Nuclear thermal rocket#Risks

{{More citations needed section|date=February 2025}}

Nuclear-powered thermal rockets are more effective than chemical thermal rockets, primarily because they can use low-molecular-mass propellants such as hydrogen.{{Cite web |date=2021-02-12 |title=Nuclear Propulsion Could Help Get Humans to Mars Faster - NASA |url=https://www.nasa.gov/solar-system/nuclear-propulsion-could-help-get-humans-to-mars-faster/ |access-date=2024-04-26}}{{Cite web |date=2018-05-25 |title=Nuclear Thermal Propulsion: Game Changing Technology for Deep Space Exploration - NASA |url=https://www.nasa.gov/directorates/stmd/tech-demo-missions-program/nuclear-thermal-propulsion-game-changing-technology-for-deep-space-exploration/ |access-date=2024-04-26}}

As thermal rockets, nuclear thermal rockets work almost exactly like chemical rockets: a heat source releases thermal energy into a gaseous propellant inside the body of the engine, and a nozzle at one end acts as a very simple heat engine: it allows the propellant to expand away from the vehicle, carrying momentum with it and converting thermal energy to coherent kinetic energy. The specific impulse (Isp) of the engine is set by the speed of the exhaust stream.{{cite book |doi=10.1016/B978-0-323-91360-7.00001-X |chapter=The scale of the problem: Interstellar distances, time, and energy considerations |title=Interstellar Travel |date=2023 |last1=Matloff |first1=Greg |last2=Gerrish |first2=Harold |pages=51–82 |isbn=978-0-323-91360-7 }} That, in turn, varies as the square root of the kinetic energy loaded on each unit mass of propellant. The kinetic energy per molecule of propellant is determined by the temperature of the heat source (whether it be a nuclear reactor or a chemical reaction). At any particular temperature, lightweight propellant molecules carry just as much kinetic energy as heavier propellant molecules and therefore have more kinetic energy per unit mass. This makes low-molecular-mass propellants more effective than high-molecular-mass propellants.

Because chemical rockets and nuclear rockets are made from refractory solid materials, they are both limited to operate below {{Convert|3000|C|F|sigfig=1}}, by the strength characteristics of high-temperature metals. Chemical rockets use the most readily available propellant, which is waste products from the chemical reactions producing their heat energy. Most liquid-fueled chemical rockets use either hydrogen or hydrocarbon combustion, and the propellant is therefore mainly water (molecular mass 18) and carbon dioxide (molecular mass 44). Nuclear thermal rockets using gaseous hydrogen propellant (molecular mass 2) therefore have a theoretical maximum specific impulse that is 3 to 4.5 times greater than those of chemical rockets.

In 1944, Stanisław Ulam and Frederic de Hoffmann contemplated the idea of controlling the power of nuclear explosions to launch space vehicles.{{cite book |last1=Corliss |first1=William R. |last2=Schwenk |first2=Francis C. |title=Nuclear Propulsion for Space, Understanding the Atom Series |date=1971 |publisher=United States Atomic Energy Commission |id={{ERIC|ED054967}} |osti=1132518 |pages=11–12 }} {{PD-notice}} After World War II, the U.S. military started the development of intercontinental ballistic missiles (ICBM) based on the German V-2 rocket designs. Some large rockets were designed to carry nuclear warheads with nuclear-powered propulsion engines. As early as 1946, secret reports were prepared for the U.S. Air Force, as part of the NEPA project, by North American Aviation and Douglas Aircraft Company's Project Rand.{{cite report |doi=10.2172/7365651 |title=LASL nuclear rocket propulsion program |date=1956 |last1=Schreiber |first1=R.E. |url=https://digital.library.unt.edu/ark:/67531/metadc1449128/ }} {{PD-notice}} These groundbreaking reports identified a reactor engine in which a working fluid of low molecular weight is heated using a nuclear reactor as the most promising form of nuclear propulsion but identified many technical issues that needed to be resolved.{{cite book|last=Serber|first=R.|title=The Use of Atomic Power for Rockets |publisher=Douglas Aircraft Company|date=5 July 1946}}H. P. Yockey, T. F. Dixon (1 July 1946), "A Preliminary study on the Use of Nuclear Power in Rocket Missiles", Report NA-46-574R. Gomog (3 August 1946), "Rocket Computations", Report NEPA-508 {{PD-notice}}L. A. Oblinger (13 August 1946), "Pilot Plant for Nuclear Powered Aircraft", Report NEEA-505L. A. Ohlinger (21 November 1946) "Controls for Nuclear Powered Aircraft", Report NEPA-511 {{PD-notice}}Feasibility of Nuclear Powered Rockets snd Ramjets, Report NA 47-15, February 1947"Nuclear-Powered Flight", LEXP-1, 30 September 1948{{cite report |last1=Redding |first1=E. M. |title=The Feasibility of Nuclear-Powered Rockets |date=8 September 1948 |publisher=Masschusetts Inst. of Tech., Cambridge. Lexington Project |osti=969679 }}

In January 1947, not aware of this classified research, engineers of the Applied Physics Laboratory published their research on nuclear power propulsion and their report was eventually classified.A. E. Ruark ed. (14 January 1947) "Nuclear Powered Flight", APL/JEU-TG-20{{cite book|last=Schreiber|first=R. E.|url=https://fas.org/sgp/othergov/doe/lanl/lib-www/la-pubs/00339473.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://fas.org/sgp/othergov/doe/lanl/lib-www/la-pubs/00339473.pdf |archive-date=2022-10-09 |url-status=live|title=The LASL nuclear rocket propulsion program|publisher=LANL|year=1956|volume=LAMS 2036}} {{PD-notice}} In May 1947, American-educated Chinese scientist Qian Xuesen presented his research on "thermal jets" powered by a porous graphite-moderated nuclear reactor at the Nuclear Science and Engineering Seminars LIV organized by the Massachusetts Institute of Technology.{{cite book|last=Tsien|first=H. S.|title=The Science and Engineering of Nuclear Power|publisher=Addison-Wesley Press|year=1949|editor-last=Goodman|editor-first=C.|volume=2|chapter=Rockets and Other Thermal Jets Using Nuclear Energy|quote=(Presented at a Massachusetts Institute of Technology seminar in 1947)}}

