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

1 Project Orion 2 Project Daedalus 3 Medusa 4 Project Longshot 5 Antimatter catalyzed nuclear pulse propulsion 6 See also

Project Orion

The first serious attempt to design a nuclear pulse rocket was Project Orion, carried out at General Atomics in the late 1950s and early 1960s. Orion used small nuclear bombs, to react against a large plate attached to the spacecraft with shock absorbing systems. Careful explosive design maximized the momentum transfer, which led to specific impulses in the range of 2,000 seconds (about four times that of the SSME) to a theoretical maximum of 100,000. Thrusts were in the millions of tons, allowing very large spacecraft to be built.

Performance would have been most impressive, single stage to Mars and back from the Earth's surface; and plans were made to visit Saturn. All this with a crew of more than 200 and a vehicle takeoff weight of several thousand tonnes.

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 nuclear weapon test bans made it illegal to explode bombs in space. There were also ethical issues with launching such a vehicle from the surface of the earth- calculations showed that between 1 and 10 people would die from each takeoff from fallout.

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 seconds.

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 300kW 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

• nuclear thermal rocket
• nuclear weapon
• PACER subterranean nuclear pulse power project
• spacecraft propulsion