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From: dsmith@HPLABSC (David Smith)
Newsgroups: net.space
Subject: Re: Scramjets
Message-ID: <8603061937.AA23798@hplabsc>
Date: Thu, 6-Mar-86 14:37:22 EST
Article-I.D.: hplabsc.8603061937.AA23798
Posted: Thu Mar  6 14:37:22 1986
Date-Received: Sat, 8-Mar-86 04:56:37 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 32

> Specific impulse is often erroneously specified in "seconds"; the correct
> units should be "meters/sec", i.e., velocity.  The error occurs because Isp
> is usually defined in English units as
> 		pounds-force of thrust x seconds
> 		--------------------------------
> 		pounds-mass of propellant
> and somebody made the mistake of "cancelling out" the pounds-force factor
> with the pounds-mass factor. A good example of how the English system of
> measurements befuddles thinking, but I digress...
> In metric units, things are much clearer:
> 		newtons of thrust x seconds
> 		---------------------------
> 		kilograms of propellant
> Since a newton is the force required to accelerate 1 kg by 1 meter/sec^2, it
> has dimensions Kg-m/sec^2. When the other factors are included, this all
> reduces to meters/second.
> This way of expressing specific impulse has a much more elegant and
> straightforward meaning: it is simply the velocity of the rocket exhaust
> relative to the rocket.

This is true enough, but note that the metric system is abused almost as
frequently as the English system.  The above English abuse is in using
pounds-mass instead of slugs.  The usual corresponding metric abuse is in
using kilograms-force.  So often we see jet and rocket engines rated in
kilograms of thrust;  or specific fuel consumption (inverse of specific
impulse) as kilograms (mass) per kilogram (force) per hour.  Phonograph
needle tracking forces are rated in grams.


			David Smith
			hplabs!dsmith
			dsmith%hp-labs@csnet-relay