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   Finds the optimal vehicle given various parameters. The routine takes
   the power to mass ratio and minimizes the mass for each value.
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   Form:
   x = OptimalVehicle( x )
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   Inputs
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   x			(:)		Data structure
                       x.dV     Total delta V capability (m/sec)
                       x.uE     Exhaust velocity (m/sec)
                       x.pR     Power to propulsion system mass ratio (w/kg)
                       x.sR     Structural mass to fuel ratio
                       x.mP     Payload mass (kg)
                       x.eff    Conversion efficiency
                       x.thrust Thrust (N)

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   Outputs
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   x             (:)   Data structure additions
                       x.mT     Total mass
                       x.mS     Structural mass
                       x.mR     Propulsion system mass
                       x.mF     Fuel mass
                       x.power  Total power (w)

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