Xref: utzoo sci.space.shuttle:3911 sci.space:14984
Newsgroups: sci.space.shuttle,sci.space
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Exhaust velocity
Message-ID: <1989Oct25.030457.15229@utzoo.uucp>
Organization: U of Toronto Zoology
References: <538.252A3A3B@mamab.FIDONET.ORG> <34577@srcsip.UUCP> <126311@sun.Eng.Sun.COM> <1307@accuvax.nwu.edu> <15596@netnews.upenn.edu> <1321@accuvax.nwu.edu> <1989Oct18.174154.23242@utzoo.uucp> <2639@ganymede.inmos.co.uk>
Date: Wed, 25 Oct 89 03:04:57 GMT

In article <2639@ganymede.inmos.co.uk> conor@inmos.co.uk (Conor O'Neill) writes:
>>... the average molecular 
>>weight of the exhaust is higher, which is bad for getting maximum velocity
>>but good for getting maximum thrust.

To get one picky issue out of the way here:  I goofed slightly.  To a first
approximation, molecular weight is not an issue for maximizing thrust.
(Should have looked it up rather than trying to figure it out myself...)

>I've seen this said before (many times) but never with a simple explanation.
>My school physics seemed to imply that it is exhaust momentum which matters,
>not simply velocity nor molecular weight...

If all you want is thrust, then momentum is the significant number, and
that is determined basically by pressures and areas in the engine.  But
for many applications propellant consumption also matters, which puts a
premium on the highest possible exhaust velocity, to get a given amount
of exhaust momentum with the minimum exhaust mass.

In a thermal rocket, where exhaust velocity is achieved by heating gases
to high temperatures and letting them expand out a nozzle, to a first
approximation exhaust velocity is set by chamber temperature and
exhaust molecular weight.  The significance of molecular weight is that
at a given temperature, the average *energy* of gas molecules is constant
regardless of their mass, so the ones with the lowest molecular weight
move fastest.
-- 
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry henry@zoo.toronto.edu