Project Orion (nuclear propulsion) - AbsoluteAstronomy.com

Project Orion was a study of a spacecraft

Spacecraft

A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

intended to be directly propelled by a series of explosions of atomic bombs

Nuclear weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

behind the craft (Nuclear pulse propulsion

Nuclear pulse propulsion

Nuclear pulse propulsion 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...

). Early versions of this vehicle were proposed to have taken off from the ground with significant associated nuclear fallout

Nuclear fallout

Fallout is the residual radioactive material propelled into the upper atmosphere following a nuclear blast, so called because it "falls out" of the sky after the explosion and shock wave have passed. It commonly refers to the radioactive dust and ash created when a nuclear weapon explodes...

; later versions were presented for use only in space.

A 1955 Los Alamos Laboratory document states (without offering references) that general proposals were first made by Stanislaw Ulam in 1946, and that preliminary calculations were made by F. Reines and Ulam in a Los Alamos memorandum dated 1947. The actual project, initiated in 1958, was led by Ted Taylor at General Atomics

General Atomics

General Atomics is a nuclear physics and defense contractor headquartered in San Diego, California. General Atomics’ research into fission and fusion matured into competencies in related technologies, allowing the company to expand into other fields of research...

and physicist Freeman Dyson

Freeman Dyson

Freeman John Dyson FRS is a British-born American theoretical physicist and mathematician, famous for his work in quantum field theory, solid-state physics, astronomy and nuclear engineering. Dyson is a member of the Board of Sponsors of the Bulletin of the Atomic Scientists...

, who at Taylor's request took a year away from the Institute for Advanced Study

Institute for Advanced Study

The Institute for Advanced Study, located in Princeton, New Jersey, United States, is an independent postgraduate center for theoretical research and intellectual inquiry. It was founded in 1930 by Abraham Flexner...

in Princeton

Princeton, New Jersey

Princeton is a community located in Mercer County, New Jersey, United States. It is best known as the location of Princeton University, which has been sited in the community since 1756...

to work on the project.

By using energetic nuclear power, the Orion concept offered high thrust and high specific impulse

Specific impulse

Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

, or propellant efficiency, at the same time. As a qualitative comparison, traditional chemical rockets—such as the Saturn V

Saturn V

The Saturn V was an American human-rated expendable rocket used by NASA's Apollo and Skylab programs from 1967 until 1973. A multistage liquid-fueled launch vehicle, NASA launched 13 Saturn Vs from the Kennedy Space Center, Florida with no loss of crew or payload...

that took the Apollo program to the Moon—produce high thrust with low specific impulse, whereas electric ion engines produce a small amount of thrust very efficiently. Orion would have offered performance greater than the most advanced conventional or nuclear rocket engines then under consideration. Supporters of Project Orion felt that it had potential for cheap interplanetary travel

Interplanetary travel

Interplanetary spaceflight or interplanetary travel is travel between planets within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the Solar System....

, but it lost political approval over concerns with fallout from its propulsion. The Partial Test Ban Treaty

Partial Test Ban Treaty

The treaty banning nuclear weapon tests in the atmosphere, in outer space and under water, often abbreviated as the Partial Test Ban Treaty , Limited Test Ban Treaty , or Nuclear Test Ban Treaty is a treaty prohibiting all test detonations of nuclear weapons...

of 1963 is generally acknowledged to have ended the project.

Basic principles

The Orion nuclear pulse drive combines a very high exhaust velocity, from 20 to 30 km/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

Spacecraft propulsion

Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...

for more speculative systems). Specific impulse

Specific impulse

measures how much thrust can be derived from a given mass of fuel, and is the standard figure of merit for rocketry.

Since weight is no limitation, an Orion craft can be extremely robust. An unmanned craft could tolerate very large accelerations, perhaps 100 g

G-force

The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

. A human-crewed Orion, however, must use some sort of damping system behind the pusher plate to smooth the instantaneous acceleration to a level that humans can comfortably withstand – typically about 2 to 4 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 (T_c) 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

Tungsten

Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

. The bomb's geometry and materials focused the X-ray

X-ray

X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s and plasma from the core of nuclear explosive to hit the reaction mass. In effect each bomb would be a nuclear shaped charge

Shaped charge

A shaped charge is an explosive charge shaped to focus the effect of the explosive's energy. Various types are used to cut and form metal, to initiate nuclear weapons, to penetrate armor, and in the oil and gas industry...