In 1948 and 1949, physicist Leslie Shepherd and rocket scientist Val Cleaver produced a series of groundbreaking scientific papers that considered how nuclear technology might be applied to interplanetary travel. The papers examined both nuclear-thermal and nuclear-electric propulsion.{{cite journal|last1=Shepherd|first1=L. R.|last2=Cleaver|first2=A. V.|date=September 1948|title=The Atomic Rocket I|journal=Journal of the British Interplanetary Society|volume=7|issn=0007-084X|pages=185–194}}{{cite journal|last1=Shepherd|first1=L. R.|last2=Cleaver|first2=A. V. |date=November 1948|title=The Atomic Rocket II|journal=Journal of the British Interplanetary Society|volume=7|issn=0007-084X|pages=234–241}}{{cite journal|last1=Shepherd|first1=L. R. |last2=Cleaver |first2=A. V.|date=January 1949|title=The Atomic Rocket III|journal=Journal of the British Interplanetary Society|volume=8|issn=0007-084X|pages=23–27}}{{cite journal|last1=Shepherd|first1=L. R. |last2=Cleaver|first2=A. V.|date=March 1949|title=The Atomic Rocket IV|journal=Journal of the British Interplanetary Society|volume=8|issn=0007-084X|pages=59–70}}

Through Project Rover, Los Alamos National Laboratory began developing nuclear thermal engines as soon as 1955 and tested the world's first experimental nuclear rocket engine, KIWI-A, in 1959.{{cite web | url=https://www.ans.org/news/article-4640/review-of-rover/ | title=Project Rover: The original nuclear-powered rocket program }} This work at Los Alamos was then continued through the NASA's NERVA program (1961–1973). NERVA achieved many successes and improved upon the early prototypes to create powerful engines that were several times more efficient than chemical counterparts. However, the program was cancelled in 1973 due to budget constraints. To date no nuclear thermal propulsion system has ever been implemented in space.{{cite report|title=Nuclear Thermal Propulsion Ground Test History-The Rover/NERVA Program |url=https://ntrs.nasa.gov/citations/20140008805 |date=February 25, 2014 |website=NASA |author=Harold P. Gerrish Jr |access-date=March 8, 2025}}

A nuclear thermal rocket can be categorized by the type of reactor, ranging from a relatively simple solid reactor up to the much more difficult to construct but theoretically more efficient gas core reactor. As with all thermal rocket designs, the specific impulse produced is proportional to the square root of the temperature to which the working fluid (reaction mass) is heated. To extract maximum efficiency, the temperature must be as high as possible. For a given design, the temperature that can be attained is typically determined by the materials chosen for reactor structures, the nuclear fuel, and the fuel cladding. {{citation_needed|date=September 2022}} Erosion is also a concern, especially the loss of fuel and associated releases of radioactivity.{{cite web |last1=Hall |first1=Loura |title=Nuclear Thermal Propulsion: Game Changing Technology |url=https://www.nasa.gov/directorates/spacetech/game_changing_development/Nuclear_Thermal_Propulsion_Deep_Space_Exploration |publisher=NASA |access-date=21 September 2022 |date=21 May 2018 |quote=Past NERVA research found that graphite composite fueled engines exhibited unwanted erosion and cracking }}

File:NASA-NERVA-diagram.jpg solid-core design]]

Solid core nuclear reactors have been fueled by compounds of uranium that exist in solid phase under the conditions encountered and undergo nuclear fission to release energy. Flight reactors must be lightweight and capable of tolerating extremely high temperatures, as the only coolant available is the working fluid/propellant.{{cite news|last=Cain|first=Fraser |url=https://www.universal-sci.com/headlines/2019/7/3/earth-to-mars-in-100-days-the-power-of-nuclear-rockets|title=Earth To Mars in 100 Days? The Power of Nuclear Rockets|publisher=Universal Sci|date=3 July 2019|access-date=24 August 2019|quote=The first tests of nuclear rockets started in 1955 with Project Rover at the Los Alamos Scientific Laboratory. The key development was miniaturizing the reactors enough to be able to put them on a rocket. Over the next few years, engineers built and tested more than a dozen reactors of different sizes and power outputs.}} A nuclear solid core engine is the simplest design to construct and is the concept used on all tested NTRs.[https://beyondnerva.com/nuclear-thermal-propulsion/solid-core-ntr/ Solid Core NTR] Beyond Nerva. Retrieved 4 May 2022

Using hydrogen as a propellant, a solid core design would typically deliver specific impulses (I_sp) on the order of 850 to 1000 seconds, which is about twice that of liquid hydrogen-oxygen designs such as the Space Shuttle main engine. Other propellants have also been proposed, such as ammonia, water, or LOX, but these propellants would provide reduced exhaust velocity and performance at a marginally reduced fuel cost. Yet another mark in favor of hydrogen is that at low pressures it begins to dissociate at about 1500 K, and at high pressures around 3000 K. This lowers the mass of the exhaust species, increasing I_sp.

Early publications were doubtful of space applications for nuclear engines. In 1947, a complete nuclear reactor was so heavy that solid core nuclear thermal engines would be entirely unableAlvarez, Luis, "There Is No Obvious Or Simple Way To Use Atomic Energy For Space Ships", U.S. Air Services, January 1947, pp. 9-12 to achieve a thrust-to-weight ratio of 1:1, which is needed to overcome the gravity of the Earth at launch. Over the next twenty-five years, U.S. nuclear thermal rocket designs eventually reached thrust-to-weight ratios of approximately 7:1. This is still a much lower thrust-to-weight ratio than what is achievable with chemical rockets, which have thrust-to-weight ratios on the order of 70:1. Combined with the large tanks necessary for liquid hydrogen storage, this means that solid core nuclear thermal engines are best suited for use in orbit outside Earth's gravity well, not to mention avoiding the radioactive contamination that would result from atmospheric use (if an "open-cycle" design was used, as opposed to a lower-performance "closed cycle" design where no radioactive material was allowed to escape with the rocket propellant.{{cite web|url=http://www.projectrho.com/public_html/rocket/enginelist2.php#id--Nuclear_Thermal|title=Engine List 2 - Atomic Rockets|website=projectrho.com}})

