FLowThrust2D:

--------------------------------------------------------------------------
   This function is for the planar orbit problem which includes
   radial position, radial velocity and tangential velocity only.
   This function is specifically for the minimum time problem. It is designed
   to be called by ode113. 

   Since version 7.
--------------------------------------------------------------------------
   Form:
   varargout = FLowThrust2D( t, x, flag, d )
--------------------------------------------------------------------------

   ------
   Inputs
   ------
   t          (1,1)  Time
   x          (8,1)  States [r;u;v;m;lambdaR;lambdaU;lambdaV;v/r]
   flag       (1,1)  Flag from ode113
   d          (1,1)  Structure to pass to the integration RHS

   -------
   Outputs
   -------
   xDot       (7,1)  State derivatives d[r;u;v;m;lambdaR;lambdaU;lambdaV;v/r]/dt

   or

   vnew       (1,1)  Value of constraint
   isterminal (1,1)  1 if a terminal constraint  
   direction  (1,1)  Direction  

--------------------------------------------------------------------------