I found your applet very usefull and clear. I am an astronautical engineer and I am coping for the first time with Java. I used to program in MATLAB/SIMULINK environment, therefore the point of view is quite different because of the Java Object Orientation. I would like to ask to the author of the applet some additional detail about the integration of the equations of motion:

1. What kind of numerical integrator did you use (RK4, Euler,...)?
2. Is it possible to integrate non linear ODE of the second order (i.e. the equations of motion for the restricted three body problem) or it is necessary to transform every equation of the second order in two equations of the first order?
3.I went through the source code of the applet, but it is quite confusing for a new Java user; is it possible to point out the section of the code where the integration is performed?

I do apologize if these questions may seem elementary, but the approach needed to program in Java is completely different from that one needed in MATLAB/SIMULINK, where the integrators are black boxes.

Many thanks in advance for any help.

Best regards,