A rocket engine is a type of jet engine[1] 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. Compared to other types of jet engines, rocket engines have the highest thrust, are by far the lightest, but are the least propellant efficient (have the lowest specific impulse). The ideal exhaust is hydrogen, the lightest of all gases, but chemical rockets produce a mix of heavier species, reducing the exhaust velocity. Rocket engines become more efficient at high velocities (due to greater propulsive efficiency and Oberth effect). Since they do not require an atmosphere, they are well suited for uses at very high altitude and in space. Terminology[edit] Here, "rocket" is used as an abbreviation for "rocket engine". Chemical rockets are powered by exothermic chemical reactions of the propellant. Thermal rockets use an inert propellant, heated by a power source such as electric or nuclear power. Solid-fuel rockets (or solid-propellant rockets or motors) are chemical rockets which use propellant in a solid state. Liquid-propellant rockets use one or more liquid propellants fed from tanks. 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. Monopropellant rockets use a single propellant decomposed by a catalyst. The most common monopropellants are hydrazine and hydrogen peroxide. Principle of operation[edit] Rocket engines produce thrust by the expulsion of an exhaust fluid which has been accelerated to a high speed through a propelling nozzle. The fluid is usually a gas created by high pressure (150-to-2,900-pound-per-square-inch (10 to 200 bar)) combustion of solid or liquid propellants, consisting of fuel and oxidiser components, within a combustion chamber. The nozzle uses the heat energy released by expansion of the gas to accelerate the exhaust to very high (supersonic) speed, and the reaction to this pushes the engine in the opposite direction. Combustion is most frequently used for practical rockets, as high temperatures and pressures are desirable for the best performance, permitting a longer nozzle, giving higher exhaust speeds and better thermodynamic efficiency. An alternative to combustion is the water rocket, which uses water pressurised by compressed air, carbon dioxide, nitrogen, or manual pumping, for model rocketry. Propellant[edit] Rocket propellant is mass that is stored, usually in some form of propellant 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 most commonly used, which undergo exothermic chemical reactions which produce hot gas which is used by a rocket for propulsive purposes. Alternatively, a chemically inert reaction mass can be heated using a high-energy power source via a heat exchanger, and then no combustion chamber is used. A solid rocket motor. Solid rocket propellants are prepared as a mixture of fuel and oxidising components called 'grain' and the propellant storage casing effectively becomes the combustion chamber. Injection[edit] Liquid-fuelled rockets force separate fuel and oxidiser components into the combustion chamber, where they mix and burn. Hybrid rocket engines use a combination of solid and liquid or gaseous propellants. Both liquid and hybrid rockets use injectors to introduce the propellant into the chamber. These are often an array of simple jets - holes through which the propellant escapes under pressure; but sometimes may be more complex spray nozzles. When two or more propellants are injected, the jets usually deliberately cause the propellants to collide as this breaks up the flow into smaller droplets that burn more easily. Combustion chamber[edit] Main article: Combustion chamber For chemical rockets the combustion chamber is typically just a cylinder, and flame holders are rarely used. The dimensions of the cylinder are such that the propellant is able to combust thoroughly; different rocket propellants require different combustion chamber sizes for this to occur. This leads to a number called L^*: L^* = \frac {V_c} {A_t} where: V_c is the volume of the chamber A_t is the area of the throat L* is typically in the range of 25–60 inches (0.64–1.52 m). The combination of temperatures and pressures typically reached in a combustion chamber is usually extreme by any standards.