A nuclear rocket engine is a rocket engine, the principle of which is based on a nuclear reaction or radioactive decay, while energy is released that heats the working fluid, which can be the reaction products or some other substance, such as hydrogen.
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There are several types of rocket engines using the above-described principle of operation: nuclear, radioisotope, thermonuclear. Using nuclear rocket engines, specific impulse values can be obtained significantly higher than those that can be obtained from chemical rocket engines. The high value of the specific impulse is explained by the high speed of the outflow of the working fluid - about 8-50 km / s. The thrust force of a nuclear engine is comparable to that of chemical engines, which will make it possible in the future to replace all chemical engines with nuclear ones.
The main obstacle to complete replacement is the radioactive contamination of the environment caused by nuclear rocket engines.
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They are divided into two types - solid and gas phase. In the first type of engines, fissile matter is placed in rod assemblies with a developed surface. This allows you to efficiently heat the gaseous working fluid, usually hydrogen acts as the working fluid. The outflow rate is limited by the maximum temperature of the working fluid, which, in turn, directly depends on the maximum allowable temperature of structural elements, and it does not exceed 3000 K. In gas-phase nuclear rocket engines, fissile matter is in a gaseous state. Its retention in the working area is carried out through the action of an electromagnetic field. For this type of nuclear rocket engines, structural elements are not a deterrent, therefore, the velocity of the working fluid can exceed 30 km / s. They can be used as first stage engines, regardless of fissile material leakage.
In the 70s. XX century in the United States and the Soviet Union, nuclear rocket engines with solid-phase fissile matter were actively tested. In the United States, a program was developed to create an experimental nuclear rocket engine under the NERVA program.
The Americans developed a liquid hydrogen-cooled graphite reactor that was heated, vaporized, and ejected through a rocket nozzle. The choice of graphite was dictated by its temperature resistance. According to this project, the specific impulse of the resulting engine was to be twice the corresponding indicator typical for chemical engines, with a thrust of 1100 kN. The Nerva reactor was supposed to work as part of the third stage of the Saturn V launch vehicle, but due to the closure of the lunar program and the absence of other tasks for rocket engines of this class, the reactor was never tested in practice.
A gas-phase nuclear rocket engine is currently under theoretical development. In a gas-phase nuclear engine, it is intended to use plutonium, a slowly moving gas stream of which is surrounded by a faster flow of cooling hydrogen. Experiments were carried out on the orbiting space stations MIR and ISS, which can give impetus to the further development of gas-phase engines.
Today it can be said that Russia has slightly "frozen" its research in the field of nuclear propulsion systems. The work of Russian scientists is more focused on the development and improvement of basic units and assemblies of nuclear power plants, as well as their unification. The priority direction of further research in this area is the creation of nuclear power propulsion units capable of operating in two modes. The first is the mode of a nuclear rocket engine, and the second is the mode of installing generating electricity to power the equipment installed on board the spacecraft.