.

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.

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

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), V_e is the nuclear pulse unit plasma debris velocity, and g_n is the standard acceleration of gravity (9.81 m/s²; this factor is not necessary if I_sp 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.

The smaller the bomb, the smaller each impulse will be, so the higher the rate of impulses and more than will be needed to achieve orbit. Smaller impulses also mean less g shock on the pusher plate and less need for damping to smooth out the acceleration.

The optimal Orion drive bomblet yield (for the human crewed 4,000 ton reference design) was calculated to be in the region of 0.15 KT, with approx 800 bombs needed to orbit and a bomb rate of approx 1 per second.

Sizes of Orion vehicles

The following can be found in George Dyson

George Dyson (science historian)

George Dyson is a scientific historian, the son of Freeman Dyson and Verena Huber-Dyson, brother of Esther Dyson, and the grandson of Sir George Dyson. He is the father of Lauren Dyson. When he was sixteen he went to live in British Columbia in Canada to pursue his interest in kayaking and...

's book pg. 55 published in 2002. The figures for the comparison with Saturn V are taken from this section and converted from metric (kg) to US short tons.

	Orbital Test	Interplanetary	Advanced interplanetary	Saturn V
Ship mass	880 t	4,000 t	10,000 t	3,350 t
Ship diameter	25 m	40 m	56 m	10 m
Ship height	36 m	60 m	85 m	110 m
Bomb yield (sea level)	0.03 kt	0.14 kt	0.35 kt	n/a
Bombs (to 300 mi Low Earth Orbit Low Earth orbit A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km... )	800	800	800	n/a
Payload (to 300 mi LEO)	300 t	1,600 t	6,100 t	130 t
Payload (to Moon soft landing)	170 t	1,200 t	5,700 t	52 t
Payload (Mars orbit return)	80 t	800 t	5,300 t	–
Payload (3yr Saturn return)	–	–	1,300 t	–

In late 1958 / early 1959, it was realized that the smallest practical vehicle would be determined by the smallest achievable bomb yield. The use of 0.03 kT (sea-level yield) bombs would give vehicle mass of 880 tons. However, this was regarded as too small for anything other than an orbital test vehicle and the team soon focused on a 4,000 ton "base design".

At that time, the details of small bomb designs were shrouded in secrecy. Many Orion design reports had all details of bombs removed before release. Contrast the above details with the 1959 report by General Atomics, which 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	3000 t

The biggest design above is the "super" Orion design; at 8 million tonnes, it could easily be a city. In interviews, the designers contemplated the large ship as a possible interstellar ark

Interstellar ark

An interstellar ark is a conceptual space vehicle that some have speculated could be used for interstellar travel. The concept was first developed by Dr...

. 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 thousand tonnes of each of the "super" Orion's propulsion units would be inert material such as polyethylene

Polyethylene

Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...

, or boron

Boron

Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

salts, used to transmit the force of the propulsion units 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

Transuranium element

In chemistry, transuranium elements are the chemical elements with atomic numbers greater than 92...

so that upon reaching a nearby star system the plate could be converted to nuclear fuel.

Interplanetary 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.

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.

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, such an unmanned mission would eliminate the need for 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.

Interstellar missions

Freeman Dyson performed the first analysis of what kinds of Orion missions were possible to reach Alpha Centauri

Alpha Centauri

Alpha Centauri is the brightest star in the southern constellation of Centaurus...

, the nearest star system to the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. His 1968 paper "Interstellar Transport" (Physics Today, October 1968, p. 41–45) retained the concept of large nuclear explosions but Dyson moved away from the use of fission bombs and considered the use of one megaton deuterium

Deuterium

Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

fusion explosions instead. His conclusions were simple: the debris velocity of fusion explosions was probably in the 3000–30,000 km/s range and the reflecting geometry of Orion's hemispherical pusher plate would reduce that range to 750–15,000 km/s.

