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A rocket engine, also known as a rocket motor, is a reaction engine, producing thrust in accordance with Newton's third law by ejecting reaction mass rearward, usually a high-speed jet of high-temperature gas produced by the combustion of rocket propellant stored inside the rocket.

Rocket vehicles carry their own oxidiser, unlike most combustion engines such as pulse engines or jet engines, so rocket engines can be used in a vacuum, and they can achieve great speed, beyond escape velocity if enough delta V is supplied. Vehicles commonly propelled by rocket engines include missiles, artillery shells, ballistic missiles, and space vehicles. Compared to other types of jet engines, rocket engines typically have the highest thrust, but are the least propellant-efficient (they have the lowest specific impulse). For thermal rockets, pure hydrogen, the lightest of all elements, gives the highest exhaust velocity, but practical chemical rockets produce a mix of heavier species, reducing the exhaust velocity.

Here, "rocket" is used as an abbreviation for "rocket engine". Hybrid rockets use a solid propellant in the combustion chamber, to which a second liquid or gas oxidiser or propellant is added to permit combustion.

Rocket engines produce thrust by the expulsion of gas that has been accelerated to high speed through a nozzle.

Rocket propellant is mass that is stored, usually in some form of tank, or within the combustion chamber itself, prior to being ejected from a rocket engine in the form of a fluid jet to produce thrust. Chemical rocket propellants are the most commonly used.

Hybrid rocket engines use a combination of solid and liquid or gaseous propellants.

The temperatures and pressures typically reached in a rocket combustion chamber in order to achieve practical thermal efficiency are extreme compared to a non-afterburning airbreathing jet engine.

For a rocket engine to be propellant efficient, it is important that the maximum pressures possible be created on the walls of the chamber and nozzle by a specific amount of propellant; as this is the source of the thrust. By comparison, at room temperature the speed of sound in air is about 340 m/s while the speed of sound in the hot gas of a rocket engine can be over 1700 m/s; much of this performance is due to the higher temperature, but additionally rocket propellants are chosen to be of low molecular mass, and this also gives a higher velocity compared to air.

The most important metric for the efficiency of a rocket engine is impulse per unit of propellant, this is called specific impulse (usually written

Rocket engine nozzles are surprisingly efficient heat engines for generating a high speed jet, as a consequence of the high combustion temperature and high compression ratio.

Since the establishment of the first liquid-propellant rocket engine company (Reaction Motors, Inc.) in 1941 and the first government laboratory (GALCIT) devoted to the subject, the US liquid-propellant rocket engine (LPRE) industry has undergone significant changes.

If none of the previously developed rocket engines met the needs of the mission, a new rocket engine with specific requirements would be contracted to another DB specialised in LPRE development (oftentimes each DB had expertise in specific types of LPREs with different applications, propellants, or engine sizes). However, when two parallel engine development programs were supported in order to select the superior one for a specific application, several qualified rocket engine models were never used.

Regenerative cooling: The fuel (and possibly, the oxidiser) of a liquid rocket engine is routed around the nozzle before being injected into the combustion chamber or preburner. Regeneratively and film cooled combustion chamber and nozzle: V-2 Rocket Engine Regeneratively cooled combustion chamber with an ablatively cooled nozzle extension: The LR-91 rocket engine

Rocket jets vary depending on the rocket engine, design altitude, altitude, thrust and other factors.

Quick Facts

• Rocket propellant is mass that is stored, usually in some form of tank, or within the combustion chamber itself, prior to being ejected from a rocket engine in the form of a fluid jet to produce thrust.
• Rocket vehicles carry their own oxidiser, unlike most combustion engines such as pulse engines or jet engines, so rocket engines can be used in a vacuum, and they can achieve great speed, beyond escape velocity if enough delta V is supplied.
• Here, "rocket" is used as an abbreviation for "rocket engine".
• Compared to other types of jet engines, rocket engines typically have the highest thrust, but are the least propellant-efficient (they have the lowest specific impulse).
• Rocket jets vary depending on the rocket engine, design altitude, altitude, thrust and other factors.

Source material: Wikipedia - "Rocket engine". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.