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From: roger@telesoft.com (Roger Arnold @prodigal)
Newsgroups: sci.space.shuttle,sci.space
Subject: Re: Exhaust velocity
Summary: energy vs. momentum
Message-ID: <571@telesoft.com>
Date: 24 Oct 89 19:08:00 GMT
References: <538.252A3A3B@mamab.FIDONET.ORG> <34577@srcsip.UUCP> <2639@ganymede.inmos.co.uk>
Organization: TeleSoft, San Diego, CA.
Lines: 35

conor@inmos.co.uk (Conor O'Neill) writes:
> henry@utzoo.uucp (Henry Spencer) writes:
> >In general, correct.  For one thing, it's easier to build solid motors
> >in large sizes (i.e. high thrusts).  For another, the average molecular 
> >weight of the exhaust is higher, which is bad for getting maximum velocity
> >but good for getting maximum thrust.
> 
> 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. Could someone please elaborate.
> 
> -- 
> Conor O'Neill, Software Group, INMOS Ltd., UK.
> UK: conor@inmos.co.uk		US: conor@inmos.com

You're correct that it's exhaust momentum which matters.  Momentum is
mass times velocity; energy, however, is proportional to mass times 
velocity *squared*.  For a given amount of power (energy per second),
you can increase the thrust (momentum per second) by increasing the
mass flow and reducing the exhaust velocity. 

The tradeoff between thrust and exhaust velocity is most relevant in 
optimizing the performance of ion engines for interplanetary missions.
In that case, you're dealing with an external power source, as opposed
to using the energy in the fuel itself.  Henry's example for solid
fuel rockets is a little misleading.  Power, in a solid fuel rocket,
is a function of the burn rate, and that's controlled by things like 
the fuel composition, the exposed surface area, and the grain size.  
It's pretty easy to design for the maximum burn rate, and hence thrust,
that the rocket casings will take.  There's no reason to deliberately
reduce exhaust velocity in a solid fuel rocket; you want whatever the
most energetic combination of solid fuel and oxidizer you can manage
to develop will deliver.

- Roger Arnold				ucsd!telesoft!roger