Fuel, also called propellant, is an expendable resource used to operate reaction engines. Some fuels require another resource for the engine using them to operate, and others operate alone. Unlike some of these companion resources, there are no stock parts which collect or generate fuel[outdated]; its only source is tanks added to craft during construction in the Editor.
The types of fuel are:
All fuels can be transferred between compatible types of fuel storage tanks except solid fuel, which is built into the same part as the engines which utilize it.
Liquid fuel is the most commonly used and must be combusted with a resource representing oxygen. For rocket engines it must be combined with onboard oxidizer. For jet engines it must be combined with sufficient intake air continuously collected inside an oxygen-rich atmosphere.
Solid fuel works alone and is built into solid rocket boosters (SRBs).
Monopropellant works alone, most often used in Reaction Control System thrusters.