Fuel

Fuel, also called propellant, is the key resource used to propel craft. All fuels except xenon gas are combined with oxidizer and burned in the engine's combustion chamber; xenon gas is simply accelerated by electromagnetic forces.

Fuel either references all resources, including electric charge, which are used by engines; or alternatively, and more commonly, describes only the resource which is used while the engine is burning. The latter definition describes:

• Liquid fuel
• Solid fuel
• Xenon gas
• Monopropellant

The first definition would also include oxidizer and intake air, both of which supply oxygen to liquid fuel. In wider terms, electric charge may also be covered, since it is required by ion engines.