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A propellant is a material that is used to move an object by applying a motive force. This may or may not involve a chemical reaction. It may be a gas, liquid, plasma, or, before the chemical reaction, a solid. Common chemical propellants consist of a fuel, like gasoline, jet fuel and rocket fuel, and an oxidizer.

In aerosol spray cans, the propellant is simply a pressurized vapour in equilibrium with its liquid. As some gas escapes to expel the payload, more liquid evaporates, maintaining an even pressure. (See aerosol spray propellant for more information.)

In ballistics and pyrotechnics, a propellant is a generic name for explosives used for propelling projectiles from guns and other firearms, in order to distinguish them from the more violent explosives as used in shells and mines to produce a blasting effect. Some explosive substances can be used both as propellants and as bursters, as for example gunpowder, and some of the ingredients of a propellant may be similar, though differently proportioned and combined, to those of a " high explosive."

A propellant burns very rapidly but controllably, to produce thrust by gas pressure and thus accelerate a projectile or rocket. In this sense, common or well known propellants include, for firearms, artillery and solid propellant rockets:

• Gun propellants, such as:
  • Gunpowder (black powder)
  • Nitrocellulose-based powders
  • Cordite
  • Smokeless powders

• Composite propellants made from a solid oxidizer such as ammonium perchlorate or ammonium nitrate, a rubber such as HTPB or PBAN, and usually a powdered metal fuel such as aluminum.
• Some amateur propellants use potassium nitrate, combined with sugar, epoxy, or other fuels / binder compounds.
• Potassium perchlorate has been used as an oxidizer, paired with asphalt, epoxy, and other binders.

Technically, the word propellants is used for the chemicals combined in a rocket engine to make it move by reactive force. However, amongst the English-speaking lay public, used to having fuels propel vehicles on Earth, the word fuel is inappropriately used. In Germany, the word Treibstoff—literally "drive-stuff"—is used; in France, the word ergols is used; it has the same Greek roots as hypergolic, a term used in English for propellants which combine spontaneously and do not have to be set ablaze by auxiliary ignition system.

Most common are bipropellant combinations, which use two chemicals, a fuel and an oxidiser. There is the possibility of a tripropellant combination, which takes advantage of the ability of substances with smaller atoms to attain a greater exhaust velocity, and hence propulsive efficiency, at a given temperature. Although not used in practice, the most developed theory involves adding a third propellant tank containing liquid hydrogen to do this. In practice, a hydrogen-oxygen engine can take advantage of this by simply adding more hydrogen than would obtain at the stoichiometric ratio.

Common propellant combinations used for liquid propellant rockets include:

• RFNA and kerosene or RP-1
• RFNA and UDMH
• Dinitrogen tetroxide and UDMH, MMH and/or Hydrazine
• Liquid oxygen and kerosene or RP-1
• Liquid oxygen and liquid hydrogen
• Hydrogen Peroxide and alcohol or RP-1
• Chlorine pentafluoride & Hydrazine

(incomplete)

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

• Fuel
• Spacecraft propulsion
• Specific impulse