XPost: sci.space.shuttle   
   From: henry@spsystems.net   
      
   In article <40151B68.6BD94019@mediasec.de>,   
   Jan C. =?iso-8859-1?Q?Vorbr=FCggen?=  wrote:   
   >> ...The gyrodynes (I think) that actually produce   
   >> torque to rotate Hubble have been reliable.   
   >   
   >I thought HST is using magnetic torque bars for manoeuvering?   
      
   Magnetorquers are difficult to use all by themselves for maneuvering,   
   because they can generate torque around only some axes.  If I've   
   visualized this right, the axis of the torque has to be perpendicular to   
   the current local direction of Earth's magnetic field -- the torquers   
   cannot generate rotation around the field axis.   
      
   What they *are* useful for is to solve a secondary problem.  Any of the   
   systems using wheels basically just stores angular momentum in the wheels.   
   This is fine if disturbances or maneuvering motions sum to zero in the   
   long run.  But if there is anything that constantly adds angular momentum   
   on some axis, and never takes it away again, you're in trouble.  With   
   reaction wheels (the simplest case), the wheels have to spin faster and   
   faster to take up the added momentum; there are analogous but more subtle   
   bad behaviors in the other approaches.  And torques are never perfectly   
   cyclic:  there's always some imbalance somewhere that causes such momentum   
   buildup eventually.   
      
   So any wheel system needs, in addition to the wheels, some way of   
   "dumping" momentum -- a way to exchange angular momentum with the outside   
   world, so it doesn't build up forever.  If you're in LEO, or elsewhere   
   with a substantial external magnetic field, magnetorquers are a very good   
   choice for that.  They use nothing but electric power, and you can usually   
   get at least a small torque around the desired axis *eventually* -- the   
   wheels will store angular momentum temporarily, so there is no hurry -- by   
   just waiting for the right point in your orbit.  The major alternative is   
   thrusters, which give results without waiting, but use up fuel and tend to   
   pollute the neighborhood.   
      
   MOST, the spacecraft alluded to in my signature, has reaction wheels and   
   magnetorquers.  With minor variations on the exact wheel type, this is a   
   fairly standard approach for LEO spacecraft.   
   --   
   MOST launched 30 June; science observations running     |   Henry Spencer   
   since Oct; first surprises seen; papers pending.        | henry@spsystems.net   
      
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