To estimate the upper and lower limits of what could be done using contemporary technology (in 1968), Dyson considered two starship designs. The more conservative energy limited pusher plate design simply had to absorb all the thermal energy of each impinging explosion (4×10¹⁵ joules, half of which would be absorbed by the pusher plate) without melting. Dyson estimated that if the exposed surface consisted of copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

with a thickness of 1 mm, then the diameter and mass of the hemispherical pusher plate would have to be 20 kilometers and 5 million metric tons, respectively. 100 seconds would be required to allow the copper to radiatively cool before the next explosion. It would then take on the order of 1000 years for the energy-limited heat sink

Heat sink

A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator in a car...

Orion design to reach Alpha Centauri.

In order to improve on this performance while reducing size and cost, Dyson also considered an alternative momentum limited pusher plate design where an ablation coating of the exposed surface is substituted to get rid of the excess heat. The limitation is then set by the capacity of shock absorbers to transfer momentum from the impulsively accelerated pusher plate to the smoothly accelerated vehicle. Dyson calculated that the properties of available materials limited the velocity transferred by each explosion to ~30 meters per second independent of the size and nature of the explosion. If the vehicle is to be accelerated at 1 Earth gravity (9.81 m/s) with this velocity transfer, then the pulse rate is one explosion every three seconds. The dimensions and performance of Dyson's vehicles are given in the table below

	"Energy Limited" Orion	"Momentum Limited" Orion
Ship diameter (meters)	20,000 m	100 m
Mass of empty ship (metric tons)	10,000,000 t (incl.5,000,000 t copper hemisphere)	100,000 t (incl.50,000 t structure+payload)
+Number of bombs = total bomb mass (each 1MT bomb weighs 1 metric ton)	30,000,000	300,000
=Departure mass (metric tons)	40,000,000 t	400,000 t
Maximum velocity (kilometers per second)	1000 km/s (=0.33% of the speed of light)	10,000 km/s (=3.3% of the speed of light)
Mean acceleration (Earth gravities)	0.00003 g (accelerate for 100 years)	1 g (accelerate for 10 days)
Estimated cost	1 year of U.S. GNP GNP Gross National Product is the market value of all products and services produced in one year by labor and property supplied by the residents of a country... (1968)	0.1 year of U.S. GNP

Later studies indicate that the top cruise velocity that can theoretically be achieved by a thermonuclear Orion starship, assuming no fuel is saved for slowing back down, is about 8% to 10% of the speed of light

Speed of light

The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

(0.08-0.1c). An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of light. A nuclear pulse drive starship powered by matter-antimatter

Antimatter

In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...

pulse units would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light

Speed of light

.

At 0.1c, Orion thermonuclear starships would require a flight time of at least 44 years to reach Alpha Centauri, not counting time needed to reach that speed (about 36 days at constant acceleration of 1g or 9.8 m/s²). At 0.1c, an Orion starship would require 100 years to travel 10 light years. The late astronomer Carl Sagan

Carl Sagan

Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...

suggested that this would be an excellent use for current stockpiles of nuclear weapons.

Later developments

A concept similar to Orion was designed by the British Interplanetary Society

British Interplanetary Society

The British Interplanetary Society founded in 1933 by Philip E. Cleator, is the oldest space advocacy organisation in the world whose aim is exclusively to support and promote astronautics and space exploration.-Structure:...

(B.I.S.) in the years 1973–1974. Project Daedalus

Project Daedalus

Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible unmanned interstellar spacecraft. Intended mainly as a scientific probe, the design criteria specified that the spacecraft had to use current or near-future technology and had to...

was to be a robotic interstellar probe to Barnard's Star

Barnard's star

Barnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately six light-years away from Earth in the constellation of Ophiuchus . In 1916, the American astronomer E.E...

that would travel at 12% of the speed of light. In 1989, a similar concept was studied by the U.S. Navy and NASA in Project Longshot

Project Longshot

Project Longshot was a conceptual design for an interstellar spacecraft, an unmanned probe intended to fly to Alpha Centauri powered by nuclear pulse propulsion. Developed by the US Naval Academy and NASA from 1987 to 1988, Longshot was designed to be built at Space Station Freedom, the precursor...

. 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.

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

Deuterium

or tritium pellets) detonated by electron beam inertial confinement. This is the same principle behind inertial confinement fusion

Inertial confinement fusion

Inertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....

