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   Design an electric transfer vehicle.

   Type TVDesign for a demo.
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   Form:
   x = TVDesign( mPayload, dV, thrust, eff, sR, pR )
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   Inputs
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   mPayload  (1,1)    Mass of the payload
   dV        (1,1)    Total delta-V capacity (km/sec)
   thrust    (1,1)    Total vehicle thrust (N)
   eff       (1,1)    Efficiency
   sR        (1,1)    Structural Ratio
   pR        (1,1)    Power ratio