The island of Tinian is somewhat of an amazing thing. This small — 41 square miles – member of the Marianas served the the jumping-off point for the late-World War II American bomber raids against Japan. In this capacity, it was also the launching spot for the B-29s that used the first atomic bombs.
During World War II, Tinian was nothing less than a gigantic airbase carved onto a tiny, relatively flat island, one that had been hard-taken from the Japanese. There was hardly anything else to the island but runways, troop housing, and all of the buildings required for a massive military airport. "Massive" doesn't really cut it, though: imagine an airport that had to support a thousand B-29 Superfortresses, taking off for raids of 100 planes at a time.
The Manhattan Project sent a contingent of some 50 people to Tinian for the atomic bomb work. At least a third of them were civilian scientists. It was an impressive group: two of the scientists involved in bomb assembly would later win Nobel Prizes. One of these was Norman F. Ramsey, who passed away only last November, the assistant chief to the Los Alamos contingent.
Sometime after the Nagasaki run, Ramsey wrote a 12-page, handwritten letter to J. Robert Oppenheimer with his thoughts on how the operation had gone, and his thoughts for future bomb improvements. (The letter is undated, but it is clearly written sometime after August 20.) It's a gripping read, one that conjures up the grittiness of the Tinian experience. The parts that are most interesting are those that concern Ramsey's fears on the days of the bombings.1
The letter begins with Ramsey lamenting the fact that he can't communicate with Los Alamos. As the chief scientist of the Tinian operation, Ramsey had sent numerous reports to Los Alamos, some even to Oppenheimer by name. It was evident, though, that Los Alamos was not receiving them — they were asking questions that had been explicitly already answered. So Ramsey resorted to these hand-written, hand-carried letters, "only means of communication with you in which I have any real confidence." I'm not sure what caused the communication glitch — DC would have been a complete mess at that point in things.
Ramsey then turns to the real meat of the letter:
Our experience in the delivery of the Fat Man has convinced almost all of us of the importance of one much needed improvement. It is in my opinion essential that any atomic bomb to be used in any fair quantity must be capable of being completely protected against even a slight possibility of a nuclear explosion being detonated by fire in take off of the aircraft. This will be particularly true later when atomic bombs are available in sufficient quantity that one can not safely gamble the safety of the base on merely the low probability of a fire on a single takeoff and when one can afford even a small loss of reliability to ensure the protection of the home base.
This "one much needed improvement" is a biggie — Ramsey was pointing out that if an atomic bomb of the Manhattan Project vintage caught on fire, it would very possibly detonate with a nuclear yield. This was no trivial matter. The Little Boy bomb was notoriously unsafe (not only could it easily accidentally detonate, but if it merely was dropped into salt water it would become a dangerous, uncontrolled nuclear reactor), and the Fat Man bomb, even with its complex firing mechanism, was still not very safe by later standards. Los Alamos would actually spend quite a long time trying to make sure that its bombs were reasonably safe from accidental fires or plane crashes.
This concern was rooted in Ramsey's personal experience of life on Tinian:
Only twice since I have been here have I been even slightly worried or nervous but both of these times the intensity of my worry made up for the relative calm of the other periods. ...[T]he worst period was that between the time the B-29 engines with the Fat Man were cranked up and the time the plane was well clear of the island. The night before the takeoff four planes in succession crashed in takeoff at the other end of the island — in fact the situation got so bad a mission of 100 planes was cancelled after only 30 got off the ground. Since I have been here I have watched several fires resulting from crashes. By actual timing a very intense gasoline fire continues for over twenty minutes. Six of eight fire engines working on such a fire don't even making a dent. After witnessing such fires and after having sweated out one FM [Fat Man] atomic bomb take off, I can’t urge too strongly the importance of complete nuclear safety in take off for future models. … The one FM take off has been my most unpleasant experience since joining the project. … We all aged ten years until the plane cleared the Island.
Ramsey was worried about atomic bombs catching on fire because in his experience it seemed like an awfully real possibility.
