Editing Propellant (section)

{{Short description|Chemical substance used for propulsion}}
{{More footnotes needed|date=May 2023}}
A '''propellant''' (or '''propellent''') is a [[reaction mass|mass]] that is expelled or expanded in such a way as to create a [[thrust]] or another [[Net force|motive force]] in accordance with [[Newton's third law of motion]], and "propel" a vehicle, [[projectile]], or [[fluid]] payload. In vehicles, the engine that expels the propellant is called a [[reaction engine]]. Although technically a propellant is the reaction mass used to create thrust, the term "propellant" is often used to describe a substance which contains both the reaction mass and the fuel that holds the energy used to accelerate the reaction mass. For example, the term "propellant" is often used in [[Rocket engine|chemical rocket design]] to describe a combined fuel/propellant, although the propellants should not be confused with the [[fuel]] that is used by an engine to produce the energy that expels the propellant. Even though the byproducts of substances used as fuel are also often used as a reaction mass to create the thrust, such as with a chemical rocket engine, propellant and fuel are two distinct concepts.

Vehicles can use propellants to move by ejecting a propellant backwards which creates an opposite force that moves the vehicle forward. Projectiles can use propellants that are expanding gases which provide the motive force to set the projectile in motion. Aerosol cans use propellants which are fluids that are compressed so that when the propellant is allowed to escape by releasing a valve, the energy stored by the compression moves the propellant out of the can and that propellant forces the aerosol payload out along with the propellant. Compressed fluid may also be used as a simple vehicle propellant, with the potential energy that is stored in the compressed fluid used to expel the fluid as the propellant. The energy stored in the fluid was added to the system when the fluid was compressed, such as [[Compressed-air energy storage|compressed air]]. The energy applied to the pump or thermal system that is used to compress the air is stored until it is released by allowing the propellant to escape. Compressed fluid may also be used only as energy storage along with some other substance as the propellant, such as with a [[water rocket]], where the energy stored in the compressed air is the fuel and the water is the propellant.

In [[electrically powered spacecraft propulsion|electrically powered spacecraft]], electricity is used to accelerate the propellant. An [[electrostatic force]] may be used to expel positive ions, or the [[Lorentz force]] may be used to expel negative ions and electrons as the propellant. [[Electrically powered spacecraft propulsion#Electrothermal|Electrothermal]] engines use the [[electromagnetic force]] to heat low molecular weight gases (e.g. hydrogen, helium, ammonia) into a plasma and expel the plasma as propellant. In the case of a [[resistojet rocket]] engine, the compressed propellant is simply heated using [[Joule heating|resistive heating]] as it is expelled to create more thrust.

In chemical rockets and aircraft, fuels are used to produce an energetic gas that can be directed through a [[nozzle]], thereby producing thrust. In rockets, the burning of [[rocket fuel]] produces an exhaust, and the exhausted material is usually expelled as a propellant under pressure through a [[nozzle]]. The exhaust material may be a [[gas]], [[liquid]], [[Plasma (physics)|plasma]], or a [[solid]]. In powered aircraft without propellers such as [[Jet aircraft|jets]], the propellant is usually the product of the burning of fuel with atmospheric oxygen so that the resulting propellant product has more mass than the fuel carried on the vehicle.

Proposed [[photon rocket]]s would use the [[Momentum#Four-vector formulation|relativistic momentum]] of photons to create thrust. Even though photons do not have mass, they can still act as a propellant because they move at relativistic speed, i.e., the speed of light. In this case [[Newton's laws of motion|Newton's third Law of Motion]] is inadequate to model the physics involved and [[relativistic physics]] must be used.

In chemical rockets, chemical reactions are used to [[Chemical energy|produce energy]] which creates [[fluid dynamics|movement of a fluid]] which is used to expel the products of that chemical reaction (and sometimes other substances) as propellants. For example, in a simple hydrogen/oxygen engine, hydrogen is burned  (oxidized) to create {{H2O-nl}} and the energy from the chemical reaction is used to expel the water (steam) to provide thrust. Often in chemical rocket engines, a higher molecular mass substance is included in the fuel to provide more reaction mass.

Rocket propellant may be expelled through an expansion nozzle as a cold gas, that is, without energetic mixing and combustion, to provide small [[delta-v|changes in velocity]] to spacecraft by the use of [[cold gas thruster]]s, usually as maneuvering thrusters.

To attain a useful density for storage, most propellants are stored as either a solid or a liquid.