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From: ST401385@BROWNVM.BITNET
Newsgroups: net.space
Subject: Specific Impulse
Message-ID: <8603070058.AA06014@s1-b.arpa>
Date: Thu, 6-Mar-86 14:36:07 EST
Article-I.D.: s1-b.8603070058.AA06014
Posted: Thu Mar  6 14:36:07 1986
Date-Received: Sat, 8-Mar-86 05:11:26 EST
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Organization: The ARPA Internet
Lines: 25


     The metric unit (newton seconds thrust per kilogram propellant)
which is measured in meters per second, is properly defined as the
exhaust velocity, Ve, *not* the specific impulse.

          Ve = g Isp

Where Isp is specific impulse, and g is the acceleration of gravity.
This is true in any system of units.
     However, you are right that Ve is a much more convenient way to
specify propellant characteristics, since you can plug it right into
the rocket equation,

                    Delta V = Ve ln(Mi/Mf)

     Where Mi/Mf is the mass ratio.
     Specific impulse (and exhaust velocity) are most useful for
systems where the energy and the reaction gas are the same.
In this case, it is *always* to your advantage to increase the
specific impulse (or exhaust velocity).   For
objects like nuclear engines, where the energy comes from a different
source than the reaction gas, it is a less useful concept.

                   --Geoffrey A. Landis, Brown University
                     Reply to: ST401385%BROWNVM.BITNET@WISCVM.ARPA