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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: sci.space.shuttle
Subject: Re: Throttling the Challenger
Message-ID: <8855@utzoo.UUCP>
Date: Fri, 30-Oct-87 14:13:26 EST
Article-I.D.: utzoo.8855
Posted: Fri Oct 30 14:13:26 1987
Date-Received: Fri, 30-Oct-87 14:13:26 EST
References: <187@scdpyr.UUCP> <5270001@hpisof0.HP.COM>, <9298@tekecs.TEK.COM>
Organization: U of Toronto Zoology
Lines: 22

> ... 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?

1. The mixture has to be hydrogen-rich to protect the engine walls against
	attack by hot high-pressure oxygen.  (For the same reason, the
	hydrogen and oxygen supplies would not run out at the same instant
	if the engines burned to fuel exhaustion [normally they shut down
	a bit before that] -- the hydrogen tank is deliberately filled a
	little fuller than it has to be, to make *sure* the oxygen runs
	out first no matter what.)

2. Adding hydrogen lowers the average molecular weight of the exhaust, so a
	modest excess of hydrogen can improve performance even though it
	does not add energy.

3. Adding hydrogen lowers the exhaust temperature, which reduces energy
	absorption by thermal dissociation of H2O.  Again, in small doses
	this can be a net win.
-- 
PS/2: Yesterday's hardware today.    |  Henry Spencer @ U of Toronto Zoology
OS/2: Yesterday's software tomorrow. | {allegra,ihnp4,decvax,utai}!utzoo!henry