Engine
An engine is a part to accelerate a craft. There are three different type of engines available which have different properties.
Reaction engine
- → Main article: Reaction engine
A reaction engine (also rocket engine) simply pushes mass (usually called fuel) out of a nozzle. Because of Newton's third law it propels the craft with the same force it pushes the mass out. There are four different type of rocket engines which only differ in the kind of fuel they use:
Those engines work in vacuum and weightlessness. They don't have a specific speed limit except for the speed of light, which isn't implemented in KSP.
Jet engine
- → Main article: Jet engine
A jet engine intakes air, mixes it with liquid fuel, and after combustion pushes exhaust gas out of a nozzle. They could work in weightlessness but need an oxygenated atmosphere to work, which is impossible to recreate in KSP. They are more efficient than rocket engines as the oxygen for combusting their fuel is gathered in-flight from outside air, not oxidizer carried onboard adding to total mass. These engines use only liquid fuel.
Rover wheels
A completely different type of engine are rover wheels. They can accelerate a craft in vacuum but they need friction with an surface to move along this surface, so they won't work in weightlessness but with artificial gravitation they can drive on the inner side of a centrifuge in weightlessness. Usually they are very slow compared to the fuel consuming engines but run only on electricity so that they can run forever as long as there is a generator on board.