One way to increase the working temperature of the reactor is to change the nuclear fuel elements. This is the basis of the particle-bed reactor, which is fueled by several (typically spherical) elements that "float" inside the hydrogen working fluid. Spinning the entire engine could prevent the fuel element from being ejected out the nozzle. This design is thought to be capable of increasing the specific impulse to about 1000 seconds (9.8 kN·s/kg) at the cost of increased complexity. Such a design could share design elements with a pebble-bed reactor, several of which are currently generating electricity.{{citation_needed|date=June 2019}} From 1987 through 1991, the Strategic Defense Initiative (SDI) Office funded Project Timberwind, a non-rotating nuclear thermal rocket based on particle bed technology. The project was canceled before testing.{{cite book |title=Priorities in space science enabled by nuclear power and propulsion |date=2006 |publisher=National Academies Press |location=Washington, D.C. |isbn=978-0-309-10011-3 |page=114 |doi=10.17226/11432 |url=https://nap.nationalacademies.org/catalog/11432/priorities-in-space-science-enabled-by-nuclear-power-and-propulsion |access-date=21 September 2022 |archive-url=https://web.archive.org/web/20220713012855/https://nap.nationalacademies.org/catalog/11432/priorities-in-space-science-enabled-by-nuclear-power-and-propulsion |archive-date=13 July 2022|url-status=live|quote="Preliminary designs had been selected but no prototype components had been tested before the program was canceled. No system was ever launched."}}

{{Main|Pulsed nuclear thermal rocket}}

File:pulsedrocketAri.jpg

In a conventional solid core design, the maximum exhaust temperature of the working mass is that of the reactor, and in practice, lower than that. That temperature represents an energy far below that of the individual neutrons released by the fission reactions. Their energy is spread out through the reactor mass, causing it to thermalize. In power plant designs, the core is then cooled, typically using water. In the case of a nuclear engine, the water is replaced by hydrogen, but the concept is otherwise similar.{{fact|date=March 2025}}

Pulsed reactors attempt to transfer the energy directly from the neutrons to the working mass, allowing the exhaust to reach temperatures far beyond the melting point of the reactor core. As specific impulse varies directly with temperature, capturing the energy of the relativistic neutrons allows for a dramatic increase in performance.{{cite book |doi=10.2514/6.2016-4685 |chapter=On the Use of a Pulsed Nuclear Thermal Rocket for Interplanetary Travel |title=52nd AIAA/SAE/ASEE Joint Propulsion Conference |date=2016 |last1=Arias |first1=Francisco J. |isbn=978-1-62410-406-0 }}

To do this, pulsed reactors operate in a series of brief pulses rather than the continual chain reaction of a conventional reactor. The reactor is normally off, allowing it to cool. It is then turned on, along with the cooling system or fuel flow, operating at a very high power level. At this level the core rapidly begins to heat up, so once a set temperature is reached, the reactor is quickly turned off again. During these pulses, the power being produced is far greater than the same sized reactor could produce continually. The key to this approach is that while the total amount of fuel that can be pumped through the reactor during these brief pulses is small, the resulting efficiency of these pulses is much higher.{{fact|date=March 2025}}

Generally, the designs would not be operated solely in the pulsed mode but could vary their duty cycle depending on the need. For instance, during a high-thrust phase of flight, like exiting a low earth orbit, the engine could operate continually and provide an Isp similar to that of traditional solid-core design. But during a long-duration cruise, the engine would switch to pulsed mode to make better use of its fuel.{{fact|date=March 2025}}

Liquid core nuclear engines are fueled by compounds of fissionable elements in liquid phase. A liquid-core engine is proposed to operate at temperatures above the melting point of solid nuclear fuel and cladding, with the maximum operating temperature of the engine instead of being determined by the reactor pressure vessel and neutron reflector material. The higher operating temperatures would be expected to deliver specific impulse performance on the order of 1300 to 1500 seconds (12.8-14.8 kN·s/kg).{{citation_needed|date=June 2019}}

A liquid-core reactor would be extremely difficult to build with current technology. One major issue is that the reaction time of the nuclear fuel is much longer than the heating time of the working fluid. If the nuclear fuel and working fluid are not physically separated, this means that the fuel must be trapped inside the engine while the working fluid is allowed to easily exit through the nozzle. One possible solution is to rotate the fuel/fluid mixture at very high speeds to force the higher-density fuel to the outside, but this would expose the reactor pressure vessel to the maximum operating temperature while adding mass, complexity, and moving parts.{{citation_needed|date=June 2019}}

An alternative liquid-core design is the nuclear salt-water rocket. In this design, water is the working fluid and also serves as the neutron moderator. Nuclear fuel is not retained, which drastically simplifies the design. However, the rocket would discharge massive quantities of extremely radioactive waste and could only be safely operated well outside the Earth's atmosphere and perhaps even magnetosphere.{{citation_needed|date=June 2019}}

File:Gas Core light bulb.png

File:Gas Core open cycle.png

The final fission classification is the gas-core engine. This is a modification to the liquid-core design which uses rapid circulation of the fluid to create a toroidal pocket of gaseous uranium fuel in the middle of the reactor, surrounded by hydrogen. In this case, the fuel does not touch the reactor wall at all, so temperatures could reach several tens of thousands of degrees, which would allow specific impulses of 3000 to 5000 seconds (30 to 50 kN·s/kg). In this basic design, the "open cycle", the losses of nuclear fuel would be difficult to control, which has led to studies of the "closed cycle" or nuclear lightbulb engine, where the gaseous nuclear fuel is contained in a super-high-temperature quartz container, over which the hydrogen flows. The closed-cycle engine has much more in common with the solid-core design, but this time is limited by the critical temperature of quartz instead of the fuel and cladding. Although less efficient than the open-cycle design, the closed-cycle design is expected to deliver a specific impulse of about 1500 to 2000 seconds (15 to 20 kN·s/kg).{{citation_needed|date=June 2019}}

