Space nuclear propulsion: Humanity’s route to the solar system

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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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.

3 Responses to Space nuclear propulsion: Humanity’s route to the solar system

  1. James Greenidge

    Really informative article, especially since unlike many others, you aren’t apologetic about using nukes in space! I remember a early sci-fi film (think it was “Destination Moon”) supposedly supported by von Braun himself, which used superheated water instead of chem fuels driving through its nuclear reactor! Be nice to see where that concept went! Hope you’ll also consider exploring ways that the U.N.’s too hasty Nuclear Test Ban Treaty can be lifted to allow super-heavy ultra-swift vehicles using the nuclear pulse drive system of Project Orion.

    James Greenidge
    Queens NY

  2. Thanks for making this so easy to understand, Wes.

  3. Nathan Wilson

    When employed as an expendable upper stage engine for deep space missions, a nuclear thermal rocket does greatly boost the payload capacity of the rocket (by about 100%). However, it fundamentally does the same thing as a chemical rocket.
    One the other hand, when employed on a distant world like Mars, nuclear power is the only way to achieve global mobility (just as is the case for submarine propulsion).

    The ultimate example is the Mars Hopper. As described by Mars pioneer Robert Zubrin, the Mars Hopper is a vertical take-off and landing Nuclear thermal rocket using Indigenous Martian Fuel (or NIMF), which expels heated carbon dioxide for thrust. It would fly a thousand or so miles in a ballistic or gliding trajectory; following each landing, it refills its CO2 tank by compressing the Martian atmosphere and is ready for the next hop.

    Even simple wheeled rovers allow Mars explorers to have an exploration radius of hundreds of miles when used in the energy-rich environment of a base that has a nuclear power supply. Frequent long distance roving uses a lot of fuel, which would be prohibitively expensive if hauled all the way from Earth. But where energy is cheap, fuel can be made cheaply from the Martian atmosphere. Maybe carbon-monoxide and oxygen bi-propellant, or with a little hydrogen from Earth, methane and oxygen bi-propellant can be made (for a fuel cell or combustion engine).