- The nuclear pulse drive would power a spacecraft with a series of nuclear explosions.
- Project Orion, led by Freeman Dyson, planned a four-year round trip to the moons of Saturn.
- After Orion was abandoned, pulse drive has led to several concept studies.
- It has only flown as a prototype using conventional explosives.
In both novels, the characters adopt nuclear pulse drive because they have a pressing need to operate spacecraft too big to be powered by a rocket, but they only have contemporary technology to work with. Therein lies a clue that however futuristic it appears, the nuclear pulse drive concept is not a progression of rocketry but was originally developed in parallel with it. It never got beyond the concept stage, but it’s hard to read Footfall or Ark without wondering whether, beneath the elephantine aliens and mysterious floods, the authors are pointing us at what could have been and indeed, what could still be.
The glimmering of an idea
The concept of the pulse drive goes back to the very first nuclear bomb test, in the Nevadadesert in 1945. The bomb was detonated on top of a 200-foot tower, which everyone assumed would be vaporised by heat as intense as the surface of the sun. In fact, the tower was blown to fragments and scattered around the desert. It occurred to Ted Taylor, one of the Manhattan Project engineers, that if the power of the blast could be harnessed and directed, a force of destruction could be turned into a force of propulsion.
It was Stanislaw Ulam who developed the pulse drive concept in a series of papers, many of which are still classified as they were part of the American nuclear weapon program, but summarised in a 1955 paper he co-authored with CJ Everett. The paper describes an interplanetary spacecraft powered by successive nuclear blasts, carrying a smaller, more manoeuvrable rocket-powered spacecraft that could be released it its destination. Far from the advanced Alcubierre drive that provides the second stage propulsion in Ark, Ulam and Everett envisaged the second stage being a V-2 rocket: the first vehicle ever to leave the earth’s atmosphere.
In 1958, pulse drive became more than an idea that physicists kicked around in their spare time. The year before, the Soviet Union had launched the Sputnik satellite and the USA was, rather belatedly, flinging resources into a space race that they had started from well behind their rivals. Taylor approached Freeman Dyson, then working at Princeton, and brought him in to lead ‘Project Orion’ at General Atomics.
Much of what we now know about Orion now is due Project Orion, written by by Dyson’s son George, which was summarised in a BBC documentary on the subject:
Dyson and Taylor knew that the problems they faced were formidable, but then Dyson has been described as ‘one of the few true geniuses I’ve ever met’ by Arthur C Clarke, which is high praise indeed. They also knew that they would be able to recruit the best nuclear physicists and engineers in the USA. Many of them had worked on the Manhattan Project and disliked working on weapons of mass destruction. As Dyson put it:
Nobody liked the mass murder aspect of bombs but nevertheless, they loved playing around withbombs. So this was a way of having your cake and eating it too, that you could be playing around with bombs but not be killing people, to be exploring the universe.
The first problem they faced was that while their colleagues working on rocketry were trying to squeeze more energy out of liquid or solid fuel, Project Orion had to deal with the fact that an atom bomb produced far too much energy. Taylor’s observations of the bomb test tower led them to place a steel or aluminium ‘pusher plate’ between the explosion and the spacecraft, and they found that a large enough plate could absorb successive nuclear blasts.
Going large to slow down
A bigger problem was that Ulam and Everett’s paper describes a spaceship accelerating at around 10,000g, which is similar to the acceleration of an artillery shell in the barrel of a howitzer. Even if they could design a spacecraft that did not collapse at that acceleration, a human crew could not survive it.
The first solution was to damp down the acceleration with a large spacecraft. They envisaged Orion weighing 8000 tons at launch, which is about two and a half times the weight of the Saturn V that was still a twinkle in Wernher von Braun’s eye. A key difference was that while the Saturn V used nearly all of that mass to get a mere 50 tons of payload into an orbit that intersected the moon, Orion would carry several hundred tons of payload beyond the earth’s gravity well and to a planet of the team’s choosing.
They found that they could design shock absorbers to spread the acceleration, reducing it to a level that a human body could survive. At the same time, the military was developingnuclear devices small enough to be fired as artillery shells. As it was envisaged, Orion would lift off on a succession of 1,000 0.1kt bombs, and then accelerate using 20kt bombs once it was clear of the earth’s atmosphere.
On paper, Orion could launch an expedition to the moons of Saturn and return in four years. To put that into context, that’s a similar timescale that is currently imagined for a manned mission to Mars using chemical rockets that have yet to be invented, over several times the distance.
What the team needed was to move the project off paper and into practice. They started small with a prototype lifted by conventional explosives. To build it, they approached a man whose wartime experience had been less backroom and more front line than most of them: Jaromir Astl had learned his expertise with explosives as a saboteur with the Czechoslovakian resistance. With Astl on the team, they flew several models of what they called the ‘putt-putt’ on a succession of conventional explosives:
Commenting on the lack of safety protocols and the enthusiasm for explosives, Dyson says:
We were a bunch of crazies in a certain way and it was certainly an unusual time when crazy people were actually given a chance to do their stuff.
