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 propellants are gasoline, jet fuel and rocket fuel.

Aerosol sprays

In aerosol cans, the propellant is simply a pressurized gas. If the can was simply filled with compressed gas, either it would need to be at a dangerously high pressure, or the amount of gas in the can would be small, and it would soon run out. Hence usually the gas is the vapour of a liquid with boiling point slightly lower than room temperature. This means that inside the pressurized can, the vapour can exist in equilibrium with its bulk liquid at a pressure that is higher than atmospheric pressure (and thus able to expel the payload), but not dangerously high; yet, as gas escapes, it is immediately replaced by more liquid evaporating.

Chlorofluorocarbons (CFCs) were once often used, but have been replaced in recent years due to the negative effects CFCs have on Earth's atmospheric ozone layer. The most common replacements are mixtures of volatile hydrocarbons, typically propane, n-butane and isobutane. Dimethyl ether (DME) and methylethyl ether are also used. All these have the principle disadvantage of being quite flammable. Nitrous oxide is also used as a propellant to deliver foodstuffs (e.g. whipped cream). Medicinal aerosols such as asthma inhalers use hydrofluoroalkanes (HFA): either HFA 134a (1,1,1,2,-tetrafluoroethane) or HFA 227 (1,1,1,2,3,3,3-heptafluoropropane) or a combination of the two.

Solid fuelled rockets and projectiles

In ballistics and pyrotechnics, a propellant is a material which 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 fuel rockets:

• Gunpowder
• Nitrocellulose
• Cordite and other smokeless powders
• Special fuels made from ammonium perchlorate and synthetic rubbers

Liquid fuelled rockets

Common propellants for liquid fuelled rockets include:

• RFNA and kerosene or RP-1
• RFNA and UDMH
• Dinitrogen tetroxide and UDMH
• Liquid oxygen and kerosene or RP-1
• Liquid oxygen and liquid hydrogen

See also

• Spacecraft propulsion
• Specific impulse
• Fuel