NUCLEAR ROCKETRY

The great advantage of nuclear over chemical fuels for rocket propulsion was realized early. It has advantages for providing electric power on board satellites and spacecraft, too, but this is something I will leave for another issue; this month we will only take a quick look at the use of nuclear energy for the propulsion of spacecraft.

The first try to use nuclear propulsion was the attempted development of a long-range bomber with a nuclear-powered jet engine in the 1950s. The problems of weight and size imposed by the shielding of the reactor were immense and were never overcome; the development of the chemically fueled B-52 bomber, and above all the shift to missiles for carrying bombs, made further development pointless, and in 1962 President Kennedy killed the unwieldy project.

But for spacecraft the story is quite different. At least three types of nuclear-powered rocket engine have been developed and successfully tested on the ground, though none of them have been actually deployed in space.

Nuclear reactors can be made fairly lightweight and the vastly higher energy concentration of their fuel soon pays off. (Chemical fuels are not very light, either: the ill-fated space shuttle carried 14 tons of fuel.) One of the methods of harnessing nuclear power for spacecraft is "ion propulsion," in which positively and negatively charged ions are expelled to the rear of the vehicle; they can be electrically accelerated to higher velocities than can be done with neutral molecules by mere mechanical pressure, so that the momentum imparted to the spaceraft is higher. By the time a nuclear rocket has left the earth's gravitational field, it therefore has a far higher velocity than a chemically fueled vehicle of the same weight. For example, to put a spacecraft in orbit round Neptune with chemical fuel would require a flight time of 20 years; nuclear-electric propulsion would cut the time in half.