"When the nuclear device is exploded, the channel filler absorbs the radiation emitted and rises to a high temperature," explained Bud Pyatt in one of Project Orion's later reports. "The radiation case serves to contain the energy released by the explosion so that more energy is absorbed by the channel filler than is emitted into the solid angle it subtends relative to the source. The high pressure achieved in the heated channel filler then drives a strong shock into the propellant, which vaporizes the propellant material and accelerates it toward the pusher."[146] By this time—1963—tungsten had been chosen for the propellant, beryllium oxide for the channel filler, and uranium for the radiation case. Tungsten—2.5 times heavier than steel—allows for a very thin pancake, producing an optimally narrow jet; beryllium, a strong absorber of neutrons, captures maximum energy from the bomb; uranium is highly opaque to X rays, making it difficult for the bomb's initial burst of radiation to escape.
The expansion of the bomb and subsequent compression of the tungsten pancake takes a few millionths of a second. During this time, the channel filler and propellant absorb neutrons and X rays emitted by the bomb. This reduces the shielding required to protect the Orion crew, and transforms as much as possible of the bomb's output into kinetic energy that can be intercepted to drive the ship. The re-expansion of the propellant gives the pusher a brief but intense kick. "It's like what happens down at the ocean on a very, very stormy day, and suddenly a lot of churned-up sand hits against you," says Pyatt. "Or if you are suddenly hit with a very strong firehose." For the original 4,000-ton, 135-foot diameter Orion design, there was almost one-third of an acre of pusher plate. All this pressure—on the order of 50,000 pounds per square inch—adds up.
The propellant slab, after being compressed to about one-quarter of its original thickness, expands as a jet of plasma moving at some 150 km/sec (300,000 mph) toward the ship. It takes about 300 microseconds to make the trip. During this time the expanding propellant cools to about 10,000 degrees—a temperature described by physicists as roughly one electron volt. Within another few hundred microseconds the propellant cloud hits the pusher (or the advancing front of the reflected shock wave produced by the initial collision) and is suddenly recompressed. For less than a millisecond the stagnating propellant reaches a temperature of between 100,000 and 120,000 degrees—about ten times the temperature of the visible surface of the sun, but only a small fraction of the temperature of a bomb. In space, with no atmosphere to produce a fireball, die explosion would appear quite different from what we usually picture as an atomic bomb. "The debris goes out from the bomb essentially invisibly," explains Freeman Dyson. "You don't see anything until the stuff is stopped. Around the bomb you have a lot of cold stuff, which absorbs the energy so the debris comes out forwards and backwards and that won't produce anything very spectacular in the way of a flash until it hits the ship. Then all its energy is converted into heat and so you get about a millisecond or so of intense white flash. And very little else."
Orion's feasibility depends on what happens during those few hundred microseconds as the hot plasma piles up against the plate. After two or three thousand impacts would there be any spaceship left? It was the amount of ablation, or eroding away, of the pusher's surface that placed Orion in a regime that was completely unexplored. Later in the project, jets of high-speed plasma were fired at sample targets, attempting to validate the predictions of successive generations of computer codes. But in making the proposal to ARPA in 1958 optimism rested precariously on untested theory and the one-shot evidence from Lew Allen's Balls.
