A propellant is a chemical 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). Explosives 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 also 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.
What is a propellant?
Rockets work because every action has an equal and opposite reaction (according to Sir Issac Newton's third principle). In order for the rocket to rush forward, something has to rush backwards. That thing is the propellant. The propellant is a material that spews out of the back of the spacecraft giving it thrust, or a push forward.
Often the propellant is a kind of fuel which is burned with an oxidizer to produce large volumes of very hot gas. These gasses expand until they rush out of the back of the rocket, making thrust. Sometimes the propellant is not burned, but pushed directly out of the spacecraft, making thrust. In ion propulsion, the propellant is made of electrically charged atoms, which are magnetically pushed out of the back of the spacecraft. For smaller attitude control thrusters, a compressed gas is pushed out of the spacecraft.