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Limitations of NDSolve

I have tried using Mathematica to numerically solve the equations of
motion for a vehicle in flight about a round Earth.  The flight is in
two dimensions and the equations consist of a set of five nonlinear
ordinary differential equations.  I have tried the same thing without
problems using computer programs and Excel.  Even the Runge-Kutta
package supplied with Mathematica does fine.  Unfortunately, NDSolve
will not work, instead reporting an infinite expression.  The initial
conditions normally provide a small, nonzero value for the velocity
to eliminate the problem of velocity in the denominator in the fourth
equation.  This has been something I've tried off and on a couple of
times in the past, and in talking with others I have found that NDSolve
has had trouble with other equations when those equations have not 
presented problems to other means of computing numerical solutions,
like writing a program in Fortran or C.  What sort of limitations are
there on NDSolve with regard to the problems it can handle?  The system
of equations I use are included below, along with initial conditions and
the parameters necessary for solution.

  th'[t]==-(v[t] Cos[gam[t]]/r[t]),
  r'[t]==v[t] Sin[gam[t]],
  v'[t]==(-drag+thrust Cos[al+eps]-g m[t]*
  gam'[t]==(lift-m[t] g Cos[gam[t]]+ m[t]*
	v[t]^2 Cos[gam[t]]/r[t]+thrust Sin[al+eps])/
  m'[t]==-thrust/(Isp g)

Initial Conditions:

Other important functions:
  lift=0;      (* This is a rocket, so there is little lift - neglect it now *)
  g=9.81;      (* surface gravitational acceleration *)
  m0=10000;    (* Initial mass *)
  r0=6376000;  (* Radius of the Earth *)
  area=10;     (* Rocket frontal area *)
  cd=.1;       (* Drag coefficient *)
  Isp=400;     (* Engine specific impulse *)
  eps=0;       (* Engine gimbal angle, try 0 for now to see if this works *)
  al=0;        (* Angle of attack, set to zero to see if this works *)
  rho=.00276;  (* Air density, try a constant for now *)

David Garza
dgarza at

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