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Re: Changing AO-40's attitude



Doug Braun NA1DB wrote:

> 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!

True (I suppose) about the gyroscopic effect, but at least the lack
of spacecraft rotation would have the benefit of stopping the
repetitive interruption of telemetry.

However, Richard D. Burgan (WC8J) wrote:

> As I understand it the spin rate MUST BE ZERO before the momentum
> wheels can be started.  I assume this is because that is how the
> controller is designed.  Turning the wheels on full blast while
> the spacecraft is spinning at 17.7 RPM would probably destroy
> AO-40.  I also doubt that the wheels could "absorb" a rotation
> rate of 17.7 RPM.

Oh well, Murphy wins again!  :-P

I guess it should be possible to compute if the wheels could or
couldn't absorb the 17.7 RPM rotation rate in theory (if it
wouldn't be destructive, and if it wouldn't be prevented by the
controller hardware, and if it wouldn't be prevented by prudence).

My Physics is a bit rusty, but I think that the wheel would need
to spin at a rate of:

       (Mass of AO-40) - (Mass of that reaction wheel)
17.7 x -----------------------------------------------
               (Mass of that reaction wheel)

The various masses are known, and the maximum allowable spin rate
of the wheels has also been set.  I don't have those figures at
this location, however.

(Conservation of angular momentum, unless I've omitted something
else needed by the calculation such as the diameter of the spinning
masses in question, or some such thing.)

And, of course, Doug also wrote:

> 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.

Well, SOMEDAY the wheels will be turned on for the first time, so
the experiment will (hopefully!) eventually occur.  But I agree with
the sentiment of moving cautiously, concentrating on approaches that
are relatively well-understood.  We'd certainly hate to toss our
sick baby into an uncontrollable spin that prevented us from sending
ANY control commands back up to correct things!

Oh well, I can relate to the frustration felt by the command team.
I've been tasked to repair a local repeater when the temperature was
20 degrees below zero Fahrenheit (-28.9 C) and the snowy and icy path
up the hill to the repeater site hadn't been plowed.  How much worse
it is to have your patient 60,000 Km away, where you have to diagnose
and treat him by something akin to mental telepathy (ok, radio waves)!

73 de KB0ZEV
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