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   Compute the fuel mass needed to reach a velocity given mass properties and ISP.   

   Type SS2O for a demo.

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   Form:
   [mF, mT, mS, e] = SS2O( Isp, mP, fS, dV )
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   Inputs
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   Isp          (1,1) Specific impulse    (sec)
   mP           (1,1) Payload mass
   fS           (1,1) Structural fraction
   dV           (1,:) Required delta v    (km/sec)

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   Outputs
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   mF          (1,:) Fuel mass
   mT          (1,:) Total mass				    
   mS          (1,:) Structural mass				    
   e           (1,:) Fraction of the launcher that is the payload

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