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Nuclear pulse propulsion

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An artist's conception of a spacecraft powered by nuclear pulse propulsion
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An artist's conception of a spacecraft powered by nuclear pulse propulsion

Nuclear pulse propulsion (or External Pulsed Plasma Propulsion, as it is termed in one recent NASA document) is a proposed method of spacecraft propulsion that uses nuclear explosions for thrust. It was first developed as Project Orion by ARPA, after a suggestion by Stanislaw Ulam in 1957. Newer designs using inertial confinement fusion have been the baseline for most post-Orion designs, including the famous Project Daedalus and the less well-known Project Longshot.

Contents

Project Orion

The first serious attempt to design a nuclear pulse rocket was Project Orion (See the article for details, including the vehicle sizes, problems, propulsion cycle and shielding). The design effort was carried out at General Atomics in the late 1950s and early 1960s. Orion reacted 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 six times that of the SSME). With refinements a theoretical maximum of 100,000 seconds (1 MNs/kg) might be possible. Thrusts were in the millions of short tonnes, allowing spacecraft larger than 8x10^6 short tonnes to be built with 1958 materials.

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 tonnes. This low-tech single-stage reference design would reach Mars and back in four weeks from the Earth's surface (compare to 12 months for NASA's current chemically-powered reference mission). The same craft could visit Saturn's moons in a seven-month mission (compare 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 (good) and pusher-plate lifetime (which was unlimited). The system appeared to be entirely workable when the project was shut down in 1965, the main reason being given that the Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space, and Under Water made it illegal. There were also ethical issues with launching such a vehicle within the Earth's magnetosphere. Calculations showed that between 1 and 10 people would die from each takeoff from fallout. Enthusiasts want to launch outside the magnetosphere, or use clean fusion explosives, which are technically, but not politically feasible.

One useful mission for this near-term technology would be to deflect an asteroid that could collide with the earth. The extremely high performance would permit even a late launch to succeed, and the vehicle could effectively transfer a large amount of kinetic energy to the asteroid by simple impact. Also, an automated mission would eliminate the most problematic issues of the design: the shock absorbers.

Orion's technology is also one of very few known interstellar space drives that could be constructed with known technology.

Some authorities say that President Kennedy initiated the Apollo program to buy off the technical enthusiasts backing the Orion program. The recent book by George Dyson says that one design proposal presented to Kennedy was a space-going nuclear battleship, which so offended him that he decided to end the program.

Project Daedalus

Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society (BIS) to design a plausible interstellar unmanned spacecraft that could reach a nearby star within one human lifetime (set to be 50 years). A dozen scientists and engineers led by Alan Bond worked on the project. At the time fusion research appeared to be making great strides, and in particular, inertial confinement fusion (ICF) appeared to be adaptable as a rocket engine.

ICF uses small pellets of fusion fuel, typically Li6D with a small deuterium/tritium "trigger" at the center. The pellets are thrown into a reaction chamber where they are hit on all sides by lasers or another form of beamed energy. The heat generated by the beams explosively compresses the pellet, to the point where fusion takes place. The result is a hot plasma, and a very small "explosion" compared to the minimum size bomb that can be created using conventional means.

For Daedalus, this process was run within a large electromagnet which formed the rocket engine. After the reaction, ignited by electron beams in this case, the magnet funnelled the hot gas to the rear for thrust. Some of the energy was also collected to run the ship's systems and engine. In order to make the system safe and energy efficient, Daedalus was powered by a Helium-3 fuel that would have had to be collected from Jupiter.

Medusa

The "Medusa" design is a type of nuclear pulse propulsion which shares more in common with solar sails than with conventional rockets. It was proposed in the 1990s in another BIS project when it became clear that ICF did not appear to be able to run both the engine and the ship, as previously believed.

A Medusa spacecraft would deploy a large sail ahead of it, attached by cables, and then launch nuclear explosives forward to detonate between itself and its sail. The sail would be accelerated by the impulse, and the spacecraft would follow.

Medusa performs better than the classical Orion design because its "pusher plate" intercepts more of the bomb's blast, its shock-absorber stroke is much longer, and all its major structures are in tension and hence can be quite lightweight. It also scales down better. Medusa-type ships would be capable of a specific impulse between 50,000 and 100,000 lbfs/lb (500 to 1000 kNs/kg).

The Jan 1993 and June 1994 issues of JBIS have articles on Medusa. (There is also a related paper in the Nov/Dec 2000 issue.)

Project Longshot

Project Longshot was a NASA-sponsored research project carried out at the US Navy Naval Academy in the early 1990s. Longshot was in some ways a development of the basic Daedalus concept, in that it used magnetically-funneled ICF as a rocket. The key difference was that they felt that the reaction could not power both the rocket and the systems, and instead included a 300 kW conventional nuclear reactor for running the ship. The added weight of the reactor reduced performance somewhat, but even using LiD fuel it would be able to reach Alpha Centauri in 100 years.

Antimatter catalyzed nuclear pulse propulsion

In the mid-1990s research at the Pennsylvania State University led to the concept of using antimatter to catalyze nuclear reactions. In short, anti-protons would react inside the nucleus of uranium, causing a release of energy that breaks the nucleus apart as in conventional nuclear reactions. Even a small number of such reactions can start the chain reaction that would otherwise require a much larger volume of fuel to sustain. Whereas the "normal" critical mass for plutonium is about 11.8 kilograms, with antimatter catalyzed reactions this could be well under one gram.

Several rocket designs using this reaction were proposed, ones using all-fission for interplanetary missions, and others using fission-fusion (effectively a very small version of Orion's bombs) for interstellar ones. See antimatter catalyzed nuclear pulse propulsion for details.

See also

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