Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!zaphod.mps.ohio-state.edu!rpi!uupsi!rodan!amichiel
From: amichiel@rodan.acs.syr.edu (Allen J Michielsen)
Newsgroups: sci.space.shuttle
Subject: Re: Launch scrubbed
Message-ID: <2907@rodan.acs.syr.edu>
Date: 11 Apr 90 19:01:36 GMT
References: <4076@nmtsun.nmt.edu> <1990Apr10.192721.31497@uokmax.uucp>
Reply-To: amichiel@rodan.acs.syr.edu (Allen J Michielsen)
Organization: Syracuse University, Syracuse, NY
Lines: 35


>In article <4076@nmtsun.nmt.edu> dbriggs@nrao.edu (Daniel Briggs) writes:
>>Are the
>>APUs just fuel cells ala Apollo, or are they something more
>>sophisticated?  (BTW, I am under the impression that this is an
>>acronym for Auxiliary Power Unit.  Am I right?)  Has any of the NASA
>

   While avoiding a whole big discussion of these buggers, and since I am
not the best qualified to give technical details on this, I will try for
a very simple explaination.
   The APU's for the shuttle which we are currently discussing could & should
be thought of as the following: The APU is very much like a chemical turbine
similiar to/not unlike a jet engine.  Instead of jet fuel (kerosene) & atmos
air being brought together & burned, the seperate chemicals (all pretty
nasty, toxic, & dangerous stuff) are brought together creating a exothermic
reaction (explosion).  The explosion drives turbines in both cases & the
energy is used to drive pumps. generators, compressors, or whatever is hooked
up to it. Where jet engines get rid of the excess heat in the exhaust gas 
stream, the APU's use a heat exchanger & the excess heat goes into the radiator
portion of the cargo bay doors, which basically have to be opened after launch
to dissipate this energy.  A jet engine is a open system & the APU's are closed
for obvious reasons.  The waste stream is used a differently for a engine than
the APU, but the principle is the same.
   As I listened to the audio feed, of the aborted launch, one of the APU's was
reporting too many RPM's.  The problem couldn't be isolated to 1 of the most
likely 2 causes & the launch was scrubbed because of the risk involved.  The
2 most likely causes were 1. bad rpm pickup 2. bad/faulty/error in the fuel
control loop.  The APU could have been shut down & if it went down o.k., just
done without.  This could be bad if another APU developed a in flight problem.
Trying to guess what sensor was wrong or control loop was bad would have been
dangerous to the entire deal.  Too much fuel or too bad a mix & the APU would
be a bomb.  I really think that NASA did the right thing pulling the plug &
fixing the problem, the bad PR would be much worse IF another fatal accident
were to occur.