Page "Nuclear thermal rocket" Paragraph 14

These engines are currently considered very difficult to build.

The reaction time of the nuclear fuel is much higher than the heating time of the working fluid, requiring a method to trap the fuel inside the engine while allowing the working fluid to easily exit through the nozzle.

Most liquid-phase engines have focused on rotating the fuel / fluid mixture at very high speeds, forcing the fuel to the outside due to centripetal force ( uranium is heavier than hydrogen ).

In many ways the design mirrors the particle-bed design, although operating at even higher temperatures.