"To calculate the ablation you needed some pretty good physics, and that Rosenbluth was able to do," Freeman explains. "The most important thing is how opaque the stuff is. This whole business of opacity is the central problem both in stars and in bombs. The opacity is like the resistivity of a metal except you are dealing with radiation instead of electrons. It tells you how hard it is for the radiation to get through." Opacity is where Orion either succeeds or fails. "It just kept coming up," says Ted Taylor. "Opacity was repeated ten times a day." The benefits of high opacity are manifold. If the material is opaque, it prevents harmful radiation from reaching the surface of the pusher. It also blocks secondary radiation produced by the collision between the plasma and the pusher from escaping back through the layer of stagnating propellant, thus doubling the kick. "If it is sufficiently opaque so you don't lose that energy by radiation, then it bounces back and you get the momentum doubled. If it is transparent the heat radiates away and you lose it; it is just like a slab of mud being thrown against something and you get the momentum it had originally, nothing more." The initial estimates were rough. "I just did some more opacity calculations and got results differing from Marshall by a factor 4 (which is not significant) in the pessimistic direction," Freeman reported to Ted on May 2, 1958.[147]
Nature appeared to be on the side of Orion. "If you have, roughly speaking, a bomb that is a hundred meters away from the ship with a yield of a kiloton, the temperature works out at a hundred thousand degrees," Freeman explains. "This was an unusual temperature, which had never been thought about much because stars are generally cooler and bombs are usually hotter. So this was an intermediate range. What Rosenbluth understood was that this is a good range for getting high opacity. It's essentially just ultraviolet radiation, soft X rays, which is easily absorbed. Almost anything you put there is opaque. And that's why the thing works, because the more opaque it is then the less the radiation eats into the surface."
Opacity increases as the plasma piles up. "The densities we were talking about were, roughly speaking, one gram per liter, or normal air density, which is unusual for something that hot. The more dense it is the more opaque it gets; if you squeeze the stuff together it gets blacker. Nobody had calculated this before." Opacities had been studied intensively and in secret at both Los Alamos and at RAND. The opacity of heavy elements like uranium at high temperatures was essential to hydrogen-bomb design, and the opacity of air at lower temperatures was critical to an understanding of fireball development in order to either survive or maximize the effects of nuclear bombs. The domain in question for Orion—a hydrocarbon plasma hitting an iron pusher plate at a temperature of 10 electron volts—fell somewhere in between. The Orion physicists were unsure at first whether these numbers would be classified or not. "The question came up very quickly after we started doing this work at General Atomic, what's classified and what isn't in this field, because all during the war the work on opacity was as dark as it could be, it was kept hush-hush," says Pyatt. "So there was an arbitrary decision made that if it was lighter than iron it was not classified. If it was heavier than iron it was classified, at temperatures above 10 electron volts. And to this day that pretty much still holds."
Orion depended on how the numbers turned out. "If the opacity of the propellant is not sufficiently high to contain the radiation near the pusher then one loses the factor of 2 from reflected momentum and this hurts the whole scheme very seriously," Don Mixson and Lew Allen reported after a visit to General Atomic in July 1958.[148] Harris Mayer, the leading authority on opacity, was brought in as a consultant; Dyson and Rosenbluth went to meet with Arthur Kantrowitz at a company called AVCO, which was at the forefront of designing ablative nose cones for ICBMs; the computer programmers started adapting weaponeering codes. Mayer remembers Dyson taking an approach that "was more than mathematical," looking at maximum possible opacity to start things off. "He said, never mind calculating opacities, let's see what the largest opacities could be. And he had a very simple theorem for this, which was well based. Now I'd worked many years on opacities. I never thought in that way."
The opacity of a material across a radiation spectrum is characterized by lines and windows. Lines are where the radiation is absorbed and windows are where the radiation gets through. "To describe where the lines were, how broad they were and how much of the window regions would be obscured by the lines—how much they overlapped, how much they were split by various interactions—was a very, very messy quantitative problem," says Burt Freeman. Astrophysicist John C. Stewart was brought in to focus on the opacities of light elements at relatively low temperature, which, with ingenuity, would eventually intersect with enough computer power to perform a calculation rather than a guess. "What was unique about John Stewart's work," says Burt Freeman, "is that it was a detailed description of a region where the electronic structure was sufficiently simple so that you could do a quantitative calculation."