File:Kiwi-A Prime Atomic Reactor - GPN-2002-000141.jpg

The Soviet RD-0410 went through a series of tests at the nuclear test site near Semipalatinsk Test Site.{{cite web|url=http://www.astronautix.com/engines/rd0410.htm|title=RD-0410|last=Wade|first=Mark|publisher=Encyclopedia Astronautica|access-date=2009-09-25|archive-url=https://web.archive.org/web/20090408122011/http://www.astronautix.com/engines/rd0410.htm|archive-date=8 April 2009}}{{cite web|title="Konstruktorskoe Buro Khimavtomatiky" - Scientific-Research Complex / RD0410. Nuclear Rocket Engine. Advanced launch vehicles|publisher=KBKhA - Chemical Automatics Design Bureau |url=http://www.kbkha.ru/?p=8&cat=11&prod=66 |access-date=2009-09-25 |archive-date=30 November 2010 |archive-url=https://web.archive.org/web/20101130084749/http://www.kbkha.ru/?p=8&cat=11&prod=66}}

In October 2018, Russia's Keldysh Research Center confirmed a successful ground test of waste heat radiators for a nuclear space engine, as well as previous tests of fuel rods and ion engines.{{cite web |title=В России успешно испытали ключевой элемент космического ядерного двигателя |trans-title= Russia has successfully tested a key element of a space nuclear engine|url=https://ria.ru/20181029/1531649544.html |website=РИА Новости |publisher=RIA Novosti |access-date=21 September 2022 |archive-url=https://web.archive.org/web/20220211081720/https://ria.ru/20181029/1531649544.html |archive-date=11 February 2022 |language=ru |date=3 March 2020 |url-status=live}}

File:DOE video about nuclear thermal propulsion rockets.ogg

Development of solid core NTRs started in 1955 under the Atomic Energy Commission (AEC) as Project Rover and ran to 1973. Work on a suitable reactor was conducted at Los Alamos National Laboratory and Area 25 (Nevada National Security Site) in the Nevada Test Site. Four basic designs came from this project: KIWI, Phoebus, Pewee, and the Nuclear Furnace. Twenty individual engines were tested, with a total of over 17 hours of engine run time.{{cite book |last1=Dewar |first1=James A. |title=To The End of the Solar System: The Story of the Nuclear Rocket |date=2007 |publisher=Apogee Books |isbn=978-1-894959-68-1 |edition=2nd }}{{pn|date=March 2025}}

When NASA was formed in 1958, it was given authority over all non-nuclear aspects of the Rover program. To enable cooperation with the AEC and keep classified information compartmentalized, the Space Nuclear Propulsion Office (SNPO) was formed at the same time. The 1961 NERVA program was intended to lead to the entry of nuclear thermal rocket engines into space exploration. Unlike the AEC work, which was intended to study the reactor design itself, NERVA's goal was to produce a real engine that could be deployed on space missions. The {{cvt|334|kN}} thrust baseline NERVA design was based on the KIWI B4 series.{{citation_needed|date=June 2019}}

Tested engines included Kiwi, Phoebus, NRX/EST, NRX/XE, Pewee, Pewee 2, and the Nuclear Furnace. Progressively higher power densities culminated in the Pewee. Tests of the improved Pewee 2 design were canceled in 1970 in favor of the lower-cost Nuclear Furnace (NF-1), and the U.S. nuclear rocket program officially ended in the spring of 1973. During this program, the NERVA accumulated over 2 hours of run time, including 28 minutes at full power. The SNPO considered NERVA to be the last technology development reactor required to proceed to flight prototypes.{{citation_needed|date=June 2019}}

Several other solid-core engines have also been studied to some degree. The Small Nuclear Rocket Engine, or SNRE, was designed at the Los Alamos National Laboratory (LANL) for upper stage use, both on uncrewed launchers and the Space Shuttle. It featured a split-nozzle that could be rotated to the side, allowing it to take up less room in the Shuttle cargo bay. The design provided 73 kN of thrust and operated at a specific impulse of 875 seconds (8.58 kN·s/kg), and it was planned to increase this to 975 seconds, achieving a mass fraction of about 0.74, compared with 0.86 for the Space Shuttle main engine (SSME).{{fact|date=March 2025}}

A related design that saw some work, but never made it to the prototype stage, was Dumbo. Dumbo was similar to KIWI/NERVA in concept, but used more advanced construction techniques to lower the weight of the reactor. The Dumbo reactor consisted of several large barrel-like tubes, which were in turn constructed of stacked plates of corrugated material. The corrugations were lined up so that the resulting stack had channels running from the inside to the outside. Some of these channels were filled with uranium fuel, others with a moderator, and some were left open as a gas channel. Hydrogen was pumped into the middle of the tube and would be heated by the fuel as it traveled through the channels as it worked its way to the outside. The resulting system was lighter than a conventional design for any particular amount of fuel.{{Citation needed|date=January 2012}}

Between 1987 and 1991, an advanced engine design was studied under Project Timberwind, under the Strategic Defense Initiative, which was later expanded into a larger design in the Space Thermal Nuclear Propulsion (STNP) program. Advances in high-temperature metals, computer modeling, and nuclear engineering, in general, resulted in dramatically improved performance. While the NERVA engine was projected to weigh about {{Convert|6803|kg|lb}}, the final STNP offered just over 1/3 the thrust from an engine of only {{Convert|1650|kg|lb}} by improving the I_sp to between 930 and 1000 seconds.{{Citation needed |reason=Reference needed. Also, the SNTP Program Final Report cited in the Project Timberwind article seems to indicate the final Isp was 930 seconds.|date=April 2018}}

File:Destruction of KIWI Nuclear Reactor - GPN-2002-000145.jpg

KIWI was the first to be fired, starting in July 1959 with KIWI 1. The reactor was not intended for flight and was named after the flightless bird, Kiwi. The core was simply a stack of uncoated uranium oxide plates onto which the hydrogen was dumped. The thermal output of 70 MW at an exhaust temperature of 2683 K was generated. Two additional tests of the basic concept, A1 and A3, added coatings to the plates to test fuel rod concepts.{{citation_needed|date=June 2019}}

