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Fission-fragment rocket
The fission-fragment rocket is a rocket engine design that directly harnesses hot nuclear fission products for thrust, as opposed to using a separate fluid as working mass. The design can, in theory, produce very high specific impulses while still being well within the abilities of current technologies.
== 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 the average temperature of the fuel, the vast majority of which is not reacting at any given instant. 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 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 converted to heat in the surrounding fuel mass.
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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