Opacity was a perfect exercise for someone fluent in QED. "We started doing a much better job than anyone had done before, doing it atom by atom, not just using averages," says Freeman Dyson. "These atoms all have very complicated spectra and everything depends on windows because it's where the atom doesn't absorb that the radiation gets through. The important thing is to get the exact shape of the windows right. It's a delicate calculation. And to fill in the windows it's important to have a mixture of things: carbon and nitrogen and oxygen, which have windows in different places so they fill in each other's windows. And you need the hydrogen just in order to have the chemical compounds that are easy to handle, like polyethylene, which is good stuff physically and also reasonably opaque. You prefer to have something with nitrogen and oxygen as well. But we generally thought of polyethylene as being good enough. It turned out the opacities were pretty high, even just for carbon by itself. The results always turned out to be rather good from the point of view of feasibility."
The results also turned out well from the point of view of economy—and at that time large-scale commercialization of Orion was what Ted and Freeman had in mind. "The best propellant worked out being something like equal amounts of hydrogen, carbon, nitrogen, and oxygen," says Freeman. "Urea would be the ideal substance, it has just about the right proportions." This had two implications for extended interplanetary voyages: 1) ordinary nitrogen-based high explosive, minimizing consumption of expensive plutonium by reducing the critical mass of fissile material required for each bomb, would become excellent propellant when its ionized remnants hit the ship, and 2) shipboard waste could be recycled as propellant instead of as drinking water, an alternative cited in General Dynamics mission studies as a factor affecting crew morale.
The next step was to execute numerical simulations of a cloud of propellant hitting a plate, following the process step by step in time, first as a one-dimensional calculation and then in two dimensions, looking at what happens at a surface being ablated not only by a vertical impact but also by a horizontal wind. The initial shock wave and rarefaction wave were followed by complex interactions as the incoming plasma begins to mix with material being evaporated from the surface of the plate. "The question is, when is that stable and when is it unstable," says Freeman. "The answer was that it was generally stable, but you couldn't be sure."
Convection or turbulence between the layers of stagnating propellant and ablating pusher might defeat the self-protection of the pusher, with disastrous results. "I did a calculation looking at the worst case," says Freeman. "If the thing was totally unstable and convective then how bad would the ablation be? And it turned out even in that case it wasn't terribly bad. Because the time is so short, convection only has time to go around once or twice, so even in the worst case the stuff doesn't ablate more than is tolerable. It was on the whole quite encouraging." Turbulent ablation remained one of the unknowns that could be decided only by a nuclear test. "We just said, 'We'll see when we do the trials whether that happens or not.' "
In 1958, the pusher plate was envisioned as a heavily reinforced 1,000-ton steel or aluminum disk about 120 feet in diameter, lens-shaped so that its mass distribution matched the momentum distribution from the bomb. Fiberglass was also considered and given preliminary tests. "General Atomic may require fiberglass impregnated plastics to be subjected to extremes of temperature and stress," one of Orion's project officers wrote to the Air Force materials laboratory in September 1958. "The material may have a very important application. We are interested in manufacturing methods and techniques for the production of massive slabs weighing up to 1,000 tons. The need for this information is urgent."[149]
After receiving an ambivalent reply—"We are not familiar with any work that has been conducted on molding massive slabs of the size mentioned"—Carroll Walsh, the project's all-around logistical trouble-shooter, enlisted a local surfboard maker willing to help.[150] "We got them to make us a big piece, three-inch-thick fiberglass that had never been done before, but they did it. And then we busted the hell out of that." After the expenditure of considerable high explosives, adds Ted, "fiberglass was abandoned on the grounds that it was hard to ensure it would be in the same state after a few shots that it was at the beginning." A good way to envision Orion is as a ship that surfs through space on waves of plasma generated by atomic bombs.
Pusher tests were first performed by detonating a few pounds of high explosive a short distance from target plates. This approximated the mechanical stress on the pusher but did not get anywhere near either the velocity or temperature of the bomb debris that would hit the ship. Brian Dunne thought that they should see how close they could get. Having worked with Ted and Freeman on building the TRIGA reactor, Dunne joined Orion almost from the start. "Ted Taylor called me down to Barnard Street shortly after Sputnik and asked me to contribute to this proposal that he and Rosenbluth and Loomis were haranguing over," remembers Dunne, who had a knack for bridging the gaps between theorists and experimentalists, turning this to his advantage early on. "When I wrote proposals, I learned to include both theory and experiments. When those proposals go before the committee, there are going to be both experimentalists and theorists. And the theorists are impressed with the experiments, while the experimentalists are impressed with the theory. This almost always works." Dunne became Project Orion's chief experimentalist, but he is a theorist in disguise. "Experimentally that's what I do—I picture things," he says. "You picture things and then go work with the thing that is most soothing to the nerves."
