Nuclear Space Propulsion

Julian Villalpando
February 7, 2017

Submitted as coursework for PH241, Stanford University, Winter 2017


Fig. 1: Mars Rover powered by an RTG on Mars (Source: Wikimedia Commons)

Nuclear space propulsion is nothing new to aerospace engineers. In fact, they've already deployed several forms of it. However, newer forms of harnessing nuclear power offer much higher returns in energy. RTG, Fission and Fusion are the three methods at the forefront, and I will explore them in this paper.

Radioisotope Thermoelectric Generators (RTG)

Radioisotope Thermoelectric Generators produces heat from the radioactive decay of plutonium. The generator takes advantage of the heat with thermocouples, a device that generates electrical energy from differences in temperature. [1] RTGs have proven successful through their use the renowned Voyager spacecrafts and the Mars Rover show in Fig. 1. Furthermore, they have proven reasonably safe. In the two cases of reentry to the Earth's atmosphere, the RTG successfully burned up, or contained its plutonium to ensure safety. [2]


Fusion produces much more energy that Fission. It is the process of combining two nuclei into a larger one, with the release of energy as a byproduct. While Fusion reactions have the potential to achieve high amounts of energy release, and they have observed in labs, no lab has been able to get more energy out than they put in. This is because the reactions can require temperatures as high as 150 million degrees Celsius. While the energy return on Fusion reactions has not yet reached a favorable yield, such reactions are being considered for future spacecraft designs. [3]


In many ways Fission is the antithesis of Fusion. It is the splitting of a nucleus into smaller nuclei, and the release of energy in the process. Fission does not generate as much energy as Fusion, but its energy outputs are significant.

Fission was first observed in 1939, when Meitner and Frisch observed a nucleus observing an electron, and then destabilizing, splitting in two, and releasing 200 million volts in the process. For the purposes of space propulsion, the desired process is a slow, chain result, where the released neutrons, split off themselves, and heat is produced. A quick, sudden reaction must be avoided at all costs, in order to avoid large scale explosions. [4]

The extreme temperature levels of nuclear reactions, prove difficult to deal with. However, in 2012 a research team demonstrated the use of a water-based heat pipe to extract heat from Uranium. While the parameters of the experiment closely reflect those in the context of a space engine, the input temperature would be even hotter in practice. [5]

Fission-Fusion Hybrids

Fission-Fusion hybrids, take advantage of how well the two reactions play off each other. In theory, one could ignite a thermonuclear deuterium, which would induce fission in a uranium liner, which would then heat the fusion plasma. This method would effectively capitalize on the strengths and weaknesses of both methods. Fission produces vast amounts of heat that is difficult to use effectively, and Fusion requires large amounts of energy to receive its large amount of output. [6]

© Julian Villalpando. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] V. Chirayath, "Advances in Thermophotovoltaic Radioisotope Generators for Deep Space Exploration," Physics 241, Stanford University, Winter 2015.

[2] "Space Radioisotope Power Systems, U.S. Department of Energy, January 2008.

[3] S. Aldousary, "Nuclear Fusion: An Abundant Source of Energy and Recent Developments," Physics 241, Stanford University, Winter 2016.

[4] A. Crerend, "Nuclear Fission in the Context of Pressurized Water Reactors," Physics 241, Stanford University, Winter 2015.

[5] "Researchers Test Novel Power System For Space Travel," Los Alamos National Laboratory, 26 Nov 12.

[6] F. Tew, "Is the Fusion-Fission Hybrid Necessary?," Physics 241, Stanford University, Winter 2016.