OptimalPayloadRatio

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   Compute the optimal payload ratios for a multi-stage rocket.

   The objective is to maximize the overall payload ratio. In other words,
   maximize the payload mass given a total takeoff mass.

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   Form:
   pR = OptimalPayloadRatio( VBO, Ve, sR )
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   Inputs
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   VBO           (1,1)    Burnout velocity (km/s)
   Ve            (1,N)    Exhaust velocity at each stage (km/s)
   sR            (1,N)    Structural ratio for each stage
                             sR(k) = mSk / (mSk + mPk)
                                where mSk = structure mass of stage k
                                      mPk = propellant mass of stage k

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   Outputs
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   pR            (1,N)    Optimal set of payload ratios.
   pRTot         (1,1)    Total (overall) payload ratio. Product of all
                          elements of pR.
   nIter         (1,1)    Number of iterations.

   See also: BurnoutVelocity, M2EP
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   References:   Weisel, "Spaceflight Dynamics", 2nd Edition, 1995. 
                    pp. 212-214.
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