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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.
A nuclear salt-water rocket (or NSWR) is a proposed type of nuclear thermal rocket designed by Robert Zubrin that would be fueled by water bearing dissolved salts of Plutonium or U235. These would be stored in tanks that would prevent a critical mass from forming by some combination of geometry or neutron absorption (for example: long tubes made out of boron in an array with considerable spacing between tubes). Thrust would be generated by nuclear fission reactions from the nuclear salts heating the water and being expelled through a nozzle. The water would serve as both a neutron moderator and propellant.
Because of their ability to harness the power of what is essentially a continuous nuclear fission explosion, NSWRs would have both very high thrust and very high exhaust velocity, a rare combination of traits in the rocket world, meaning that the rocket would be able to accelerate quickly as well as be extremely efficient in terms of propellant usage. One design would generate 13 meganewtons of thrust at 66 km/s exhaust velocity (compared to ~4.5 km/s exhaust velocity for the best chemical rockets of today). Another design would achieve much higher exhaust velocities (4,700 km/s) and use 2,700 tonnes of highly enriched Uranium salts in water to propel a 300 tonne spacecraft up to 3.6% of the speed of light.
WASHINGTON: The defence department first proposed Star Wars. Now it wants Star Trek. DARPA, the Pentagon's research agency that helped foster the internet, wants someone to dream up a way to send people to a star.