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   Computes the orbit radius.
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   Form:
   r = ROrb( e, rp, f )
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   ------
   Inputs
   ------
   e                 Eccentricity
   rp                Perigee radius
   f                 True anomaly

   -------
   Outputs
   -------
   r	                Radius

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	References: Bate, R. R., Fundamentals of Astrodynamics. pp. 20-40.
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