Fission-fragment rocket
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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.
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 lbf·s/lb (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.
One such design was worked on to some degree by the Idaho National Engineering Laboratory and Lawrence Livermore National Laboratory. In their design the fuel was placed into a number of very thin carbon bundles, each one normally sub-critical. Bundles were collected and arranged like spokes on a wheel, and the entire wheel (or stack of them) was rotated such that some bundles were always in a reactor core where additional surrounding fuel made the bundles go critical. The fission fragments at the surface of the bundles would break free and be channeled for thrust, while the lower-temperature un-reacted fuel would eventually rotate out of the core to cool. The system thus automatically "selected" only the most energetic fuel to become the working mass.
The efficiency of the system is surprising; specific impulses of greater than 100,000 are possible. This is high performance, although not that which the technically daunting antimatter rocket could achieve, and the weight of the reactor core and other elements would make the overall performance of the fission-fragment system lower. Nonetheless, the system provides the sort of performance levels that would make an interstellar precursor mission possible.