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George Dyson 1 Jan 2001

Project Orion

In 1957, tail fins, not seat belts, were standard equipment on American cars.

Tail fins reached a peak in popularity with the 1957 Chevrolet Bel-Air. Powered by a 235-cubic inch straight six or a 283-cubic inch V-8, with either manual overdrive or powerglide automatic transmission, the Bel-Air had a two-tone exterior, accented by anodized aluminum suggesting space-age Los Angeles rather than iron-age Detroit. Optional equipment, besides seat belts, included power windows, six-way power seats, and a built-in electric razor. The Russians were ahead in space, but General Motors was ahead on the road.

Preface -Bel-Air-

This book is the story of Project Orion . In 1957, a small group of scientists, led by physicist Theodore B. Taylor and including my father, Freeman J. Dyson, launched a serious attempt to build an interplanetary spaceship propelled by nuclear bombs. This account, as best as I can reconstruct it, is the story my father could only tell me in fragments at the time.

Orion was a sibling of both Sputnik and the Chevrolet Bel-Air. When my father joined Project Orion , he was driving a 1949 Ford. After a year in La Jolla, California, it was time to give up the Ford. Our poor old car finally gave up the ghost, he reported in his weekly letter home. So on Friday night we took the old car out for its last run. We went to a big car-dealer in San Diego. We looked at a lot of cars, drove three, and finally bought a 2-year-old Chevrolet at 9 P.M. A turquoise and white 1957 Bel-Air.

To a five-year-old from New Jersey, La Jolla (the jewel) was paradise found. General Atomic, the project's contractor, provided a house with a swimming pool and citrus orchard, draped in bougainvillea and overlooking the Pacific Ocean, which we scanned at sunset for the green flash. Winter swells broke over the reef at Windansea, where a surfing culture as tenacious as the inshore kelp beds had taken hold. Theodore Geisel, better known as Dr. Seuss, visited our three-room schoolhouse at La Jolla Cove. Abalone big enough to resist a tire iron could be gathered at low tide. My father and later my thirteen-year-old sister Katarina joined the local glider club and spent Saturdays trying to stay aloft in a fabric-covered sailplane winched into the updrafts above the cliffs at Torrey Pines. Jack Kerouac published On the Road.

The tail fins on the Chevrolet matched those on the Atlas intercontinental ballistic missiles that Convair Astronautics, a branch of the same corporate family as General Atomic, was building at a new $40-million facility four miles inland. In July of 1958 Convair held an open house, providing free hot dogs and the hourly flight of a model ICBM, which, the local paper announced, will emit a trail of smoke and will complete its trip with a big red flash, simulating the detonation of a warhead. Real Atlas missiles, with a range of 5,000 miles, carried thermonuclear warheads yielding one hundred Hiroshimas each. The delivery of hydrogen bombs to civilian targets was celebrated with an open house, while Orion, a spaceship that would use bombs to deliver civilians to Mars, Jupiter, and Saturn, was so encumbered by secrecy that until July of 1958 even the existence of the project remained publicly unknown.

Much of the record of Project Orion is still classified Secret & Restricted Data even though most of what kept the project secret in 1958 is now in the open, except for a few specific technical details. Any danger of Project Orion literature being used for destructive purposes is outweighed by the possibility that knowledge of Orion may be useful in ways that we cannot now predict or understand. Eventually, we will outgrow the use of nuclear energy as a weapon. Project Orion is a monument to those who once believed, or still believe, in turning the power of these weapons into something else.

All the people I visited or revisited in gathering this account believe they contributed to a dream that was nonetheless important for having failed. The years they spent working on Orion were the most exciting of their lives. Would they do it again? Definitely yes. Should we do it now? Probably not.

We had a wonderfully free time, before any of that fallout stuff came down, says Orion's lead experimentalist, Brian Dunne. It was a crazy era. All of our values were tweaked because of the cold war. It was a closed society, and all kinds of strange ideas were able to grow.


