Category Archives: American Nuclear Society

By Wesley Deason

Part III: Nuclear Thermal Propulsion

Today’s post is the final installment of a series concerning space nuclear propulsion (Part I) (Part II). Previous posts discussed nuclear reactor safety and nuclear electric propulsion. Today I will focus on the other extensively researched nuclear space propulsion method: nuclear thermal propulsion.

Nuclear thermal propulsion

Nuclear thermal propulsion (NTP) involves the direct heating and expulsion of a propellant using nuclear power. To accomplish this, nuclear thermal rockets (NTRs) normally consist of three components: a propellant tank, a nuclear power generator, and a nozzle. As in nuclear electric rocket systems, the component that sets various NTRs apart is the type of nuclear generator used.

Most systems that have been designed and tested have used a nuclear reactor to provide heat, while some others have examined the concept of radioisotope power. In the end, the determining factor for which nuclear power generator type should be used is the purpose for which the system was designed. If a nuclear thermal rocket is intended to power a mission to Mars or beyond, a nuclear reactor is a necessity as a power source.

History

The concept of the nuclear thermal rocket was first developed in the 1950s as a solution for safe and reliable travel to Mars. The research program subsequently developed in the late 1950s and 1960s was unprecedented for space nuclear technology. Through the program, many NTRs were designed, built, and tested. The test site for these systems was Jackass Flats, a location adjacent to what is now the Nevada National Security Site, which lies about 65 miles northwest of Las Vegas.

Famous tests in the program included PHOEBUS 2A, the most powerful nuclear reactor ever to be operated, and NRX-A2, a reactor that was purposefully placed under a very fast power transient to prove its safety. Later NTRs were designed with a specific application in mind, as they were considered for the eventual final stage for the famous Saturn V rocket. Unfortunately, funding for the NTRs, and even the Saturn V rocket, eventually vanished due to a change in the nation’s priorities after the Apollo lunar landings. Despite this change, the program is today considered a technical success, as the tests showed that a system could be safely built and operated.

Some Reactors tested in Rover Program -- Space Nuclear Power by Angelo and Buden

Advantages

But why choose nuclear thermal rockets—and nuclear propulsion in general—over chemical propulsion technology, which has been used for carrying payload from earth to space for over 50 years? The answer lies in the tremendous energy density present in nuclear power, and its inherent flexibility in application. NTRs are able to heat any propellant that is pushed through its core, unlike chemical rockets that must rely on the combustion of propellant for energy transfer. Because of this feature, NTRs can heat and expel the most efficient propellant possible, which is hydrogen gas, allowing for a large reduction in the overall mass that must be carried from earth’s surface to orbit.

In addition, all nuclear propulsion methods are inherently capable of providing long-term electricity production. Bimodal NTRs (BNTRs) can accomplish this by coupling a dynamic power conversion system to the reactor system. These systems are designed to run an additional coolant through selected channels in the reactor core, spinning a turbine, and producing electricity. Unlike solar power, nuclear power can operate independent of its location and orientation in space, providing electricity for energy intensive life support systems and scientific equipment.

Humanity’s route to the solar system

Nuclear power offers an unmatched capability for producing the massive amounts of energy required to travel in and out of the gravity wells of our solar system. Whether nuclear power is applied as a means of heating a propellant, as in nuclear thermal propulsion—or as a generator of electricity, as in nuclear electric propulsion—nuclear power stands as humanity’s route to the solar system.

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Part II: Electric propulsion and fission power generation in space

(Part I, “Space nuclear reactor safety,” is here)

By Wesley Deason

Ever since man set foot on Earth’s moon, explorers have envisioned traveling out of Earth’s orbit and into space beyond. To do so, however, will require a propulsion device capable of traveling farther than any used before. These devices will be powered by nuclear energy. In this post, I will discuss nuclear electric propulsion, one of the two primary nuclear propulsion concepts considered by engineers for near-term space travel. Nuclear thermal propulsion, the other primary concept, will be explored in a later post.

