A method of propulsion by which, in accordance with Isaac Newton's third law of motion 'to every action there is an equal and opposite reaction', an object is propelled forwards by a stream of gas or liquid (the jet) expelled in the opposite direction.
Within the animal kingdom, creatures such as squid or octopuses make use of short bursts of jet propulsion to move quickly. In general, water is taken into a muscular sac and is expelled rapidly through a small opening to provide a rapid acceleration in the opposite direction. The simplest example of jet propulsion in the underwater world is jellyfish. Although not all species of jellyfish use jet propulsion to travel, many species fill the umbrella section with water and then push the water out in a stream of short duration. However, the jellyfish has little control over direction. Squid, on the other hand, are able to control the direction of the jet, allowing them to move in a definite direction. Some squid are able to reach speeds high enough to shoot them out of the water and onto the deck of a ship.
Several shelled animals such as clams or scallops can use a more primitive form of jet propulsion to rapidly escape from enemies. They achieve this by bringing the two halves of their shells together rapidly, although like the jellyfish they have little control over the direction of the expelled liquid, and therefore little control over the direction of their movement.
Outside of the animal kingdom, the most common use of jet propulsion is the widely-known jet engine or turbojet. The turbojet is a kind of gas turbine, in which air passes through a forward-facing intake, is compressed and fed into a combustion chamber. Fuel is sprayed in and ignited, producing a rapidly expanding ball of hot gas which proceeds rearwards, spinning a turbine in the process, which drives the compressor. The hot gas is finally ejected from the rear of the unit, usually a nozzle or some form of tail-pipe, at very high speed. This is the simplest form of gas turbine, used in supersonic aircraft.
The turboprop, used for moderate speeds and altitudes, adds extra stages that absorb most of the energy from the gas stream to drive a propellor shaft. The turbofan is best suited to high subsonic speeds, and features an extra compressor in front. Some of the airflow bypasses the core engine and mixes with the jet exhaust stream, providing a lower temperature and velocity. Compared to the turbojet this modification results in improvements in economy, efficiency and quietness, yet provides a higher speed compared to the turboprop.
The turboshaft is a form of jet engine used to drive the rotors of helicopters, as well as providing propulsion to hovercraft, ships and trains. Essentially a turboprop without a propellor, power from the extra turbine instead being delivered to an output shaft or a reduction gearbox.
The ramjet is used for a variety of missiles. When running at twice the speed of sound (Mach 2), the pressure in the front intake of the ramjet engine is such that no compressor or turbine is required. It is cheap and light, but consumes a high amount of fuel, which is burnt in the widest section of what is essentially an open-ended barrel-shaped tube. The lack of a turbine, however, means that the rocket must be boosted to operational speed before the ramjet engine will function.
Variants of the jet engine include vectored thrust engines, allowing vertical takeoff, reverse thrust, used to slow down a jet plane on landing, and reheat or afterburning, used in military aircraft to provide short-duration increases in thrust.