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 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 US and Soviet Union had already detonated 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 (a claim that has been disputed: see radiation hormesis).
One useful mission for this near-term technology would be to deflect an asteroid that could collide with the earth, depicted dramatically in the 1998 film Deep Impact. 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, and in the event of an imminent asteroid impact a few predicted deaths from fallout would probably not be considered prohibitive. Also, an automated mission would eliminate the most problematic issues of the design: the shock absorbers.
Orion is one of very few interstellar space drives that could theoretically be constructed with available technology, as discussed in a 1968 paper, Interstellar Transport by Freeman Dyson.
"In the early 1960s, Sakharov, I remember, once invited us into his office and told us about this idea for an interstellar space ship that would be propelled through space by micro nuclear explosions."—Former USSR Minister and current Russian Deputy Minister of Atomic Energy (MinAtom) Victor Mikhailov recalling discussing nuclear powered rockets with Andrei Sakharov ca. 1961.[4][5][6]
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 scientist's working 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 would be required to instead create the necessary amount of fission.
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 diverted to run the ship's systems and engine. In order to make the system safe and energy efficient, Daedalus was to be powered by a Helium-3 fuel that would have had to be collected from Jupiter.
As of 2009, designing an ICF system efficient enough for a Daedalus design is still considerably beyond current technical capabilities. However some designs are on the drawing board awaiting confirmation.
The "Medusa" design is a type of nuclear pulse propulsion which has 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 (500 to 1000 kN·s/kg).
The January 1993 and June 1994 issues of the Journal of the British Interplanetary Society (JBIS) have articles on Medusa. (There is also a related paper in the Nov/Dec 2000 issue.)[7]
Project Longshot was a NASA-sponsored research project carried out in conjunction with the US Naval Academy in the late 1980s[8] . 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, (approx. velocity of 13,411 km/s, or 30,000,000 mph)
In the mid-1990s research at the Pennsylvania State University led to the concept of using antimatter to catalyze nuclear reactions. In short, antiprotons 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.
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