Editing Nuclear thermal rocket (section)

== Solid core fission designs in practice ==
[[File:Kiwi-A Prime Atomic Reactor - GPN-2002-000141.jpg|thumb|upright=1.0|right|The KIWI A prime nuclear thermal rocket engine]]

=== Soviet Union and Russia ===
The Soviet [[RD-0410]] went through a series of tests at the nuclear test site near [[Semipalatinsk Test Site]].<ref name="astronautix1">{{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}}</ref><ref name="KBKhA">{{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}}</ref>

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 thruster|ion engines]].<ref>{{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}}</ref>

=== United States ===
[[File:DOE video about nuclear thermal propulsion rockets.ogg|thumb|upright=1.0|right|A United States Department of Energy video about nuclear thermal rockets.]]

Development of solid core NTRs started in 1955 under the [[United States Atomic Energy Commission|Atomic Energy Commission]] (AEC) as [[Project Rover]] and ran to 1973.<ref name=unisci20190703/> 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.<ref name="dewar">{{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}}</ref>

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.<ref name="dewar"/> 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.<ref name=unisci20190703/> 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&nbsp;kN of thrust and operated at a specific impulse of 875 seconds (8.58&nbsp;kN·s/kg), and it was planned to increase this to 975 seconds, achieving a [[Propellant mass fraction|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<sub>sp</sub> 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}}

==== Test firings ====
[[File:Destruction of KIWI Nuclear Reactor - GPN-2002-000145.jpg|thumb|upright=1.0|right|A KIWI engine being destructively tested.]]

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 [[Kiwi (bird)|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&nbsp;[[Watt|MW]] at an exhaust temperature of 2683&nbsp;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]] (UO<sub>2</sub>) 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|NERVA/Rover]] project was eventually canceled in 1972 with the general wind-down of NASA in the post-[[Project Apollo|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}}

==== Kiwi-TNT destructive test ====
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]].<ref>{{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}}</ref>

=== United Kingdom ===
As of January 2012, the propulsion group for [[Project Icarus (Interstellar Probe Design Study)|Project Icarus]] was studying an NTR propulsion system,<ref>{{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}}</ref> but has seen little activity since 2019.{{fact|date=March 2025}}

=== Israel ===
In 1987, Ronen & Leibson<ref name=Ronen1987>Ronen, 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</ref><ref name="Ronen1988">{{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 }}</ref> published a study on applications of <sup>242m</sup>Am (one of the [[isotopes of americium]]) as nuclear fuel to [[Nuclear power in space|space nuclear reactors]], noting its extremely high [[Neutron cross section|thermal cross section]] and [[energy density]]. Nuclear systems powered by <sup>242m</sup>Am require less fuel by a factor of 2 to 100 compared to conventional [[nuclear fuel]]s.{{fact|date=March 2025}}

[[Fission-fragment rocket]] using <sup>242m</sup>Am was proposed by [[George Chapline Jr.|George Chapline]]<ref name=Chapline1988>{{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 }}</ref> 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.<ref name=Ronen/> demonstrate that <sup>242m</sup>Am can maintain sustained nuclear fission as an extremely thin metallic film, less than 1/1000 of a millimeter thick. <sup>242m</sup>Am requires only 1% of the mass of <sup>235</sup>U or <sup>239</sup>Pu to reach its critical state. Ronen's group at the [[Ben-Gurion University of the Negev]] further showed that nuclear fuel based on <sup>242m</sup>Am could speed space vehicles from Earth to Mars in as little as two weeks.<ref>{{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}}</ref>

The <sup>242m</sup>Am as a nuclear fuel is derived from the fact that it has the highest thermal fission cross section (thousands of [[Barn (unit)|barns]]), about 10x the next highest cross section across all known isotopes. The <sup>242m</sup>Am is [[fissile]] (because it has an odd number of [[neutron]]s) and has a low [[critical mass]], comparable to that of [[plutonium-239|<sup>239</sup>Pu]].<ref>{{cite web |title=Critical Mass Calculations for <sup>241</sup>Am, <sup>242m</sup>Am and <sup>243</sup>Am|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}}</ref><ref>{{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 }}</ref>

It has a very high [[Nuclear cross section|cross section]] for fission, and if in a nuclear reactor is destroyed relatively quickly. Another report claims that <sup>242m</sup>Am can sustain a chain reaction even as a thin film, and could be used for a novel type of [[nuclear rocket]].<ref name=Ronen>{{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}}</ref><ref name="Ronen2">{{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}}</ref><ref name="Ronen2000">{{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 }}</ref><ref name="Ronen2006">{{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 }}</ref>

