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Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!petrus!karn
From: karn@petrus.UUCP
Newsgroups: net.space
Subject: Re: Funny units in SF book
Message-ID: <312@petrus.UUCP>
Date: Fri, 22-Mar-85 01:44:21 EST
Article-I.D.: petrus.312
Posted: Fri Mar 22 01:44:21 1985
Date-Received: Sat, 23-Mar-85 01:09:00 EST
References: <1083@mordor.UUCP>
Organization: Bell Communications Research, Inc
Lines: 28

The "true" units of specific impulse is force-time/mass, ie, how much
thrust can it produce for how long for a given amount of propellant. 
In metric units, this is simply newton-seconds/kg, which simplifies to
meters/sec (the exhaust velocity). However, in archaic English units
this is written as poundsforce-seconds/poundsmass, and the dissimilar
pounds units are incorrectly canceled out, leaving so-called "seconds".
Because a pound of mass corresponds to a pound of force only in a 1G
acceleration field, the earth's surface gravitational acceleration is
factored into this so-called "seconds" figure. To convert to exhaust
velocity, just multiply by 9.8 m/sec^2 or 32 ft/sec^2.

The number you really want to use is exhaust velocity, in the classic rocket
equation

delta-v = Ve * ln(mass loaded/mass dry)

I agree, metric units are the only way to go. I always feel a twinge of
embarassment every time I hear the NASA commentator use "knots",
"feet per second" and "statute miles" in the same breath during a shuttle
launch. The press kits on the Shuttle are even worse. Every measure is given
in about three or four sets of units, and many of the conversions are wrong.
You don't know which to believe unless there are enough versions to allow
majority voting.

ESA may use three languages, but at least they have their measurement
act together.

Phil