🚀 go-pugleaf

-From: jtk@s1.gov (Jordin Kare)
-Subject: Re: Aluminum as rocket fuel?
-Date: 29 Dec 92 22:18:33 GMT
-Organization: LLNL

-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?

Maybe they'll have to get some technical assistance from the John Deere
company (maker of agricultural combines). :-) [Combines use a mechanical
feed to transport the harvested grain.]

For final injection, it occurs to me that using gaseous oxygen to "blow"
the powdered aluminum in might be an option.

The final mix probably needs to be oxygen-rich if you want to maximize
specific impulse.

-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 :-(.  

I suspect they'll eventually have to go for something like ceramic linings.

-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.

According to Henry, there may also be problems with Al2O3 building up deposits
on the inside of the engine. Aluminum-oxygen engines will probably be 
single-use for the forseeable future.

-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.

A linear launcher (or laser launcher - plug, plug :-) might be better in the
long run. Aluminum engines might have an advantage in the short-to-medium
run by virtue of lower startup costs. The first lunar launch systems will
probably use fuel imported from off-moon. Combinations of the above might
be useful. Other factors - possible eventual availability of volatiles
from the asteroids, and dual-use laser systems for both launching and
landing payloads on the moon.

John Roberts
roberts@cmr.ncsl.nist.gov