Since the thermal [[absorption cross section]] of <sup>242m</sup>Am is very high, the best way to obtain <sup>242m</sup>Am is by the capture of [[Neutron temperature#Fast|fast]] or [[Neutron temperature#Epithermal|epithermal]] neutrons in [[Americium-241]] irradiated in a [[Fast-neutron reactor|fast reactor]]. However, [[Fast neutron reactor|fast spectrum reactors]] are not readily available. Detailed analysis of <sup>242m</sup>Am breeding in existing [[pressurized water reactor]]s (PWRs) was provided.<ref>{{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 }}</ref> [[Treaty on the Non-Proliferation of Nuclear Weapons|Proliferation]] resistance of <sup>242m</sup>Am was reported by the [[Karlsruhe Institute of Technology]] 2008 study.<ref>{{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 }}</ref>

=== Italy ===
In 2000, [[Carlo Rubbia]] at [[CERN]] further extended the work by Ronen<ref name="Ronen1988"/> and [[George Chapline Jr.|Chapline]]<ref name=Chapline1988/> on a [[Fission-fragment rocket]] using <sup>242m</sup>Am as a fuel.<ref name=Rubbia2000>Rubbia, Carlo. "Fission fragments heating for space propulsion" No. SL-Note-2000-036-EET. CERN-SL-Note-2000-036-EET, 2000</ref> Project 242<ref>{{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 }}</ref> based on Rubbia design studied a concept of <sup>242m</sup>Am based Thin-Film Fission Fragment Heated NTR<ref>{{cite report |last1=Davis |first1=Eric W |title=Advanced Propulsion Study |id={{DTIC|ADA426465}} |publisher=Warp Drive Metrics |date=2004 }}</ref> 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.<ref>{{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 }}</ref> 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 <sup>242m</sup>Am in conventional thermal nuclear reactors.<ref>{{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}}</ref>

=== European Space Agency ===

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.<ref>{{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}}</ref><ref>{{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}}</ref>

=== Current research in the US since 2000 ===
[[File:Orion docked to Mars Transfer Vehicle.jpg|thumb|upright=1.0|right|Artist's impression of bimodal NTR engines on a [[Mars Transfer Vehicle]] (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|thumb|right|Artist's concept of the Demonstration Rocket for Agile Cislunar Operations (DRACO).]]

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.<ref>{{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}}</ref>

As of 2013, an NTR for [[interplanetary spaceflight|interplanetary travel]] from Earth orbit to Mars orbit is being studied at [[Marshall Space Flight Center]] with [[Glenn Research Center]].<ref>{{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}}</ref> In historical ground testing, NTRs proved to be at least [[specific impulse|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,<ref>{{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}}</ref> compared to 6–9 months using chemical engines,<ref>{{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}}</ref> would reduce crew exposure to potentially harmful and difficult to [[radiation shielding|shield]] [[cosmic ray]]s.<ref>{{cite web|url=http://www.adastrarocket.com/aarc/NEP-Mars|title=How Fast Could (Should) We Go to Mars? &#124; 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}}</ref><ref name="arc.aiaa.org">{{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 }}</ref><ref name=NTP2012>{{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}}</ref><ref>{{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}}</ref> NTR engines, such as the [[NERVA|Pewee]] of [[Project Rover]], were selected in the [[Mars Semi-Direct|Mars Design Reference Architecture]] (DRA).<ref name="arc.aiaa.org"/><ref name=NTP2012/><ref name="MarsRoadmap">{{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}}</ref><ref>{{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}}</ref>

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.<ref name=NTP-2017-08>{{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}}</ref>

In 2019, an appropriation bill passed by the [[United States Congress|U.S. Congress]] included US$125 million<ref name=unisci20190703/> in funding for nuclear thermal propulsion research, including planning for a flight demonstration mission by 2024.<ref name=SpaceNews>{{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}}</ref>

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 [[Human mission to Mars|mission to Mars]].<ref name=S-2020-Gry>{{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}}</ref> [[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,<ref name=S-2020-Gry/> and another award in April 2021 to General Atomics for US$22 million, both for preliminary designs for the reactor.<ref name=SN-2021-GA>{{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}}</ref> Two conceptual spacecraft designs by Blue Origin and Lockheed Martin were selected. Proposals for a flight demonstration of nuclear thermal propulsion in [[Fiscal year|FY]]2026 were due on 5 August 2022.<ref>[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</ref>

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.<ref>{{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}}</ref> 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.<ref>{{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}}</ref>