Nuclear pulse propulsion or external pulsed plasma propulsion is a hypothetical method of spacecraft propulsion that uses nuclear explosions for thrust. [1] It originated as Project Orion with support from DARPA, after a suggestion by Stanislaw Ulam in 1947. [2] Newer designs using inertial confinement fusion have been the baseline for most later designs, including Project Daedalus and Project Longshot.
Calculations for a potential use of this technology were made at the laboratory from and toward the close of the 1940s to the mid-1950s. [3]
Project Orion was the first serious attempt to design a nuclear pulse rocket. A design was formed at General Atomics during the late 1950s and early 1960s, with the idea of reacting 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 s (59 km/s) seconds, or about thirteen times that of the Space Shuttle main engine. With refinements a theoretical maximum specific impulse of 100,000 s (980 km/s) (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. [4]
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 single-stage reference design would reach Mars and return 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). Notable engineering problems that occurred were related to crew shielding and pusher-plate lifetime.
Although the system appeared to be workable, the project was shut down in 1965, primarily because the Partial Test Ban Treaty made it illegal; in fact, before the treaty, the US and Soviet Union had already separately detonated a combined number of at least nine nuclear bombs, including thermonuclear, in space, i.e., at altitudes of over 100 km (see high-altitude nuclear explosions). Ethical issues complicated the launch of such a vehicle within the Earth's magnetosphere: calculations using the (disputed) linear no-threshold model of radiation damage showed that the fallout from each takeoff would cause the death of approximately 1 to 10 individuals. [5] 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. [6] [7] The use of less efficient clean nuclear bombs for achieving orbit and then more efficient, higher yield dirtier bombs for travel would significantly reduce the amount of fallout caused from an Earth-based launch.
One useful mission would be to deflect an asteroid or comet on collision course with the Earth, depicted dramatically in the 1998 film Deep Impact . The 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. [8] The prospect of an imminent asteroid impact would obviate concerns over the few predicted deaths from fallout. An automated mission would remove the challenge of designing a shock absorber that would protect the crew.
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 was a study conducted between 1973 and 1978 by the British Interplanetary Society (BIS) to design an interstellar uncrewed spacecraft that could reach a nearby star within 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 to be run within a large electromagnet that formed the rocket engine. After the reaction, ignited by electron beams, 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 collected from Jupiter.
The Medusa design has more in common with solar sails than with conventional rockets. It was envisioned by Johndale Solem [9] in the 1990s and published in the Journal of the British Interplanetary Society (JBIS). [10]
A Medusa spacecraft would deploy a large sail ahead of it, attached by 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 a fisher, generating electricity at the "reel". The spacecraft would use some of the generated electricity to reel itself up toward the sail, constantly smoothly accelerating as it goes. [11]
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, stored previously at a stable Lagrange point. [12]
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 its major structures are in tension and hence can be quite lightweight. Medusa-type ships would be capable of a specific impulse of 50,000–100,000 s (490–980 km/s) (500 to 1000 kN·s/kg).
Medusa became widely known to the public in the BBC documentary film To Mars By A-Bomb: The Secret History of Project Orion. [13] 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". [14]
Project Longshot was a NASA-sponsored research project carried out in conjunction with the US Naval Academy in the late 1980s. [15] Longshot was in some ways a development of the basic Daedalus concept, in that it used magnetically funneled ICF. The key difference was that they felt that the reaction could not power both the rocket and the other 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 neighboring star Alpha Centauri 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).
In the mid-1990s, research at Pennsylvania State University led to the concept of using antimatter to catalyze nuclear reactions. Antiprotons would react inside the nucleus of uranium, releasing 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.
 Concept graphic of a fusion-driven rocket powered spacecraft arriving at Mars | |
| Designer | MSNW LLC |
|---|---|
| Application | Interplanetary |
| Status | Theoretical |
| Performance | |
| Specific impulse | 1,606 s to 5,722 s (depending on fusion gain) |
| Burn time | 1 day to 90 days (10 days optimal with gain of 40) |
| References | |
| References | [16] |
| Notes |
|
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. [17]
The rocket uses a form of magneto-inertial fusion to produce a direct thrust fusion rocket. Magnetic fields cause large metal rings to collapse around the deuterium-tritium plasma, triggering fusion. The energy 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 accelerate or decelerate the spacecraft. [18] The fusion reaction is not self-sustaining and requires electrical energy to explode each pulse. With electrical requirements estimated to be between 100 kW to 1,000 kW (300 kW average), designs incorporate solar panels to produce the required energy. [16]
Foil Liner Compression creates fusion at the proper energy scale. The proof of concept experiment in Redmond, Washington, was to use aluminum liners for compression. However, the ultimate design was to use lithium liners. [19] [20]
Performance characteristics are dependent on the fusion energy gain factor achieved by the reactor. Gains were expected to be between 20 and 200, with an estimated average of 40. Higher gains produce 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.
