Fuel, also called propellant, is an expendable resource used by reaction engines to generate thrust. Some fuels require another resource for the engine using them to operate, and others operate alone. The Convert-O-Tron 125 or Convert-O-Tron 250 on a craft can be used to convert Ore that has been drilled to fuel, but there are no other ways to generate fuel [outdated]; its only source is tanks added to craft during construction in the Editor.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 in a 9:11 ratio of fuel to oxidizer. Fuel and oxidizer could be liquid hydrogen and liquid oxygen, as stated in the Convert-O-Tron 250's description when it says "This mobile processing plant can take raw materials containing even trace amounts of oxygen and hydrogen, and crack them into useful fuel products." For jet engines each unit of fuel must be combined with 11 units of 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.