An engine is a part to accelerate a craft. There are three different type of engines available which have different properties.
- → Main article: Reaction engine
A reaction engine is based on Newton's third law: “for every action there is an equal and oppposite reaction”. Reaction engines expel reaction mass outward along the exact opposite vector as the craft is accelerating along. This force can generally be called thrust.
To be more intuitive to KSP, this wiki considers reaction engines only to be those where thrust comes directly from expending fuel. There are five major types of reaction engine, one of which is technically a sub-type of another:
- Jet engines, which expend liquid fuel and intake air
- Liquid fuel rocket engines, which expend liquid fuel and oxidizer
- or Monopropellant engines, which expend monopropellant
- Solid fuel rocket engines, which expend solid fuel
- Ion engines, which expend xenon gas utilizing electric charge
Of note, jet engines can only work inside of an oxygen-rich atmosphere, as intake air cannot be stored in any practical way. The rest are considered rocket engines as they can function in vacuum using only resources stored aboard the craft.
- → 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.
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.