The United States justly gets the lion’s share of credit for making and using the atomic bomb. The Manhattan Project was an international effort, we all know, but the U.S. bore the greatest amount of the labor, the cost, and the ultimately responsibility. When people get mad about the bomb, they get mad at the United States. It makes sense.

The importance of the United Kingdom, in some narratives of the bomb, was mostly in pushing the idea, sending a handful of scientists to help, and signing off on the use of the bomb. (Let’s not even get into Canada, who, according to this narrative, just provided some uranium and a lab that wasn’t allowed to communicate with the rest of the Manhattan Project.)

That handful of scientists, though, included some fairly important people. James Tuck did crucial work relating to the development of the explosive lens in the Trinity and Nagasaki bombs. Klaus Fuchs, aside from being the most significant Soviet spy on the project, helped tremendously with the design of the implosion bomb and the neutron initiators. And so on.

William Penney (later Baron Penney), who later ran the British atomic bomb project, did a whole variety of interesting things. He was crucial in developing means of assessing the effects of the bomb, was the only member of the British delegation to witness the bombing of Japan (he was on an observation plane on the Nagasaki mission), and was one of the first Allied scientists who went into Hiroshima and Nagasaki after the war.

One of the other topics he worked on was the question of what height the bombs should be detonated at for maximum effect.

This was a non-trivial issue: prior to nuclear weapons, it was pretty common for a bomb to detonate just when it actually physically made contact with its target. With atomic bombs, though, that isn’t ideal. Set one off too high, and much of the blast is going off into outer space and not the target. Too low, and a lot of it is going off straight into the ground. There is no truly “right” height, in the sense that any height will produce different effects that may or may not be desirable, but if the goal is the destruction of civilian buildings, then there are heights that work much better for that than others.

All of which Penney wrote up in a charming December 1944 report on “The Height of Burst of the Gadget.”

It’s a chilling — and fascinating — document for a few reasons. One is that it goes against any myth that the height of the bomb was set to minimize fallout exposure. This is discussed in great length by Sean Malloy in a recent article in Diplomatic History, which I will be posting a review of fairly soon. The entire reasoning for the height of the bomb detonation was about blast and fire effects. Radiation didn’t come into it. Later, when it was questioned as to whether there were fallout issues, the Manhattan Project principals said that they thought the height of the blast would avoid a lot of fallout — which is somewhat true — but this isn’t what caused them to set the height of the blast.

Another interesting bit is that Penney actually wants you to set the bomb off very low — 500 feet or so above the ground — rather than very high. The reason, though, is that Penney is assuming a much smaller bomb than actually was built: his calculation is based on the notion that the bomb will only be about 1 kiloton in yield, whereas previous guesses had been that it would be 10 kilotons or so in yield.

Penney’s document is remarkable in its frankness about the point of the bomb: to destroy civilian houses. He also makes a big distinction between the types of targets available:

There is a significant difference in the blast resisting characteristics of German towns and of Japanese towns. While in Germany a broad distinction may be made between industrial buildings and the remainder, it is true that there is no pronounced contrast in the strength of one building or another, with the exception of the multi-floored reinforced concrete structures which are relatively few in number. In Tokyo or Yokahama, some areas consist entirely of extremely strong steel framed and concrete structures built to resist earthquakes, whereas other much larger areas contain only comparatively flimsy wooden houses with tiled roofs. … Therefore, it may be said that if the gadget is to be used in area attack against a German town, only one height of fusing is required for attack anywhere. Such is not the case for area attack on Tokyo. If the accuracy of delivery can be guaranteed within 500 yards, then the bursting height for attack on wooden houses can be set twice as high as for attack on the business and shopping areas. If this accuracy cannot be guaranteed, then some suitable compromise must be worked out from tactical and statistical arguments.

In other words, all German buildings would be more or less affected in the same way in an atomic bomb blast. Japanese buildings, however, vary between very strong industrial buildings and very flimsy civilian buildings. If the goal is to target civilians, you have much more leeway in the blast height.

Two very interesting things here: One, an atomic attack on Germany is still being explicitly discussed. Two, the “area attack” being discussed is one that contemplates the direct and purposeful targeting of civilians — perhaps even at a detriment to targeting industrial facilities.

Penney then moves on to consider what kind of damage you’re trying to maximize: “complete destruction” versus “severe but not unrepairable damage.” Penney notes that “opinion in England” has gone from favoring “severe” damage to favoring “complete destruction.” Why? Because it turns out that “severe” damage is pretty quick to repair, as the Londoners discovered after the V-1 attacks (“the robot attacks,” as Penney calls them), and because “complete destruction” “implies many casualties, and this in turn has a very serious effect on the efficiency of fire fighting.”

Dwell on that for a moment. The more people you kill, the less people who can put out the fires. Lest you think this is perhaps just a moment of remarkable cruelty, Penney actually elaborates on this theme at length later in the report. He makes a distinction here between the “Fire Force” and “fire guards,” which, if I understand it correctly, is the distinction between professional and amateur fire prevention forces:

The explosion of a gadget in either Germany or Japan, causing large amount of [“complete destruction” class] damage will almost certainly result in fires. While the general impression among the Fire Force in England is that the Japanese are likely to prove the most efficient fire guards in the world (because their ordinary lives gives them continual experience), the large number of casualties associated with [this class of] damage may well lead to such confusion in Japan that the critical incubation period of the fires [to become a firestorm] is passed unobserved. Thereafter the fire guards are useless, and only the Fire Force counts. The possibility of eliminating a large fraction of the Fire Force of a Japanese town by getting the fireman into the radioactive contaminated area to fight fires is attractive and realistic. The success of a follow-up attack may be greatly increased in this way. … If a gadget can be followed or accompanied by small [incendiary devices] the probability of a devastating fire, spreading well beyond the limits of the blast damage, will be greatly increased.

Gristly thoughts, no? This is also the only area of the report where radioactivity is mentioned: as a way to kill firefighters, so you can encourage a massive conflagration to develop.

In reality, a massive firestorm did erupt in Hiroshima and Nagasaki, and was responsible for the majority of the deaths in each. Making the bombs go off at just the right height for maximizing this sort of damage was a high priority — it even required the development of new kinds of bomb fuses just for this purpose.

The bomb that went off above Hiroshima, 67 years ago this week, was set off at 600 meters above the ground. The Nagasaki bomb was detonated at 500 meters. Both of those heights were chosen to maximize damage — especially for the “flimsy” wooden houses of Japanese civilians.

It’s easy to demonize the people who made these kind of decisions — especially when they describe the pre-irradiation of firefighters as “attractive.” But this is the logic of total war, when you’ve given up on the idea of a morality of war fighting and decide that the ends — and not the means — are all that matter.

Whether one thinks the bombing of Hiroshima and Nagasaki were justified or not, it must be remembered that along with whatever else they were they were massacres of civilians. It was not an incidental or accidental side-effect: it was what they were planned to be. When the scientists at Los Alamos made plans for how to use the atomic bomb, they optimized them for the burning of civilians. This should not be forgotten or glossed over, even if one thinks it was still the right thing to do.

Tags: 1940s, Hiroshima, Manhattan Project, United Kingdom