Editing Interplanetary spaceflight (section)

====Nuclear thermal and solar thermal rockets====
[[File:Nuclear thermal rocket en.svg|thumb|250px|Sketch of nuclear thermal rocket]]
In a [[nuclear thermal rocket]] or [[solar thermal rocket]] a working fluid, usually [[hydrogen]], is heated to a high temperature, and then expands through a [[nozzle|rocket nozzle]] to create [[thrust]]. The energy replaces the chemical energy of the reactive chemicals in a traditional [[rocket engine]]. Due to the low [[molecular mass]] and hence high thermal velocity of hydrogen these engines are at least twice as fuel efficient as chemical engines, even after including the weight of the reactor.{{Citation needed|date=April 2007}}

The US [[United States Atomic Energy Commission|Atomic Energy Commission]] and NASA tested a few designs from 1959 to 1968. The NASA designs were conceived as replacements for the upper stages of the [[Saturn V]] launch vehicle, but the tests revealed reliability problems, mainly caused by the vibration and heating involved in running the engines at such high thrust levels. Political and environmental considerations make it unlikely such an engine will be used in the foreseeable future, since nuclear thermal rockets would be most useful at or near the Earth's surface and the consequences of a malfunction could be disastrous. Fission-based thermal rocket concepts produce lower exhaust velocities than the electric and plasma concepts described below, and are therefore less attractive solutions. For applications requiring high thrust-to-weight ratio, such as planetary escape, nuclear thermal is potentially more attractive.<ref>{{cite web |url=https://x-energy.com/why/nuclear-and-space/nuclear-thermal-propulsion |title=Nuclear Thermal Propulsion |author=<!--Not stated--> |website=X-Energy |access-date=2024-02-07 |quote=One of the main benefits of nuclear thermal propulsion is its efficiency. A nuclear thermal rocket can achieve more than twice the efficiency compared to a conventional chemical rocket because it's propellant is brought to a far higher temperature than can be achieved in a conventional combustion chamber. |archive-date=2024-02-07 |archive-url=https://web.archive.org/web/20240207175946/https://x-energy.com/why/nuclear-and-space/nuclear-thermal-propulsion |url-status=live }}</ref>