From: henry@spsystems.net   
      
   In article <1117152984.609403.15160@z14g2000cwz.googlegroups.com>,   
   Mark Adler  wrote:   
   >By "desired velocity after escaping", I assume you mean v-infinity...   
      
   Correct.   
      
   >...Departing directly from Deimos is about 1.8 km/s.  Departing   
   >from a 200 km altitude periapsis (with apoapsis at Deimos), plus the   
   >periapsis reduction maneuver to get there, is about 1.6 km/s.   
      
   That sounds plausible.  A modest gain, but a useful one.   
      
   >Of course, this all applies in the reverse direction, as a good way to   
   >rendezvous with Deimos in the first place, if you're coming from Earth.   
      
   In that direction, you can improve it a bit further with aerobraking:  do   
   your arrival burn at the lowest altitude you think safe, and just barely   
   achieve capture, entering an elliptical orbit with very low perigee(*) and   
   very high apogee, and then aerobrake at perigee to bring the apogee down.   
      
   (* The pedantic may substitute "periapsis" etc. if they wish. )   
      
   >So is there a three-maneuver strategy that's better still?   
      
   For departure, yes.  Boost from Deimos into an elliptical orbit with   
   perigee at Deimos orbit and very high apogee.  At apogee, do a small burn   
   to lower perigee down to the edge of the atmosphere.  Do the final   
   departure burn at perigee.   
      
   You still don't see a net gain unless Vinfinity is significant, but the   
   Vinfinity threshold is now lower.  (For an extremely high apogee, it can   
   approach zero.)   
      
   As a bonus, if your desired departure is not in the plane of Deimos's   
   orbit -- which in general, it won't be -- you can do quite large plane   
   changes quite cheaply as part of the apogee burn.   
      
   One downside is that you need careful navigation for the apogee burn -- a   
   slight overburn and you do an MCO -- although errors can be corrected   
   quite cheaply if you catch them early.  One possibly-useful tactic is to   
   do two apogee burns:  do *most* of the apogee burn at the first apogee, do   
   one full orbit around Mars to measure the exact achieved perigee, and then   
   do a second apogee burn to finish the job.  If you're feeling paranoid but   
   patient, you can even do the perigee lowering in several steps.   
      
   The reverse works for arrival too, although there is no opportunity for   
   aerobraking with this strategy.  You'd have to run the numbers to decide   
   whether you're better off with two burns and aerobraking, or three with   
   no aerobraking.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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