Editing Nuclear thermal rocket (section)

=== 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>