Electric propulsion, also commonly referred to as “ion thrust propulsion,” uses electrical power to accelerate ions to very high speeds to provide thrust for a spacecraft. Nuclear electric propulsion is electric propulsion whose power source is fission reactor based, or radioisotope decay based. Electric propulsion is not a new technology, and is well understood. Currently, solar powered electric propulsion devices are used at a small scale to keep satellites in their correct orbit. For operation at a larger scale, however, where much higher thrust values will be needed, or operation at a distance from the sun where solar power is incapable of providing the necessary energy intensity, nuclear electric propulsion will be required.

Aside from the ion thruster used to provide thrust for a nuclear electric propelled spacecraft, its other defining characteristic is the type of nuclear electric generator needed to provide power. Nuclear electric generators used for power production in space can come in many shapes and sizes, depending on the power requirements and spacecraft dimensions. For small power production needs in missions requiring a low thrust, radioisotope power systems can be used. In these systems, the radioactive decay heat from a radioisotope is converted to electricity through the use of a heat-to-work conversion device, or a heat engine. The most common of these are Radioisotope Thermoelectric Generators (RTGs) and Advanced Stirling Radioisotope Generators (ASRGs). You can find out more about these systems by reading my previous post on plutonium-based radioisotope power systems.

SNAP 10-A, fission-based space power system launched in 1965

For larger thrust requirements, however, fission-based power systems become a necessity. Missions requiring such high thrust will be manned and/or carry a large onboard capacity for conducting science. These mission requirements are also often outside the capability of chemical (or even nuclear thermal) propulsion. Individually, the previously stated requirements are not difficult to meet. For example, earth-based power reactors generate enough power to light a large city, but the thought of launching them into space to produce power is absurd. Alternatively, fission power systems have been proposed that are about the size of a small car, which is a relatively small payload to put into low earth orbit. Unfortunately, these systems can produce only a fraction of the electric power that could be produced by that same small car. The ideal space nuclear electric generator would meet both requirements of size and power. To evaluate competitive designs for nuclear electric propulsion systems, engineers seek the smallest system mass possible for a given power production level.

SNAP 10-A

To meet these system requirements, engineers must consider different technologies from those used in earth-based nuclear reactors. For example, the first and only fission-based space power system to be flown by the United States, the SNAP-10A spacecraft, used thermoelectrics, which is the same power conversion technology used by RTGs to produce electricity. Thermoelectrics, however, while dependable, are very inefficient, and excess heat produced by the reactor must be rejected away from the spacecraft. In space, this heat rejection can only be in the form of radiative energy. For those unfamiliar with methods of heat transfer, radiative heat transfer is how heat lamps heat food at a local fast food restaurant, how heat is lost from a vacuum sealed Thermos, and even how the earth is heated by the sun. This may seem unintuitive at first but if you think about it, there are no lakes or rivers of water in space to sweep away excess heat like earth-based power systems. In space nuclear power systems, large panels are heated to high temperatures in order to reject this excess heat. Thus the size, and accordingly the temperature, of these radiator panels drive the power system to be as efficient and high temperature as possible.

Many technologies have been discussed as being capable of achieving such power production goals, with some being invented primarily for this purpose. One of the simpler systems may use a combination of helium and xenon gas as coolant, which can spin a turbine to produce electricity. This was the system designed and proposed for use in the Jupiter Icy Moons Orbiter (JIMO), a space exploration program under serious consideration only a few years ago. More complicated systems propose boiling potassium to spin a turbine, although the zero gravity environment of space makes the task more difficult to accomplish. Lastly, some propose suspending the fuel in a gaseous form, allowing it to flow through a magnetohydrodynamic generator (MHD), which uses the ionized fuel particles to produce electricity. The best way to explain an MHD generator is to think of it as a reverse ion thruster, where charged particles induce a current to produce electricity.

Prometheus nuclear electric Deep Space Vehicle, incorporating JIMO Mission Module

Nuclear electric propulsion has great potential. Its ability to provide propulsion to anywhere in the solar system makes it a viable competitor when the human race decides to explore beyond the gravity well of earth. Like most nuclear technologies, research will continue and technological advancements will continue to be made in the meantime.

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Deason

Wes Deason is a graduate student in nuclear engineering at Oregon State University working on the safety analysis of vented fuel systems for gas-cooled fast breeder reactors. He is a former summer fellow for the Center for Space Nuclear Research and the current student liaison for the Aerospace Nuclear Science and Technology Division of the American Nuclear Society.