George Dyson: "Project Orion — The Atomic Spaceship 1957-1965", Penguin Books, 2003, ISBN 0-140-27732-3
To visualize Orion, imagine an enomous one-cylinder external combustion engine: a single piston reciprocating within the combustion chamber of empty space. The ship itself, egg-shaped and the height of a twenty-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.
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 rockets'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 burned. Even on ambitious interplanetary mission, involving 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.
Orion's external combustion engine escapes the temperature limitation, developing 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.
Lew Allen performed a similar series of experiments in Nevada, hanging spheres of material from shot towers in the desert during the Teopot test series in April 1955. [...] At a February 1957 conference, Livermore physicist Tom Wainwright noted that nonmetallic material such as Bakelite suffered markedly less ablation, a phenomenon that became the key to protecting Orion's pusher plate from repeated blasts. [...] says Bud Pyatt [...]: "You can go and see these famous iron balls that, in terms of temperature, were within the 150,000 degrees Kelvin range of the fireball. The phenomena of the self-protection from ablation through the creation of a hot layer that was opaque enough to protect the remainder of the ball from any of the radiation were important observations in terms of could we create a layer of pusher that could exist that close to a nuclear explosion?"
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 pacity 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." [...]
Nature appeared to be 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 hundreed thousand degrees," Freeman explains. "This was an unusual termperature [...] 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 are 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; [...]"
Orion depends 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 [...]."
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. [...] "The best propellant worked out being something like equal amounts of hydrogen, carbon, nitrogen, and oxygen," [...]
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 of 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. [...]"
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."
Rosenbluth then produced, as he describes it, some "real quick and dirty calculations, the way a physicist would do the problem" concerning the capabilities of shock absorbers, and whether a bomb-driven ship would be stable in flight. "Far from whether you could really engineer it," he adds, "I could have proven it was utterly impossible, but it came out that it was possible, but you would have to avoid goofs like the bomb that didn't go off or unbalanced shock absorbers and things like that." He saw that the worst thing for Orion, worse than a complete dud, might be a bomb whose high explosive detonated without the bomb going nuclear, throwing shrapnel rather than plasma at the ship.
"That remains a very serious question," says Ted [Taylor].
Bill Vulliet is still doing physics [...]. Unlike his former colleagues who believe the project was technically sound, Vulliet now thinks that Orion could never have survived intact. "Opacity was only part of the problem," he says. "The other part of the problem is spallation from the violent shock waves that go through that pusher. Any time a shock wave meets a surface, a rarefied surface, like air or gas on one side, metal plate on the other side, it goes roaring through there, it comes to this air/steel interface, reflects, starts going back the other way and reflects as a rarefaction wave. This shock wave is strong enough that nothing would survive! There's no way you could design a pusher to do that job. It's nice to have specific impulse, but you don't want to grind the whole ship into powder on the first two or three shots!"
Boosting Orion vehicles above the atmosphere with chemical rockets reduces the immediate fallout, and it was suggested that with later, hybrid versions of Orion the fallout problem had been solved. Space is a high-radiation environment, and there is no reason to fear that fission products that stay in space would do anyone any harm. Unfortunately for Orion, a significant fraction of fission products released anywhere in Earth's magnetosphere—not just within Earth's atmosphere—will slowly spiral in along magnetic field lines and eventually reach the ground.
"The variety of conceivable space engines is huge; we have so far worked hard on only a very small fraction of the possible ones," he [Ted Taylor] explained in 1966. "I have made a morphological outline of possible space propulsion systems, classifying them according to whether the energy release is pulsed or continuous, the type of energy sources that are used, the number and types of energy conversion stages in the engine, and so on. If one randomly permutes the elements of this outline, one generates more than 10e22 different space propulsion concepts, each of which makes logical sense! [...] Random generation of propulsion concepts from Table III is practically guaranteed to produce a concept that no one has ever hought of before," he reported. "I have found it impossible to reject, as clearly nonsensical, any of the dozen or so concepts which I have seen derived that way, mostly by my children. But every one of them has been a strange idea indeed."
In the early 1970s a revival of interest in Orion at Los Alamos resulted in a number of advances, including an experimental investigation of pusher-plate ablation at higher energy densities, and a proposal by Ted P. Cotter for a "rotating-cable pusher." Instead of a massive pusherplate backed by shock absorbers, the ship, spinning slowly around its central axis, would unreel a large number of steel cables, radiating outward like the arms of a giant squid. The cables, with flattened extremities, would absorb mementum from the explosions, transmitting it gently to the main body of the ship. Cotter credited this design to a still-classified proposal of Freeman Dyson's, circulated in November 1958 under the title The Bolo and the Squid.
The squid's latest incarnation at Los Alamos is a concept named Medusa by its inventor; Johndale Solem, coordinator for advanced concepts at the theoretical division. "Orion is mind-blowing compared to any other kind of spacecraft," he says. [...] Solem took a fresh look at the entire problem and came up with a small, lightweight spacecraft pulled along on elastic tethers behind a large, parachute-like canopy, billowing out under the pressure from the explosion of very small, low-yield bombs.
I make contact with Huntsville, thinking that NASA must surely have excavated the original Orion technical reports, and will by now have found Wernher von Braun's 1964 paper arguing the merits of Orion, which repeted Freedom of Information Act requests have failed to unearth. Unfortunately, NASA has had difficulty obtaining even the basic Orion literature, and wants to obtain copies—as soon as possible—from me! [...]
Several months later, I receive a draft NASA report, External Pulsed Plasma Propulsion and Its Potential for the Near Future, by J. A. Bonometti, P. J. Morton, and G. R. Schmidt. It takes the reader straight back to 1958: [...] "The physics behind creating a highly efficient fission burst is well understood, and in a vacuum, it produces a shell of ionized particles with an extremely high radial velocity. Thus, this concept of "riding on a plasma wave" is appropriately termed External Pulsed Plasma Propulsion or EPPP. EPPP provides a technology that would allow us to seriously consider missions to the outer planets."