The Scientific Gamer

If you follow UK news at all you’ll know that recently there was a bit of a brouhaha over the Royal Navy’s failed Trident II missile test just off the coast of Florida. Some of the more hysterical accounts of the incident have the missile veering towards the US mainland before self-destructing; these sound a little dubious, but there’s at least a sense of irony to the idea as Trident is a US-developed weapons system. The UK abandoned its own nuclear weapons development program back in 1958 in favour of simply buying the technology from the Americans, and there are some very good reasons why this is so. One of them is Violet Club.

The UK’s first serious attempts to develop a nuclear weapon actually predate the Manhattan Project, but after it became apparent just how much industrial capacity the US was willing to devote to producing a working fission bomb the British sensibly dropped their parallel development and instead swung their resources behind the American program. The British contribution consisted of both materials and scientists and significantly accelerated the completion of the bomb, but unfortunately one of the scientists in question was the spy Klaus Fuchs, who handed over a vast quantity of atomic data to the Soviet Union. Once this came out it really turned the Americans off of the idea of post-war collaboration on nukes, and after Roosevelt died and the only American copy of the agreement detailing post-war cooperation between the UK and US was lost (the British copy was sent to them, but certain suspicious figures in the US government weren’t convinced it was authentic) the US unilaterally broke off all collaboration, forcing the British to resume independent development of nuclear weapons.

British efforts culminated in the shipborne Operation Hurricane test detonation in 1952 and the first operational nuclear bomb — Blue Danube, which was essentially a copy of the Manhattan Project Fat Man — being tested in 1953. Unfortunately by this point both the US and the Soviets had tested their first thermonuclear weapons – if you want an explanation of the difference between nuclear and thermonuclear weapons you can read either the Wikipedia article or my own blog post on the subject, but a quick primer would be that a thermonuclear weapon uses a regular fission bomb as a first stage to kick-start nuclear fusion in hydrogen isotopes, which releases several times more energy than nuclear fission and creates a commensurately bigger bang.  Thermonuclear weapons effectively rendered pure fission bombs obsolete, as there were hard limits on the amount of fissile fuel that could be converted to energy by the chain reaction in the tiny fraction of a second before that energy was released and the bomb destroyed itself – much of the nuclear material inside a fission bomb therefore ends up being wasted, and cramming more uranium or plutonium into your weapon results in diminishing returns in terms of yield because it ends up being scattered into the shockwave as fallout material rather than being released as explosive power.

This was something of a problem for the British military establishment, as it was additionally becoming very clear that the delivery system of choice for a nuclear weapon was going to be a missile rather than a bomber. A missile has a far smaller payload capability than a bomber, which made the small, efficient thermonuclear devices ideal in comparison to large wasteful fission bombs. Unfortunately for them they’d expected fission bombs to have a longer lifespan and were stockpiling large amounts of highly-enriched uranium to build them. Not only would there be a gap in capability while they developed their own thermonuclear weapons, but said weapons would only require small amounts of this fissile material to trigger the second stage. As they were unwilling to see this expensive nuclear material go to waste, and using a smaller (yet saner) amount of plutonium would have been even more expensive, it was eventually proposed that an “Interim Megaton Device” be constructed using uranium to fill the perceived need for a megaton-grade weapon until the UK could build its own hydrogen bombs. Hence Violet Club.

Even on paper Violet Club sounds like a tremendously stupid idea. It’s a classic implosion design: a sphere of highly-enriched uranium surrounded by a set of high explosive lenses. When triggered the lenses explode inwards, compressing the uranium core so that it achieves critical mass; the uncompressed uranium core on its own is sub-critical and can’t go nuclear unless the explosive lenses trigger, and the explosive lenses are very difficult to trigger accidentally, making this a relatively safe design for fission weapons up until this point. The problem with Violet Club was that the target yield of one megaton was so much larger than a typical fission yield (which at this point was on the order of a hundred kilotons or so) that they had to cram in a truly ridiculous amount of uranium to achieve it. So much uranium, in fact, that the uncompressed mass of the uranium sphere was actually greater than one critical mass. The reason it wasn’t instantly going nuclear was because the Violet Club core was spread out into a thin, hollow uranium shell – critical mass is a bit of a misleading term and should be thought of as more like critical density as it relies on having a lot of fissile atoms in very close proximity so that they can bounce neutrons off one another — and the big gap in the middle was enough to stop the chain reaction from running out of control.