On October 4, 1957, Earth's first artificial satellite, weighing 184 pounds, was launched. Sputnik I circled the earth every 90 minutes for the next three months. Sputnik II followed on November 3 and weighed 1,120 pounds, including Laika, the pioneer of space-faring dogs. Earth's third artificial satellite was sent into orbit on January 31, 1958. Launched by a 32-ton Jupiter-C rocket built by the Chrysler Corporation, Explorer I weighed 31 pounds.

The race for space had begun. In Washington, D.C., the Advanced Research Projects Agency (then ARPA, now DARPA) was given a small office in the Pentagon and assigned the task of coordinating United States efforts—both civilian and military—to catch up. NASA did not exist until July of 1958. All three branches of the United States military had competing designs on space. If it flies, that's our department, claimed the Air Force. But they're called space-ships, replied the Navy. OK, but the Moon is high ground, answered the Army, who had already enlisted rocket pioneer Wernher von Braun.

Sputnik caught the American public, but not the United States aerospace establishment, by surprise. American scientists were well aware of the Soviet effort and several United States space programs—including the Atlas and Titan Intercontinental Ballistic Missiles, the Explorer and Vanguard satellite programs, the Rover nuclear rocket project at Livermore and Los Alamos, and even plans for a moon landing—were under way before the Russian Sputniks went up. ARPA's mission was to consolidate existing aerospace projects, differentiate military from civilian objectives, and consider all alternatives, however far-fetched. Things nuclear were viewed with enthusiasm. This was the era of unrestricted atmospheric bomb tests, with the equivalent of several thousand Hiroshimas being exploded from one year to the next.

One of ARPA's alternatives, code-named Project Orion , was an interplanetary space ship powered by nuclear bombs. Orion was the offspring of an idea first proposed, as an unmanned vehicle, by Los Alamos mathematician Stanislaw Ulam shortly after the Trinity atomic bomb test at Alamogordo, New Mexico, on July 16, 1945. It was typical of Ulam to be thinking about using bombs to deliver missiles, while everyone else was thinking about using missiles to deliver bombs.

To visualize Orion, imagine an enormous one-cylinder external combustion engine: a single piston reciprocating within the combustion chamber of empty space. The ship, egg-shaped and the height of a 20-story building, is the piston, armored by a 1,000-ton pusher plate attached by shock absorbing legs. The first two hundred explosions, fired at half-second intervals with a total yield equivalent to some 100,000 tons of TNT, would lift the ship from sea level to 125,000 feet. Each kick adds about 20 miles per hour to the ship's velocity, an impulse equivalent to dropping the ship from a height of 15 feet. Six hundred more explosions, gradually increasing in yield to 5 kilotons each, would loft the ship into a 300-mile orbit around the earth. I used to have a lot of dreams about watching the flight, the vertical flight, says Ted Taylor, Ulam's younger colleague of who founded Project Orion and, as the designer of both the smallest and the largest fission bombs in the United States repertoire, was uniquely qualified to dream where nightmares alone have otherwise led. The first flight of that thing doing its full mission would be the most spectacular thing that humans had ever seen.

The performance of a conventional rocket is governed by the velocity of its exhaust gases. This is limited by the energy content of the fuel, the efficiency in converting this into kinetic energy via the propellant, and the temperature that the combustion chamber and rocket nozzles can withstand. For a chemical rocket, maximum exhaust velocity—about 3km/sec (6,000 mph)— is limited by the speed to which the energy released by rearranging electrons in a chemical reaction can make the combustion products fly apart. The only way to propel the rocket faster is to leave part of the rocket behind as the next stage continues on. To reach low earth orbit (7km/sec) requires at least two stages and to reach escape velocity (11km/sec) at least three—which will get you away from Earth but will not get you back. For a given final payload, each stage adds a factor of about four to the initial mass. Some 16 tons of chemical rocket are required to place a one-ton payload in low earth orbit. A round-trip visit to the Moon, requiring five stages, takes almost a thousand tons for every ton that makes it there and back. Voyages to Jupiter, Saturn, or even Mars become prohibitive, which is why we have yet to go beyond the Moon except with one-way, unmanned probes. To really explore the solar system, on a time scale compatible with the careers of individual adventurers, requires high acceleration to get to the places that are interesting, fuel for the return trip, and good brakes so you can stop.

