propellant

A propellant is a material that produces pressurized gas that:

• can be directed through a nozzle, thereby producing thrust (rocket propellant used in a rocket motor);
• fills the interior of an ammunition cartridge or the chamber of a gun or cannon, leading to the expulsion of a bullet or shell (gunpowder, smokeless powder, and large gun propellants);
• can fill an expansible bag or membrane, such as an automotive airbag (gas generator propellants).

Common chemical propellants are energetic materials and consist of a fuel; like gasoline, jet fuel, rocket fuel, and an oxidizer. Chemical propellants are burned or otherwise decomposed to produce the propellant gas. Other propellants are simply liquids that can readily be vaporized into the desired pressurized gas. Such propellants include the nitrous oxide that is dissolved in the butterfat canned whipped cream, and the dimethyl ether or low-boiling alkane used in hairspray.

Propellants are used in pressurised dispensing systems, such as aerosol sprays, to force a material through a nozzle.

In rockets, the propellant produces an exhaust and the exhausted material is usually expelled under pressure through a nozzle. The pressure may be from a compressed gas, or a gas produced by a chemical reaction. The exhaust material may be a gas, liquid, plasma, or, before the chemical reaction, a solid, liquid or gelled.

In aircraft, the propellant is usually a fuel and is combusted to generate energy to power the aircraft's flight.

Aerosol sprays

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

Propellant used for propulsion

Technically, the word propellant is the general name for chemicals used to create thrust. For vehicles, the term propellant refers only to chemicals that are stored within the vehicle prior to use, and excludes atmospheric gas or other material that may be collected in operation.

Amongst the English-speaking laymen, used to having fuels propel vehicles on Earth, the word fuel is inappropriately^{[dubious – discuss]} 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.

In rockets, the most common combinations are bipropellants, 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 tripropellant systems involves adding a third propellant tank containing liquid hydrogen to do this.

Solid propellant

In ballistics and pyrotechnics, a propellant is a generic name for chemicals used for propelling projectiles from guns and other firearms.

Propellants are usually made from low explosive materials, but may include high explosive chemical ingredients that are diluted and burned in a controlled way (deflagration) rather than detonation. The controlled burning of the propellant composition usually produces thrust by gas pressure and can accelerate a projectile, rocket, or other vehicle. 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
  • Ballistite
  • Smokeless powders
• Composite propellants made from a solid oxidizer such as ammonium perchlorate or ammonium nitrate, a rubber such as HTPB, or PBAN (may be replaced by energetic polymers such as polyglycidyl nitrate or polyvinyl nitrate for extra energy) , optional high explosive fuels (again, for extra energy) such as RDX or nitroglycerin, 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.

Propellants that explode in operation are of little practical use currently, although there have been experiments with Pulse Detonation Engines.

Grain

Propellants are used in forms called grains. A grain is any individual particle of propellant regardless of the size or shape. The shape and size of a propellant grain determines the burn time, amount of gas and rate produced from the burning propellant and consequently thrust vs time profile.

There are three types of burns that can be achieved with different grains.

Progressive Burn

Usually a grain with multiple perforations or a star cut in the center providing a lot of surface area.

Digressive Burn

Usually a solid grain in the shape of a cylinder or sphere.

Neutral Burn

Usually a single perforation; as outside surface decreases the inside surface increases at the same rate.

Composition

There are four different types of solid propellant compositions:

Single Based Propellant: A single based propellant has nitrocellulose as its chief explosives ingredient. Stabilizers and other additives are used to control the chemical stability and enhance the propellant’s properties.

Double Based Propellant: Double based propellants consist of nitrocellulose with nitroglycerin or other liquid organic nitrate explosives added. Stabilizers and other additives are used also. Nitroglycerin reduces smoke and increases the energy output. Double based propellants are used in small arms, cannons, mortars and rockets.

Triple Based Propellant

Triple based propellants consist of nitrocellulose, nitroquanidine, nitroglycerin or other liquid organic nitrate explosives. Triple based propellants are used in cannons.

Composite

Composites contain no nitrocellulose, nitroglycerin, nitroquanidine or any other organic nitrate. Composites usually consist of a fuel such as metallic aluminum, a binder such as synthetic rubber, and an oxidizer such as ammonium perchlorate. Composite propellants are used in large rocket motors.

Liquid propellant

Common propellant combinations used for liquid propellant rockets include:

• Red fuming nitric acid (RFNA) and kerosene or RP-1
• RFNA and Unsymmetrical dimethylhydrazine (UDMH)
• Dinitrogen tetroxide and UDMH, MMH, and/or hydrazine
• Liquid oxygen and kerosene or RP-1
• Liquid oxygen and liquid hydrogen
• Liquid oxygen and ethanol
• Hydrogen peroxide and alcohol or RP-1
• Chlorine pentafluoride and hydrazine

Common monopropellant used for liquid rocket engines include:

• Hydrogen peroxide
• Hydrazine
• Red fuming nitric acid (RFNA)

See also

• Fuel
• Propellant depot
• Spacecraft propulsion
• Specific impulse

References

(incomplete)

•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. 
• Clark, John D. (1972). Ignition! An Informal History of Liquid Rocket Propellants. Rutgers University Press. pp. 214. ISBN 0813507251. 

External links

Wikimedia Commons has media related to: Propellants

Look up propellant in Wiktionary, the free dictionary.

• Rocket Propellants