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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.space
Subject: Re: Terraforming vs. Space Stations --> - (nf)
Message-ID: <3501@utzoo.UUCP>
Date: Tue, 31-Jan-84 17:56:20 EST
Article-I.D.: utzoo.3501
Posted: Tue Jan 31 17:56:20 1984
Date-Received: Tue, 31-Jan-84 17:56:20 EST
References: <2319@fortune.UUCP>
Organization: U of Toronto Zoology
Lines: 17

Heinlein's catapults in "The Moon Is A Harsh Mistress" are properly
and correctly called "catapults", not "mass-drivers".  The distinction
is that a mass-driver uses recirculating buckets, so that the moving
part of the magnetic system is not ejected along with the payload.  This
was O'Neill's key invention, which nobody had thought of before.

Heinlein's catapults are probably unworkable, in fact.  The problem is
that they are linear induction motors.  An induction motor is about the
best you can do if the moving part of the magnetic system can't include
things like coils, but induction motors do not scale well to large sizes
and high accelerations.  Nobody is seriously considering induction motors
for space propulsion nowadays.  O'Neill mass-drivers are linear synchronous
motors, which work *much* better but do require the moving part to have
a strong magnetic field that they can push/pull against.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry