Science: The Nuclear Rockets

When spaceships start using nuclear power, they will have to take off from deserts with no unsheltered humans for miles around. Only the crewmen in their cabins will be fully shielded. As the ship departs for space it will blast a considerable area with gamma rays, neutrons and radioactive exhaust, and a new, unpoisoned site may have to be found for the next takeoff. But designers of nuclear rockets do not worry much about this sort of thing. In Nucleonics, a group of experts tell about current projects to soar into space by atom power.

Light Molecules. The simplest kind of atomic engine uses a nuclear reactor to heat a gaseous propellant and shoot it out of a nozzle. Its chief advantage over chemical rocket engines: its propellant can be liquid hydrogen, whose molecules are light and therefore move faster at a given temperature. The best possible chemical combination (hydrogen and ozone), burning at 5,000° F. and 500 lbs.-per-sq.-in. chamber pressure, gives an exhaust velocity of 13,000 ft. per sec. A nuclear rocket, using hydrogen at the same pressure and only 3,000° F., shoots it out the tail pipe at 19,000 ft. per sec. If the working temperature rises to 4,500°, the exhaust velocity approaches 24,000 ft. per sec.

Since the efficiency of a rocket engine depends largely on its exhaust velocity, the nuclear engine has a big initial advantage, but it has to pay a high price. The engine itself, which must be cooled elaborately by the liquid hydrogen, will be about as complicated as a conventional chemical engine (see diagram). Its controls will be even more complicated, and all its delicate parts will have to perform perfectly in spite of intense gamma rays striking through them at takeoff.

The worst problem will be the reactor itself. The core will have to be small, probably a cylinder a few feet in diameter, but it will have to generate something like 100 times the energy of the massive reactor of Britain’s Calder Hall nuclear power station. This means that it will run very hot, and will be kept from flashing into vapor only by the stream of liquid hydrogen forced rapidly through it. On the other hand, the core need work for only a few minutes. By that time the propellant will have been exhausted, and the rocket will be on its way into deep space.

Kiwi-A. This sort of engine, which nuclear engineers consider a first step only, has been in development at Los Alamos Scientific Laboratory for three years under the code name of Project Rover. The first experimental engine, the Kiwi-A (which is not expected to fly, hence the name), is scheduled for testing in Nevada late this year, and an elaborate test setup is being built at Jackass Flats, a safe 20 miles (and many mountains) west of the Atomic Energy Commission’s main test base.

In spite of its comparative feebleness, Kiwi-A will do its reacting all by itself. The nearest humans will be at a control point 1½ miles away. When the test is over, Kiwi-A, still intensely radioactive, will be drawn along a railroad track by a remotely controlled locomotive and tucked into a shielded area where it can be inspected by nonhuman hands controlled from behind thick shields.

Kiwi-A will be only a small beginning. Later will come more ambitious engines of the same general type. The chances are that they will not be used for military purposes; chemical rockets can toss H-bombs cheaper and better. The role of nuclear rockets will be to carry large payloads to orbits around the earth or to the nearer parts of the solar system. They will be particularly good for ferrying supplies to an orbiting space-station. The engine will use only a small part of its uranium fuel during each trip, so if the space-ferry is recoverable, it can make several trips on the same charge.

For Deep Space. Nuclear rocket enthusiasts are not really satisfied with an engine that works in so simple a way. They are already dreaming of more sophisticated schemes for long-distance flights. One of these is an engine whose nuclear fuel is a uranium-rich gas mixed with the hydrogen propellant. When the nuclear reaction starts, both gases will get hot and blast out of the nozzle. This would produce a magnificent short-duration thrust, but the wasted uranium would cost something like $150 million per takeoff. The way around this little difficulty would be some system to keep the heavy uranium atoms in the reaction chamber while permitting the hydrogen to escape. No one now knows how to do it.

Another possibility is a rocket engine that uses nuclear fusion of heavy hydrogen instead of fission of uranium. No controlled fusion reactor has yet been constructed for any purpose, and making a light one for rockets will be much harder than making a heavy one for power stations. But the nuclear enthusiasts are not discouraged. Deuterium is cheap, they say, and even if the entire stock were shot out of the nozzle, the fuel for a flight would cost only $150,000.

Getting off the ground, say the nuclear rocketmen, is only part of the space-flight problem. After the earth has been left behind, and the ship is moving essentially in gravity-free space, it will need an engine that can exert a small thrust for a long time. Several nuclear systems look good for this purpose. A small stream of propellant could be heated by an electric arc, shooting out of the nozzle at very great speed. Or the propellant could be ionized and shot away from the rocket by electrical repulsion. The thrust of this system would be extremely low, but it would use little material. Ten Ibs. of thrust working for 1.5 years would speed a 50-ton spaceship to 135,000 m.p.h. At the end of this time it would have covered one billion miles, or beyond the orbit of Saturn.