XPost: sci.space.shuttle   
   From: henry@spsystems.net   
      
   In article ,   
   Gordon D. Pusch  wrote:   
   >>> The problem is that Hubble only has, what, 3 of it's   
   >>> original 6 gyroscopes still working...   
   >   
   >In this particular context, "gyro" actually means "reaction wheel."   
      
   No, actually, "gyro" means "gyro" here.  It's the high-precision sensors   
   that are giving trouble.  The gyrodynes (I think) that actually produce   
   torque to rotate Hubble have been reliable.   
      
   (Reaction wheels, momentum wheels, and gyrodynes aka control moment gyros,   
   are three different things, despite some similarities, although the   
   terminology is not entirely standardized.  Reaction wheels are wheels   
   whose initial speed is nominally roughly zero, and whose speed is varied   
   to exert torque.  Momentum wheels nominally run at constant speed and   
   their purpose is to provide gyroscopic stiffness around the other two   
   axes, although often some degree of reaction-wheel functionality is added,   
   with some variation in speed permitted for exerting small torques around   
   the wheel axis.  Gyrodynes run at constant speed, and unlike the other   
   two, are mounted in gimbals; torques are produced by rotating the gimbals,   
   pushing against the wheels' gyroscopic stiffness, roughly speaking.  If   
   memory serves, Hubble uses gyrodynes, although they are sometimes called   
   reaction wheels.)   
   --   
   MOST launched 30 June; science observations running     |   Henry Spencer   
   since Oct; first surprises seen; papers pending.        | henry@spsystems.net   
      
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