The Orion Project
In the 1960s NASA began testing designs to superheat hydrogen gas inside a nuclear reactor in order to produce thrust from a rocket nozzle. Termed the NERVA project, this idea seemed to produce practical solutions for space travel as long as the temperatures were kept low enough inside the reactor. To approach relativistic velocity, however, such a reactor would have to be heated far beyond the melting point, so NERVA was not considered a practical solution for interstellar propulsion. In 1955 an alternative to using a nuclear reactor was devised, one which could utilize the upper theoretical limits of nuclear energy. In the 1960s this idea became a highly classified project under NASA and the Airforce because of its nature as a spacecraft propelled by atomic bombs. It was called Orion.
The Orion concept also utilized superheated hydrogen gas as propellant. Instead of a reactor heating the gas inside the spacecraft, Orion ejected atomic bombs at proper intervals encased in a hydrogen-rich plastic casing at the rear of the spacecraft. When detonated at a suitable distance some of the superheated plasma would inpinge upon a large inertial plate at the rear of the spacecraft, which would propel Orion forward. Made of metal, the pusher plate was only exposed to extreme temperatures for a tiny fraction of a second, thereby preventing the plate from vaporizing beyond the surface.
An interstellar version of Orion called for a 40 ton spacecraft powered by 10 million bombs. The beauty of the design was its high rate specific impulse, which was theoretically as high as 1,000,000 seconds and therefore capable of reaching .1c or 10% the speed of light. In terms of mission time Orion has the potential to reach Alpha Centauri in roughly 40 years.
Unfortunately for the Orion project, which was under the guise of the Airforce, the United States signed the nuclear test-ban treaty with Britain and the Soviet Union in 1963. Overshadowed by the Apollo moon program, Orion was ignored by NASA as a peaceful means of space exploration and disappeared from history.
There were no actual serious challenges to the economic or technical feasibility in the Orion concept. Its major weakness was it's use of nuclear weapons, which were seriously harmful to the environment. Originally Orion was designed to blast off from the ground. Later a much more environmentally practical design was introduced, which would be lifted into orbit via Saturn V rockets and then assembled.
References
Dyson, F. "Death of a Project," Science, 149, 141-44 (1965)
Dyson, F. "Interstellar Transport," Physics Today, 41 (October 1968).
"Orion Project" The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. 06/23/2003 <http://www.angelfire.com/on2/daviddarling/OrionProj.htm>
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