I think Leigh and I are saying the same thing and we are just using
different words to say it. Leigh says there is a gyrocompass fixed to the
stars, plus there are "reaction wheels" acting as "reservoirs" for angular
momentum.
The different thing I said was to label the "reaction wheels" as another set
of gyros. I described two sets of "gyros," one gimbaled for maintaining a
position reference, and another larger set not gimbaled for use as an
angular mometum reservoir.
Leigh's wording is much better than mine because gyro (implying gyrocompass)
would imply gimbal-mounted rotors. Therefore my use of gyro to describe the
rotors that act as "angular momentum reservoirs" was a poor choice. Now
that Leigh has used the word "reaction wheels" I remember this wording does
appear in some descriptions I have read of the Hubble Telescope. The
following quotation is taken from a web page at www.stsci.edu
<begin quote>
HST utilizes electrically driven reaction wheels to perform all maneuvering
required for guide-star acquisition and pointing control. A separate set of
rate gyroscopes is used to provide attitude information to the pointing
control system. The servicing mission restored or replaced three gyros that
had failed since the original launch, so that the spacecraft currently has a
total of six operational gyros. Any three of these are the minimum required
for telescope pointing control.
<end quote>
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817
Leigh Palmer wrote:
I've never done so either, but I've heard a little about this matter
from one of my kids. The gyros on spacecraft are simply reference
devices for gross pointing adjustments. Star sensors are used for
the final references. One does not turn the spacecraft *against* the
gyromechanical moment of a gyroscope, one typically uses "reaction
wheels" which act as reservoirs for angular momentum which is built
up unintentionally by the interaction of the spacecraft with the
vagaries of space - atmospheric drag, electromagnetic drag, etc. The
reaction wheels are eventually despun by means of vernier rockets or
else (in the case of Earth orbiting satellites) by torquing against
Earth's magnetic field. I don't know how commonly they are used
(they may only be in design stages now), but ring lasers can replace
mechanical gyroscopes in these applications.