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   Designs a blowdown system. 
   If gas constant is omitted, Helium will be assumed.
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   Form:
   [v, mP] = BDDesign( mF, rhoF, tNom, pI, pF, rP )
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   Inputs
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   mF            (1,:)    Mass of fuel (kg)
   rhoF          (1,1)    Density of fuel (kg/m^3)
   tNom          (1,1)    Nominal temperature (K)
   pI            (1,1)    Initial pressure (N/m^2)
   pF            (1,1)    Final pressure (N/m^2)
   rP            (1,1)    Gas constant of pressurant

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   Outputs
   -------
   v             (1,:)    Volume (m^3)
   mP            (1,:)    Required mass of pressurant (kg)

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