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From: kwan@smeagol.UUCP (Richard Kwan)
Newsgroups: net.space
Subject: Re: Scramjets (specific impulse)
Message-ID: <640@smeagol.UUCP>
Date: Wed, 5-Mar-86 23:46:45 EST
Article-I.D.: smeagol.640
Posted: Wed Mar  5 23:46:45 1986
Date-Received: Sun, 9-Mar-86 00:26:53 EST
References: <344@vger.UUCP> <617@smeagol.UUCP> <34@petrus.UUCP> <623@smeagol.UUCP> <739@jplgodo.UUCP>
Organization: JPL, Pasadena CA
Lines: 30
Summary: I think Phil Karn is still basically right.  To obtain mass ratio with his derivation requires folding the constant 'g' into exhaust velocity.

In article <739@jplgodo.UUCP>, ted@jplgodo.UUCP (Ted Sweetser x4989 156/224) writes:
> 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 ... 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.
Yes, but not by much.  If you assume that
	exhaust-V = g*Isp
since 'g' is a constant, you get
	mass ratio = exp(-(delta-V)/(exhaust-V))
Now, of my reading from the ancient past, I only recall once seeing this
relationship between Isp and exhaust velocity; so I don't know how widely
accepted this is.  (Furthermore, that was in my decidedly dumber days :-})

-- 
		Rick Kwan
		JPL Spacecraft Data Systems
		sdcrdcf!smeagol!kwan (UUCP)
		ia-sun2!smeagol!kwan@csvax.caltech.EDU (ARPA)