George Dyson: "Project Orion — The Atomic Spaceship 1957-1965", Penguin Books, 2003, ISBN 0-140-27732-3
[page 2]
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.
[page 3]
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.
[page 4]
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.
[page 71]
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?"
[page 122]
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. [...]"
[page 129]
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."
[page 90]
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].
[page 280]
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!"
[page 228]
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.
[page 97]
"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."
[page 283]
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.
[page 292]
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."