Within
one microsecond of the bomb's explosion, the
propellant is compressed to a high temperature and density before
expanding as
a jet of plasma that takes about 300 microseconds to reach the ship.
Within
another 300 microseconds, the propellant cloud has stagnated against
the pusher
plate, giving the vehicle a kick.
Dunne knew that you could never duplicate the effects of a nuclear explosion with chemical explosives, but if you began heading, experimentally, in that direction you could check whether the mathematical models of the plasma hitting the pusher were on the right track or not. As a graduate student, he had worked with shock tubes—evacuated cylinders in which a high-speed shock wave is propagated from end to end—and was also familiar, from Los Alamos, with shaped high-explosive charges whereby a jet of material can be accelerated intensely enough to penetrate an armored tank. He put the two concepts together and came up with high-explosive-driven plasma guns—lead-lined evacuated cylinders encased in a thick sleeve of high explosive, up to 40 pounds per shot. The implosion produces effects similar to squeezing a ripe banana out of its skin—with 50,000 mph banana plasma as the result. Dunne remembers trying everything to get to 107 cm/sec but the best they could do was 1 or 2 x 106. "That's just the kind of problem I wanted to be working on," he explains. These velocities were lower than an Orion plasma but the densities were higher, producing conditions at the target that were close enough to keep the theorists honest and instill confidence in the mathematical models that were evolving, at the hands of Charles Loomis and others, into the SPUTTER ablation code, which sought to predict how much pusher plate would be ablated with each shot. "The theory and the experiment coupling was very similar to what it had been at Los Alamos," says Ted. "You use these very fancy calculations to bridge the gap between actual tests."
"The explosive jets were able to cover only part of the range of temperatures, pressures, and durations that were of interest for the full-scale ship," says Freeman Dyson, "but they provided a detailed check of the theoretical calculations within the overlapping part of the range and gave us confidence that the theory had not overlooked anything essential. You couldn't really fit all the parameters, but the experiments gave a feeling that some of the things we were saying were right. They tended to ablate a tenth of a millimeter or something, which looked very much like what we had in mind for the full-scale ship. It couldn't be much more than that."[151] During the course of the project, with hundreds of charges fired, there was only one accident. "We were working at night on shaped charges, measuring the speed of the jet, doing it optically with a two-spark camera," remembers Dunne. "Somebody pushed the firing switch and nothing happened. Perry Ritter went over the revetment to see what happened and tripped over a wire and the thing went off. He was completely dazed. He gradually came back into focus, with a punctured eardrum and a concussion to one side of his head."
The ablation problem remained under active investigation long after the termination of Project Orion in 1965. In the early 1970s, Los Alamos investigated a possible reincarnation of Orion based on tiny laser-ignited fusion bombs. When the problem of pusher-plate ablation came up, the Los Alamos team dug out the old Orion research, reconstructed the SPUTTER code, and consulted with Brian Dunne. This time, they were able to build an electrically driven plasma gun that produced velocities as high as 1.6 x 107 cm/sec (350,000 mph) and pressures of 2.8 kbar (40,000 psi). The effects on targets of aluminum, polyethylene, phenolic, and greasy coatings were encouraging and indicated "that even much higher energy pulses would not cause ablation severe enough to significantly degrade the performance of a pulsed-propulsion space vehicle." The tests reached energy densities where the self-protection that the Orioneers had hoped for started to take effect. "In the ORION experiments the energy fluxes were insufficient to evaporate an optically thick layer of ablative material, and the radiation diffusion phase was never reached," the study concluded. "Relying solely on experimental results, one would conclude that the amount of material ablated scales as the energy flux and would extrapolate accordingly to the conditions of the actual devices. The results of such an extrapolation would probably be much too high because calculations indicate that 70 to 80% of the ablation actually occurs during the radiation diffusion-dominated phase where the rates are small."[152]
Early in the project it was recognized that a sacrificial, ablative coating—known as "anti-ablation oil" or "anti-ablation grease"—could be applied either to or through the pusher plate. According to Harris Mayer, "Sometime during 1958 it was apparent that you could have a transpiration layer of oil coming off, coating the surface, and this would ablate away. And that meant that the structure of the plate was independent of the wear and tear on it. That was one of the key ideas."
