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.
For example, in a simple hydrogen/oxygen engine, hydrogen is burned (oxidized) to create H2O and the energy from the chemical reaction is used to expel the water (steam) to provide thrust.
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, usually as maneuvering thrusters.
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.
For example, in a simple hydrogen/oxygen engine, hydrogen is burned (oxidized) to create H2O and the energy from the chemical reaction is used to expel the water (steam) to provide thrust.
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, usually as maneuvering thrusters.
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.
Low molecular weight gases (e.g. hydrogen, helium, ammonia) are preferred propellants for this kind of system.
The pressures and energy densities that can be achieved, while insufficient for high-performance rocketry and firearms, are adequate for most applications, in which case compressed fluids offer a simpler, safer, and more practical source of propellant pressure.
Additionally, a hand pump to compress air can be used for its simplicity in low-tech applications such as atomizers, plant misters and water rockets.
However, compressed gases are impractical as stored propellants if they do not liquify inside the storage container, because very high pressures are required in order to store any significant quantity of gas, and high-pressure gas cylinders and pressure regulators are expensive and heavy.
This pressure is high enough to provide useful propulsion of the payload (e.g. aerosol paint, deodorant, lubricant), but is low enough to be stored in an inexpensive metal can, and to not pose a safety hazard in case the can is ruptured.
As the payload is depleted, the propellant vaporizes to fill the internal volume of the can.
This is usually insignificant, although it can sometimes be an unwanted effect of heavy usage (as the system cools, the vapor pressure of the propellant drops).
However, in the case of a freeze spray, this cooling contributes to the desired effect (although freeze sprays may also contain other components, such as chloroethane, with a lower vapor pressure but higher enthalpy of vaporization than the propellant).
The most common replacements of CFCs are mixtures of volatile hydrocarbons, typically propane, n-butane and isobutane.
Nitrous oxide and carbon dioxide are also used as propellants to deliver foodstuffs (for example, whipped cream and cooking spray).
[9] The practicality of liquified gas propellants allows for a broad variety of payloads.
Uniquely, in the case of a gas duster ("canned air"), the only payload is the velocity of the propellant vapor itself.