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From: David.Smith@CMU-CS-IUS.ARPA
Newsgroups: net.space
Subject: Re: Rocket thrust
Message-ID: <988@sri-arpa.UUCP>
Date: Mon, 18-Jun-84 13:13:00 EDT
Article-I.D.: sri-arpa.988
Posted: Mon Jun 18 13:13:00 1984
Date-Received: Thu, 21-Jun-84 01:06:09 EDT
Lines: 37

Let's look at the impulse (momentum transfer) derived from burning a small
parcel of fuel/oxidizer.  The parcel has mass m and chemical energy E
(which we assume is used with perfect efficiency).  Equate the chemical
energy to the kinetic energy of the parcel as it exits the engine (in
the frame of the rocket).  This gives us the exhaust velocity,

	v = sqrt( 2E/m ).

The impulse to the engine is the same as the momentum imparted to the
exhaust:

	I = mv = sqrt( 2mE )

>From these equations, it is clear that throwing extra inert mass into
the engine (raising m without raising E) will lower the exhaust
velocity, while increasing impulse (and thrust with it).  This is the
reason that turbofans (and turboprops) are more fuel-efficient than
turbojets, at least up to the speed at which shock waves form on the
blades.  Suppose that m is raised not by dumping mass into the
chamber, but by putting the rocket into the atmosphere and letting
the exhaust entrain the air.  More thrust, no?

Of course, if the vehicle has to carry the extra mass to the point of
use, it would be better to have it in the form of propellant, so that E
is also raised.  Carrying dead mass is pretty expensive.  And in space,
you have to carry your dead mass with you.

				David Smith

P.S.  As I stated in my original message, rockets really do generate
more thrust in space than in the atmosphere.  Nasa has stated this, and
they ought to know, having operated engines in both places.  I am still
hoping that someone  can either show the flaw in my reasoning or tell
what other effects are operating.  Perhaps energy is lost to sideways
turbulence?

				DRS