. 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 behind a pusher-plate mounted on the bottom of a spacecraft and exploded. The shock wave and radiation from the detonation would impact against the underside of the pusher plate, giving it a powerful "kick." The pusher plate would be mounted on large two-stage shock absorber

Shock absorber

A shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. It is a type of dashpot.-Nomenclature:...

s that would smoothly transmit acceleration to the rest of the spacecraft.

During take-off, there were concerns of danger from fluidic shrapnel being reflected from the ground. One proposed solution was to use a flat plate of conventional explosives spread over the pusher plate, and detonate this to lift the ship from the ground before going nuclear. This would lift the ship far enough into the air that the first focused nuclear blast would not create debris capable of harming the ship.

A preliminary design for the explosives was produced. It used a shaped-charge fusion-boosted fission explosive. The explosive was wrapped in a beryllium oxide

Beryllium oxide

Beryllium oxide , also known as beryllia, is an inorganic compound with the formula BeO. This colourless solid is a notable electrical insulator with a higher thermal conductivity than any other non-metal except diamond, and actually exceeds that of some metals. As an amorphous solid, beryllium...

"channel filler," which was surrounded by a uranium

Uranium

Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

radiation mirror. The mirror and channel filler were open ended, and in this open end a flat plate of tungsten

Tungsten

propellant was placed. The whole thing was built into a can with a diameter no larger than 6 inches (15 cm) and weighed just over 300 lb (140 kg) so it could be handled by machinery scaled-up from a soft-drink vending machine (indeed, Coca-Cola was consulted on the design).

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 vaporized. The flat plate of propellant formed 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 shaped distribution profile and low density of the plasma reduces the instantaneous shock to the pusher plate.

The pusher plate's thickness would 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

Compton scattering

In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...

could potentially harm the crew and a radiation refuge would be necessary anyway on long missions to survive solar flare

Solar flare

A solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...

s. Radiation shielding effectiveness increases exponentially with shield thickness (see gamma ray

Gamma ray

Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

for a discussion of shielding), so 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 would provide more than adequate shielding for the crew.

Stability was initially thought to be a problem due to inaccuracies in the placement of the bombs, but it was later shown that the effects would tend to cancel out.

Numerous model flight tests (using conventional explosives) were conducted at Point Loma in 1959. On November 14, the one-meter model, called "Hot Rod" (or "putt-putt"), first flew using RDX

RDX

RDX, an initialism for Research Department Explosive, is an explosive nitroamine widely used in military and industrial applications. It was developed as an explosive which was more powerful than TNT, and it saw wide use in WWII. RDX is also known as cyclonite, hexogen , and T4...

(chemical explosives) in a controlled flight for 23 seconds to a height of 56 meters. Film of the tests has been transcribed to video shown on the BBC TV program "To Mars by A-Bomb" in 2003 with comments by Freeman Dyson and Arthur C. Clarke

Arthur C. Clarke

Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...

. The model landed by parachute undamaged and is in the collection of the Smithsonian National Air and Space Museum.

The first proposed shock absorber was merely a ring-shaped airbag. However, it was soon realized that, should an explosion fail, the 500 to 1000 ton pusher plate would tear away the airbag on the rebound. So a two-stage, detuned spring/piston shock absorber design was developed. On the reference design, the first stage mechanical absorber was tuned 4.5 times the pulse frequency whilst the second stage gas piston was tuned to 1/2 times the pulse frequency. This permitted timing tolerances of 10 ms in each explosion.

The final design coped with bomb failure by overshooting and rebounding into a 'center' position. Thus, following a failure (and on initial ground launch) it would be necessary to start (or restart) the sequence with a lower yield device. In the 1950s methods of adjusting bomb yield

Variable yield

Variable yield — or dial-a-yield — is an option available on most modern nuclear weapons. It allows the operator to specify a weapon's yield, or explosive power, allowing a single design to be used in different situations...

were in their infancy and considerable thought was given to providing a means of 'swapping out' a standard yield bomb for a smaller yield one in a 2 or 3 second time frame (or to provide an alternative means of firing low yield bombs). These days the yield of a standard device would be 'tuned down', as needed, 'on the fly'.