Orion escapes both energy and temperature limitations, because nuclear fission releases a million times the energy of burning chemicals and because burning the fuel in discrete pulses and at a distance avoids high temperatures within the ship. In a chemical rocket, the fuel, heated by its own combustion, becomes the propellant. Orion's propellant, distinct from the uranium or plutonium used as fuel, can be almost any cheap, inert material that is placed between the pusher and the bomb. It might be as light as polyethylene or as heavy as tungsten, and, on a long voyage might also include shipboard waste in addition to ice, frozen methane, or other material obtained from the surface of Mars, among the rings of Saturn, or elsewhere the ship decides to stop.

The propellant is vaporized into a jet of plasma by the bomb. In contrast to a rocket, which pushes the propellant away from the ship, Orion pushes the ship away from the propellant—by ejecting slow-moving propellant, igniting the bomb, and then bouncing some of the resulting fast-moving propellant off the bottom of the ship. The bomb debris hits the pusher at roughly a hundred times the speed of a rocket's exhaust, producing temperatures that no rocket nozzle could withstand.

For about one three-thousandth of a second the plasma stagnates against the pusher plate at a temperature of about 120,000 degrees. The time is too short for heat to penetrate the pusher, so the ship is able to survive an extended series of pulses, the way someone can run barefoot across a bed of coals without getting burnt. Even on an ambitious interplanetary mission, after several thousand explosions, the total plasma-pusher interaction time amounts to less than one second. The high temperatures are safely isolated, in both time and distance, from the ship.

The feature that sold Orion to ARPA in 1958, to the Air Force for seven years, and briefly to NASA in 1963, is its specific impulse—the standard measure for comparing the performance of different forms of propulsion in space. Formally defined as the propellant exhaust velocity divided by the acceleration of gravity, g, specific impulse (Isp) is measured in seconds and can be visualized, informally, as the length of time for which one pound of fuel can generate one pound (on Earth) in thrust. The best chemical rockets achieve a specific impulse of about 430, and reactor-driven nuclear rockets, using existing technology, might reach 1,000. Specific impulse varies as the square root of exhaust temperature, so an internal combustion rocket cannot achieve a much higher specific impulse without melting the ship.

Orion's external combustion engine escapes this restriction, allowing far higher Isp: 2,000 to 3,000 for first-generation designs, 4,000 to 6,000 for larger vehicles using existing bombs, possibly an order of magnitude higher if the state of the art was advanced. Other technologies, such as nuclear-electric or solar-electric ion propulsion, offer high specific impulse, but only at very low thrust. Chemical rockets produce high thrust but low specific impulse. Only Orion offers both. And the larger the ship, the higher the Isp. Even the first-generation versions of Orion could, as aerospace historian Scott Lowther describes it, go from downtown Jackass Flats to Saturn orbit back to low earth orbit in a single stage.

Project Orion took place from 1957 to 1965 at General Atomic, a division of the General Dynamics Corporation established to develop peaceful uses for atomic energy, usually assumed to include everything nuclear except bombs. General Atomic was founded, in 1955, by Frederic de Hoffmann, a young physicist turned entrepreneur who sought to recapture the freewheeling spirit he had known at Los Alamos during the war. General Atomic attracted not only theoretical and experimental talent but the backing of politicians, financiers, and industrialists who, after the success of the Manhattan Project and its hydrogen-bomb successors, were eager to see what de Hoffmann's colleagues might come up with next. There was a narrow window of opportunity between the launch of Sputnik and the commitment of the United States to an exclusively chemical approach to space. It was only a time like this and a place like General Atomic that gave a proposal as unorthodox as Orion a chance. Where else could a thirty-two-year-old physicist show up for work the day after Sputnik, start daydreaming about how many bombs it would take to put something the size of a nuclear submarine into orbit and spend the next seven years—with the support of General Dynamics, the AEC, the Air Force, and, to a small extent, even NASA—making a serious effort to get the idea off the ground?

