ThrustAngleGravity:

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   Gravity turn thrust angle (0 degrees) for 2D simulations.
   Since version 2015.1
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   Form:
   angle = ThrustAngleGravity( x, d, params )
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   ------
   Inputs
   ------
   x        (4+n,1)   State
   d           (.)    Data structure, unused
   params      (.)    Parameters
                        None.
   -------
   Outputs
   -------
   angle    (1,1)    Commanded thrust angle (zero)

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