Nuclear pulse propulsion

Nuclear pulse propulsion or external pulsed plasma propulsion is a hypothetical 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. Newer designs using inertial confinement fusion have been the baseline for most post-"Orion" designs, including Project "Daedalus" and Project "Longshot".
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 utilizing a variant of the Teller–Ulam two-stage bomb design 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, or about thirteen 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 tons 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 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 using the now disputed linear no-threshold model of radiation damage showed that the fallout from each takeoff would kill between 1 and 10 people. In a threshold model, such extremely low levels of thinly distributed radiation would have no associated ill-effects, while under hormesis models, such tiny doses would be negligibly beneficial. With the possible use of less efficient clean nuclear bombs for achieving orbit and then more efficient higher yield dirty bombs for travel would bring down the amount of fallout caused from an Earth-based launch by a significant factor.
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", even though it was a comet in that particular film. 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.
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 scientist's working lifetime or about 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 lithium deuteride (6Li2H) 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.
"Medusa".
The "Medusa" design is a type of nuclear pulse propulsion which has more in common with solar sails than with conventional rockets. It was envisioned by Johndale Solem in the 1990s and published in the "Journal of the British Interplanetary Society" (JBIS).
A "Medusa" spacecraft would deploy a large "spinnaker" sail ahead of it, attached by separate independent cables, and then launch nuclear explosives forward to detonate between itself and its sail. The sail would be accelerated by the plasma and photonic impulse, running out the tethers as when a fish flees the fisherman, and generating electricity at the "reel". The spacecraft would then use some of the generated electricity to reel itself up towards the sail, constantly smoothly accelerating as it goes.
In the original design, multiple tethers connected to multiple motor generators. The advantage over the single tether is to increase the distance between the explosion and the tethers, thus reducing damage to the tethers.
For heavy payloads, performance could be improved by taking advantage of lunar materials, for example, wrapping the explosive with lunar rock or water, likely stored previously at a stable Earth-Moon Lagrange point to be subsequently acquired by the "Medusa" spacecraft.
"Medusa" performs better than the classical Orion design because its sail intercepts more of the explosive impulse, its shock-absorber stroke is much longer, and all its major structures are in tension and hence can be quite lightweight. "Medusa"-type ships would be capable of a specific impulse between 50,000 and 100,000 seconds (500 to 1000 kN·s/kg).
"Medusa" is widely known to the public in the BBC documentary film "To Mars By A-Bomb: The Secret History of Project Orion". A short film shows an artist's conception of how the "Medusa" spacecraft works "by throwing bombs into a sail that's ahead of it".
Project Longshot.
Project Longshot was a NASA-sponsored research project carried out in conjunction with the US Naval Academy in the late 1980s. "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, the closest solar system to our own, in 100 years (approx. velocity of 13,411 km/s, at a distance of 4.5 light years - equivalent to 4.5% of light speed).
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, 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 (for a sphere at standard density), with antimatter catalyzed reactions this could be well under one gram.
Several rocket designs using this reaction were proposed, some which would use all-fission reactions for interplanetary missions, and others using fission-fusion (effectively a very small version of Orion's bombs) for interstellar missions.
MSNW magneto-inertial fusion driven rocket.
NASA funded MSNW LLC and the University of Washington in 2011 to study and develop a fusion rocket through the NASA Innovative Advanced Concepts NIAC Program.
The rocket uses a form of magneto-inertial fusion to produce a direct thrust fusion rocket. Powerful magnetic fields cause large metal rings (likely made of lithium, where a set for one pulse has a total mass of 365 grams) to collapse around the deuterium-tritium plasma, compressing it to a fusion state. Energy from these fusion reactions heats and ionizes the shell of metal formed by the crushed rings. The hot, ionized metal is shot out of a magnetic rocket nozzle at a high speed (up to 30 km/s). Repeating this process roughly every minute would propel the spacecraft. The fusion reaction is not self-sustaining and requires electrical energy to induce fusion. With electrical requirements estimated to be between 100 kW to 1,000 kW (300 kW average), spacecraft designs incorporate solar panels to produce the electrical energy needed for the fusion engine. 
This approach uses Foil Liner Compression to create a fusion reaction of the proper energy scale to be used for space propulsion. The proof of concept experiment in Redmond, Washington, will use aluminum liners for compression. However, the actual rocket design will run with lithium liners. MSNW released a visual simulation of the process to demonstrate the principles of the engine.
The performance characteristics of the engine are highly dependent on the Fusion energy gain factor achieved by the reactor. Gains are expected to be between a factor of 20 and 200, with an estimated average of 40. With higher fusions gains comes higher exhaust velocity, higher specific impulse and lower electrical power requirements. The table below summarizes different performance characteristics for a theoretical 90-day Mars transfer at gains of 20, 40 and 200.
By April 2013, MSNW had demonstrated subcomponents of the systems: heating deuterium plasma up to fusion temperatures and have concentrated the magnetic fields needed to create fusion. They planned to put the two technologies together for a test before the end of 2013.
They could later be scaled up in power and plan to add the necessary fusion fuel (deuterium) by the end (Sept 2014) of the NIAC Study.
Pulsed Fission-Fusion Propulsion.
Pulsed Fission-Fusion (PuFF) propulsion, a system reliant on principles similar to those of a magneto-inertial fusion driven rocket, aims to solve the problem of the extreme stress induced on containment architecture by an Orion-like motor by ejecting the plasma obtained from small fuel pellets that undergo autocatalytic fission and fusion reactions initiated by a Z-pinch. It is a theoretical propulsion system currently being researched through the NIAC Program by the University of Alabama in Huntsville. It is in essence a fusion rocket that uses a Z-pinch configuration, but coupled with a fission reaction to boost the fusion process for a more efficient system. 
A PuFF fuel pellet, around 1 cm in diameter, can be divided into two components: A deuterium-tritium (D-T) cylinder of plasma, called the "target", which undergoes fusion and a surrounding U-235 sheath which undergoes fission enveloped by a lithium liner. Liquid lithium, serving as a moderator, fills the space between the D-T cylinder and the uranium sheath. When a current is run through the liquid lithium, a Lorentz force is generated which then compresses the D-T plasma by a factor of 10 in what is known as a Z-pinch. The compressed plasma reaches criticality and undergoes a limited number of fusion reactions. However, the fusion energy gain ("Q") of these reactions is still far below breakeven ("Q" < 1), meaning that the reaction consumes more energy than it produces. 
In a PuFF design, the fast neutrons released by the initial fusion reaction induce fission in the U-235 sheath. The resultant heat causes the sheath to expand, increasing its implosion velocity onto the D-T core and compressing it further, making it emit even more fast neutrons. Those again amplify the fission rate in the sheath, rendering the process autocatalytic. It is hoped that this results in a complete burn up of both the fission and fusion fuels, making PuFF more efficient than other nuclear pulse concepts. Much like in a magneto-inertial fusion rocket, the performance of the engine would be highly dependent on the degree to which the fusion gain of the D-T target could be increased.
One "pulse" consist of the injection of a fuel pellet into the combustion chamber, its complete consumption through a series of fission-fusion reactions, and finally the ejection of the released plasma through a magnetic nozzle, thus generating thrust. A single pulse is expected to take only a fraction of a second to complete.