Now, a key feature of nuclear weapons, even back then, was that they should be fail-safe. If the weapon is not armed then it should be impossible for it to go off, even if e.g. the bomber it’s being carried on gets shot down and crashes, or if there’s a fire on-base and the building that it’s being stored in collapses. Violet Club was very emphatically not fail-safe, however; if the uranium core was crushed or damaged in any way that led to that hollow gap in the middle being squeezed out of the sphere, the chain reaction would start and the bomb would detonate. It wouldn’t be as destructive as if it were triggered intentionally as without the explosive lenses to compress the core the reaction would be even more inefficient than it already was, but it would certainly have been enough to ruin the day of anyone caught within a mile or two of the epicentre. This is why the bomb designers included a rather dubious safety feature: a small hole was bored through the uranium shell through to the hollow gap inside through which 133,000 steel ball-bearings were inserted. The ball-bearings were then sealed inside with a plastic bung. The theory went that as long as the ball-bearings were present inside the weapon, that hollow gap that prevented it from going nuclear couldn’t be crushed out of the core and the bomb would remain safe.

That was the theory, anyway. In practice the ball-bearings were even dumber than the bomb itself:

• Having to remove 133,000 pieces of steel from the interior of your weapon before it can be considered armed turned out to take some time – a minimum of half an hour, in fact (and up to 90 minutes during bad weather), which was far too slow considering the increasingly truncated warning time that was expected to precede any hostile nuclear action.

• The ball-bearings increased the weight of the Violet Club bomb assembly by half a tonne. The V-bomber force of the time (so-called as the UK’s nuclear deterrent consisted of Valiant, Vulcan and Victor bombers) could not carry a bomb that heavy, so the ball-bearings had to be removed from the bomb before takeoff. This ensured that Violet Club would always be armed while it was in the air, and this made it far too dangerous to be flown on exercises or even sent to a dispersal base to mitigate the effects of an expected nuclear strike.

• The bombs had to be stored inverted when not in use, as otherwise there was a risk that the plastic bung would fall out and the ball-bearings would exit the core, arming the bomb.

• By their nature nuclear bombers spent a lot of time sitting around on exposed airstrips waiting for an alert, and so did the bombs they carried. If it was a particularly cold day the ball-bearings would freeze solid inside the bomb, rendering it useless.

And so on. The Violet Club bombs were quite possibly the most dangerous and impractical nuclear weapons ever made, and the RAF knew it: 12 were ordered, but only 5 were delivered, and the ground crew who had to handle them were absolutely terrified of them. To add insult to injury, it was estimated (we’ll never know for sure as Violet Club was deemed too dangerous to even test) that thanks to the inherent inefficiency of pure fission bombs Violet Club would have a yield of around 400 kilotons – far short of the one megaton target yield.  In short, it was not an ideal outcome for the UK’s homegrown nuclear weapons development program, and god knows what else we might have come up with if we’d been allowed to continue bodging together the safety features that kept our nuclear deterrent from prematurely exploding.

Fortunately for us the Operation Grapple tests of 1958 proved to the US that we had the capability to build thermonuclear warheads of our own, and at that point the Americans figured that if we were going to get them anyway, they might as well make a few bucks by giving us their warhead blueprints and then selling us the delivery systems to go with them. The 1958 Mutual Defense Agreement swiftly followed and ensured Violet Club was the last purely British nuclear weapon ever deployed; Red Beard (the more prevalent — and conventional — successor to Blue Danube) remained in service until the switch to a primarily submarine-based deterrent force and the first Polaris patrol in 1968. And whatever else you can say about the American designs, at least they aren’t armed by physically yanking out a plastic bung from the warhead casing.