The KIWI B series was fueled by tiny uranium dioxide (UO₂) spheres embedded in a low-boron graphite matrix and coated with niobium carbide. Nineteen holes ran the length of the bundles, through which the liquid hydrogen flowed. On the initial firings, immense heat and vibration cracked the fuel bundles. The graphite materials used in the reactor's construction were resistant to high temperatures but eroded under the stream of superheated hydrogen, a reducing agent. The fuel species was later switched to uranium carbide, with the last engine run in 1964. The fuel bundle erosion and cracking problems were improved but never completely solved, despite promising materials work at the Argonne National Laboratory.{{citation_needed|date=June 2019}}

NERVA NRX (Nuclear Rocket Experimental), started testing in September 1964. The final engine in this series was the XE, designed with flight representative hardware and fired into a low-pressure chamber to simulate a vacuum. SNPO fired NERVA NRX/XE twenty-eight times in March 1968. The series all generated 1100 MW, and many of the tests concluded only when the test-stand ran out of hydrogen propellant. NERVA NRX/XE produced the baseline {{cvt|334|kN}} thrust that Marshall Space Flight Center required in Mars mission plans. The last NRX firing lost {{cvt|38|lb|kg|order=flip}} of nuclear fuel in 2 hours of testing, which was judged sufficient for space missions by SNPO.{{citation_needed|date=June 2019}}

Building on the KIWI series, the Phoebus series were much larger reactors. The first 1A test in June 1965 ran for over 10 minutes at 1090 MW and an exhaust temperature of 2370 K. The B run in February 1967 improved this to 1500 MW for 30 minutes. The final 2A test in June 1968 ran for over 12 minutes at 4000 MW, at the time the most powerful nuclear reactor ever built.{{citation_needed|date=June 2019}}

A smaller version of KIWI, the Pewee was also built. It was fired several times at 500 MW to test coatings made of zirconium carbide (instead of niobium carbide) but Pewee also increased the power density of the system. A water-cooled system is known as NF-1 (for Nuclear Furnace) used Pewee 2's fuel elements for future materials testing, showing a factor of 3 reductions in fuel corrosion still further. Pewee 2 was never tested on the stand and became the basis for current NTR designs being researched at NASA's Glenn Research Center and Marshall Space flight Center.{{citation_needed|date=June 2019}}

The NERVA/Rover project was eventually canceled in 1972 with the general wind-down of NASA in the post-Apollo era. Without a human mission to Mars, the need for a nuclear thermal rocket is unclear. Another problem would be public concerns about safety and radioactive contamination.{{fact|date=March 2025}}

In January 1965, the U.S. Rover program intentionally modified a Kiwi reactor (KIWI-TNT) to go prompt critical, resulting in immediate destruction of the reactor pressure vessel, nozzle, and fuel assemblies. Intended to simulate a worst-case scenario of a fall from altitude into the ocean, such as might occur in a booster failure after launch, the resulting release of radiation would have caused fatalities out to {{cvt|600|ft|m|sigfig=1|order=flip}} and injuries out to {{cvt|2000|ft|m|sigfig=1|order=flip}}. The reactor was positioned on a railroad car in the Jackass Flats area of the Nevada Test Site.{{cite journal|last1=Fultyn|first1=R. V. |title=Environmental Effects of the Kiwi-TNT Effluent: A Review and Evaluation|journal=LA Reports: U.S. Atomic Energy Commission|pages=1–67|date=June 1968|pmid=5695558|id=LA-3449|location=Los Alamos |url=https://fas.org/sgp/othergov/doe/lanl/docs1/la-3449.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://fas.org/sgp/othergov/doe/lanl/docs1/la-3449.pdf |archive-date=2022-10-09 |url-status=live}} (Pages 35-36 contain the cited material) {{PD-notice}}

As of January 2012, the propulsion group for Project Icarus was studying an NTR propulsion system,{{cite web|url=https://www.centauri-dreams.org/2012/01/26/project-bifrost-return-to-nuclear-rocketry/comment-page-1/|title=Project Bifrost: Return to Nuclear Rocketry|first=Paul|last=Gilster|date=26 January 2012|access-date=5 July 2019}} but has seen little activity since 2019.{{fact|date=March 2025}}

In 1987, Ronen & LeibsonRonen, Yigal, and Melvin J. Leibson; "An example for the potential applications of americium-242m as a nuclear fuel" Trans. Israel Nucl. Soc. 14 (1987): V-42{{cite journal |last1=Ronen |first1=Yigal |last2=Leibson |first2=Melvin J. |title=Potential Applications of 242m Am as a Nuclear Fuel |journal=Nuclear Science and Engineering |date=July 1988 |volume=99 |issue=3 |pages=278–284 |doi=10.13182/NSE88-A28998 |bibcode=1988NSE....99..278R }} published a study on applications of ^242mAm (one of the isotopes of americium) as nuclear fuel to space nuclear reactors, noting its extremely high thermal cross section and energy density. Nuclear systems powered by ^242mAm require less fuel by a factor of 2 to 100 compared to conventional nuclear fuels.{{fact|date=March 2025}}

Fission-fragment rocket using ^242mAm was proposed by George Chapline{{cite journal |last1=Chapline |first1=George |title=Fission fragment rocket concept |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |date=August 1988 |volume=271 |issue=1 |pages=207–208 |doi=10.1016/0168-9002(88)91148-5 |bibcode=1988NIMPA.271..207C }} at Lawrence Livermore National Laboratory (LLNL) in 1988, who suggested propulsion based on the direct heating of a propellant gas by fission fragments generated by a fissile material. Ronen et al. demonstrate that ^242mAm can maintain sustained nuclear fission as an extremely thin metallic film, less than 1/1000 of a millimeter thick. ^242mAm requires only 1% of the mass of ²³⁵U or ²³⁹Pu to reach its critical state. Ronen's group at the Ben-Gurion University of the Negev further showed that nuclear fuel based on ^242mAm could speed space vehicles from Earth to Mars in as little as two weeks.{{cite press release |url=https://www.sciencedaily.com/releases/2001/01/010103073253.htm|title=Extremely Efficient Nuclear Fuel Could Take Man To Mars in Just Two Weeks|date=28 December 2000|publisher=Ben-Gurion University of the Negev}}