| Total gain | Gain of 20 | Gain of 40 | Gain of 200 |
|---|---|---|---|
| Liner mass (kg) | 0.365 | 0.365 | 0.365 |
| Specific impulse (s) | 1,606 | 2,435 | 5,722 |
| Mass fraction | 0.33 | 0.47 | 0.68 |
| Specific mass (kg/kW) | 0.8 | 0.53 | 0.23 |
| Mass propellant (kg) | 110,000 | 59,000 | 20,000 |
| Mass initial (kg) | 184,000 | 130,000 | 90,000 |
| Electrical power required (kW) | 1,019 | 546 | 188 |
By April 2013, MSNW had demonstrated subcomponents of the systems: heating deuterium plasma up to fusion temperatures and concentrating the magnetic fields needed to create fusion. They planned to put the two technologies together for a test before the end of 2013. [16] [21] [22]
Pulsed fission-fusion (PuFF) propulsion is reliant on principles similar to magneto-inertial fusion. It aims to solve the problem of the extreme stress induced on containment 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 researched through the NIAC Program by the University of Alabama in Huntsville. [23] It is in essence a fusion rocket that uses a Z-pinch configuration, but coupled with a fission reaction to boost the fusion process.
A PuFF fuel pellet, around 1 cm in diameter, [24] consists of two components: A deuterium-tritium (D-T) cylinder of plasma, called the target, which undergoes fusion, and a surrounding U-235 sheath that 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. 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 fusion reactions. However, the fusion energy gain (Q) of these reactions is 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, releasing 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. [25] [26] Much like in a magneto-inertial fusion rocket, the performance of the engine is dependent on the degree to which the fusion gain of the D-T target is increased.
One "pulse" consist of the injection of a fuel pellet into the combustion chamber, its 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.
Interstellar travel is the hypothetical travel of spacecraft from one star system, solitary star, or planetary system to another. Interstellar travel is expected to prove much more difficult than interplanetary spaceflight due to the vast difference in the scale of the involved distances. Whereas the distance between any two planets in the Solar System is less than 55 astronomical units (AU), stars are typically separated by hundreds of thousands of AU, causing these distances to typically be expressed instead in light-years. Because of the vastness of these distances, non-generational interstellar travel based on known physics would need to occur at a high percentage of the speed of light; even so, travel times would be long, at least decades and perhaps millennia or longer.
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric entry.
A nuclear thermal rocket (NTR) is a type of thermal rocket where the heat from a nuclear reaction replaces the chemical energy of the propellants in a chemical rocket. In an NTR, a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor and then expands through a rocket nozzle to create thrust. The external nuclear heat source theoretically allows a higher effective exhaust velocity and is expected to double or triple payload capacity compared to chemical propellants that store energy internally.
A nuclear electric rocket is a type of spacecraft propulsion system where thermal energy from a nuclear reactor is converted to electrical energy, which is used to drive an ion thruster or other electrical spacecraft propulsion technology. The nuclear electric rocket terminology is slightly inconsistent, as technically the "rocket" part of the propulsion system is non-nuclear and could also be driven by solar panels. This is in contrast with a nuclear thermal rocket, which directly uses reactor heat to add energy to a working fluid, which is then expelled out of a rocket nozzle.
The nuclear salt-water rocket (NSWR) is a theoretical type of nuclear thermal rocket designed by Robert Zubrin. In place of traditional chemical propellant, such as that in a chemical rocket, the rocket would be fueled by salts of plutonium or 20 percent enriched uranium. The solution would be contained in a bundle of pipes coated in boron carbide. Through a combination of the coating and space between the pipes, the contents would not reach critical mass until the solution is pumped into a reaction chamber, thus reaching a critical mass, and being expelled through a nozzle to generate thrust.
Beam-powered propulsion, also known as directed energy propulsion, is a class of aircraft or spacecraft propulsion that uses energy beamed to the spacecraft from a remote power plant to provide energy. The beam is typically either a microwave or a laser beam and it is either pulsed or continuous. A continuous beam lends itself to thermal rockets, photonic thrusters and light sails, whereas a pulsed beam lends itself to ablative thrusters and pulse detonation engines.
A fusion rocket is a theoretical design for a rocket driven by fusion propulsion that could provide efficient and sustained acceleration in space without the need to carry a large fuel supply. The design requires fusion power technology beyond current capabilities, and much larger and more complex rockets.
The Bussard ramjet is a theoretical method of spacecraft propulsion for interstellar travel. A fast moving spacecraft scoops up hydrogen from the interstellar medium using an enormous funnel-shaped magnetic field ; the hydrogen is compressed until thermonuclear fusion occurs, which provides thrust to counter the drag created by the funnel and energy to power the magnetic field. The Bussard ramjet can thus be seen as a ramjet variant of a fusion rocket.
