A rocket engine is a reaction engine that can be used for spacecraft propulsion as well as terrestrial uses, such as missiles.
Rocket engines take their reaction mass from within the vehicle and form it into a high speed jet, obtaining thrust in accordance with Newton's third law.
Most rocket engines are internal combustion engines, although non combusting forms exist.
Classic rocket engines produce a high temperature, hypersonic gaseous exhaust.
This is achieved by the combustion of solid, liquid or gaseous propellant, containing oxidiser and a fuel, within a combustion chamber at high pressure.
The hot gas produced is then allowed to escape through a narrow hole (the 'throat'), into a high-expansion ratio nozzle.
The effect of the nozzle is to dramatically accelerate the mass, converting most of the thermal energy into kinetic energy.