Nuclear pulse propulsion

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Nuclear pulse propulsion (or External Pulsed Plasma Propulsion, as it is termed in one recent NASA document[1]) is a proposed method of spacecraft propulsion that uses nuclear explosions for thrust. It was first developed as Project Orion by DARPA, after a suggestion by Stanislaw Ulam in 1947.[2] Newer designs using inertial confinement fusion have been the baseline for most post-Orion designs, including Project Daedalus and Project Longshot.

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Project Orion

Project Orion was the first serious attempt to design a nuclear pulse rocket. The design effort was carried out at General Atomics in the late 1950s and early 1960s. The idea of Orion was to react small directional nuclear explosives against a large steel pusher plate attached to the spacecraft with shock absorbers. Efficient directional explosives maximized the momentum transfer, leading to specific impulses in the range of 6,000 seconds (about twelve times that of the Space Shuttle main engine). With refinements a theoretical maximum of 100,000 seconds (1 MN·s/kg) might be possible. Thrusts were in the millions of tons, allowing spacecraft larger than 8×106 tons to be built with 1958 materials.[3]

The reference design was to be constructed of steel using submarine-style construction with a crew of more than 200 and a vehicle takeoff weight of several thousand tons. This low-tech single-stage reference design would reach Mars and back in four weeks from the Earth's surface (compared to 12 months for NASA's current chemically-powered reference mission). The same craft could visit Saturn's moons in a seven-month mission (compared to chemically-powered missions of about nine years).

A number of engineering problems were found and solved over the course of the project, notably related to crew shielding and pusher-plate lifetime. The system appeared to be entirely workable when the project was shut down in 1965, the main reason being given that the Partial Test Ban Treaty made it illegal (however, before the treaty, the U.S. and Soviet Union had already exploded at least nine nuclear bombs, including thermonuclear bombs, in "space," i.e., at altitudes over 100 km: see high altitude nuclear explosions). There were also ethical issues with launching such a vehicle within the Earth's magnetosphere. Calculations showed that the fallout from each takeoff would kill between 1 and 10 people[citation needed] (a claim that has been disputed: see radiation hormesis).

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