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From: David.Smith@CMU-CS-IUS.ARPA
Newsgroups: net.space
Subject: Re: Rocket thrust
Message-ID: <918@sri-arpa.UUCP>
Date: Fri, 15-Jun-84 17:05:00 EDT
Article-I.D.: sri-arpa.918
Posted: Fri Jun 15 17:05:00 1984
Date-Received: Fri, 22-Jun-84 05:38:32 EDT
Lines: 38


	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
	analogous to the exhaust gasses, not the rocket. It is the rifle
	that is analogous to the rocket.

I am not yet convinced that the analogy is specious.  The gases
push laterally on the sides of the barrel, as on the sides of the
rocket.  To a first approximation, it doesn't matter whether the barrel
is fixed to the breech or travels with the bullet (or it wouldn't if
the barrel had zero weight but retained it strength).  The gases push
equally on both bullet and breech, and it doesn't matter which one you
consider to be engine and which exhaust.

But suppose we go ahead and consider the rifle to correspond to the engine.
Which way will you get a bigger kick in the shoulder?
    1. Fire the charge without a bullet, producing maximal exhaust velocity.
    2. Obstruct exit of the exhaust by forcing it to drive a bullet out.


	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, ...

To the extent that the air being accelerated ahead of the bullet drags
on the barrel, OK.  But besides that (and it's not applicable to the
rocket anyway), I'll still argue that it does not impart greater impulse
in space, even though the bullet velocity is higher.

	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.

You would if you applied pressure to the air on one side which was not
matched on the other.  The rocket exhaust does this.

				David Smith