# Project Orion (nuclear propulsion)

An artist's conception of the NASA reference design for the Project Orion spacecraft powered by nuclear propulsion.

Project Orion was the first engineering design study of a spacecraft powered by nuclear pulse propulsion, an idea first proposed by Stanisław Ulam in 1947. The project, initiated in 1958, envisioned the explosion of atomic bombs behind the craft and was led by Ted Taylor at General Atomics and physicist Freeman Dyson, who at Taylor's request took a year away from the Princeton Institute for Advanced Study to work on the project. The first such think-tank of its kind since the Manhattan Project, Project Orion is recalled by many of its team as representing the best years of their lives.

By using energetic nuclear power, Orion offered both high thrust and high specific impulse — the holy grail of spacecraft propulsion. It offered performance greater than the most advanced conventional or nuclear rocket engines now under study. Cheap interplanetary travel was the goal of the Orion Project. Its supporters felt that it had great potential for space travel, but it lost political approval because of concerns with fallout from its propulsion. This concern could be partially addressed by building it in orbit.[1] The Partial Test Ban Treaty of 1963 is generally acknowledged to have ended the project.

## Nuclear power

Stanisław Ulam realized that nuclear explosions could not yet be realistically contained in a combustion chamber. Such a project did briefly exist, named Helios, but its theoretical performance was so poor that it never got beyond the drawing board.

Instead, the Orion design would have worked by dropping fission or thermonuclear explosives out the rear of a vehicle, detonating them 200 feet (60 m) out, and catching the blast with a thick steel or aluminum pusher plate.

Large multi-storey high shock absorbers (pneumatic springs) were to have absorbed the impulse from the plasma wave as it hit the pusher plate, spreading the millisecond shock wave over several seconds and thus giving an acceptable acceleration speed. The long arm pistons proved one of the most difficult design features. Low pressure gas bags were also proposed as a primary shock absorber. The two sets of shock absorption systems were tuned to different frequencies to avoid resonance.

One aspect of the proposed vessel seems counter-intuitive today: because of the force involved in the thermonuclear detonations and the need to absorb the energy without harm, massive vessel designs were needed. Early designs had crew compartments and storage areas that were several stories tall, as opposed to contemporary chemical rockets whose height was almost all multi-stage fuel tanks with relatively little payload.

Reaction mass for Orion would have been built into the bombs or dropped between 'pulses' to provide thrust. Polyethylene masses, garbage and sewage were all considered for use as reaction mass.

The smallest 4000 ton model planned for ground launch from Jackass Flats, Nevada had each blast add 30 mph (50 km/h, 13.89m/s) to the craft's velocity. A graphite based oil was to be sprayed on the pusher plate before each explosion to prevent ablation of the pusher plate. This sequence would be repeated thousands of times, like an atomic pogo stick.

Orion's potential performance was stunning, at least compared to today's chemical or even other nuclear designs. Jerry Pournelle, who is acquainted with the project and its ex-team leader Freeman Dyson, has been quoted as saying that a single mission could have provided us with a large permanent moon base. Alternatively, an Orion could reach Pluto and return to Earth inside of a year. Single stage to Mars and back also seemed to be possible.

## Background

In the 1954 Operation Castle nuclear test series at Bikini Atoll, a crucial experiment by Lew Allen proved that nuclear explosives could be used for propulsion. Two graphite-covered steel spheres were suspended near the test article for the Castle Bravo shot. After the explosion, they were found intact some distance away, proving that engineered structures could survive a nuclear fireball.

## Performance

The Orion nuclear pulse drive combines a very high exhaust velocity, from 20,000 to 30,000 m/s, with meganewtons of thrust. Many spacecraft propulsion drives can achieve one of these or the other, but nuclear pulse rockets are the only proposed technology that could potentially deliver both (see spacecraft propulsion for more speculative systems). Specific impulse measures how much thrust can be derived from a given mass of fuel, and is the standard figure of merit for rocketry.

Unmanned Orion-style nuclear pulse rockets can tolerate very large accelerations. A human-crewed Orion, however, must use damped springs behind the pusher plate to smooth the instantaneous acceleration to a level that humans can withstand–typically about 1–3 g.

The high performance depends on the high exhaust velocity, in order to maximize the rocket's force for a given mass of propellant. The velocity of the plasma debris is proportional to the square root of the change in the temperature (Tc) of the nuclear fireball. Since fireballs routinely achieve ten million degrees Celsius or more in less than a millisecond, they create very high velocities. However, a practical design must also limit the destructive radius of the fireball. The diameter of the nuclear fireball is proportional to the square root of the bomb's explosive yield.

