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   Function to be minimized by OptimalVehicle.m
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   Form:
   y = FMinUE( uE, x )
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   ------
   Inputs
   ------
   uE           (1,1) Exhaust velocity
   x            (:)   Data structure
                           .dV     Total delta V capability (m/sec)
                           .uE     Exhaust velocity (m/sec)
                           .pR     Power to propulsion system mass ratio (w/kg)
                           .sR     Structural mass to fuel ratio
                           .mP     Payload mass (kg)
                           .thrust Thrust (N)
                           .eff    Efficiency
                       

   -------
   Outputs
   -------
   y            (:)   Output

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