AI and FM are the real killers, particularly combined. TO move 3D objects, even very detailed 3D objects, in 3D space is not that tough - FPS's do it all the time. To make the behave physically realistically particularly with bullets that need to behave physically realistically and planes that need to take damage and then fly physically realistically for their damage - now that's quite tough on a CPU. And then to make an AI that can fly and fight intelligently taking into account the accurate behaviour of his plane - now that's very tough on a CPU!