The shape of the bomb's reaction mass is critical to efficiency. The original project designed bombs with a reaction mass made of tungsten. The bomb's geometry and materials focused the x-rays and plasma from the core of nuclear explosive to hit the reaction mass. In effect each bomb would be a nuclear shaped charge.

A bomb with a cylinder of reaction mass expands into a flat, disk-shaped wave of plasma when it explodes. A bomb with a disk-shaped reaction mass expands into a far more efficient cigar-shaped wave of plasma debris. The cigar shape focuses much of the plasma to impinge onto the pusher-plate.

A 10 kiloton of TNT equivalent atomic explosion will achieve a plasma debris velocity of about 100,000 m/s, and the destructive plasma fireball is only about 100 meters in diameter. A 1 megaton of TNT explosion will have a plasma debris velocity of about 10,000,000 m/s but the diameter of the plasma fireball will be about 1000 m.

The maximum effective specific impulse, Isp, of an Orion nuclear pulse drive generally is equal to:

$I_{sp} = \frac{C_0 \cdot V_e}{g_n}$

where C0 is the collimation factor (what fraction of the explosion plasma debris will actually hit the impulse absorber plate when a pulse unit explodes), Ve is the nuclear pulse unit plasma debris velocity, and gn is the standard acceleration of gravity (9.81 m/s²; this factor is not necessary if Isp is measured in N·s/kg or m/s). A collimation factor of nearly 0.5 can be achieved by matching the diameter of the pusher plate to the diameter of the nuclear fireball created by the explosion of a nuclear pulse unit.

## Sizes of Orion vehicles

A 1959 report by General Atomics[2] explored the parameters of three different sizes of hypothetical Orion spacecraft:

"Satellite"
Orion
"Midrange"
Orion
"Super"
Orion
Ship diameter 17–20 m 40 m 400 m
Ship mass 300 t 1000–2000 t 8,000,000 t
Number of bombs 540 1080 1080
Individual bomb mass 0.22 t 0.37–0.75 t 3.00 t

The biggest design above is the "super" Orion design; at 8 million tons, it could easily be a city.[3] In interviews, the designers contemplated the large ship as a possible interstellar ark. This extreme design could be built with materials and techniques that could be obtained in 1958 or were anticipated to be available shortly after. The practical upper limit is likely to be higher with modern materials.

Most of the three tons of each of the "super" Orion's propulsion units would be inert material such as polyethylene, or boron salts, used to transmit the force of the propulsion unit's detonation to the Orion's pusher plate, and absorb neutrons to minimize fallout. One design proposed by Freeman Dyson for the "Super Orion" called for the pusher plate to be composed primarily of uranium or a transuranic element so that upon reaching a nearby star system the plate could be converted to nuclear fuel.

## Applications

The Orion nuclear pulse rocket design has extremely high performance. Orion nuclear pulse rockets using nuclear fission type pulse units were originally intended for use on interplanetary space flights.

The top cruise velocity that can theoretically be achieved by a thermonuclear Orion starship is about 8% to 10% of the speed of light (0.08-0.1c).[1] An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of light. A nuclear pulse drive starship powered by matter-antimatter pulse units would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light.

Missions that were designed for an Orion vehicle in the original project included single stage (i.e., directly from Earth's surface) to Mars and back, and a trip to one of the moons of Saturn.[3]

One possible modern mission for this near-term technology would be to deflect an asteroid that could collide with 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 shock absorbers, the most problematic issue of the design.

Nuclear fission pulse unit powered Orions could provide fast and economical interplanetary transportation with useful human crewed payloads of several thousand tonnes.

At 0.1c, Orion thermonuclear starships would require a flight time of at least 44 years to reach Proxima Centauri, the nearest star to the Sun, not counting time needed to reach that speed. At 0.1c, an Orion starship would require 100 years to travel 10 light years. The late astronomer Carl Sagan suggested that this would be an excellent use for current stockpiles of nuclear weapons.[4]

## Later developments

A concept similar to Orion was designed by the British Interplanetary Society (B.I.S.) in the years 1973-1974. Project Daedalus was to be a robotic interstellar probe to Barnard's Star that would travel at 12% of the speed of light (0.12c). In 1989, a similar concept was studied by the U.S. Navy and NASA in Project Longshot. Both of these concepts require significant advances in fusion technology, and therefore cannot be built at present, unlike Orion.

