Gone with fission.

Gone with fission 

Both fission and fusion reactions involving atomic nuclei throw off energetic debris in the form of fragments of nuclei. For some time scientists and technologists have wondered whether such flying fragments could be used to provide thrust for rocket engines. George Chapline of Lawrence Livermore (Calif.) National Laboratory said that he hated to be the bearer of bad news at a meeting of fusion enthusiasts, but he thinks a fusion rocket is impossible. However, he described a fission rocket that he thinks would work.

To do it, the reactor fuel has to be arranged so that the fission fragments can escape. Thus he suggests using layers of thin wires of americium--an uncommon metal, but more efficient than the more common plutonium--spaced out in a volume 10 meters by 10 meters by 1 meter. The fuel area would be surrounded by electric currents producing a magnetic field in the shape of a "magnetic mirror,' which would gather up the fission fragments and bounce them back and forth.

This would be a leaky mirror, and some of the fragments would escape out the end--at least 50 percent, and Chapline hopes possibly 90 percent--producing a thrust that could have a specific impulse up to 1 million seconds. However, the wires of fuel would have to be mounted on wheels and rotated to dissipate the tremendous heat produced. Otherwise the arrangement would have to be too large for a practical spacecraft.

Such an engine might be useful for deep-space missions. For example, for a 50-year trip to Alpha Centauri, the nearest star, Chapline calculates it would take 200 tons of americium. The reactor fuel would last for 25 years, accelerating the spacecraft to one-eighth the velocity of light in that time. The craft would then be about 3 light-years from Alpha Centauri (or 1 light-year from earth). It would coast for 25 more years to finish its trip.