A basic attitude instability of spacecraft with imperfect momentum wheels

On a few occasions, spacecraft with momentum wheels have exhibited an undesired, and at first unexplained, attitude wobble. The present paper suggests a possible cause and also suggests that with the present trend in momentum wheel design using magnetic wheel bearings, the problem may get worse.A momentum wheel in practice will run in bearings which comply to the loads placed on them, to a very tiny extent. The compliance will be reduced to zero by the bearing reaction forces precessing the wheel back to its undeflected position. However, in doing so the combination of the very small compliance angle with the angular velocity of the spacecraft will cause an energy flow which will enhance the nutation of the spacecraft which caused the deflection in the first place.