes like this have to run very oxygen-rich, so there is
still a reasonable gas flow left after the alumina (or whatever)
solidifies.  It's a bit like a nuclear rocket:  the alumina is basically 
just an energy source, the exhaust gas has to be supplied separately.

>...What are you making your combustion chamber out of?

Well, yes, that's the fun part.  Oxide refractories, presumably.

>Hey, I know!  How about making these things out of a slush?  Divide 
>aluminum into a fine dust... and mix liberally with LOX.  Put it in an 
>ablatively lined tank - with extra LOX to keep the molecular weight down 
>and your expansion up.  Light it up... and stand back.  
>Or just treat the LOX/slush mixture as a monopropellant...

I think I'll stand way, way back.  LOX-based monopropellants are also
known as "explosives".   Maybe one could passivate the surfaces well
enough, but *I'm* not going to do the experiments.  People have tried
remarkably hard for energetic monopropellants by this kind of route,
and not all of them survived the discovery that it doesn't work.
-- 
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |   henry@zoo.toronto.edu