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   Generates a blowdown curve of mass vs. pressure.

   Type BloDownMass for a demo.
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   Form:
   mFuel = BloDownMass( mPress, rhoFuel, vTank, rPress, T, p )
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   Inputs
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   mPress      (1,1)    Mass of pressurant
   rhofuel     (1,1)    Density of the fuel
   vTank       (1,1)    Tank volume
   rPress      (1,1)    Gas constant of pressurant (default is helium)
   T           (1,1)    Temperature (deg-K)
   p           (1,:)    Pressure 

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   Outputs
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   mFuel        (1,:)   Mass of the fuel

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