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 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. However, non-combusting forms such as cold gas thrusters, nuclear thermal rockets, and ion engines exist. Rocket vehi Rocket engines produce thrust by the expulsion of gas that has been accelerated to high speed through a nozzle. The fluid is usually a gas created by high pressure (150-to-4,350-pound-per-square-inch (10 to 300 bar)) combustion of solid or liquid propellants, consisting of fuel and oxidiser components, within a combustion chamber. As the gases expand through the nozzle, they are accelerated to very high (supersonic) speed, and the reaction to this pushes the vehicle (rocket) in the opposite direction. Combustion is most frequently used for practical rockets, as the laws of thermodynamics (more specifically Carnot's theorem) dictate that high temperatures and pressures are desirable for the best thermal efficiency. Nuclear thermal rockets are capable of higher efficiencies, but have low thrust, thanks to the low mass of the propellants used, and also have environmental problems which preclude their routine use in the Earth's atmosphere and cislunar space. For model rocketry, an available alternative to combustion is a water rocket pressurized by compressed air, carbon dioxide, nitrogen, or any other readily available, inert gas. The hot gas produced in the combustion chamber is permitted to escape through a narrow space, called the throat, to increase the velocity until it reaches Mach 1, and then continues to accelerate through a diverging expansion section. When sufficient pressure is provided to the nozzle (about 2.5–3 t The most common monopropellants are hydrazine and hydrogen peroxide. == Principle of operation == Rocket engines produce thrust by the expulsion of gas that has been accelerated to high speed through a nozzle. The fluid is usually a gas created by high pressure (150-to-4,350-pound-per-square-inch (10 to 300 bar)) combustion of solid or liquid propellants, consisting of fuel and oxidiser components, within a combustion chamber. As the gases expand through the nozzle, they are accelerated to very high (supersonic) speed, and the reaction to this pushes the vehicle (rocket) in the opposite direction. About half of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber, and the rest comes from the pressures acting against the inside of the nozzle (see diagram). As the gas expands (adiabatically) the pressure against the nozzle's walls forces the rocket engine in one direction while accelerating the gas in the other. The most commonly used nozzle is the de Laval nozzle, a fixed geometry nozzle with a high expansion-ratio. The large bell- or cone-shaped nozzle extension beyond the throat gives the rocket engine its characteristic shape. The temperatures reached by combustion in rocket engines often substantially exceed the melting points of the nozzle and combustion chamber (See § Nozzle above for temperatures in nozzle). In rockets the coolant methods include: Ablative: The combustion chamber inside walls are lined with a material that traps heat and carries it away with the exhaust as it vaporizes. Radiative cooling: The engine is made of one or several refractory materials, which take heat flux until its outer thrust chamber wall glows red- or white-hot, radiating the heat away. Dump cooling: A cryogenic propellant, usually hydrogen, is passed around the nozzle and dumped. This cooling method has various issues, such as wasting propellant. It is only used rarely. 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. This is the most widely applied method of rocket engine cooling. Film cooling: The engine is designed with rows of multiple orifices lining the inside wall through which additional propellant is injected, cooling the chamber wall as it evaporates. This method is often used in cases where the heat fluxes are especially high, likely in combination with regenerative cooling. A more efficient subtype of film cooling is transpiration cooling, in which propellant passes through a porous inner combustion chamber wall and transpirates. So far, this method has not seen usage due to various issues with this concept. Rocket engines may also use several cooling methods. Some fuel/oxidiser combinations ignite on contact (hypergolic), and non-hypergolic fuels can be "chemically ignited" by priming the fuel lines with hypergolic propellants (popular in Russian engines). Gaseous propellants generally will not cause hard starts, with rockets the total injector area is less than the throat thus the chamber pressure tends to ambient prior to ignition and high pressures cannot form even if the entire chamber is full of flammable gas at ignition. Solid propellants are usually ignited with one-shot pyrotechnic devices and combustion usually proceeds through total consumption of the propellants. Once ignited, rocket chambers are self-sustaining and igniters are not needed and combustion usually proceeds through total consumption of the propellants. Indeed, chambers often spontaneously reignite if they are restarted after being shut down for a few seconds. Unless designed for re-ignition, when cooled, many rockets cannot be restarted without at least minor maintenance, such as replacement of the pyrotechnic igniter or even refueling of the propellants. == Jet physics == Rocket jets vary depending on the rocket engine, design altitude, altitude, thrust and other factors.