The bombs had to be launched behind the pusher plate fast enough to explode 20 to 30 m beyond it every 1.1 seconds or so. Numerous proposals were investigated, from multiple guns poking over the edge of the pusher plate to rocket propelled bombs launched from 'roller coaster' tracks, however the final reference design used a simple gas gun to shoot the devices through a hole in the center of 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

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

and hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

minimize heating. The design temperature of the shockwave, 67,000 °C, emits ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

. 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 or not 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 spall

Spall

Spall are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure...

ing—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

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 1960s that with conventional nuclear weapons, each launch would cause on average between 0.1 and 1 fatal cancers from the fallout. Danger to human life was not a reason given for shelving the project – those included lack of mission requirement (no-one in the US Government could think of any reason to put thousands of tons of payload into orbit), the decision to focus on rockets (for the Moon mission) and, ultimately, the signature of the Partial Test Ban Treaty

Partial Test Ban Treaty

in 1963. The danger to electronic systems on the ground (from electromagnetic pulse

Electromagnetic pulse

An electromagnetic pulse is a burst of electromagnetic radiation. The abrupt pulse of electromagnetic radiation usually results from certain types of high energy explosions, especially a nuclear explosion, or from a suddenly fluctuating magnetic field...

) is insignificant from the sub-kiloton blasts proposed.

Orion-style nuclear pulse rockets can be launched from above the magnetosphere

Magnetosphere

A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

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 6,000 short ton

Short ton

The short ton is a unit of mass equal to . In the United States it is often called simply ton without distinguishing it from the metric ton or the long ton ; rather, the other two are specifically noted. There are, however, some U.S...

(5,500 metric ton) Orion is only equal to a ten-megaton

TNT equivalent

TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...

(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

Pure fusion weapon

A pure fusion weapon is a hypothetical hydrogen bomb design that does not need a fission "primary" explosive to ignite the fusion of deuterium and tritium, two heavy isotopes of hydrogen . Such a weapon would require no fissile material and would therefore be much easier to build in secret than...

could be constructed; however, a pure fusion explosive has yet to be successfully developed according to declassified US government documents.

The vehicle and its test program would violate the Partial Test Ban Treaty

Partial Test Ban Treaty

of 1963 as currently written, which prohibited all nuclear detonations except those 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 Project 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

Low Earth orbit

A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

would generate an electromagnetic pulse

Electromagnetic pulse

that could cause significant damage to computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

s and satellite

Satellite

In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

s, as well as flooding the van Allen belt

Van Allen radiation belt

The Van Allen radiation belt is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field. It is believed that most of the particles that form the belts come from solar wind, and other particles by cosmic rays. It is named after its discoverer, James...

s 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 tether

Electrodynamic tether

Electrodynamic tethers are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy...

s 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 Lofstrom launch loop

Launch loop

A launch loop or Lofstrom loop is a proposed system for launching objects into space orbit using a moving cable-like system attached to the earth at two ends and suspended above the atmosphere in the middle...

or a space elevator

Space elevator

A space elevator, also known as a geostationary orbital tether or a beanstalk, is a proposed non-rocket spacelaunch structure...

hypothetically provides an excellent solution, although in the case of the space elevator existing carbon nanotubes composites do not yet have sufficient tensile strength

Tensile strength

Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

. All chemical rocket designs are extremely inefficient (and expensive) when launching mass into orbit, however could be employed if the result was seen as worth the cost (for example, the alternative being the impact of an asteroid of size similar to that of the Cretaceous-Tertiary extinction event

Cretaceous-Tertiary extinction event

The Cretaceous–Paleogene extinction event, formerly named and still commonly referred to as the Cretaceous-Tertiary extinction event, occurred approximately 65.5 million years ago at the end of the Maastrichtian age of the Cretaceous period. It was a large-scale mass extinction of animal and plant...

).

Adverse public reaction to any use of nuclear explosives of any type is likely to remain a stumbling block even if 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. 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

Escape velocity

In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...

. The plate was never found, but 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.

Basic principles

Sizes of Orion vehicles

Interplanetary applications

Interstellar missions

Later developments

Economics

Vehicle architecture

Potential problems

Operation Plumbbob

See also

Further reading

External links