HeliopauseSailAngle:

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   Generates a constant angle to get a velocity reversal trajectory.

   Since version 7.
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   Form:
   angle = HeliopauseSailAngle( x, t )
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   ------
   Inputs
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   x          (:,:)  States
   t          (1,:)  TIme

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   Outputs
   -------
   angle      (1,:)  Sail in plane angle (rad)  

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   Reference: http://en.wikipedia.org/wiki/Fast_solar_sailing
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