Saturn by 1970

That was the motto of Project Orion, a very cool, hugely impractical rocket and politically impossible technology. It's motor was nuclear weapons thrown out the back and exploded, with the explosion pushing the ship.

Here is George Dyson, son of Freeman Dyson, one of the scientist who came up with Orion discussing it's history.



Here's an article on Orion. [Link]

Project Orion was a space vehicle propulsion system that depended on exploding atomic bombs roughly two hundred feet behind the vehicle (1). The seeming absurdity of this idea is one of the reasons why Orion failed; yet, many prominent physicists worked on the concept and were convinced that it could be made practical. Since atomic bombs are discrete entities, the system had to operate in a pulsed rather than a continuous mode. It is similar in this respect to an automobile engine, in which the peak combustion temperatures far exceed the melting points of the cylinders and pistons. The engine remains intact because the period of peak temperature is brief compared to the combustion cycle period.

The idea of an "atomic drive" was a science-fiction cliche by the 1930's, but it appears that Stanislaw Ulam and Frederick de Hoffman conducted the first serious investigation of atomic propulsion for space flight in 1944, while they were working on the Manhattan Project (2). During the quarter-century following World War II, the U.S. Atomic Energy Commission (replaced by the Department of Energy in 1974) worked with various federal agencies on a series of nuclear engine projects with names like Dumbo, Kiwi, and Pluto, culminating in NERVA (Nuclear Engine for Rocket Vehicle Application) (3). Close to producing a flight prototype, NERVA was cancelled in 1972 (4). The basic idea behind all these engines was to heat a working fluid by pumping it through a nuclear reactor, then allowing it to expand through a nozzle to develop thrust. Although this sounds simple the engineering problems were horrendous. How good were these designs? A useful figure for comparing rocket engines is specific impulse (Isp), defined as pounds of thrust produced per pound of propellant consumed per second. The units of Isp are thus seconds. The best chemical rocket in service, the cryogenic hydrogen-oxygen engine, has an Isp of about 450 seconds (5). NERVA had an Isp roughly twice as great (6), a surprisingly small figure considering that nuclear fission fuel contains more than a million times as much energy per unit mass as chemical fuel. A major problem is that the reactor operates at a constant temperature, and this temperature must be less than the melting point of its structural materials, about 3000 K (7).

A number of designs were proposed in the late 1940's and 1950's to get around the temperature limitation and to exploit the enormous power of the atomic bomb, estimated to be on the order of 10 billion horsepower for a moderate-sized device (8). The Martin Company designed a nuclear pulse rocket engine with a "combustion chamber" 130 feet in diameter. Small atomic bombs with yields under 0.1 kiloton (a kiloton is the energy equivalent of 1000 tons of the high explosive TNT) would have been dropped into this chamber at a rate of about one per second (9); water would have been injected to serve as propellant. This design produced the relatively small Isp of 1150 seconds, and could have yielded a maximum velocity change for the vehicle of 26,000 feet/second. The vehicle would have been boosted to an altitude of 150 miles by chemical rockets, and the extra 8000 ft/sec or so thus provided would have allowed it to escape the Earth's gravity (10). The Lawrence Livermore Laboratory produced a similar although much smaller design called Helios at about the same time (11).

In a classified 1955 paper (12), Stanislaw Ulam and Cornelius Everett eliminated the combustion chamber entirely. Instead, bombs would be ejected backwards from the vehicle, followed by solid-propellant disks. The explosions would vaporize the disks, and the resulting plasma would impinge upon a pusher plate. The advantage of this system is that no attempt is made to confine the explosions, implying that relatively high-yield (hence high-power) bombs may be used. Such a system is neither temperature- nor power-limited. Ulam may have been influenced by experiments conducted at the Eniwetok proving grounds, where graphite-covered steel spheres were suspended thirty feet from the center of an atomic explosion. The spheres were later found intact; a thin layer of graphite had been ablated from their surfaces (13).

Project Orion was born in 1958 at General Atomics in San Diego. The company, now a subsidiary of defense giant General Dynamics, was founded by Frederick de Hoffman to develop commercial nuclear reactors. The driving force behind Orion was Theodore Taylor, a veteran of the Los Alamos weapons programs. De Hoffman persuaded Freeman Dyson, a theoretical physicist then at the Institute for Advanced Study in Princeton, New Jersey, to come to San Diego to work on Orion during the 1958-1959 academic year. Dyson says that Taylor adopted a specific management model for the project: the Verein fur Raumschiffahrt (VfR), the German rocket society of the 1920's and 1930's which numbered among its members Werner von Braun. The VfR had little structure: no bureaucracy and essentially no division of labor between its members; it accomplished much before it was taken over by the German army. Orion at first was similar: scientists did practical engineering and engineers built working scale models, all on a shoestring budget (14).

Here's a video of a small model propelled by small explosive charges.