Rocket engines create thrust by discharging high-velocity gas in the opposite direction of travel. They use propellant mass to produce a propulsive jet, unlike other jet engines. Chemical rockets are the most commonly used type, with solid, liquid, and hybrid fuels. Thermal rockets use external heat sources, including hot water, electric fields, solar energy, and nuclear power. Solid-fuel rockets are still used for fireworks and model rockets, while liquid-fuel and hybrid designs are more efficient for spaceflight.
A rocket engine is a type of jet engine, meaning it is a jet engine that creates thrust by discharging a high-velocity stream of gas in the direction opposite to the desired direction of travel, propelling itself forward due to conservation of quantity of motion. The distinctive feature of a rocket is that its propulsive jet is produced entirely by the propellant mass of the engine, with none of it being taken from the external environment. This differs from other forms of jet engines, such as turbojets, turbofans and ramjets, which mix their fuel with compressed air from the atmosphere to burn their fuel and produce a jet. Rocket engine technology is essential to spaceflight because rockets can operate outside of an atmosphere. Rockets are also used for purposes such as fireworks, weapons, and high-speed aircraft.
There are several forms of rocket engine. The most commonly used type is called a chemical rocket. A chemical rocket is propelled forward by heat-producing chemical reactions in its propellant, producing a high-velocity exhaust stream that is discharged out the rear of the rocket. Each chemical rocket carries a flammable propellant as a fuel reserve. This is combined with an even more flammable substance, called an initiator or igniter. The initiator is ignited, usually by an electric spark or pyrotechnic charge, and the heat in turn ignites the propellant, which burns to produce a propulsive exhaust jet.
Propellant chemicals can be solids, liquids, or solids combined with liquids or gases. In a solid fuel rocket, the solid propellant, called grain, is stored together with oxidizing chemicals that act as the initiator, while liquid fuel rockets store the liquid propellant and initiator in separate tanks until it is time to release them into the fuel tank. combustion chamber for mixing. Hybrid fuel rockets use solid propellant, which is then mixed with a liquid or gaseous initiator stored in a separate tank until it is ready to be used.
The most common solid fuel used today is called an ammonium perchlorate composite propellant (APCP), which refers to a number of different chemical mixtures that incorporate both propellant and initiator. APCP commonly includes the oxidizer ammonium perchlorate (NH4ClO4), elastic polymers called elastomers, and powdered aluminum or other metals. Liquid rocket fuels are often composed of liquid oxygen blended with refined kerosene or liquid hydrogen, or of dinitrogen tetroxide (N2O4) blended with hydrazine (N2H4) or one of its derivatives.
Solid-fuel rockets were the first form of rocket engine, but have largely been superseded by more efficient liquid-fuel and hybrid designs. They are still commonly used for purposes such as fireworks and model rockets, however, and are sometimes used in spaceflight to launch small payloads into orbit or as supplements to a liquid fuel rocket in order to increase payload capacity . For example, the Space Shuttle uses a single large liquid-fuel rocket flanked by two smaller solid-fuel rockets to achieve orbit.
A thermal rocket uses propellant heated by an external heat source rather than chemical reactions in the propellant itself. Hot water rockets, also called steam rockets, use water as a propellant by heating it to produce jets of steam. These are often used in very high speed land vehicles, such as drag racers. Electrothermal rockets use electric fields to produce heated plasma, which then heats the propellant to produce a jet. Electrothermal rockets are useful for producing short bursts of thrust and commonly used for purposes such as altitude control in satellites.
Several other types of thermal rockets have been proposed and could eventually be used. A solar thermal rocket would use solar energy as a heat source, either by directly exposing the propellant to solar radiation or by using the solar energy to power a heat exchanger which would heat the propellant. Solar energy would be collected and concentrated through mirrors or lenses to provide enough concentrated heat. A thermal rocket engine could also be powered by energy transmitted to it from an external source via laser beams or microwaves. A nuclear-powered thermal rocket could heat its propellant with energy from a nuclear reactor or the decay of radioactive isotopes.
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