Editing Nuclear thermal rocket (section)

=== 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}}