Editing Nuclear thermal rocket (section)

=== 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>