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   RHS for planar equations.
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   Form:
   xDot = RHSMinTOrbitInjection( x, t, beta, g, a )
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   Inputs
   ------
   x           (4,1)[x;y;vX;vY]
   beta        (1,1) Thrust vector angle from horizontal (rad)
   g           (1,1) Graviational acceleration.
   a           (1,1) Acceleration

   -------
   Outputs
   -------
   xDot        (4,1) State derivative

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