From 1998 to the present, the nuclear engineering department at Pennsylvania State University has been developing two improved versions of the Daedalus design known as Project Ican and Project Aimstar.[5]

## Economics

The expense of the fissionable materials required was thought high, until the physicist Ted Taylor showed that with the right designs for explosives, the amount of fissionables used on launch was close to constant for every size of Orion from 2,000 tons to 8,000,000 tons. The larger bombs used more explosives to super-compress the fissionables, reducing fallout. The extra debris from the explosives also serves as additional propulsion mass.

Project Daedalus later proposed fusion explosives (deuterium or tritium pellets) detonated by electron beam inertial confinement. This is the same principle behind inertial confinement fusion. However, theoretically, it might be scaled down to far smaller explosions, and require small shock absorbers.

## Vehicle architecture

From 1957 until 1964 this information was used to design a spacecraft propulsion system called "Orion" in which nuclear explosives would be thrown through a pusher-plate mounted on the bottom of a spacecraft and exploded underneath. The shock wave and radiation from the detonation would make an impact against the underside of the pusher plate, giving it a powerful "kick", and the pusher plate would be mounted on large two-stage shock absorbers which would transmit the acceleration to the rest of the spacecraft in a smooth manner.

Radiation shielding for the crews was thought to be a problem, but on ships with mass greater than a thousand tons, the structural bulk of the ship, its stores, and the mass of the bombs and propellant provides shielding for the crew from most of the explosives' radiation. Radiation shielding effectiveness increases exponentially with shield thickness (see gamma ray for a discussion of shielding).

At low altitudes, during take-off, the fallout would be highly radioactive, and there was a grave danger of fluidic shrapnel being reflected from the ground. The solution was to use a flat plate of conventional explosives spread over the pusher plate, and detonate these to lift the ship from the ground before going nuclear. This would lift the ship far enough into the air that a focused nuclear blast would avoid harming the ship.

A preliminary design for the explosives was produced. It used a fusion-boosted fission explosive. The explosive was wrapped in a beryllium oxide "channel filler", which was surrounded by a uranium radiation mirror. The mirror and channel filler were open ended, and in this open end a flat plate of tungsten propellant was placed. The whole thing was wrapped in a can so that it could be handled by machinery scaled-up from a soft-drink vending machine.

At 1 microsecond after ignition, the gamma bomb plasma and neutrons would heat the channel filler, and be somewhat contained by the uranium shell. At 2-3 microseconds, the channel filler would transmit some of the energy to the propellant, which would vaporize. The flat plate of propellant would form a cigar-shaped explosion aimed at the pusher plate.

The plasma would cool to 14,000 °C, as it traversed the 25 m distance to the pusher plate, and then reheat to 67,000 °C, as (at about 300 microseconds) it hit the pusher plate and recompressed. This temperature emits ultraviolet, which is poorly transmitted through most plasmas. This helps keep the pusher plate cool. The cigar shape and low density of the plasma reduces the shock to the pusher plate.

The pusher plate's thickness was to decrease by about a factor of 6 from the center to the edge, so that the net velocity of the inner and outer parts of the plate are the same, even though the momentum transferred by the plasma increases from the center outwards.

At low altitudes where the surrounding air is dense, gamma scattering could potentially harm the crew. The plan to solve this was to have takeoff stations in inner rooms shielded by supplies and equipment. Such a radiation refuge is necessary anyway on long missions to survive solar flares.

Stability was thought to be a problem due to random placement errors of the bombs, but it was later shown that over time the random errors would tend to cancel out.

A one-meter model using RDX (chemical explosives), called "putt-putt", flew a controlled flight for 23 seconds to a height of 56 meters at Point Loma.

The shock absorber was at first merely a ring-shaped airbag. However, if an explosion should fail, the 1000 ton pusher plate would tear away the airbag on the rebound. A two-stage, detuned shock absorber design proved more workable. On the reference design, the mechanical absorber was tuned to 1/2 the pulse frequency, and the air-bag absorber was tuned to 4.5 times the pulse frequency.

Another problem was finding a way to push the explosives past the pusher plate fast enough that they would explode 20 to 30 m beyond it, and do so every 1.1 seconds. The final reference design used a gas gun to shoot the devices through a hole in the pusher plate.

