# Re: Superbird A secondary flashes

rkresken@esoc.esa.de
Wed, 24 Jun 1998 09:37:07 +0100

```> Energy dissipation will make any object that spins in an uncontrolled way
> spin about the body axis with the highest moment of inertia. In the case
of
> Superbird A, this axis must be perpendicular to the solar panels.

>>What kind of 'energy dissipation' process are you thinking of? I've
always
>>wanted to find a good text on what the nature of these is. I've never
>>found an article/book that explains these, except for assuming that
>>they are there.

Every process that turns kinetic energy into heat causes "energy
dissipation",
like the internal friction that causes solar panel jitter damping. A decent
mathematical description of flat spin and nutation damping can be found in
"Spacecraft Attitude Determination and Control" by James R. Wertz.

>To go farther, I also wonder if the opposite effect is occurring:
Superbird
>will go from a flat spin to a wobble.  Those big solar panels experience a
>lot of solar radiation pressure, and if the pressure is not applied
>precisely through the satellite's center of mass then nutation would be
>induced.  I believe Rob Matson has reported that Superbird's axis does
>drift.

Flat spin only has an influence on the orientation of the spin vector in
the
reference frame of the spacecraft. Every external torque on a body in flat
spin
will cause a change of the angular momentum vector in inertial space and a
certain
nutation which will damp out eventually, leaving the body in flat spin
again.
So flat spin and spin axis motion are no contradiction.

I am not sure about the effect of eddy currents in upper stages, but one
effect
the could cause energy dissipation here would be sloshing of remaining
fuel.
The torques caused by eddy currents might be "external" magnetic torques,
but I
have to think about this a bit.

Rainer Kresken
```