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From: ted@jplgodo.UUCP (Ted Sweetser x4989 156/224)
Newsgroups: net.space
Subject: Re: Scramjets (specific impulse)
Message-ID: <739@jplgodo.UUCP>
Date: Wed, 5-Mar-86 21:14:03 EST
Article-I.D.: jplgodo.739
Posted: Wed Mar  5 21:14:03 1986
Date-Received: Sun, 9-Mar-86 00:26:13 EST
References: <344@vger.UUCP> <617@smeagol.UUCP> <34@petrus.UUCP> <623@smeagol.UUCP>
Organization: Jet Propulsion Labs, Pasadena, CA
Lines: 24
Summary: specific impulse definition

In article <623@smeagol.UUCP>, kwan@smeagol.UUCP (Richard Kwan) writes:
> And in <34@petrus.UUCP>, Phil Karn responded:
> > Specific impulse is often erroneously specified in "seconds"; the correct
> > units should be "meters/sec", i.e., velocity.
> I don't know what got into me.  You are correct.  "Seconds" is the
> accepted units in the English system.

Wait a minute, I don't think Mr. Karn *is* right.  The best definition of 
specific impulse (Isp) is "thrust / (weight of propellent mixture used per 
unit time)".  With this definition the units for Isp are seconds in both the 
metric and English systems and no conversion factor is needed for Isp 
between the two systems.  Furthermore, this definition is needed to make 
the following form of the rocket equation:
    mass ratio = exp(-(delta-V)/(g*Isp))
work in any consistent system of units; if you use a "meter/second" Isp
then you have to use a different form of the rocket equation in the metric
system.

A short history of the term can be found in "Comment on 'Definition of
Specific Impulse'", _J._Spacecraft_, vol.12(1975), no.9, p.576, by 
Alfred Africano, one of the originators of the concept.  Unfortunately, 
textbook writers have been consistently inconsistent on Isp.

			Ted Sweetser (...smeagol!jplgodo!ted)