## Potential problems

Exposure to repeated nuclear blasts raises the problem of ablation (erosion) of the pusher plate. However, calculations and experiments indicate that a steel pusher plate would ablate less than 1 mm if unprotected. If sprayed with an oil, it need not ablate at all (this was discovered by accident; a test plate had oily fingerprints on it, and the fingerprints suffered no ablation). The absorption spectra of carbon and hydrogen minimize heating. The design temperature of the shockwave, 67,000 °C, emits ultraviolet. Most materials and elements are opaque to ultraviolet, especially at the 340 MPa pressures the plate experiences. This prevents the plate from melting or ablating.

One issue that remained unresolved at the conclusion of the project was whether the turbulence created by the combination of the propellant and ablated pusher plate would dramatically increase the total ablation of the pusher plate. According to Freeman Dyson, during the 1960s they would have had to actually perform a test with a real nuclear explosive to determine this; with modern simulation technology, this could be determined fairly accurately without such empirical investigation.

Another potential problem with the pusher plate is that of spalling - shards of metal - potentially flying off the top of the plate. The shockwave from the impacting plasma on the bottom of the plate passes through the plate and reaches the top surface. At that point spalling may occur, damaging the pusher plate. For that reason, alternative substances (e.g., plywood and fiberglass) were investigated for the surface layer of the pusher plate, and thought to be acceptable.

If the conventional explosives in the nuclear bomb detonate, but a nuclear explosion does not ignite (a dud), shrapnel could strike and potentially critically damage the pusher plate.

True engineering tests of the vehicle systems were said to be impossible because several thousand nuclear explosions could not be performed in any one place. However, experiments were designed to test pusher plates in nuclear fireballs. Long-term tests of pusher plates could occur in space. Several of these tests almost flew. The shock-absorber designs could be tested at full-scale on Earth using chemical explosives.

But the main unsolved problem for a launch from the surface of the Earth was thought to be nuclear fallout. Any explosions within the magnetosphere would carry fissionables back to earth unless the spaceship were launched from a polar region such as a barge in the higher regions of the Arctic, with the initial launching explosion to be a large mass of conventional high explosive only to significantly reduce fallout; subsequent detonations would be in the air and therefore much cleaner. Antarctica is not viable, as this would require enormous legal changes as the continent is presently an international wildlife preserve. Freeman Dyson, group leader on the project, estimated back in the '60s that with conventional nuclear weapons, that each launch would cause on average between 0.1 and 1 fatal cancers from the fallout.[6] The United States Government concurred and decided to shelve the project because of the danger to human life and the danger to electronic systems on the ground (from electromagnetic pulse).

Orion-style nuclear pulse rockets can be launched from above the magnetosphere so that charged ions of fallout in its exhaust plasma are not trapped by the Earth's magnetic field and are not returned to Earth.

The fallout for the entire launch of a 6000 short ton (5500 metric ton) Orion is only equal to a ten-megaton (40 petajoule) blast, assuming the use of pure fission weapon-type nuclear explosives.

With special designs of the nuclear explosive, Ted Taylor estimated that it could be reduced tenfold, or even to zero if a pure fusion explosive could be constructed; however, a pure fusion explosive has yet to be successfully developed.[6]

The vehicle and its test program would violate the Partial Test Ban Treaty of 1963 as currently written, which prohibited all nuclear detonations except those which were conducted underground, both as an attempt to slow the arms race and to limit the amount of radiation in the atmosphere caused by nuclear detonations. There was an effort by the US government to put an exception into the 1963 treaty to allow for the use of nuclear propulsion for spaceflight, but Soviet fears about military applications kept the exception out of the treaty.

One way around the restrictions of the treaty would be to use a form of the Daedalus fusion microexplosion rocket. Daedalus class systems use pellets of one gram or less ignited by particle or laser beams to produce very small fusion explosions with a maximum explosive yield of only 10–20 tons of TNT equivalent.

The launch of such an Orion nuclear bomb rocket from the ground or from low Earth orbit would generate an electromagnetic pulse that could cause significant damage to computers and satellites, as well as flooding the van Allen belts with high-energy radiation. This problem might be solved by launching from very remote areas, because the EMP footprint would be only a few hundred miles wide. The Earth is well-shielded by the Van Allen belts. In addition, a few relatively small space-based electrodynamic tethers could be deployed to quickly eject the energetic particles from the capture angles of the Van Allen belts.

