Gyroscopes

[Urwumpe]

Well... from what i understand you shouldn't need 3.
Since you can well rotate the gyro, each gyro should be able to provide control over two axis. So you'd need two GMC for tri-axial control.
Am i wrong? Guess so...

Yes, you are wrong. There is the fun experiment for engineers, of packing a gyroscope into a suitcase, and then let a friend lift the suitcase on it's handle (so it would have to rotate by 90° by gravity).

Thanks to physics, you have a transfer of rotational momentum when you lift the case - you try to tilt the axis, but the gyro acts against this. But instead of just keeping the horizontal attitude, the gyro makes the suitcase to dance with you: It rotates in a weird way.

The reason behind are Euler's Equations (which also orbiter uses... you would have to ask martins about the details, but in the earlier versions, the rotation equations had been painfully wrong very often, because of numerical errors)

http://en.wikipedia.org/wiki/Precession

 

[tblaxland]

Well... from what i understand you shouldn't need 3.
Since you can well rotate the gyro, each gyro should be able to provide control over two axis. So you'd need two GMC for tri-axial control.
Am i wrong? Guess so...

Actually, you are correct. Because of failures in the CMG system, the ISS was actually running on two gyros in 2005:
http://www.nasawatch.com/archives/2005/03/iss_cgm2_is_off.html

One important consideration when only running two gyros is that there is a nasty control singularity (a gimbal lock) that occurs if the axis of the two gyros line up. In this case you are left with only single axis of control (that may not line up with a vessel primary axis). Trying to rotate around any other axis only results in momentum transfer between the CMGs. Theoretically, you can steer out of this singularity if your vessel has sufficient polhode motion, but it is best not to get there in the first place.

A minimum of three single axis CMGs are required for three axis control.