One-meter-diameter tethered model suspended from the Atlas missile test tower at Point Loma in preparation for single-shot high-explosive tests.
Stanislaw Ulam, whose role in the Teller-Ulam invention remains disputed by Edward Teller, is securely on record as the principal inventor of space propulsion by nuclear bombs. "The day after Trinity he found himself thinking about propelling something into a ballistic trajectory," says Ted. In 1959, the AEC patented the concept of a bomb-propelled space vehicle in his and Cornelius Everett's name. "It does not matter," Ulam once said in response to a summary of Project Orion that omitted his paternity from the account. "After all, it is MY patent!" Nuclear pulse propulsion was only one of Ulam's progeny of ideas, from pure mathematics through weapons physics to what is now called the science of complexity, and is associated, thanks in part to Ulam, with his adopted home of Santa Fe.[28]
"He was a maverick, a very complicated man, a Pole, and, above all, a study in contrasts and contradictions, says his wife, Francoise. "He lived mainly in the confines of his mind."[29] He was also gregarious. "Many of us at the laboratory who were associated with him knew how much he disliked being alone, how he would summon us at odd times to be rescued from the loneliness of some hotel room, or from the four walls of his office, after he had exhausted his daily round of long-distance calls," says his mathematical colleague Gian-Carlo Rota. "One day I mustered the courage to ask him why he constantly wanted company and his answer gave him away 'When I am alone,' he admitted, I am forced to think things out.' "[30]
"Ulam knew something about everything," says Brian Dunne, who remembers driving the Ulams down to Point Loma from La Jolla in his Volkswagen to watch the launch of one of the explosive-driven models of Orion that were being tested in 1959. The flights depended on precisely timed sequential charges, and Ulam helped obtain some of the low-jitter detonators developed at Los Alamos to ensure the perfectly symmetrical implosion that triggers the explosion of a bomb. "He was a real singularity in many ways," confirms Bruno Augenstein, a RAND analyst and architect of the United States ICBM program who intersected periodically with Ulam during the H-bomb years. "He was simultaneously one of the smartest people that I've ever met and one of the laziest—an interesting combination." Francoise disagrees: "With his aristocratic nonchalance he gave the appearance of being lazy, but in reality he pushed himself mentally, all the time." Claire Ulam, age nine in 1953, was overheard telling a friend that "all my father does is think, think, think!"
Ulam emigrated from Poland to the United States in 1935 to join von Neumann at the Institute for Advanced Study. In 1941, upon acquiring United States citizenship, he tried to enlist in the Air Force, hoping to become a navigator if not a pilot, but due to his age and imperfect eyesight his application was turned down. In 1943 he asked von Neumann how he might assist in the war effort, and, as Ulam recalls, "Johnny answered with an intimation that there was interesting work going on—he could not tell me where." Ulam then received an invitation, signed by Hans Bethe, "to join an unidentified project that was doing important work, the physics having something to do with the interior of stars." He accepted, on the strength of Bethe's reputation, without knowing what he had agreed to, or where. "Soon after, other people I knew well began to vanish one after the other. Finally I learned that we were going to New Mexico, to a place not far from Santa Fe."[31]
The Ulams, with daughter Claire on the way, arrived in Los Alamos in February 1944. "In the entire history of science there had never been anything even remotely approaching such a concentration," he noted, astonished at the team of scientists that Oppenheimer had assembled on the mesa above Santa Fe. "At thirty-four I was already one of the older people." Ulam found wartime Los Alamos a refreshing contrast to academic life. "People here were willing to assume minor roles for the sake of contributing to a common enterprise, he noted. "Jules Verne had anticipated this when he wrote about the collective effort needed for his 'Voyage to the Moon.' "[32] Ulam's exposure to physics revealed gifts that otherwise might have gone unrecognized this late in a mathematical career. "I found out that the main ability to have was a visual, and also an almost tactile, way to imagine the physical situation, rather than a merely logical picture of the problems. One can imagine the subatomic world almost tangibly, and manipulate the picture dimensionally and qualitatively, before calculating more precise relationships."[33]