De Hoffmann and Taylor secured a $5,000 study contract from the Albuquerque office of the Atomic Energy Commission, a formality that provided access to the classified information necessary to work on anything having to do with nuclear bombs. On November 3, 1957, the day that Sputnik II (with Laika aboard) was launched, General Atomic issued T. B. Taylor's Note on the Possibility of Nuclear Propulsion of a Very Large Vehicle at Greater than Earth Escape Velocities. The new project was named Orion—for no particular reason, says Taylor, who just picked the name out of the sky. Marshall Rosenbluth suggested the code name be spelled O'Ryan—to throw others off the trail.

By the beginning of 1958, plans to build Orion were taking form. Ted Taylor's proposal, submitted to ARPA in early 1958, envisioned a 4,000-ton vehicle, carrying up to 2,600 bombs and capable of orbiting a payload of 1,600 tons. Dr. Taylor estimated that a fully completed spaceship could be achieved by 1963-1964 and would cost approximately $500,000,000, Second Lieutenant Ronald Prater, one of ARPA's contract monitors, noted after a visit to General Atomic in November 1958. Suggested missions ranged from the ability to deliver a hydrogen warhead so large that it would devastate a country one-third the size of the United States to a grand tour of the solar system that Orion's chief scientists envisioned as an extension of Darwin's voyage of the Beagle: a four-year expedition to the moons of Saturn including a two-year stay on Mars. Saturn by 1970, announced the physicists. Whoever controls Orion will control the world, claimed General Thomas Power, commander-in-chief of the Strategic Air Command.

In the early spring of 1958, General Atomic began moving from temporary quarters in the Barnard Street School in downtown San Diego to a spectacular facility on 300 acres of mesa above the beaches of La Jolla near Torrey Pines. The centerpiece was a circular technical library, two stories high and 135 feet in diameter—exactly the diameter of the 4,000-ton Orion design. The library, which included a cafeteria, provided a sense of scale. Ted Taylor would point to a car or a delivery truck, the size of existing space vehicles, and say, This is the one for looking through the keyhole. Then he would point to the library and say, And this is the one for opening the door.

Most technical references to Project Orion remain classified to this day. Even the titles of documents were often classified, leaving only occasional clues to their existence, for instance a reference to Ted Taylor and Marshall Rosenbluth's original report on the possibility of Orion, identified in declassified Air Force correspondence only as GA-292, and by Air Force historians who note that it included all the necessary practical working features for a very large space vehicle... which was sent to ARPA and AFSWC early in 1958. Some of the reasons for this secrecy, such as Orion's potential as a strategic deep-space weapons platform, are now obsolete. Other secrets, especially the details of how to build miniature, directed-energy nuclear explosives using very small amounts of plutonium, remain as sensitive as ever today.

Taylor could supplement the available staff at General Atomic with outside consultants, as long as they had up-to-date Q clearance, the special level of security administered by the AEC under the Atomic Energy Act of 1954. One of the consultants was my father, Freeman J. Dyson, of the Institute for Advanced Study in Princeton, New Jersey, who disappeared periodically to General Atomic with little hint as to what he was working on, though it was possible to guess.

I have nothing very original to say about Sputniks. I feel very cheerful about them, he wrote to his parents in England on New Year's Day, 1958, flying from New York to San Diego to spend his first ten days consulting on Project Orion with Ted. It seems to me clear that the Soviet government does not intend to throw bombs at anybody but does intend to dominate the earth by rapid scientific and industrial growth. This will in turn stimulate the Americans to undertake a whole lot of major projects which they would be too parsimonious to do otherwise. There is no question that colonization of the Moon and planets will be one of them. I expect eventually to take a hand in this myself.

