A rocket engine is a type of jet engine that uses only stored rocket propellant mass for forming its high
speed propulsive jet. Rocket engines are reaction engines, obtaining thrust in accordance with Newton’s third
law. Most rocket engines are internal combustion engines, although non-combusting forms (such as cold gas
thrusters) also exist. Vehicles propelled by rocket engines are commonly called rockets. Since they need no
external material to form their jet, rocket engines can perform in a vacuum and thus can be used to propel
spacecraft and ballistic missiles.