For fifty years, we have separated the uses of nuclear energy into two distinct regimes: reactors and bombs. The nuclear establishment does its best to preserve the distinction: making sure that bombs do blow up and that reactors don't. The spectrum between reactors and bombs, however, is a continuum. Some of the earliest ideas for nuclear propulsion fell in the middle, intermediate between an overheated reactor and a fizzling fission bomb. The minutes of an informal meeting on nuclear rockets, held at Los Alamos on January 17, 1949, and recorded by Frederic de Hoffmann, note that "the group took as its premise that an effective method for the delivery of nuclear bombs had to be available" and considered at least one design where the "nuclear motor becomes a bomb on landing," concluding that the resulting hybrid would be inefficient both as a rocket and as a bomb. "It would also be feasible to shoot at the moon and thus obtain interesting physics data," the group added. Attendees included George Gamow, Edward Teller, and Fred Reines as well as Ulam and de Hoffmann.[34]
"The idea of nuclear propulsion of space vehicles was born as soon as nuclear energy became a reality," Ulam recalls. "It was an obvious thought to try and use its more powerful concentration of energy to propel vehicles with a very large payload for ambitious voyages of space exploration or even for excursions to the moon. I think Feynman was the first in Los Alamos during the war to talk about using an atomic reactor which would heat hydrogen and expel the gas at high velocity."[35] These ideas became the basis of separate Livermore and Los Alamos efforts, later consolidated as Project Rover (named, by Herbert York, after the fictional Rover Boys) and the NERVA (Nuclear Energy for Rocket Vehicle Application) program, which remained active until 1973. All these nuclear rockets—and the parallel project to build a nuclear-powered airplane—would have been dirty at the best of times and disastrous if anything went wrong. "Fission product decay heat would cause melting and/or vaporization of the reactor within about 30 seconds after shutdown if the coolant is shut off or exhausted," noted R. W. Bussard in reviewing the prospects for a nuclear-powered Atlas missile in 1956.[36]
Ulam began thinking about propulsion by external nuclear explosions in 1946. Preliminary calculations were recorded in a still-classified memorandum coauthored with Fred Reines in 1947. Johndale Solem, a Los Alamos physicist who has been studying next-generation Orion-type vehicles for deflection of asteroids on collision course with Earth, confirms having seen the original memo when cleaning out the T-Division safe. "I asked Fred Reines about it and he said, 'Well, Stan talked me into that one.' He wasn't about to admit he was involved in the beginning of Orion. The stuff we were throwing out here went to archives somewhere."
"I heard Stan talk about this in—maybe it was 1948," remembers Harris Mayer, a Los Alamos colleague whose insights into the opacity of matter at high temperatures were relevant both to Orion and to the hydrogen bomb. "We knew a lot about nuclear bombs. At that time we didn't know about hydrogen bombs. But his idea was very simple. If you threw a nuclear bomb out the back of a rocket ship, it exploded and gave it a kick. Now he was thinking of a rocket ship of the conventional size and class, something like the Atlas; the whole ship is maybe 100 tons. We were just brainstorming, that was the level of it, and recognized immediately that this was not a manned ship. The accelerations would just crush a person into a blot. So we didn't worry about all the other things, radioactivity and so on. And nobody did anything about it."
