Types Of Nuclear Rocket Propulsion

We can distinguish three different types of nuclear energy sources that can be used to heat a working fluid, usually liquid hydrogen, which can be expanded in a nozzle and therefore, accelerated to high ejection velocities (6,000 to 10,000 m/s). However, none of them can be considered fully developed and none has flown. They are the fission reactor, the radioactive isotope decay source and the fusion reactor.

All three types of follow the same principle as liquid rocket engines do: the heating of a gas in order to eject it at the highest velocity possible. However, in nuclear engines, the heating of the gas i accomplished by energy derived from transformations within the nuclei of atoms. In chemical rockets, the energy is obtained from within the propellants, but in nuclear rockets the power source is usually separate from the propellant.

NUCLEAR FISSION ROCKET ENGINE

HOW DOES FISSION WORK?

Nuclear fission is a reaction wherein a heavy nucleus is bombarded by neutrons and thus becomes unstable, which causes it to decompose into two nuclei with equivalent size and magnitude, with a great detachment of energy and the emission of two or three neutrons.

These neutrons can cause more fissions by interacting with new fissionable nuclei that will then emit new neutrons and so forth. This multiplying effect is known as chain reaction. In a small fraction of a second, the number of fissioned nuclei releases an energy a million times greater than that obtained from burning a block of carbon or exploding a block odf dynamite of the same mass.


Credit: Fastfission (Wikipedia)

In this image a neutron is absorbed by a uranium-235 nucleus, turning it briefly into an excited uranium-236 nucleus. The uranium-236, which is unstable, it splits into fast-moving ligther elements and releases a small amout of free neutrons. This process releases huge amount of energy.

HOW DOES A FISSION ROCKET ENGINE WORK?

In a nuclear fission reactor rocket, the heat is generated by the fission of uranium in the solid reactor material and it is transferred to the working fluid (eg. liquid hydrogen). The nuclear fission rockets have high-thrust (above 40,000 N) with a specific impulse values up to 900 s. There were ground test in the 1960s. The ground tests with hydrogen as a working fluid culminated in a thrust of 980,000 N at a graphite core nuclear reactor of 4100 MW with an equivalent altitude-specific impulse of 848s and hydrogen temperature above 2500 K.

Sketch of a nuclear fission rocket engine.
Credit: Tokino

In recent years there has been renewed interest with this type of technology primarily for a potential manned planetary exploration mission. Studies have show specific impulses up to 1100s. This would allow shorter interplanetary trips, smaller vehicles and more flexibility in the launch time when planets are not in their optimum relative position.

Artist’s concept of a Bimodal Nuclear Thermal Rocket in Low Earth Orbit. This rocket could be used to travel to Mars in 100 days!
Credit: NASA

ISOTOPE DECAY ENGINE

HOW DOES AN RADIOISOTOPE THERMOELECTRIC GENERATOR WORK?

A Radioisotope Thermoelectric Generator (RTG) is an electrical generator that uses an array of thermocouples to convert the heat released by the decay of a suitable radioactive material into electricity by the Seebeck effect (basically it converts heat directly into electricity). This generator has no moving parts.

(Ficar alguna foto)

HOW DOES AN ISOTOPE DECAY ENGINE WORK?

In the isotope decay engine, a radioactive material gives off radiation, which is converted into heat (as stated above, an RTG). Isotope decay engines have been used for generating electrical power in space vehicles such as deep space probes like Cassini. The released energy could be used to heat a working fluid (eg. liquid hydrogen) like a fission rocket engine does or to power an electric propulsion system. When used with a working fluid, it usually provides a lower thurst and a lower temperature that the other types of rockets. These type of engines haven’t been developed or flow.

Sketch of a RTG rocket engine.
Credit: Tokino
Cassini spacecraft Radioisotope Thermoelectric Generator (RTG)
Credit: Unknown

FUSION

HOW DOES FUSSION WORK?

Nuclear fusion is an atomic reaction in which multiple atoms combine to create a single, more massive atom. The resulting atom has a slightly smaller mass than the sum of the masses of the original atoms. The difference in mass is released in the form of energy during the reaction, according to the Einstein formula E = ?mc^2 , where E is the energy in joules, m is the mass difference in kilograms, and c is the speed of light (approximately 300,000,000 or 3x10^8 meters per second).

Artist’s concept of a Bimodal Nuclear Thermal Rocket in Low Earth Orbit. This rocket could be used to travel to Mars in 100 days!
Credit: NASA

Fusion of deuterium with tritium creating helium-4, freeing a neutron, and releasing 17.59 MeV as kinetic energy of the products while a corresponding amount of mass disappears, in agreement with kinetic E = ?mc^2, where ?m is the decrease in the total rest mass of particles.

HOW DOES A FUSSION ROCKET ENGINE WORK?

Fusion is a method of creating nuclear energy that could be used to heat a working fluid. A number of different concepts have been studied, however, to date none have been tested and many concepts are not yet feasible or practical.


Written by David Esteba Fàbrega on 2019-09-30.

TEST YOURSELF WITH THIS SURVEY!

In which nuclear process a heavy nucleus is bombarded by neutrons to make it to decompose into two smaller nuclei?

  • Combustion
  • Fission
  • Fussion
  • Vaporization

EXPLANATION

Nuclear fission is a reaction wherein a heavy nucleus is bombarded by neutrons and thus becomes unstable, which causes it to decompose into two nuclei with equivalent size and magnitude, with a great detachment of energy and the emission of two or three neutrons.

Not agree? Send us a request!

Which of these types of rocket engines has a higher isp?

  • Isotope Decay Engine
  • Solid rocket motor
  • Liquid rocket engine
  • Fission rocket engine

EXPLANATION

Fission rocket engines have higher ISP than all the other types of rocket engines listed. They usually have an ISP of around 900 s, while solid and liquid rocket engines usually are between 250 and 450 s and isotope decay engines has lower values than fission engines have.

Not agree? Send us a request!

In which nuclear process multiple atoms combine to create a single more massive atom?

  • Fussion
  • Fission
  • Combustion
  • Ionization

EXPLANATION

Nuclear fusion is an atomic reaction in which multiple atoms combine to create a single, more massive atom.

Not agree? Send us a request!

How does a nuclear rocket engine usually work?

  • It burns the propellant stored in the tanks.
  • It heats the propellant in order to accelerate it.
  • It decomposes the propellant into other elements.
  • It ionizes the propellant and accelerates it through an electric field.

EXPLANATION

The heat generated by the nuclear source is transferred to the working fluid (eg. liquid hydrogen).

Not agree? Send us a request!