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.
A completely different type of engine is powered wheels (not including landing gear). These are used on rovers. Motors generate torque on the wheels, and if the wheels are in contact with a surface providing friction (as all solid objects in KSP do), they can accelerate a craft. They can easily accelerate away from surfaces unless a force such as gravity or centrifugal force keeps them in contact with it. Usually they are very slow compared to aerospace engines but powered wheels require only electric charge and can thus run indefinitely without refueling by another craft.
In real-life, the surface provides a reaction mass and powered wheels could technically be considered reaction engines. However, to be more intuitive for playing KSP, this wiki requires the reaction mass to be expelled from the engine, not merely be adjacent to it.
Though far from a standard propulsion technique, as powered wheels are more easily steered and less likely to be break at higher speeds, it is entirely possible to propel a terrestrial vehicle using the torque provided by SAS and reaction wheel modules. However, this torque spins the entire craft rather than just wheels in contact with the ground. Disc-shaped, cylindrical, or spherical craft can use this torque to roll across the surface with varying levels of Kerbalness. Rotation around less rounded profiles will tend toward flip-walking across the surface. Neither is likely to fast or safe, but it can easily be fun and possibly practical in certain circumstances.