In 1955, with Cornelius Everett, Ulam produced a more detailed report, On a Method of Propulsion of Projectiles by Means of External Nuclear Explosions, issued as a classified Los Alamos document, LAMS-1955. Having the document number match the year of publication was a singularity of which Ulam was especially proud. "Repeated nuclear explosions outside the body of a projectile are considered as providing a means to accelerate such objects to velocities of the order of 106 cm/sec... in the range of the missiles considered for intercontinental warfare and even more perhaps, for escape from the earth's gravitational field, for unmanned vehicles," Ulam and Everett explained.[37] In reviewing the thinking behind these ideas, "some of which originated as long as ten years ago," Ulam and Everett followed the logical progression that leads from the energy limitations of conventional rockets to the temperature limitations of an internal combustion nuclear rocket to the breakthrough that becomes possible if you take a nuclear reactor to its extreme—the explosion of a bomb—and isolate the resulting high temperatures from the ship. "The scheme proposed in the present report involves the use of a series of expendable reactors (fission bombs) ejected and detonated at a considerable distance from the vehicle, which liberate the required energy in an external 'motor' consisting essentially of empty space. The critical question about such a method concerns its ability to draw on the real reserves of nuclear power liberated at bomb temperatures without smashing or melting the vehicle."
Ulam and Everett proposed ejecting disks of lightweight plastic propellant separately from the bombs. "The vehicle is considered to be saucer-shaped, of diameter about 10 meters, sufficient at any rate to intercept all or most of the exploding propellant. Its final mass is perhaps 12 tons.... The bombs are ejected at something like one-second intervals from the base of the rocket and are detonated at a distance of some 50 meters from the base. Synchronized with this, disk-shaped masses of propellant are ejected in such a way that the rocket-propellant distance is about 10 meters at the instant the exploding bomb hits it. The propellant is raised to high temperature, and, in expanding, transmits momentum to the vehicle."[38] The ship would carry about fifty bombs, each weighing about half a ton, of roughly one kiloton yield. "The accelerations of the order of 10,000 g are certainly large," they admitted.[39] Passengers or fragile cargo would be prohibited, and control mechanisms would have to be hardened against shock.
After hearing the news that Sputnik had been launched, Ted Taylor began thinking about adding shock absorbers to make the impulse tolerable to a human crew. "That night of thinking about the thing," he recalls, "I derived for myself the notion that if you really added together the features that you wanted of any vehicles for exploring the solar system—the whole thing, not just near in—you're led directly to energy on the scale of a lot of nuclear weapons. And having been led to that, in thinking about what might be done, I began saying, 'Gee, that's what Stan Ulam's been talking about all these years.' I had read LAMS-1955, and Stan and I, every now and then we'd get together at his house or in his office or somewhere, eating lunch in the cafeteria, and talk about his propulsion idea, and how to get people interested in it and get it off the ground."
When the proposal for Project Orion was first circulated in Washington in 1958, both Hans Bethe and Stan Ulam lent support. Ulam testified in favor of Orion before the Joint Committee on Atomic Energy, explaining to the assembled congressmen in January 1958 that "it is almost like Jules Verne's idea of shooting a rocket to the moon."[40] In April he spoke again to the committee, emphasizing that "it appears that a big ship, with payloads of the order of 1,000 or several thousands of tons, can be made to travel by such propulsion.... These planned vehicles are very large affairs. It is not a question of 'space capsules' but comfortable quarters for occupants of such a ship."[41] In response to concerns that General Atomic was absconding with a concept originated at the national weapons lab, he wrote to Senator Clinton P Anderson, chairman of the committee, that "Dr. Taylor is one of the most inventive young scientists in this country, and if not for him, the project would probably still be in the form of a purely theoretical scheme."[42]
"The spaceship could transport hundreds or thousands of people," Ulam later explained. He tried to interest George Kistiakowsky, President Eisenhower's Scientific Advisor, "but his reception of it was not enthusiastic."[43] Ulam's own ideas for space travel left the limited horizons of Washington far behind. On April 1, 1958, he wrote a brief Los Alamos report, On the Possibility of Extracting Energy from Gravitational Systems by Navigating Space Vehicles, describing how a spacecraft might operate as a gravitational "Maxwell's demon," amplifying a limited supply of fuel and propellant by using computational intelligence to select a trajectory that harvested energy from celestial bodies as it passed by.
One-meter-diameter
tethered model suspended from the Atlas missile test tower at Point
Loma in
preparation for single-shot high-explosive tests.