By springtime, Freeman was spending more and more of his time on the West Coast. I find myself now in a group of not more than 12 people, all of us under 40, planning an enterprise which will inevitably grow into colossal dimensions, he wrote on April 27 from the Hotel Del Charro, near the beach at La Jolla Shores. The feeling and atmosphere we are now in must be similar to the atomic bomb project in the earliest days, before even Los Alamos was thought of, when Oppenheimer and Teller and a handful of other people were feeling their way into the problem and establishing the basic ideas for everything which came later. It is characteristic of this very early time that there is no feeling of pressure or urgency, everything is quite informal and relaxed, and we ourselves have difficulty in taking the whole situation seriously. In years to come, when huge projects and whole empires have grown out of this, the early period will have become legendary, and we ourselves will not be able to distinguish our memories of this time from the legends which will grow around us.

What I have said in this letter is no violation of secrecy, he added, since I said nothing of what we are doing. Still I would like you to even keep quiet about this, and not tell your friends I am doing anything unusual. In May of 1958, Project Orion received permission to reveal the nature of its business, confidentially, and only for recruiting purposes, since it was becoming difficult to hire talent without explaining the project's basic principle and goals. Finally, on July 2, 1958, the existence—but not the name—of the project was made public in a one-page press release issued in Washington, D.C.: Roy W. Johnson, Director of the Advanced Research Projects Agency, today announced that the Air Research and Development Command has been authorized to let a feasibility study contract with the General Dynamics Corporation's General Atomic Division, located at San Diego, California, related to the possible development of a new concept of propulsion employing controlled nuclear explosions... within the atmosphere and beyond. The initial commitment for this study calls for the expenditure of one million dollars during the fiscal year 1959.

I was five years old, and space was within reach in a way that only children of Sputnik can understand. In the summer of 1957 we had driven up to the Lick Observatory on Mount Hamilton in California, whose 26-inch refractor was at one time the most powerful telescope in the world. When my father lifted me up to the cold brass eyepiece, Saturn and its rings filled the black, starry night, big as a soup plate, as if the dome we were standing in was a spaceship and we had already traveled halfway there. Later that year we scanned the Milky Way as the first Sputniks passed overhead. Under the clear New Jersey winter sky, I learned to recognize the stars and distinguish the planets, guided by a copy of Find the Constellations by H. A. Rey, also the author of Curious George. Tonight when I came home from work it was already dark and George said, ‘I have just been outside and I could see Venus and Orion,''' Freeman reported to my grandfather in October 1957. I did not even need to check that his statement was accurate. He has his eyes very wide open for all natural phenomena, birds and butterflies, worms and clouds.'' And spaceships, large and small.

Space travel, in the 1950s, appeared imminent. As the Air Force noted in reviewing the project, The uses for ORION appeared as limitless as space itself. The children who would be disinherited in the 1960s over a colonial war in Vietnam spent the 1950s believing we would be colonists in space. When it was revealed that the United States Government was planning to build a bomb-propelled interplanetary spaceship and that my father intended to be on board, I was among the least surprised. When I drove George to school this morning I told him about the space-ship, my stepmother reported, in June 1958, when the AEC and Department of Defense first allowed General Atomic to mention the existence of the contract to the press. He was very excited, asked immediately which planet you will send it to, and whether there would be a little seat right next to you for him to come along.

When my father returned home, I had innumerable questions. How big is the spaceship? What will it look like? How does it work? Where will you be going and how long will you be gone?

I cannot tell you, he answered. But someday you will find out.

Freeman Dyson and two of his children
The illustrations in this article are gathered from US Government, General Atomic, NASA, and other sources for the Project Orion book