Assembling a pulse drive spacecraft in orbit by more conventional means and only activating its main drive at a safer distance would be a less destructive approach. The space elevator hypothetically provides an excellent solution, but is currently impossible because existing materials such as carbon nanotubes do not have sufficient tensile strength. Existing chemical rocket designs are extremely inefficient (and expensive) when launching mass into orbit. Furthermore, it is unlikely that lifting the immense Orion into orbit in one piece would even be possible, unless multiple elevators or rockets were used in tandem. Adverse public reaction to any use of nuclear explosives is likely to remain a hindrance even if all practical and legal difficulties are overcome.

## Operation Plumbbob

A test similar to the test of a pusher plate occurred as an accidental side effect of a nuclear containment test called "Pascal B" conducted on 27 August 1957.[7] The test's experimental designer Dr. Brownlee performed a highly approximate calculation that suggested that the low-yield nuclear explosive would accelerate the massive (900 kg) steel capping plate to six times escape velocity.[8] The plate was never found, and Dr. Brownlee believes that the plate never left the atmosphere (for example it could have been vaporized by compression heating of the atmosphere due to its high speed). The calculated velocity was sufficiently interesting that the crew trained a high-speed camera on the plate, which unfortunately only appeared in one frame, but this nevertheless gave a very high lower bound for the speed.

## Appearances in fiction

An early appearance of an Orion-style nuclear pulse propelled rocket in science fiction was in the science fiction novel Empire of the Atom written by A. E. van Vogt in 1956. In this novel there is a post-atomic-war interplanetary empire called the Empire of Linn that uses Orion-type nuclear rockets for interplanetary spaceflight. In the story the past atomic war was an interstellar war fought between humans and hostile aliens from another star somewhere between 800 and 8000 years before.

Early versions of 2001: A Space Odyssey had a ship (Discovery One) using this drive. The final vehicle did not use this idea since Stanley Kubrick was fed up with nuclear bombs after making Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb.

In Jerry Pournelle and Larry Niven's Hugo-nominated novel Footfall, an Orion vessel is constructed as a last-resort weapon against an alien invasion.

The Star Trek:TOS episode "For the World is Hollow and I Have Touched the Sky" features a generation ship, constructed out of a hollowed-out iron asteroid, propelled using "Orion class nuclear pulse engines" in which fission bombs were detonated in shafts. It appeared to have been traveling for about 10,000 years, and had traveled about 30 light years on its own power.

The 1998 film Deep Impact featured a spacecraft named Messiah, which utilized the "Orion drive" and appears to be a variant of nuclear detonation propulsion. In the film, the drive is accredited to the Russians.

In the books Ilium and Olympos by Dan Simmons a space ship with Orion thrust is used to travel through the Solar System over the course of a week.

The book Orion Shall Rise by Poul Anderson, part of his Maurai series of stories, featured a covert group within a resource-deprived, post-apocalyptic civilization resurrecting the Orion program to take humanity to the solar system and beyond.

The novel Anathem by Neal Stephenson features a colony ship, the Daban Urnud, able to travel between the alternate universes resulting from the many worlds interpretation of Quantum Theory that uses an Orion type device for propulsion.

Chris Berman's novel, The Hive, involves the use of a ground launched Orion spacecraft by the People's Republic of China in a gamble to reach an alien artifact in orbit between Jupiter and Saturn before the crew of a spacecraft built by the United States and Russia can reach it first.

John Varley's Steel Beach sets several scenes near or within the bulk of the "Richard Heinlein," an Orion-style ship which was built and then abandoned when humanity lapsed into apathy for stellar exploration.

## References

1. ^ a b Cosmos by Carl Sagan
2. ^ Dunne; Dyson and Treshow (1959). Dimensional Study of Orion Type Spaceships. General Atomics. GAMD-784.
3. ^ a b Dyson, George (2002). Project Orion: The True Story of the Atomic Spaceship. New York, N.Y.: Henry Holt and Co.. ISBN 0-8050-7284-5.
4. ^ Cosmos series, Episode 8
5. ^ Antimatter Space Propulsion at Penn State University (LEPS)
6. ^ a b Disturbing the Universe- Freeman Dyson
7. ^ "Operation Plumbbob". July 2003. Retrieved on 2006-07-31.
8. ^ Brownlee, Robert R. (June 2002). "Learning to Contain Underground Nuclear Explosions". Retrieved on 2006-07-31.