Well this is gonna be a hard one. In Kerbal Space Program, Physics is the name of the game. It is the most true to life space exploration simulation that is readily available to the public, but it still insanely fun to play. You control a whole space program, and decide everything that it does. You control the mission plans, creation of spacecraft, and the actual flight and maneuvers. The Physics engine in this game models real life down to the smallest details: drag, propulsion, center of mass, center of lift, this game has everything. What would be harder to write about would be what isn't realistic about this game. One of the aspects I would like to focus on however are orbits. One of the first milestones in the game is getting a spacecraft into a stable orbit around Kerbin. You have to plan your fuel consumption and staging so that you have enough fuel to adjust your orbit into a circle once you reach the correct height. This can be done by scheduled "burns" or extended periods of time where the thrusters are fully firing towards a certain direction to cause a desired orbit path. Once you finally get into orbit however, nothing is more satisfying considering you used the laws of Physics to accomplish it.