RHSMinTOrbitInjection:

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   RHS for planar equations.
   Since version 1.0
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   Form:
   xDot = RHSMinTOrbitInjection( x, t, beta, g, a )
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   Inputs
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   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

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   Outputs
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   xDot        (4,1) State derivative

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