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   Design a electric powered transfer vehicle.
   Uses the payload mass, the required delta-V, the required
   thrust, the ratio of structure to fuel mass (for tanks, etc.) and
   the power per unit mass.
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   Form:
   x = ElectricPropulsion( x )
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   Inputs
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   x         (1,1)    System data structure
                      .mPayload  (1,1)    Mass of the payload
                      .dV        (1,1)    Total delta-V capacity (km/sec)
                      .thrust    (1,1)    Total vehicle thrust (Nm)
                      .sR        (1,1)    Ratio of structure to fuel mass
                      .pR        (1,1)    Power per unit mass (W/kg)
                      .

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   Outputs
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   x         (1,1)    Data structure

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