This was discovered experimentally when it was noted that a target plate was protected from ablation by the imprint of a greasy thumb. "I was helping Brian Dunne set up an experiment with an aluminum plate," says Pyatt, "and unbeknownst to both of us I had left my thumbprint on it from some oil. So when we did the experiment, lo and behold the rest of the plate was ablated, but underneath the oil it was perfectly protected. I still have that plate. That led to a large amount of both analysis and further experiments, using a light carbonaceous material, which was light because it had relatively low energy of reaction, and carbonaceous because it turned out that at the temperatures that we were creating, carbon has very broad lines and becomes opaque very quickly to the radiation created by the stagnating plasma. So you block any radiation from reaching the surface of the metal."
As Jerry Astl remembers it, "Brian Dunne proudly showed me his tests of multiple shock waves impinging on the pusher plate and it was beautiful. But what I saw were three human fingerprints in the middle of an ablated, shiny surface—they were carbonized but intact. You could preserve them as a criminal record. And I talked with Ted about it and I said, 'Ted, it will be very easy—as the pusher plate moves up we will have to have some structural pylons to guide it, so we can put nozzles there and spray a coat of oil on it as it comes down to minimize ablation.' And lo and behold the indication was that you could control ablation completely."
Later Orion designs included tanks, plumbing, and nozzles for applying a coating of heavy oil, about 6 mils in thickness, to the pusher plate between shots. "A specially selected layer of carbonaceous material is placed below the pusher before each explosion," Pyatt and colleagues explained during a presentation to NASA officials in 1963. "No pusher material is ablated."[153]
Everyone familiar with the technical details of Orion agrees that pusher-plate ablation was the critical unknown. They may have doubts about something else—shock absorbers, bomb-ejection mechanisms, radiation shielding—but those were engineering problems that could eventually have been solved. Could a film of anything, spread as thin as paper, have protected Orion from nuclear bombs? Was anti-ablation grease realistic, or was it the 1950s equivalent of the "ghost shirts" that the last of the Sioux warriors wore into battle in 1890 before the massacre at Wounded Knee? The prophet-dreamer Wovoka had assured the Sioux they would be protected from harm. Was Orion a Ghost Dance?
Of all the original Orioneers, the physicist most familiar with ablation and anti-ablation is Moe Scharff. "There's a lot of ways to skin a cat, and sometimes the cat doesn't get skinned," he says. "I don't know what the outcome of that would have been. I can only say that subsequent experience of energy in various forms being directed at surfaces has indicated that these issues are more complicated than we were able to deal with, but that isn't to say incapable of solution. My gut reaction would be that it would have to be a lot fancier or more capable grease than we knew at the time.
"On the other hand," Scharff continues, "I think there's good news, generally speaking, in subsequent experience, not necessarily with Orion-like plasmas, but other things that are first, second, or third cousins. It is very important to choose the properties of the grease in dealing with that radiation. Do you absorb it all? Maybe. Do you try to transmit some of it? Maybe. Do you try to do something in between? Maybe. You look at all these things and you would try and choose the right opacities for that impinging material. We are interested in the conditions at high pressure, so can one take advantage of the properties of the grease under those conditions, not under ordinary conditions? Things cut both ways. Certainly what Ted tried to do was to take advantage of those features rather than succumbing to them. 'OK, we have high pressures, let's see what we can do with them.' "