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From: jheimann@BBNCCY.ARPA
Newsgroups: net.space
Subject: Re: Rocket thrust
Message-ID: <895@sri-arpa.UUCP>
Date: Fri, 15-Jun-84 15:08:42 EDT
Article-I.D.: sri-arpa.895
Posted: Fri Jun 15 15:08:42 1984
Date-Received: Tue, 19-Jun-84 01:07:39 EDT
Lines: 23

From:  John Heimann 

	The bullet/rocket analogy is specious because in fact the bullet is
part of the material which is being ejected from the rifle; i.e. is analoous 
to the exhaust gasses, not the rocket. It is the rifle that is analogous
to the rocket.  If one were to have a rocket engine with a combustion chamber 
that was open at the front, analogous to an open breech rifle, then we would
expect the rocket engine to have far less thrust (if any at all) than a closed 
chamber engine.  Alternatively, as we would expect a bullet fired from a gun
in space to have a higher velocity than one fired in the atmosphere since there 
would be no viscous drag on the bullet, and hence to impart a greater impulse to 
the rifle, so we would expect a rocket in space to eject its exhaust gasses at
a higher rate than in the atmosphere and hence have greater thrust.
	Another way to look at it:  the exhaust gasses  escape rearward at a 
rate which is dependant on the difference in pressure between the engine 
throat and the end of the exhaust nozzle.  The atmospheric "backpressure" that 
you refer to is just atmospheric pressure on the rear of the rocket, which 
is cancelled out by atmospheric pressure on the front of the rocket.  At every
moment I experience something like 5000 pounds of force on my back due to 
atmospheric pressure, yet I don't accelerate forward since there also happens 
to be the same force on my chest.

				John