Editing Rocket engine (section)

{{Short description|Non-air breathing jet engine used to propel a missile or vehicle}}
{{Use British English|date=February 2019}}
[[File:RS-68 rocket engine test.jpg|thumb|right|[[RS-68]] being tested at NASA's [[Stennis Space Center]]]]
[[File:Viking 5C rocketengine.jpg|thumb|right|[[Viking (rocket engine)|Viking 5C rocket engine]] used on [[Ariane 1]] through [[Ariane 4]]]]

A '''rocket engine''' is a [[reaction engine]], producing [[thrust]] in accordance with [[Newton's third law]] by ejecting [[reaction mass]] rearward, usually a high-speed [[Jet (fluid)|jet]] of high-temperature gas produced by the [[combustion]] of [[rocket propellant]]s stored inside the [[rocket]]. However, non-combusting forms such as [[cold gas thruster]]s and [[nuclear thermal rocket]]s also exist. Rocket vehicles carry their own [[oxidiser]], unlike most combustion engines, so rocket engines can be used in a [[vacuum]], and they can achieve great speed, beyond [[escape velocity]]. Vehicles commonly propelled by rocket engines include [[missile]]s, [[Rocket-assisted projectile|artillery shells]], [[ballistic missiles]] and [[rocket]]s of any size, from tiny [[Rocket (firework)|fireworks]] to [[Rocket (weapon)|man-sized weapons]] to huge [[Space vehicle|spaceships]].

Compared to other types of jet engine, rocket engines are the lightest and have the highest thrust, but are the least propellant-efficient (they have the lowest [[specific impulse]]). The ideal exhaust is [[hydrogen]], the lightest of all elements, but chemical rockets produce a mix of heavier species, reducing the exhaust velocity.