A gas jet is simply a device designed to
emit a stream of gas with a particular set of intended properties. Uses range from
stoves (where gas jets are intended to mix
natural gas with
atmospheric oxygen for even burning and heat generation) to
spacecraft and to
recoilless guns. In spacecraft, or on
aircraft or
missiles, such jets are intended to produce
thrust, and are either
cold gas jets (emitting gas from a
pressurized source) or hot gas jets, with gases resulting from
combustion of
propellantand
oxidizer.
In recoilless guns, the gas jets are specially designed ducts and vents which allow the hot gases from the combustion of the shell's propellant (or rocket exhaust) to escape the weapon in such a way that the impulse produced by the gas jet mitigates the recoil experienced by the weapon and shooter.
One problem with the use of gas jets on recoil neutralization systems (and muzzle brakes) is that on larger weapons the gas can itself be a hazard to the shooter, causing burns through heat and chemical action.