The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

Aproject to explore the feasibility of building a
nuclear pulse rocket powered by nuclear fission.
It was carried out by the physicist Theodore Taylor
and others over a seven-year period, beginning in
1958, with U.S. Air Force support. The propulsion
system advocated for the Orion spacecraft was based
on an idea first put forward by Stansilaw Ulam and
Cornelius Everett in a classified paper in 1955. Ulam
and Everett suggested releasing atomic bombs behind
a spacecraft, followed by disks made of solid propellant.
The bombs would explode, vaporizing the material
of the disks and converting it into hot plasma. As
this plasma rushed out in all directions, some of it
would catch up with the spacecraft, impinge upon a
pusher plate, and so drive the vehicle forward.
Project Orion originated at General Atomics in San
Diego, a company (later a subsidiary of General
Dynamics) founded by Frederick de Hoffman to
develop commercial nuclear reactors. It was de Hoffman
who persuaded Freeman Dyson to join Taylor in
San Diego to work on Orion during the 1958–1959
academic year.
Ulam and Everett’s idea was modified so that
instead of propellant disks, the propellant and bomb
were combined into a single pulse unit. Plastic was
chosen as the propellant material, not only because of
its effectiveness in absorbing the neutrons emitted by
an atomic explosion but also because it breaks down
into lightweight atoms, such as those of hydrogen and
carbon, which move at high speed when hot. This
approach, in tandem with the pusher plate concept,
offered a unique propulsion system that could simultaneously
produce high thrust and high exhaust
velocity. The effective specific impulse could theoretically
be as high as 10,000 to one million seconds.
A series of abrupt jolts would be experienced by the
pusher plate, jolts so powerful that, if these forces
were not spread out in time, they would result in
acceleration surges that were intolerable for a manned
vehicle. Consequently, a shock-absorbing system was
devised so that the impulse energy delivered to the
plate could be stored and then gradually released to
the vehicle as a whole.
Various mission profiles were considered, including
an ambitious interstellar version. This called for a 40million-ton
spacecraft to be powered by the sequen-
Project Orion
tial release of 10 million bombs, each designed to
explode roughly 60 m to the vehicle’s rear. In the
shorter term, Orion was seen as a means of transporting
large expeditions to the Moon, Mars, and Saturn.
Taylor and Dyson were convinced that chemical
rockets, with their limited payloads and high cost,
were the wrong approach to space travel. Orion, they
argued, was simple, capacious, and above all affordable.
Taylor originally proposed that the vehicle be
launched from the ground, probably from the nuclear
test site at Jackass Flats, Nevada. Sixteen stories high,
shaped like the tip of a bullet, and with a pusher plate
41 m in diameter, the spacecraft would have used a
launch pad surrounded by eight towers, each 76 m
high. Remarkably, most of the takeoff mass of about
10,000 tons would have gone into orbit. The bomb
units, ejected on takeoff at a rate of one per second,
would each have yielded 0.1 kiloton; then, as the vehicle
accelerated, the ejection rate would have slowed
and the yield increased, until 20-kiloton bombs would
have been exploding every 10 seconds.
It was a startling and revolutionary idea. At a time
when the United States was struggling to put a single
astronaut into orbit using a modified ballistic missile,
Taylor and Dyson were hatching plans to send scores
of people and enormous payloads on voyages of
exploration throughout the Solar System. The original
Orion design called for 2,000 pulse units—far
more than the number needed to reach Earth escape
velocity. In scale, Orion more closely resembled the
giant spaceships of science fiction than the cramped
capsules of Gagarin and Glenn. One hundred and
fifty people could have lived aboard in relative comfort
in a vehicle built without the need for close attention
to weight-saving measures.
One of the major technical issues was the durability
of the pusher plate, since the expanding bubble of
plasma from each explosion would have a temperature
of several tens of thousands of degrees, even at
distances of 100 m or so from the center of detonation.
For this reason, extensive tests were carried out
on plate erosion using an explosive-driven helium
plasma generator. The results showed that the plate
would be exposed to extreme temperatures for only
about one thousandth of a second during each explosion,
and that the ablation would occur only within a
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