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Re: Changing AO-40's attitude

I can think of two problems:

1: The momentum wheels are experimental and the controllers have
no real experience using them.   On the other hand, they have years of
experience magnetorquing AO-10 and AO-13, and AO-40.  So, if at the moment it
is too difficult to do magneqorquing, trying something even trickier would be
even less likely to work.

2: The spacecraft is spinning relatively fast, and has a lot of angular
momentum.  This makes it hard to change its attitude, since it is acting
like a gyroscope.  You really have to de-spin it before the momentum
wheels could be effective.  And you can't de-spin it with the momentum
wheels!  There is a Catch-22: If you despun it with a momentum wheel,
the spacecraft would no longer be spinning, but the momentum wheel would be,
and it would have the same gyroscopic effect that the spinning spacecraft
originally had!

To really understand all this, you have to appreciate the concept
of conservation of angular momentum.  This is a very powerful and
general physical law.  You can think of the conservation of
linear momentum (what they teach in science class) as merely a
special case of conservation of angular momentum.


At 10:15 AM 2/1/01 , John P. Toscano wrote:

>This implies to me that it could be possible to re-orient it in any of
>the 3 axes, by any amount, in one orbit or less, or even to change the
>current rotation speed, by judicious use of the momentum wheels.  Given
>the current (undesirable) spacecraft attitude, and given the fact that
>magnetorquing is slow now because of the unfavorable attitude, this
>might speed commissioning efforts.  Even if we were not in a position
>to place the spacecraft into full 3D-stabilized mode yet, which
>requires the momentum wheels to run continuously, plus the use of the
>magnetorquers to "dump momentum" when the wheel RPM's get close to
>their upper limit.

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