rpw3@fortune.UUCP (01/20/84)
#R:sri-arpa:-1549400:fortune:10200009:000:407
fortune!rpw3 Jan 19 20:03:00 1984
For moon-based mass drivers, also see Robert Heinlein's excellent
science-fiction novel, "The Moon Is A Harsh Mistress", where they
are called "catapults" and are long linear motors driving against
thin steel shells around the payloads.
Rob Warnock
UUCP: {sri-unix,amd70,hpda,harpo,ihnp4,allegra}!fortune!rpw3
DDD: (415)595-8444
USPS: Fortune Systems Corp, 101 Twin Dolphins Drive, Redwood City, CA 94065henry@utzoo.UUCP (Henry Spencer) (02/01/84)
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