The putt-putt showed that a vehicle could be powered on a controlled trajectory by a series of explosions but due to political rather than engineering constraints, a few hundred feet off the ground would be as far as pulse drive would ever go. When NASA was inaugurated in 1958, it had not taken on Project Orion because as a civilian agency, it could not work with a project in which so much remained classified. Project Orion fell under the auspices of the US Air Force so to the frustration of scientists who were existed by space exploration, it needed a military justification. Ideas became progressively more outlandish, including an orbital battle station, or ‘death star’ as the scientists call it in the documentary, and the placement of an enormous nuclear doomsday device over Russia. Ifdetonated, it would rip much of the atmosphere off the northern hemisphere. Perhaps someone had watched Doctor Strangelove and taken entirely the wrong message, or perhaps it was simply the logical conclusion of deterrence based on mutually assured destruction.
Death Starred to death
This was not what Dyson, who never wanted to do war work again after he had helped Bomber Command optimise the mass slaughter of German civilians, had in mind. At the same time as he was working on Orion, he was one of the architects of the Partial Test Ban Treaty that came into force in 1963. It restricted nuclear testing to underground sites, where they could not throw radioactive dust into the atmosphere. In doing so, it prevented the launch of a spacecraft that depended on letting off nuclear bombs in the atmosphere and beyond.
At around the same time, a model of the ‘death star’ was presented to President John F Kennedy who, fresh from his game of nuclear chicken now known as the Cuban Missile Crisis, vetoed it to avoid starting a nuclear arms race in space.
Dyson himself was not unhappy with the decision, if only because the team had never resolved the problem of containing radioactive fallout. The launch of one spacecraft would increase the earth’s radiation level high enough to cause between one and ten deaths by cancer, which was not acceptable to Dyson.
For better or worse, Orion was dead. As Arthur C Clarke summed up:
The idea’s not crazy. The idea that we might do it might be crazy.
We’re left with only the idea of what might have been:
Daedalus to Barnard’s Star
The idea of nuclear pulse drive has never gone away and continues to resurface from time to time. One of the more serious incarnations was Project Daedalus, a concept study by the British Interplanetary Society in the mid-1970s. Unlike the government-funded Project Orion, Daedalus was the brainchild of an informal group of enthusiasts in their spare time. Enthusiasts they may have been, but they were not amateurs: Daedalus was led by Alan Bond, who had worked in the British Blue Streak ballistic missile program and was then working on research into nuclear power.
Project Daedalus set out to design an unmanned probe that could reach Barnard’s Star, 5.9 light years away, using technology that was feasible in the immediate future. The propulsion system used a refinement of nuclear pulse drive: rather than detonating bombs under it, Daedalus’s engine would fire small quantities of deuterium and helium-3 into an electron beam. The resulting nuclear fusion explosion would be small enough to be
contained inside a reaction chamber, making it much more efficient than Orion, which only made use of the part of the explosion directed at the pusher plate.
Bond’s team calculated that they could accelerate a 54,000-ton spacecraft to 12% of the speed of light, getting it to Barnard’s Star in 50 years. On arrival, it would not slow down but would deploy 500 tons of scientific instruments to transmit data back to earth.
The idea never went further than the BIS’s papers, and they seem to have taken a rather liberal view of what constitutes feasible technology. As helium-3 is very rare on earth, they planned to mine it from Jupiter using robots with balloons, raising the whole other problem of harvesting and transporting resources between planets.
Daedalus has proved as persistent an idea as Orion. The BIS keeps it current with its follow-up project, inevitably called Project Icarus, which is still current.
Friedwardt Winterberg’s mininukes
Yet another variation on the theme was suggested in 2005 by Friedwardt Winterberg, whose earlier nuclear research had provided the starting point for Daedalus’s engine. He suggested ‘mininukes’ as a more feasible alternative to Daedalus’s helium-3 based fusion. Interestingly, his paper describes exactly the combination of small amounts of expensive uranium and plutonium and larger amounts of cheap high explosive that Dyson says should be classified in case it becomes an instruction manual for terrorists.
Winterberg’s mininuke contains a core of as little as 2g of uranium or plutonium contained in a sphere of 50kg high explosive. It would deliver the kick of around five tons of TNT, which would accelerate 1,000 tons to around 500km/h. No conventional rocket fuel comes close to that, but Winterberg says it’s still small enough that it could be contained in a parabolic reflector, harvesting more of the energy than Orion could have done byriding the edge of an expanding sphere of energy. He suggests it may be possible to scale down the mininukes even further, so they could be contained inside a closed chamber as was envisaged by Project Daedalus.
Winterberg also deals with the fallout problem: most of the energy would be released by fusion rather than fission, which does not release radioactive by-products. It’s not possible to completely eliminate the fission component of the reaction but if the fallout were to prove unacceptable, the spacecraft could be lifted into orbit with conventional rockets and the mininukes used for the interplanetary journey.
Over the last 60 years, pulse drive has received attention from some of the leading aerospace and nuclear engineers in the business but it’s never flown higher than Project Orion’s putt-putt. There is a new wave of interest in space research at the moment, led by Elon Musk’s plans for establishing colonies on Mars and Yuri Milner’s Breakthrough Starshot project for sending probes to Proxima Centauri. The time is ripe for another resurgence of interest in nuclear pulse drive and who knows, perhaps this time we’ll get to see it fly.