Question: Numeric D.E. solver help

Greetings, For a school project i am trying to calculate a trajectory of a spacecraft to mars. Using newtons equations and the numerical D.E. solver i calculated the orbits of earth and mars around the sun(which i am also using as my coordinate origin.) Then i use these results in a differential equation to find the position of a rocket launched from earth. I am shooting it in an arbitrary direction just to get started. The solver does its job and i rename the X,Y,Z coordinates of the rocket (formely called Xr,Yr,Zr) to X3,Y3,Z3. The problem is when i want a numeric answer, i.e i type in X3(400)...i get "Error, (in X3) an excessive amount of work (greater than mxstep) was done" can any one help me with this? Download
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