Design Considerations
In traditional nuclear thermal rocket and related designs, the nuclear energy is generated in some form of "reactor" and used to heat a working fluid to generate thrust. This limits the designs to temperatures that allow the reactor to remain "whole", although clever design can increase this critical temperature into the tens of thousands of degrees. A rocket engine's efficiency is strongly related to the temperature of the exhausted working fluid, and in the case of the most advanced gas-core engines, it corresponds to a specific impulse of about 7000 s (69 kN·s/kg).
The temperature of a conventional reactor design is actually the average temperature of the fuel, the vast majority of which is not actually reacting at any given instance. In fact the atoms undergoing fission are at a temperature of millions of degrees, which is then spread out into the surrounding fuel, resulting in an overall temperature of a few thousand. In the fission-fragment design, it is the individual atoms that actually undergo fission that are used to provide thrust, by extracting them from the rest of the fuel as quickly as possible before their energy is spread out into the surrounding fuel mass.
This is easier to achieve than it might sound. By physically arranging the fuel such that the outermost layers of a fuel bundle will be most likely to undergo fission, the high-temperature atoms, the fragments of a nuclear reaction, can "boil" off the surface. Since they will be ionized due to the high temperatures of the reaction, they can then be handled magnetically and channeled to produce thrust. Numerous technological challenges still remain, however.
Read more about this topic: Fission-fragment Rocket
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