An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several designs that attempt to accomplish this goal. The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket.
Antimatter-catalyzed nuclear pulse propulsion is a variation of nuclear pulse propulsion based upon the injection of antimatter into a mass of nuclear fuel to initiate a nuclear chain reaction for propulsion when the fuel does not normally have a critical mass.
An interstellar ark is a conceptual starship designed for interstellar travel. Interstellar arks may be the most economically feasible method of traveling such distances. The ark has also been proposed as a potential habitat to preserve civilization and knowledge in the event of a global catastrophe.
Project Orion was a study conducted in the 1950s and 1960s by the United States Air Force, DARPA, and NASA into the viability of a nuclear pulse spaceship that would be directly propelled by a series of atomic explosions behind the craft. Early versions of the vehicle were proposed to take off from the ground; later versions were presented for use only in space. The design effort took place at General Atomics in San Diego, and supporters included Wernher von Braun, who issued a white paper advocating the idea. Non-nuclear tests were conducted with models, but the project was eventually abandoned for several reasons, including the 1963 Partial Test Ban Treaty, which banned nuclear explosions in space, and concerns over nuclear fallout.
Nuclear propulsion includes a wide variety of propulsion methods that use some form of nuclear reaction as their primary power source. The idea of using nuclear material for propulsion dates back to the beginning of the 20th century. In 1903 it was hypothesized that radioactive material, radium, might be a suitable fuel for engines to propel cars, planes, and boats. H. G. Wells picked up this idea in his 1914 fiction work The World Set Free. Many aircraft carriers and submarines currently use uranium fueled nuclear reactors that can provide propulsion for long periods without refueling. There are also applications in the space sector with nuclear thermal and nuclear electric engines which could be more efficient than conventional rocket engines.
Laser propulsion is a form of beam-powered propulsion where the energy source is a remote laser system and separate from the reaction mass. This form of propulsion differs from a conventional chemical rocket where both energy and reaction mass come from the solid or liquid propellants carried on board the vehicle.
Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible uncrewed interstellar probe. Intended mainly as a scientific probe, the design criteria specified that the spacecraft had to use existing or near-future technology and had to be able to reach its destination within a human lifetime. Alan Bond led a team of scientists and engineers who proposed using a fusion rocket to reach Barnard's Star 5.9 light years away. The trip was estimated to take 50 years, but the design was required to be flexible enough that it could be sent to any other target star.
Project Longshot was a conceptual interstellar spacecraft design. It would have been an uncrewed starship, intended to fly to and enter orbit around Alpha Centauri B powered by nuclear pulse propulsion.
An interstellar probe is a space probe that has left—or is expected to leave—the Solar System and enter interstellar space, which is typically defined as the region beyond the heliopause. It also refers to probes capable of reaching other star systems.
ICAN-II was a proposed crewed interplanetary spacecraft that used the antimatter-catalyzed micro-fission (ACMF) engine as its main form of propulsion. The spacecraft was designed at Penn State University in the 1990s as a way to accomplish a crewed mission to Mars. The proposed ACMF engine would require only 140 nanograms of antiprotons in conjunction with traditional fissionable fuel sources to allow a one-way transit time to Mars of 30 days. This is a considerable improvement over many other forms of propulsion that can be used for interplanetary missions, due to the high thrust-to-weight ratio and specific impulse of nuclear fuels. Some downsides to the design include the radiation hazards inherent to nuclear pulse propulsion, as well as the limited availability of the antiprotons used to initialize the nuclear fission reaction. Even the small amount required by the ACMF engine is equal to the total antimatter production at the facilities CERN and Fermilab over many years, although these create antimatter only as a byproduct of physics experiments, not as a goal. ICAN-II is similar to the Project Orion design put forth by Stanislaw Ulam in the late 1950s. The Orion was intended to be used to send humans to Mars and Venus by 1968. The ICAN-II also, in a sense, utilizes nuclear "bombs" for thrust. However, instead of regular fission bombs like the Orion would utilize, ICAN-II uses what are, essentially, many tiny hydrogen bombs, set off by a stream of anti-protons. Ecological concerns would probably require that ICAN-II be assembled in space.
Mini-Mag Orion (MMO), or Miniature Magnetic Orion, is a proposed type of spacecraft propulsion based on the Project Orion nuclear propulsion system. The Mini-Mag Orion system achieves propulsion by compressing fissile material in a magnetic field, a Z-pinch, until fission occurs. This fission reaction propels the craft. MMO should be able to propel 100 tons to Mars within 3 months or to Jupiter in about one year.
Research discussed in this paper was sponsored by Air Force Special Weapons Center, Kirtland Air Force Base, New Mexico, Air Force Systems Command, USAF, under Contract AF29(601)-6214.
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