James Clerk Maxwell was the author of Maxwell's equations formalizing the concept of an electromagnetic field and namesake for the Maxwellian distribution of kinetic energy among the particles of a gas. In 1867 he conceived an imaginary being—termed "Maxwell's demon" by William Thomson (Lord Kelvin) in 1874—"whose faculties are so sharpened that he can follow every molecule in its course."[44] The demon appears to defy the second law of thermodynamics by heating a compartment in an otherwise closed system, without the expenditure of physical work, by opening and closing a vanishingly small trap door to let high-velocity molecules in and low-velocity molecules out. Resolving this apparent paradox led to advances first in thermodynamics and later in quantum mechanics. Leo Szilard showed in 1929 that even if the operation of the trap door is perfectly effortless, the cost of making the observations required to distinguish fast molecules from slow molecules assures that the laws of thermodynamics are upheld.
A Maxwellian distribution of energy among a collection of particles in thermodynamic equilibrium implies the equipartition of energy, meaning that kinetic energy tends to equalize across the particle population over time. Light particles end up moving at relatively high velocities and heavy particles end up moving relatively slowly. A 4,000-ton spaceship ends up moving faster than a planet—given enough time. If applying the kinetics of a gas to interplanetary transport seems far-fetched, remember that Maxwell first developed these ideas, later adapted to thermodynamics, in an attempt to explain the distribution, by size and velocity, of particles that make up Saturn's rings.
"As examples of the situation we have in mind," explained Ulam, "assume a rocket cruising between the sun and Jupiter, i.e., in an orbit approximately that of Mars, with an energy in reserve which would allow the kinetic energy of the vehicle to increase by a factor like 2. The question is whether, by planning suitable approaches to Jupiter and then closer approaches to the sun, it could acquire, say, 10 times more energy. It is clear, on general thermodynamic grounds, that 'in general' the equipartition of energy may take place.... Nothing is said, however, about the times necessary for effecting this. They might be of super-astronomical lengths.... With an operating intelligence perhaps this approach to near-equilibrium could be made vastly more rapid. The problem is whether, by steering the rocket, one can to some modest extent acquire the properties of a Maxwell demon, i.e., plan the changes in the trajectory in such a way as to shorten by many orders of magnitude the time necessary for acquisition of very high velocities."[45]
"I remember Stan talking about being able to make a Maxwell's demon, that it could be a possible physical thing," Ted Taylor recalls. Ulam hinted that he was thinking about the still-classified possibilities of Orion, whose cruising speed, for the 4,000-ton version, was estimated at 20 km/second. "The above discussion is, of course, intended for a purely theoretical, mathematical question," he wrote. "Even so, during the next few decades large objects may be constructed with a cruising velocity of 20 kilometers a second, and there will still be some additional energy left."[46] Although gravitational energy is available apparently for free, the intelligence required to operate such an equipartition engine would have its costs, especially when even small computers, slower than a pocket calculator of today, consumed kilowatts to operate at kilocycle speeds while weighing tons. "The computations required to plan changes in the trajectory might be of prohibitive length and complication," Ulam warned.[47]
Project Orion had Ulam's enthusiastic support. He lobbied Congress, pushed for support within Los Alamos, and encouraged Ted and the rest of the Orion gang. "Meteors, some of which come in at thirty kilometers per second, do not get excessively chewed up, even though presumably they were not especially engineered for it," he wrote to Ted in 1962, when there were worries about whether the surface of the pusher plate could withstand being hit by the propellant cloud.[48] After the death of Project Orion he continued to believe in its resurrection, perhaps as a joint U.S.-U.S.S.R. mission at some more cooperative time. Outshadowed by the success of the Teller-Ulam invention, Orion was a hope that returned to confinement after the 1963 atmospheric test-ban treaty began to close the lid on Pandora's box.
"It was such a crazy idea, really. One of those wonderful dreams," says Francoise Ulam. "Oh, but they had a good time in California, and we went and visited them once and saw one little puff. The little thing went off and then came back with a parachute, at Point Loma. They had fun, but did they really think it would go?"