The FLSpacecraft application uses physical laws (classical mechanics) to simulate a spacecraft entering and interacting with the Earth-Moon system (or Jupiter or Saturn systems). The default spacecraft is roughly the size and mass of the Apollo Command and Service modules combined, with the ability to fire the Service Modules main engine in short bursts to accelerate and decelerate along the direction of travel.
The purpose is to demonstrate how a spacecraft behaves in a gravitational field, interacting with planets and moons. It is not as simple as you might think and results may be unexpected. There is a game mode for the Earth system to get you going, but after that you might want to experiment on your own.
Starting with version 1.1.0 you have your choice of orbital system (Earth, Jupiter, or Saturn) and of spacecraft size (Apollo, asteroid-sized, or planet-sized). These choices create more complicated and unpredictable orbital interactions. The effects of the asteroid size are subtle, but the effects of the planet size can be dramatic.
Note that the simulation takes a lot of computing power, however, since the app is free you can just download it and see how it goes on your Mac. Chances are you will be fine, however if it bogs down you can try moving the speed slider at the top of the window to the "Faster" setting (which will reduce the number of recalculations for a given orbit, albeit with some loss of accuracy).
