Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!bobmon
From: bobmon@iuvax.cs.indiana.edu (RAMontante)
Newsgroups: sci.space
Subject: Re: Magellan Status for 07/17/89 (Forwarded)
Message-ID: <23767@iuvax.cs.indiana.edu>
Date: 24 Jul 89 22:35:19 GMT
References: <28782@ames.arc.nasa.gov> <805@eutrc3.urc.tue.nl> <12864@bloom-beacon.MIT.EDU> <14480@bfmny0.UUCP>
Reply-To: bobmon@iuvax.cs.indiana.edu (RAMontante)
Organization: malkaryotic
Lines: 38

<12864@bloom-beacon.MIT.EDU> pgf@space.mit.edu (Peter G. Ford) :
->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".

tneff@bfmny0.UUCP (Tom Neff) <14480@bfmny0.UUCP> :
-
-Jeez I hate to disagree with someone on the Project, but wouldn't the
-thruster burns be twisting the spacecraft in the SAME direction as the
-momentum wheels' accumulated spin, rather than in the OPPOSITE
-direction?  If you add more opposite torque you'd have to spin the
-wheels even faster to compensate.  By tweaking in the same direction,
-you allow[require] spindown of the wheels to compensate.

No, think of it this way --- You can stop the spacecraft's spin by
transferring it's angular momentum to the momentum wheel(s).  If the
craft is spinning in the same direction as the wheel, then its angular
momentum has the same sign, so transferring the ang. momentum to the
wheel increases the wheel's spin. 

So use the thrusters to give the craft a spin OPPOSITE to the wheel's
spin.  Then transfer this spin (ang. momentum) to the wheels,
incidentally causing the craft to stop spinning again.  The momentum
has the opposite sign to that of the wheel, so the transfer decreases
the net momentum, and hence the spin, of the momentum wheel.


The "opposite torque <=> spin wheels even faster to compensate" argument
would be true IF the torque came from the wheels, but it's coming from
an outside source.  The outside source, thrusters, apply a counter
torque, which just happens to be transmitted through the spacecraft
itself as an intermediary.

Yet another view:  imagine "hitting the brakes" on the momentum wheels.
This will completely stop the wheels, *relative to the spacecraft*, and
the craft will begin spinning in the same directions that the wheels
were (albeit slower).  Now fire up the thrusters to kill this spin.