Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!athena.mit.edu!pgf
From: pgf@athena.mit.edu (Peter G. Ford)
Newsgroups: sci.space
Subject: Re: Magellan Status for 07/17/89 (Forwarded)
Message-ID: <12864@bloom-beacon.MIT.EDU>
Date: 20 Jul 89 22:09:38 GMT
References: <28782@ames.arc.nasa.gov> <805@eutrc3.urc.tue.nl>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: pgf@space.mit.edu (Peter G. Ford)
Organization: MIT Center for Space Research
Lines: 50

In article <805@eutrc3.urc.tue.nl> exiphm@eutrc3.urc.tue.nl (h.munk) writes:
>In article <28782@ames.arc.nasa.gov>, yee@trident.arc.nasa.gov (Peter E. Yee) writes:
>> continues to perform twice daily momentum wheel desaturations
>                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>Forgive me my ignorance, but what is "momentum wheel desaturation",
>and why is it done ?

When mapping Venus by radar, accurate antenna pointing is very
important.  Unlike an optical imager, a synthetic aperture radar is
"pointed" electronically, by choosing a precise time and frequency
"window" within which to listen for echoes. If the antenna isn't
pointing at the correct part of the planet, you don't pick up any
useful data at all!  During each orbit of Venus, the Magellan antenna,
along with the rest of the spacecraft to which it is rigidly attached,
must slowly rotate to trace a line on the planet's surface that follows
the pre-chosen "mapping swath" for that orbit.

Unlike the earlier generation of spacecraft which used small rocket
motors for orientation, Magellan uses a set of three small gyroscopes,
called "momentum wheels", each of which spins along one of three
mutually perpendicular axes. To rotate the spacecraft about one of
these axes, it is necessary to change the rotation rate of that
particular momentum wheel IN THE OPPOSITE SENSE.  Magellan, as an
isolated body, must conserve angular momentumm, and the change in the
angular momentum of the wheel is offset by that of the rest of the
body.  A rotation about an arbitrary axis can be put together out of a
series of rotation changes of any two wheels.  Wheels are better than
rockets because they use only electric power (from the solar panels),
not precious liquid fuel.

But what, you ask, is a "momentum wheel desaturation?" Well, the wheels
are much less massive than the rest of the spacecraft (technically,
their moments of inertia are much smaller), so they must spin up to
very high rates to turn Magellan in a reasonable time. To avoid their
spinning so fast that they fly apart, there are auxiliary rockets on
Magellan that are fired from time to time to give the spacecraft a
strong twist in the opposite direction, thereby allowing the wheels to
be spun down and "desaturated".

The desaturation burns are bad news to those of us interested in the
precise location of the spacecraft--e.g. for the altimetry experiment,
or to determine the shape of the Venus gravity field--because, in
addition to a rotation, they tend to push Magellan sideways by a random
amount which must be allowed for in subsequent data processing. By a
careful combination of wheel turns, the need for desaturation burns is
minimized. Once in orbit, we hope to keep them down to less than one
per day.

Peter Ford
MIT and Magellan Project