XPost: sci.physics   
   From: henry@spsystems.net   
      
   In article <2a0cceff.0401110504.6fbd2b58@posting.google.com>,   
   Edward Green  wrote:   
   >For the benefit of groundlings, would you mind explicating exactly   
   >what "station keeping" and delta v means in this context?   
      
   Station keeping (or stationkeeping) is maintaining a particular orbit   
   despite perturbations which tend to mess it up.  For example,   
   geostationary comsats burn fuel regularly to maintain orbital positions   
   despite disturbances from (mostly) the Moon and the Sun.   
      
   Delta-v is velocity change -- it's what you get from a rocket burn.  The   
   propulsion requirements for most kinds of space maneuvers, including   
   stationkeeping, are conveniently expressed as how much velocity change   
   they require.  Given a particular spacecraft mass and rocket technology,   
   this translates directly into fuel requirements.  For example, a comsat   
   which must not drift more than half a degree north or south from GSO will   
   need a rocket burn of about 50m/s about once a year, and with rockets   
   burning hydrazine as a monopropellant, that will require about 25kg of   
   hydrazine per ton of spacecraft.   
      
   >"The time required for the amplitude of an oscillation to increase or   
   >decrease by a factor of e."   
   >Hmm...  further search reveals that this is a much-beloved term of art   
   >in orbital mechanics, but doesn't immediately clarify.   
      
   Many of the perturbations of interest in some stationkeeping problems show   
   up as gradually increasing oscillations around the desired position.  Some   
   modest amplitude will typically be tolerable, and clearly the more quickly   
   the oscillations grow, the more often maneuvers will be needed to keep   
   them from growing too far.  But there are other variables in the problem,   
   I fear; *how big* those corrections have to be is a separate issue.   
   --   
   MOST launched 30 June; science observations running     |   Henry Spencer   
   since Oct; first surprises seen; papers pending.        | henry@spsystems.net   
      
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