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Path: utzoo!dciem!nrcaer!cognos!geovision!alastair
From: alastair@geovision.UUCP (Alastair Mayer)
Newsgroups: sci.space.shuttle
Subject: Re: Throttling the Challenger
Message-ID: <216@geovision.UUCP>
Date: Tue, 3-Nov-87 16:32:35 EST
Article-I.D.: geovisio.216
Posted: Tue Nov  3 16:32:35 1987
Date-Received: Fri, 6-Nov-87 06:38:40 EST
References: <187@scdpyr.UUCP> <5270001@hpisof0.HP.COM> <9298@tekecs.TEK.COM>
Reply-To: alastair@geovision.UUCP (Alastair Mayer)
Organization: Geovision Corporation, Ottawa, Canada
Lines: 41

In article <9298@tekecs.TEK.COM> philb@maya.UUCP (Phil Biehl) writes:
>In article <5270001@hpisof0.HP.COM> campbelr@hpisof0.HP.COM (Bob Campbell) writes:
>>SSME Specifications     (Full power level)
>.
>>Flowrates:
>>   Total                 1,130 lb/s     22,557 gpm
>>   Hydrogen                160 lb/s     16,436 gpm
>>   Oxygen                  970 lb/s      6,121 gpm
>
>Does the above flowrates indicate tha actual combustion mixture present at the
>combustion chamber? If so then why is there a greater ratio than 2 parts H to
>one part O? I would think that the extra hydrogen would be unconsumed making
>the engine less efficient than it could be... What am I missing?

 Rocket engine "efficiency" is more than just optimizing the mix ratio
of the fuel and oxidizer.  It is also related to the exhaust velocity
of the combustion products.  Having the SSMEs run H2 rich means that
the average molecular weight of the exhaust is less than that of straight
H2O, hence for a given combustion temperature will reach a higher
exhaust velocity.  (If I had my textbook handy I could quote formulas
at you). 
  The *thrust* isn't as great - you need a relatively heavy exhaust
for that, but the efficiency (in terms of specific impulse) is better.
One way of looking at it is comparing the engine with, say, NERVA,
which used a nuclear heat source to heat hydrogen propellant.  MW of
the exhaust was just 2 - very efficient, except that the thermal
coupling of the reactor to propellant was poor.  The SSME uses a chemical
heat source to heat the hydrogen propellant, said source being the
H2-O2 combustion.   Thermal coupling is excellent!
  There are a couple of lesser reasons for running the engines H2-rich -
you want to maintain the exhaust as a reducing gas rather than an
oxidizing gas, and trying to run the engines right at the stoichiometric
mix ratio it would probably oscillate between reducing and oxidizing
with interesting effects on the materials in the combustion chamber
and engine bell...
  
-- 
 Alastair JW Mayer     BIX: al
                      UUCP: ...!utzoo!dciem!nrcaer!cognos!geovision!alastair

 "What we really need is a good 5-cent/gram launch vehicle."