Has any historian contemplated what would have happened if the Enola Gay, instead of making it to Hiroshima, had crashed while taking off, setting off a nuclear explosion on the most important forward airbase in the Pacific Theater? I would have done quite a number on the US bomber capabilities, to say the least. Talk about your counter-factual possibilities.
Ramsey also offered up some concrete suggestions to making the bombs safer:
The only sure ways I have been able to think of is a trap door model with a cylindrical plug through the HE so that the active material can be inserted in flight or the insertion of neutral material in the open space of a non-Christy. I realize the difficulty of this especially with a non-Christy model. However, I feel that this feature is so important that with future great abundance of active material even a loss in efficiency and reliability to achieve it is justified.
HE means high-explosives; Ramsey is suggesting that they could do in-flight insertion of the fissile material (that is, only put the plutonium in once the plane has cleared the most dangerous part of the flight — the takeoff) which is in fact what they did in the next generation of nuclear weapons. Below is a Mark 5 bomb from the early 1950s showing how easily you could open it up to plunk in the plutonium:
It's not a bad solution to the problem, except that it limits what you can do with the bombs. Most later bombs and missiles became sealed-pit weapons — you don't have someone up there on the top of the ICBM trying to insert a sphere of plutonium — which introduces its own safety concerns.
Ramsey's other suggestion was to put a solid (non-radioactive) material inside the center of a hollow bomb core. The Fat Man core was a "Christy"-type core, meaning a completely solid plutonium mass with just a tiny hole for the neutron initiator. (So-named after Robert Christy, who proved that you could do such a thing. Edward Teller later claimed to have come up with the original idea, interestingly enough, inspired by his work with George Gamow on the compressed iron core of the Earth, but there's no evidence that I've seen that he told Christy about this. I met Christy some years back and asked him if he minded having his name associated with such an invention — he said he didn't mind.)
Basically if you put a bunch of metal inside a hollow plutonium core, it won't be able to compress into a solid mass, and thus shouldn't be able to become critical. It's a clever idea — the US apparently did this with the super-dangerous Mark 18 nuclear (the all-fission Ivy King device, which had a lot of HEU in it), putting an aluminum chain inside the pit until it was ready to drop.2 Brian Burnell reports that the British definitely used this sort of system in a number of their warheads.
Ramsey continued the letter with some thoughts on future atomic basing requirements (he favors a centralized atomic base that could deploy bombs abroad when necessary), and the Nagasaki mission (which was something of a fiasco, though Ramsey concludes that it had gone, in the end, "remarkably well" — though he attributes much of that to luck). He finished on an interesting note:
Up to 19 August this was the most successful and best managed field party that I have ever seen or heard of. Everyone did a really excellent job and the whole organization worked beautifully as a unit. Unfortunately, the orders requiring us to stop on after 20 August made a bad anti-climax. However, since then we have tried to make the best of a sad situation. I hope that you can do something to get us home. Everyone deserves at least this much of a reward.
Imagine, the worst anti-climax being told not to prepare another atomic bomb for use! What I like about Ramsey's letter is it hammers home, again, how primitive the first atomic bombs were. We think of them as these paragons of sophistication, and in some ways, they were: they were built to previously-unheard of tolerances, and were as cutting edge as existed at the time. And yet, they were large, ad hoc, one-of-a-kind, dangerous devices. They required two future Nobel Prize physicists to assemble them. No surprise that the first work on the postwar stockpile was to "G.I. proof" the existing bombs — to make them something that could be assembled and used by people with considerably lesser talent.
- Letter from Norman Ramsey to J. Robert Oppenheimer (Undated, ca. August 1945), Library of Congress, J. Robert Oppenheimer Papers, Box 60, "Ramsey, Norman." Via the National Security Archive. A typed excerpt is available from them here. [↩]
- A related idea pushed by Matt Bunn is that of "pit stuffing," where you would insert a bunch of wire into stored pits, as a way of proving that you weren't planning on using them in bombs, and didn't want to make it easy for anyone else to use them in bombs. [↩]