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From: METH%USC-ISI@sri-unix.UUCP
Newsgroups: net.space
Subject: Fuses and Reaction Wheels
Message-ID: <12307@sri-arpa.UUCP>
Date: Mon, 16-Apr-84 08:35:12 EST
Article-I.D.: sri-arpa.12307
Posted: Mon Apr 16 08:35:12 1984
Date-Received: Sat, 21-Apr-84 01:31:22 EST
Lines: 30

The resaon you use fuses over circuit breakers is
just because fuses are one-shot deals.  If
your circuit is faulty you don't wan't it reset
inadvertantly by some glitch command.  The reason
you fuse circuits is as was stated in another
message, you'd prefer to permanently take out a
single circuit rather that fail the entire
satellite.

I heard an (unconfirmed) story that Solar Max's fuses
blew because the gas contained in them diffused into
space resulting in a lower current rating than
the (well behaved) circuit they were in.

Regarding reaction wheels, Space Telescope will
do its maneuvering using reaction wheels and
magnetic torquers.  The latter will permit the 
reaction wheels to operate at low average rotation
rate (there are four of them so the torquers can
despin the wheels with no net angular momentum change)
to reduce power consumption and jitter.  The reason
one does not want to use reaction jets of any kind in
such a vehicle, is beacuse the effluent produces
molecular contamination which could deposit on the
telescope mirror, severely degrading performance.

-Sheldon Meth
The BDM Coorporation
(METH@ISI.ARPA)
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