Editing Nuclear thermal rocket (section)

== Nuclear fuel types ==
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.<ref>{{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 }}</ref>

=== Solid core ===
[[File:NASA-NERVA-diagram.jpg|thumb|upright=1.0|right|A [[NERVA]] 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.<ref name=unisci20190703>{{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.''}}</ref> A nuclear solid core engine is the simplest design to construct and is the concept used on all tested NTRs.<ref>[https://beyondnerva.com/nuclear-thermal-propulsion/solid-core-ntr/ Solid Core NTR] Beyond Nerva. Retrieved 4 May 2022</ref>

Using hydrogen as a propellant, a solid core design would typically deliver specific impulses (I<sub>sp</sub>) on the order of 850 to 1000 seconds, which is about twice that of [[liquid hydrogen]]-[[Liquid oxygen|oxygen]] designs such as the [[Space Shuttle main engine]]. Other propellants have also been proposed, such as ammonia, water, or [[Liquid oxygen|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&nbsp;K, and at high pressures around 3000&nbsp;K. This lowers the mass of the exhaust species, increasing I<sub>sp</sub>.

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 unable<ref name="Alvarez">Alvarez, Luis, "There Is No Obvious Or Simple Way To Use Atomic Energy For Space Ships", [[U.S. Air Services]], January 1947, pp. 9-12</ref> 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 [[Gravitational potential|gravity well]], not to mention avoiding the [[radioactive contamination]] that would result from atmospheric use<ref name=unisci20190703/> (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.<ref name="projectrho">{{cite web|url=http://www.projectrho.com/public_html/rocket/enginelist2.php#id--Nuclear_Thermal|title=Engine List 2 - Atomic Rockets|website=projectrho.com}}</ref>)

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&nbsp;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.<ref>{{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."}}</ref>

=== Pulsed nuclear thermal rocket ===
{{Main|Pulsed nuclear thermal rocket}}

[[File:pulsedrocketAri.jpg|thumb|upright=1.0|right|Pulsed nuclear thermal rocket unit cell concept for ''I''<sub>sp</sub> amplification. In this cell, hydrogen-propellant is heated by the continuous intense neutron pulses in the propellant channels. At the same time, the unwanted energy from the fission fragments is removed by a solitary cooling channel with lithium or other liquid metal.]]

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 [[neutron]]s 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.<ref name=arias16>{{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 }}</ref>

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 ===
Liquid core nuclear engines are fueled by compounds of [[Fissile material|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&nbsp;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 of Earth|atmosphere]] and perhaps even [[magnetosphere]].{{citation_needed|date=June 2019}}

=== Gas core ===
[[File:Gas Core light bulb.png|thumb|upright=1.0|right|Nuclear gas core closed cycle rocket engine diagram, nuclear "light bulb"]]
[[File:Gas Core open cycle.png|thumb|upright=1.0|right|Nuclear gas core open cycle rocket engine diagram]]

The final fission classification is the [[Gas core reactor rocket|gas-core engine]]. This is a modification to the liquid-core design which uses rapid circulation of the fluid to create a [[toroid (geometry)|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&nbsp;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&nbsp;kN·s/kg).{{citation_needed|date=June 2019}}