Research
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 several such wheels were stacked on a common shaft to produce a single large cylinder. The entire cylinder was rotated so 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 100,000s are possible using existing materials. 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.
A newer design proposal by Rodney A. Clark and Robert B. Sheldon theoretically increases efficiency and decreases complexity of a fission fragment rocket at the same time over the bundle proposal. In their design, nanoparticles of fissionable fuel (or even fuel that will naturally radioactively decay) are kept in a vacuum chamber subject to an axial magnetic field (acting as a magnetic mirror) and an external electric field. As the nanoparticles ionize as fission occurs, the dust becomes suspended within the chamber. The incredibly high surface area of the particles makes radiative cooling simple. The axial magnetic field is too weak to affect the motions of the dust particles but strong enough to channel the fragments into a beam which can be decelerated for power, allowed to be emitted for thrust, or a combination of the two. With exhaust velocities of 3% - 5% the speed of light and efficiencies up to 90%, the rocket should be able to achieve over 1,000,000 sec Isp.
Read more about this topic: Fission-fragment Rocket
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