Editing Nuclear thermal rocket (section)

== Principle of operation ==
{{More citations needed section|date=February 2025}}

Nuclear-powered thermal rockets are more effective than chemical thermal rockets, primarily because they can use low-molecular-mass propellants such as hydrogen.<ref>{{Cite web |date=2021-02-12 |title=Nuclear Propulsion Could Help Get Humans to Mars Faster - NASA |url=https://www.nasa.gov/solar-system/nuclear-propulsion-could-help-get-humans-to-mars-faster/ |access-date=2024-04-26}}</ref><ref>{{Cite web |date=2018-05-25 |title=Nuclear Thermal Propulsion: Game Changing Technology for Deep Space Exploration - NASA |url=https://www.nasa.gov/directorates/stmd/tech-demo-missions-program/nuclear-thermal-propulsion-game-changing-technology-for-deep-space-exploration/ |access-date=2024-04-26}}</ref>

As thermal rockets, nuclear thermal rockets work almost exactly like [[Rocket engine#Chemically powered|chemical rockets]]: a heat source releases [[thermal energy]] into a gaseous [[propellant]] inside the body of the engine, and a [[nozzle]] at one end acts as a very simple heat engine: it allows the propellant to expand away from the vehicle, carrying momentum with it and converting thermal energy to coherent kinetic energy.  The [[specific impulse]] (Isp) of the engine is set by the speed of the exhaust stream.<ref>{{cite book |doi=10.1016/B978-0-323-91360-7.00001-X |chapter=The scale of the problem: Interstellar distances, time, and energy considerations |title=Interstellar Travel |date=2023 |last1=Matloff |first1=Greg |last2=Gerrish |first2=Harold |pages=51–82 |isbn=978-0-323-91360-7 }}</ref> That, in turn, varies as the square root of the kinetic energy loaded on each unit mass of propellant.  The kinetic energy per molecule of propellant is determined by the temperature of the heat source (whether it be a [[nuclear reactor]] or a [[chemical reaction]]).  At any particular temperature, lightweight propellant molecules carry just as much kinetic energy as heavier propellant molecules and therefore have more kinetic energy per unit mass.  This makes low-molecular-mass propellants more effective than high-molecular-mass propellants.

Because chemical rockets and nuclear rockets are made from refractory solid materials, they are both limited to operate below {{Convert|3000|C|F|sigfig=1}}, by the strength characteristics of high-temperature metals.  Chemical rockets use the most readily available propellant, which is waste products from the chemical reactions producing their heat energy.  Most liquid-fueled chemical rockets use either hydrogen or hydrocarbon combustion, and the propellant is therefore mainly water (molecular mass 18) and carbon dioxide (molecular mass 44).  Nuclear thermal rockets using gaseous hydrogen propellant (molecular mass 2) therefore have a theoretical maximum specific impulse that is 3 to 4.5 times greater than those of chemical rockets.