The ^242mAm as a nuclear fuel is derived from the fact that it has the highest thermal fission cross section (thousands of barns), about 10x the next highest cross section across all known isotopes. The ^242mAm is fissile (because it has an odd number of neutrons) and has a low critical mass, comparable to that of ²³⁹Pu.{{cite web |title=Critical Mass Calculations for ²⁴¹Am, ^242mAm and ²⁴³Am|url=http://typhoon.jaea.go.jp/icnc2003/Proceeding/paper/6.5_022.pdf|archive-url=https://web.archive.org/web/20110722105207/http://typhoon.jaea.go.jp/icnc2003/Proceeding/paper/6.5_022.pdf|archive-date=22 July 2011|access-date=3 February 2011}}{{cite journal |last1=Ludewig |first1=H.|display-authors=etal |title=Design of particle bed reactors for the space nuclear thermal propulsion program |journal=Progress in Nuclear Energy |date=January 1996 |volume=30 |issue=1 |pages=1–65 |doi=10.1016/0149-1970(95)00080-4|bibcode=1996PNuE...30....1L }}

It has a very high cross section for fission, and if in a nuclear reactor is destroyed relatively quickly. Another report claims that ^242mAm can sustain a chain reaction even as a thin film, and could be used for a novel type of nuclear rocket.{{cite journal|last1=Ronen|first1=Yigal|last2=Shwageraus|first2=E.|title=Ultra-thin 241mAm fuel elements in nuclear reactors|journal=Nuclear Instruments and Methods in Physics Research A|date=2000|volume=455|issue=2|pages=442–451|doi=10.1016/s0168-9002(00)00506-4 |bibcode=2000NIMPA.455..442R}}{{cite journal |last1=Ronen |first1=Y |last2=Raitses |first2=G |title=Ultra-thin 242mAm fuel elements in nuclear reactors. II |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |date=April 2004 |volume=522 |issue=3 |pages=558–567 |doi=10.1016/j.nima.2003.11.421}}{{cite journal |last1=Ronen |first1=Yigal |last2=Aboudy |first2=Menashe |last3=Regev |first3=Dror |title=A Novel Method for Energy Production Using 242 m Am as a Nuclear Fuel |journal=Nuclear Technology |date=March 2000 |volume=129 |issue=3 |pages=407–417 |doi=10.13182/NT00-A3071 |bibcode=2000NucTe.129..407R }}{{cite journal |last1=Ronen |first1=Y. |last2=Fridman |first2=E. |last3=Shwageraus |first3=E. |title=The Smallest Thermal Nuclear Reactor |journal=Nuclear Science and Engineering |date=May 2006 |volume=153 |issue=1 |pages=90–92 |doi=10.13182/NSE06-A2597 |bibcode=2006NSE...153...90R }}

Since the thermal absorption cross section of ^242mAm is very high, the best way to obtain ^242mAm is by the capture of fast or epithermal neutrons in Americium-241 irradiated in a fast reactor. However, fast spectrum reactors are not readily available. Detailed analysis of ^242mAm breeding in existing pressurized water reactors (PWRs) was provided.{{cite journal |last1=Golyand |first1=Leonid |last2=Ronen |first2=Yigal |last3=Shwageraus |first3=Eugene |title=Detailed Design of 242 m Am Breeding in Pressurized Water Reactors |journal=Nuclear Science and Engineering |date=May 2011 |volume=168 |issue=1 |pages=23–36 |doi=10.13182/NSE09-43 |bibcode=2011NSE...168...23G }} Proliferation resistance of ^242mAm was reported by the Karlsruhe Institute of Technology 2008 study.{{cite journal |last1=Kessler |first1=G. |title=Proliferation Resistance of Americium Originating from Spent Irradiated Reactor Fuel of Pressurized Water Reactors, Fast Reactors, and Accelerator-Driven Systems with Different Fuel Cycle Options |journal=Nuclear Science and Engineering |date=May 2008 |volume=159 |issue=1 |pages=56–82 |doi=10.13182/NSE159-56 |bibcode=2008NSE...159...56K }}

In 2000, Carlo Rubbia at CERN further extended the work by Ronen and Chapline on a Fission-fragment rocket using ^242mAm as a fuel.Rubbia, Carlo. "Fission fragments heating for space propulsion" No. SL-Note-2000-036-EET. CERN-SL-Note-2000-036-EET, 2000 Project 242{{cite journal |last1=Augelli |first1=M |last2=Bignami |first2=G F |last3=Genta |first3=G |title=Project 242: Fission fragments direct heating for space propulsion—Programme synthesis and applications to space exploration |journal=Acta Astronautica |date=February 2013 |volume=82 |issue=2 |pages=153–158 |doi=10.1016/j.actaastro.2012.04.007|bibcode=2013AcAau..82..153A }} based on Rubbia design studied a concept of ^242mAm based Thin-Film Fission Fragment Heated NTR{{cite report |last1=Davis |first1=Eric W |title=Advanced Propulsion Study |id={{DTIC|ADA426465}} |publisher=Warp Drive Metrics |date=2004 }} by using a direct conversion of the kinetic energy of fission fragments into increasing of enthalpy of a propellant gas. Project 242 studied the application of this propulsion system to a crewed mission to Mars.{{cite journal |last1=Cesana |first1=Alessandra|display-authors=etal |title=Some Considerations on 242 m Am Production in Thermal Reactors |journal=Nuclear Technology |date=October 2004 |volume=148 |issue=1 |pages=97–101 |doi=10.13182/NT04-A3550 |bibcode=2004NucTe.148...97C }} Preliminary results were very satisfactory, and it has been observed that a propulsion system with these characteristics could make the mission feasible. Another study focused on the production of ^242mAm in conventional thermal nuclear reactors.{{cite journal |last1=Benetti |first1=P. |display-authors=etal|title=Production of 242mAm |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |date=August 2006 |volume=564 |issue=1 |pages=482–485 |doi=10.1016/j.nima.2006.04.029}}

In 2022, the European Space Agency launched an initiative called "Preliminary European Reckon on Nuclear Electric Propulsion for Space Applications" (RocketRoll) and commissioned a consortium of companies to conduct a study on electric thrusters powered by nuclear energy, known as Nuclear Electric Propulsion. The study outlines the roadmap for the launch of a nuclear propulsion demonstrator in 2035.{{Cite web |last=Schultz |first=Isaac |date=2024-11-19 |title=New Roadmap Sets the Stage for Nuclear-Powered Spacecraft by the 2030s |url=https://gizmodo.com/new-roadmap-sets-the-stage-for-nuclear-powered-spacecraft-by-the-2030s-2000526338 |access-date=2024-11-22 |website=Gizmodo |language=en-US}}{{Cite web |last=Parsonson |first=Andrew |date=2024-11-18 |title=ESA Study Outlines 2035 Launch of Nuclear Propulsion Demonstrator |url=https://europeanspaceflight.com/esa-study-outlines-2035-launch-of-nuclear-propulsion-demonstrator/ |access-date=2024-11-22 |website=European Spaceflight |language=en-US}}

File:Orion docked to Mars Transfer Vehicle.jpg (MTV). Cold launched, it would be assembled in-orbit by a number of Block 2 SLS payload lifts. The Orion spacecraft is docked on the left.]]

