Engine

An engine is a part that can accelerate a craft. There are three different types of engines available.

Reaction engine

→ Main article: Reaction engine

A reaction engine is based on Newton's third law: “for every action there is an equal and opposite reaction”. Reaction engines expel reaction mass in one direction (the action), which accelerates a craft in the exact opposite direction (the reaction). This generates a force known as 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
  • Including Monopropellant engines, which expend monopropellant
• Solid fuel rocket engines, which expend solid fuel
• Ion engines, which expend xenon gas utilizing electric charge

Jet engines can only work inside of an oxygen-rich atmosphere, as intake air cannot be stored in any practical way. The other four categories are considered rocket engines and can function in vacuum using only resources stored aboard the craft.

Wheels

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 reaction engines, but powered wheels require only electric charge and can thus run indefinitely without refueling by another craft.

In real life, powered wheels could technically be considered reaction engines that use the surface as a reaction mass. However, to be more applicable to KSP, this wiki requires reaction engines to consume reaction mass, not merely be adjacent to it.

SAS torque

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.