Date: 29 Dec 92 22:18:33 GMT
From: Jordin Kare <jtk@s1.gov>
Subject: Aluminum as rocket fuel?
Newsgroups: sci.space

In article <1992Dec29.134016.13037@iti.org> aws@iti.org (Allen W. Sherzer) writes:
>Another source of information is the External Tank Study published
>by the Space Studies Institute. Cutting up ETs for their aluminum
>was listed as an option. Impulse of such an engine would be in the
>330 second range. Very viable as a Lunar based fuel source.

It is my understanding that there are serious technical difficulties in
making a workable aluminum/oxygen rocket.  While I'm by no means an
expert, I believe the problems include:

Injection:  How do you transport Al?  How do you get it to mix with the O2?
	Premixing them into a slurry (i.e., using the LOX to transport the
aluminum) is possible, but
a stochiometric slurry of Al powder in LOX is apparently a very
unstable system with a tendency to detonate.  It is my recollection that
Wickman Co's approach is to stabilize such a slurry with (proprietary) 
additives.  Even if you transport the Al separately (say, with a flow of
hydrogen gas) it's not clear that you can achieve stable combustion; things
like Al particle size are probably very critical.

Cooling:  The combustion temperature of Al and O2 is very high (or they 
wouldn't give even ~300 s Isp).  Conventional engines are regeneratively cooled
by the fuel.  Regenerative cooling with O2 is difficult -- O2 tends to 
oxidize engine parts :-(.  

Exhaust flow properties:  AlO2 is both refractory and abrasive.  
I don't know (and I wouldn't be surprised if no one knew) just what the
condensation properties of AlO2 would be in a rocket exhaust, but it 
seems likely that the exhaust will chew up most throat and nozzle materials
and may not provide very efficient thrust.

None of this makes an Al-O rocket impossible, just difficult -- enough so
that there are probably easier ways of getting mass off the moon.

	Jordin Kare