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   Determines performance of a cold gas rocket.
   Thrust and mDot are functions of (p0,e). All inputs are mks.
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   Form:
   [thrust, iSP, rho, mDot] = ColdGas( s, aStar, p0, pA, t0, e )
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   Inputs
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   s             (1,:) Gas to be used.
   aStar         (1,1) Throat area (m^2)
   p0            (n)   Chamber pressure after heating (N/m^2)
   pA            (1,1) Ambient pressure (N/m^2)
   t0            (1,1) Temperature after heating (deg-K)
   e             (m)   Area ratio (exit area/throat area)

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   Outputs
   -------
   thrust       (n,m)  Thrust level (N)
   iSP          (m)    Specific impulse (sec)
   rho          (n)    Gas density (kg/m^3)
   mDot         (n,m)  Mass flow (kg/sec)

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