Rocket propellant

Rocket propellant is the material expelled by a rocket engine to produce thrust, using stored chemical energy or another energy source. Chemical propellants combine fuel and oxidizer, which may be mixed or stored separately.

Rocket engine: A device that converts propellant energy into a high-speed exhaust stream to produce thrust. It burns or otherwise processes propellant and accelerates the resulting exhaust.
Rocket fuel: The energy-releasing component of a chemical rocket propellant combination. Fuel supplies chemical energy, but requires an oxidizer because rockets carry their own oxygen source.
Liquid oxygen: Oxygen cooled below its boiling point into a pale blue liquid used as an oxidizer. It is a common high-performance oxidizer paired with fuels such as hydrogen, methane, or kerosene.
Solid rocket motor: A rocket motor that burns a solid propellant grain to generate thrust. Unlike many liquid systems, it is mechanically simple but difficult to throttle or shut down.
Rocket equation: The equation relating a rocket's mass ratio and exhaust velocity to its achievable change in velocity. Propellant efficiency and the mass carried determine how much mission velocity a vehicle can gain.
Specific impulse: A measure of rocket-engine efficiency, expressed as thrust per unit weight flow of propellant. It compares how effectively different propellants and engines turn propellant into thrust.
Oxidizer: A substance that enables another substance to undergo oxidation, often by supplying oxygen or accepting electrons. Rocket oxidizers sustain combustion beyond the atmosphere, where ambient air is unavailable.
Liquid hydrogen: Hydrogen cooled into a liquid, used as a high-performance rocket fuel. Its pairing with liquid oxygen provides high exhaust efficiency but demands bulky cryogenic tanks.
Hybrid rocket: A rocket that combines a solid fuel with a separately stored liquid or gaseous oxidizer. It separates fuel and oxidizer without requiring both components to be liquids.
Cryogenic storage: The storage of materials at temperatures low enough to keep them in liquid form. Boil-off and insulation needs shape launch schedules and spacecraft tank design for cryogenic propellants.

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