The simulator fudges a bit with the rockets. A rocket is pulled on by all the other planets and moves accordingly. And when you fire a thruster, it affects the rocket's velocity in a reasonable fashion. But there is no sense that fuel itself has mass or that the ship gets lighter as the fuel is consumed. The simulator also ignores how hard a ship hits a planet when it lands.
Every landing is a safe landing.
For now, the rockets are just for fun. Customization Matters of Gravity is a safe applet that cannot access your local harddrive. However, it is still possible to customize Gravity for your own use. You will have to create your own Web page, and include the following applet tag that loads the code from my site, but which uses the system that is defined by your parameters. By the way, if you are really ambitious, you can rumage around in the temporary files created by your browser and find the 'Gravity.jar' file.