File:DRACO spacecraft.jpg

Current solid-core nuclear thermal rocket designs are intended to greatly limit the dispersion and break-up of radioactive fuel elements in the event of a catastrophic failure.{{cite web

|url=https://inldigitallibrary.inl.gov/sites/sti/sti/4731768.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://inldigitallibrary.inl.gov/sites/sti/sti/4731768.pdf |archive-date=2022-10-09 |url-status=live|publisher=Idaho National Laboratory|title=Recent Activities at the Center for Space Nuclear Research for Developing Nuclear Thermal Rockets

|website=inldigitallibrary.inl.gov|access-date=12 June 2017}} {{PD-notice}}

As of 2013, an NTR for interplanetary travel from Earth orbit to Mars orbit is being studied at Marshall Space Flight Center with Glenn Research Center.{{cite web|url=http://www.space-travel.com/reports/NASA_Researchers_Studying_Advanced_Nuclear_Rocket_Technologies_999.html|title=NASA Researchers Studying Advanced Nuclear Rocket Technologies|last=Smith|first=Rick|date=10 January 2013|website=space-travel.com}} In historical ground testing, NTRs proved to be at least twice as efficient as the most advanced chemical engines, which would allow for quicker transfer time and increased cargo capacity. The shorter flight duration, estimated at 3–4 months with NTR engines,{{cite magazine|magazine=National Security Science|url=http://www.lanl.gov/science/NSS/issue1_2011/story4full.shtml|title=Nuclear Rockets: To Mars and Beyond|author=Brian Fishbine |author2=Robert Hanrahan |author3=Steven Howe |author4=Richard Malenfant |author5=Carolynn Scherer |author6=Haskell Sheinberg |author7=Octavio Ramos Jr. |publisher=Los Alamos National Laboratory|date=December 2016|archive-url=https://web.archive.org/web/20120625011034/http://www.lanl.gov/science/NSS/issue1_2011/story4full.shtml|archive-date=25 June 2012}} {{PD-notice}} compared to 6–9 months using chemical engines,{{cite web|url=http://image.gsfc.nasa.gov/poetry/venus/q2811.html|title=How long would a trip to Mars take?|publisher=NASA|archive-url=https://web.archive.org/web/20040111085252/https://image.gsfc.nasa.gov/poetry/venus/q2811.html|archive-date=11 January 2004}} {{PD-notice}} would reduce crew exposure to potentially harmful and difficult to shield cosmic rays.{{cite web|url=http://www.adastrarocket.com/aarc/NEP-Mars|title=How Fast Could (Should) We Go to Mars? | Ad Astra Rocket|website=adastrarocket.com|archive-url=https://web.archive.org/web/20131118030419/http://www.adastrarocket.com/aarc/NEP-Mars|archive-date=18 November 2013}}{{cite book |doi=10.2514/6.2013-4076 |chapter=A One-year Round Trip Crewed Mission to Mars using Bimodal Nuclear Thermal and Electric Propulsion (BNTEP) |title=49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference |date=2013 |last1=Burke |first1=Laura M. |last2=Borowski |first2=Stanley K. |last3=McCurdy |first3=David R. |last4=Packard |first4=Thomas |isbn=978-1-62410-222-6 }}{{cite web|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120003776.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120003776.pdf |archive-date=2022-10-09 |url-status=live|title=Nuclear Thermal Propulsion (NTP): A Proven Growth Technology for Human NEO / Mars Exploration Missions|publisher=NASA|date=9 April 2012|author1=Borowski, Stanley K.|author2=McCurdy, David R.|author3=Packard, Thomas W.}} {{PD-notice}}{{cite web|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120012928.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120012928.pdf |archive-date=2022-10-09 |url-status=live|title=Nuclear Thermal Rocket/Vehicle Characteristics And Sensitivity Trades For NASA's Mars Design Reference Architecture (DRA) 5.0 Study|publisher=NASA|date=16 August 2012|author1=Borowski, Stanley K.|author2=McCurdy, David R.|author3=Packard, Thomas W.}} {{PD-notice}} NTR engines, such as the Pewee of Project Rover, were selected in the Mars Design Reference Architecture (DRA).{{cite web |url=http://www.nasaspaceflight.com/2012/01/sls-exploration-roadmap-pointing-dual-mars-approach/|publisher=NASASpaceFlight.com|access-date=26 January 2012|author=Chris Bergin|date=24 January 2012|title=SLS Exploration Roadmap evaluations provide clues for human Mars missions}}{{cite web|url=http://www.space-travel.com/reports/NASA_Researchers_Studying_Advanced_Nuclear_Rocket_Technologies_999.html |title=NASA Researchers Studying Advanced Nuclear Rocket Technologies|author=Rick Smith for Marshall Space Flight Center, Huntsville, Alabama (SPX)|date=10 January 2013}}

In 2017, NASA continued research and development on NTRs, designing for space applications with civilian approved materials, with a three-year, US$18.8 million contract.{{cite web |url=http://www.nasa.gov/centers/marshall/news/news/releases/2017/nasa-contracts-with-bwxt-nuclear-energy-to-advance-nuclear-thermal-propulsion-technology.html|title=New NASA Contract Will Advance Nuclear Thermal Propulsion Technology|first=Lee|last=Mohon|date=2 August 2017|publisher=NASA}} {{PD-notice}}

