A propellant or propellent is a chemical substance used in the production of energy or pressurized gas that is subsequently used to create movement of a fluid or to generate propulsion of a vehicle, projectile, or other object. Common propellants are energetic materials and consist of a fuel like gasoline, jet fuel, rocket fuel, and an oxidizer. Propellants are burned or otherwise decomposed to produce the propellant gas. Other propellants are simply liquids that can readily be vaporized.
In rockets and aircraft, propellants are used to produce a gas that can be directed through a nozzle, thereby producing thrust. In rockets, rocket 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 gel. In aircraft, the propellant is usually a fuel and is combusted with the air.
In firearm ballistics, propellants fill 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). Explosive material can be placed in a sealed tube and act as a deflagrant low explosive charge in mining and demolition, to produce a low velocity heave effect (gas pressure blasting).
Cold gas propellants may be used to fill an expansible bag or membrane, such as an automotive airbag (gas generator propellants) or in pressurised dispensing systems, such as aerosol sprays, to force a material through a nozzle. Examples of can propellants include nitrous oxide that is dissolved in canned whipped cream, and the dimethyl ether or low-boiling alkane used in hair spray. Rocket propellant may be expelled through an expansion nozzle as a cold gas, that is, without energetic mixing and combustion, to provide small changes in velocity to spacecraft by the use of cold gas thrusters.
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.
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.
To attain a useful density for storage, most propellants are either solid or liquid.
Solid 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:
Propellants that explode in operation are of little practical use currently, although there have been experiments with Pulse Detonation Engines. Also the newly synthesized bishomocubane based compounds are under consideration in the research stage as both solid and liquid propellants of the future.
Solid 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, as a consequence, thrust vs time profile.
There are three types of burns that can be achieved with different grains.
There are four different types of solid propellant compositions:
In rockets, three main liquid bipropellant combinations are used: cryogenic oxygen and hydrogen, cryogenic oxygen and a hydrocarbon, and storable propellants.
Propellant combinations used for liquid propellant rockets include:
Common monopropellant used for liquid rocket engines include:
The most efficient fuel and oxidizer combination commonly used today for chemical liquid rockets is hydrogen (fuel) and oxygen (oxidizer)," continued Coates. The two elements are relatively simple and they burn easily when combined—and even better, the result of their reaction is simple water.
Refined petroleum is not the most efficient thrust-producing fuel for rockets, but what it lacks in thrust production it makes up for in density. It takes less volume of RP-1 to impart the same thrust force on a vehicle, and less volume equates to reduced stage size. ... A smaller booster stage means much less aerodynamic drag as the vehicle lifts off from near sea-level and accelerates up through the more dense (thicker) part of the atmosphere near the earth. The result of a smaller booster stage is it allows a more efficient ascent through the thickest part of the atmosphere, which helps improve the net mass lifted to orbit.
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