In 2019, an appropriation bill passed by the U.S. Congress included US$125 million in funding for nuclear thermal propulsion research, including planning for a flight demonstration mission by 2024.{{cite news|url=https://spacenews.com/final-fiscal-year-2019-budget-bill-secures-21-5-billion-for-nasa/|title=Final fiscal year 2019 budget bill secures $21.5 billion for NASA|publisher=SpaceNews|date=17 February 2019|access-date=14 August 2019}}

As of 2021, there has been much interest in nuclear thermal rockets by the United States Space Force and DARPA for orbital and cis-lunar uses. In addition to the U.S. military, NASA administrator Jim Bridenstine has also expressed interest in the project and its potential applications for a future mission to Mars.{{cite web|url=https://www.space.com/darpa-nuclear-thermal-rocket-for-moon-contract|title=US military eyes nuclear thermal rocket for missions in Earth-Moon space |first=Mike |last=Wall |date=30 September 2020 |publisher=SPACE.com}} DARPA has awarded 2 contracts for their Demonstration Rocket for Agile Cislunar Operations (DRACO) program, which aims to demonstrate a nuclear thermal propulsion system in orbit: one award in September 2020 to Gryphon Technologies for US$14 million, and another award in April 2021 to General Atomics for US$22 million, both for preliminary designs for the reactor.{{cite web|url=https://spacenews.com/general-atomics-wins-darpa-contract-to-design-nuclear-reactor-to-power-missions-to-the-moon/|title=General Atomics wins DARPA contract to develop nuclear reactor to power missions to the moon|first=Sandra|last=Erwin |date=10 April 2021 |publisher=SpaceNews.com}} Two conceptual spacecraft designs by Blue Origin and Lockheed Martin were selected. Proposals for a flight demonstration of nuclear thermal propulsion in FY2026 were due on 5 August 2022.[https://spacenews.com/darpa-moving-forward-with-development-of-nuclear-powered-spacecraft/ DARPA moving forward with development of nuclear powered spacecraft] Sandra Erwin, SpaceNews. 4 May 2022

In January 2023, NASA and DARPA announced a partnership on DRACO to demonstrate an NTR engine in space, an enabling capability for NASA crewed missions to Mars.{{cite web |last1=Bardan |first1=Roxana |title=NASA, DARPA Will Test Nuclear Engine for Future Mars Missions - NASA |url=https://www.nasa.gov/news-release/nasa-darpa-will-test-nuclear-engine-for-future-mars-missions/ |publisher=NASA |date=24 January 2023}} In July 2023, U.S. agencies announced that Lockheed Martin had been awarded a $499 million contract to assemble the experimental nuclear thermal reactor vehicle (X-NTRV) and its engine.{{Cite web |last=Berger |first=Eric |date=2023-07-26 |title=The US government is taking a serious step toward space-based nuclear propulsion |url=https://arstechnica.com/space/2023/07/nasa-seeks-to-launch-a-nuclear-powered-rocket-engine-in-four-years/ |access-date=2023-07-26 |website=Ars Technica}}

An atmospheric or orbital rocket failure could result in the dispersal of radioactive material into the environment. A collision with orbital debris, material failure due to uncontrolled fission, material imperfections or fatigue, or human design flaws could cause a containment breach of the fissile material. Such a catastrophic failure while in flight could release radioactive material over the Earth in a wide and unpredictable area. The amount of contamination would depend on the size of the nuclear thermal rocket engine, while the zone of contamination and its concentration would be dependent on prevailing weather and orbital parameters at the time of re-entry.{{citation_needed|date=June 2019}}

It is considered unlikely that a reactor's fuel elements would be spread over a wide area, as they are composed of materials such as carbon composites or carbides and are normally coated with zirconium hydride.{{cite book |doi=10.1109/IECEC.1990.716860 |chapter=Safety Status of Space Radioisotope and Reactor Power Sources |title=Proceedings of the 25th Intersociety Energy Conversion Engineering Conference |date=1990 |last1=Bennett |first1=G.L. |volume=1 |pages=162–167 |isbn=0-8169-0490-1 }} Before criticality occurs, solid core NTR fuel is not particularly hazardous. Once the reactor has been started for the first time, extremely radioactive short-life fission products are produced, as well as less radioactive but extremely long-lived fission products. The amount of fission products is zero at fresh-fueled startup, and roughly proportional to (actually: limited by) the total amount of fission heat produced since fresh-fueled startup.{{cite book |doi=10.1063/1.41909 |chapter=Safety questions relevant to nuclear thermal propulsion |title=AIP Conference Proceedings |date=1992 |last1=Buden |first1=David |volume=246 |pages=648–654 |url=https://digital.library.unt.edu/ark:/67531/metadc1071526/ }}{{cite journal |last1=Sforza |first1=Pasquale |title=A safety and reliability analysis for space nuclear thermal propulsion systems |journal=Acta Astronautica |date=July 1993 |volume=30 |page=68 |doi=10.1016/0094-5765(93)90101-2 |bibcode=1992wadc.iafcQX...S }} Additionally, all engine structures are exposed to direct neutron bombardment, resulting in their radioactive activation.{{citation_needed|date=June 2019}}

{{Div col|colwidth=30em}}

• Fission-fragment rocket
• {{Annotated link|NERVA}}
• Nuclear electric rocket
• Nuclear pulse propulsion
• {{Annotated link|Project Pluto}}
• {{Annotated link|Project Prometheus}}
• {{Annotated link|Project Rover}}
• {{Annotated link|Project Timberwind}}
• Radioisotope rocket
• Spacecraft propulsion
• Thermal rocket
• {{Annotated link|UHTREX}}

{{Div col end}}

{{Reflist}}

• {{YouTube|id=eDNX65d-FBY|title=Nuclear Space Propulsion: NASA 1968}}
• [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920005899_1992005899.pdf Rover Nuclear Rocket Engine Program: Final Report - NASA 1991]
• [https://web.archive.org/web/20060112161610/http://prometheus.jpl.nasa.gov/index.cfm?pageL1=homePage Project Prometheus: Beyond the Moon and Mars]
• [https://web.archive.org/web/20090408122011/http://www.astronautix.com/engines/rd0410.htm RD-0410 USSR's nuclear rocket engine]

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