Blackett's War (25 page)

The only greater secret of the war would also soon be a matter of intense British-American collaboration. On March 19, 1940, Tizard had received a memorandum from the German refugee physicist Otto Frisch and his collaborator Rudolf Peierls, another German émigré who had ended up with Frisch at the University of Birmingham, reporting their calculations of the critical mass of uranium necessary to sustain a chain reaction. Tizard gave a subcommittee of the CSSAW the task of assessing “the possibilities of producing atomic bombs during this war.” Blackett was a member, as was Cockcroft and several other leading physicists; the chairman was G. P. Thomson, J. J. Thomson’s son and now an established physicist in his own right. Meanwhile the NDRC, as one of its first official acts, had allocated $40,000 in July 1940 for an exploratory investigation to determine if a uranium bomb was feasible. Bush was hoping to prove it was not, which would mean there was no need to fear the Germans would make one. Tizard, too, thought that the “probability of anything of real military significance is very low.”
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Blackett would be the sole member of the British committee, when it issued its findings a year later in July 1941, to dissent from the view that Britain should try to develop a bomb on its own. He thought the committee’s estimate that they could complete the job in two years at a cost of £5 million unrealistically optimistic, and instead urged that Britain discuss joining a
U.S. program. But he did not disagree with the fundamental scientific conclusion of the report, which would prove pivotal in convincing skeptical American officials that an atomic bomb was now indeed feasible:

THE LUFTWAFFE BRIEFLY
came close to toppling Britain’s last line of defense at the height of the Battle of Britain, in mid-August 1940. Concentrating their daylight attacks on the RAF’s airfields, sector stations, and radar installations, the Germans cost Fighter Command 440 planes and a quarter of its pilots in the month of August. In retaliation for nighttime raids against British cities, the RAF carried out two small night attacks on Berlin in late August. They did negligible damage but left Berliners stunned and indignant, William Shirer recorded in his diary. Goebbels at first gave instructions to the press to play down the attacks, but after the second raid ordered a different tack. Most of the Berlin newspapers carried the same headline that day: COWARDLY BRITISH ATTACK.
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On September 7, Göring sent a thousand planes, the Luftwaffe’s largest daylight raid yet, to strike the British capital. Air Vice Marshal Keith Park, the commander of the RAF’s No. 11 Group, responsible for the crucial sector in southeast England, had a single reaction: “Thank God.” The Luftwaffe’s shift from the main objective of wearing down Britain’s fighter forces and radar chain to striking cities to assuage a surge of retaliatory anger was a monumental strategic blunder, giving Fighter Command a desperately needed respite to reconstitute damaged airfields and radar stations, and build back up its dangerously depleted inventory of planes and trained pilots. A week later another huge aerial armada struck London. By the end of September the threat of a cross-Channel invasion eased as it became apparent that Hitler’s attempt to destroy the British air force had failed. In its place Britain and Germany settled into a brutish exchange of increasingly
heavy attacks on each other’s cities by night. Forty thousand British civilians would be killed, 50,000 seriously injured, over the next eight months. Any remaining illusion that this would be a war fought according to the old rules that recognized such a thing as noncombatants was gone.

A few weeks before the night raids began in earnest in September 1940, General Sir Frederick Pile, commander-in-chief of the army’s Anti-Aircraft Command, had asked A. V. Hill if he could recommend a scientist to help him improve the dismal performance of his gun batteries trying to shoot down enemy bombers during their night raids over England. Hill’s own statistical work on antiaircraft fire in World War I was one of the earliest forerunners of operational research. “Pile instantly recognized what he needed—the quick intuition of a freshman,” Hill recalled. The general had already met Blackett—accounts differ, but it may have been at a meeting of the Royal Society in early August, where Blackett delivered a paper—and been impressed by him. “Why should I not have that chap Blackett?” Pile asked Hill. Hill replied, “Why not?” Without delay, Blackett was named scientific adviser to Pile at AA Command, which shared headquarters with RAF Fighter Command at Stanmore.
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Antiaircraft guns were almost as old as aircraft but had always been woefully inaccurate, especially against high-flying bombers. In general, the best that gunners could hope to do was to throw so dense a wall of shells into the air that a plane was bound to run into one sooner or later. Blackett arrived at Anti-Aircraft Command in August just as the first radars were being delivered to antiaircraft artillery batteries around London. The idea of using radar data to calculate the proper bearing and elevation of the guns—the process known as “gun laying,” GL for short—was eminently sensible. The only problem, as Blackett quickly found, was that no one had bothered to figure out how to connect the gun-laying radars to the guns:

General Pile was more than willing to give Blackett a free hand to see if he could improve the situation. Pile was not a natural intellectual; the eldest son of an Anglo-Irish baronet, he had spent his entire adult life in the army, winning the Military Cross and DSO as an artillery officer in France in the First World War. But he was always interested in new things. On the advice of the strategist and armored warfare theorist Colonel J. F. C. Fuller he had transferred to the Royal Tank Corps in 1923. “They like bright ideas there,” Fuller told him. What impressed Pile now about Blackett was that not only was he “a first-class scientist,” but he understood the realities of fighting a war and the need to make do rather than pursue some theoretical state of perfection. Pile wrote in his memoirs: “Blackett was an ideal man for the job. He spoke his mind clearly, and was always ready to admit the fact that the most desirable things sometimes may be inadvisable.”
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Blackett was able to quickly assemble a small team mainly by hiring some young scientists he already knew. The first to arrive were A. V. Hill’s son David, who like his father was a physiologist studying the mechanics of muscles, and David’s close friend Andrew F. Huxley—a future Nobel Prize winner in medicine and half brother to two other famous Huxleys, Julian and Aldous. Like Blackett, Huxley had what one member of the team called “an incredible mixture of theoretical and practical expertise.” He could take apart one of the large mechanical “predictors” that guided the guns “like a born mechanic.” A third physiologist, Leonard Bayliss, from University College, London, became Blackett’s deputy. There were also two physicists, Frank Nabarro from Bristol University and Ivor Evans (Evans’s name had come from the Central Register); an astronomer, Hugh Butler; an expert from the Admiralty on gunnery predictors, Arthur Porter; an army second lieutenant, G. W. Raybould, who had been a surveyor in civilian life and, as Blackett found when he happened to meet him when inspecting an antiaircraft battery in the Midlands, had already worked out his own method for converting radar data to gun bearings; and “a girl mathematician,” a Miss Keast.

The group’s official status was utterly vague. Blackett was called Scientific Advisor to the Commander-in-Chief, Anti-Aircraft Command, but his salary was being paid by the Ministry of Aircraft Production. As Bayliss recalled, the scientists just decided to call themselves the Anti-Aircraft Command Research Group: “Our position was established in two ways; firstly we informed the messengers to deliver to us all correspondence so addressed, secondly we had a rubber stamp cut with the letters AACRG. It
is not known whether any more formal authorisation or recognition was ever obtained.”
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Unofficially, the group soon had another name that stuck, Blackett’s Circus.

Aiming an AA gun, even assuming the target was flying straight and level, required a complex calculation in spherical trigonometry. The plane’s height, course, and speed had to be determined and translated into an elevation angle and azimuth of the gun; at typical bomber speeds the gun needed to be aimed roughly a mile ahead of the plane’s position at the instant the gun was fired so that the shell would intercept the target as it continued forward on its course.

The Sperry Predictor was a half-ton mechanical computer designed to automate the task. A spotter would track the target, turning two dials to keep the airplane centered in a sight; the predictor, employing an array of gears, motors, and dials, would then transform the rate of change in those inputs into a continuously updated gun bearing. But the whole system was designed for visual spotting by day. There was no way to feed the radar data in directly. Blackett also found that the radar data at any given moment had a significant margin of error, so that even if the predictor could somehow be fed the data, the errors would cause the aim point of the gun to zig and zag with each updated position. Over a series of radar measurements, however, the errors tended to cancel out, and a fairly accurate track of the bomber’s course could be obtained. Blackett’s group first worked out a paper method of plotting the radar data and smoothing it out to obtain an average track that could be used to manually work out an elevation and bearing for the battery a given number of seconds later. Later they devised a modified Sperry Predictor—dubbed the “castrated” predictor—that allowed the operators to enter the radar tracking data by turning a dial originally intended to allow for wind corrections.
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About 120 guns were deployed around London, in batteries of 4 each spaced to cover the entire area of the city with overlapping fields of fire. There were only enough radar sets when the Blitz began in September 1940 to equip half of the 30 batteries. Blackett argued that there would be little to lose by grouping them into 15 eight-gun batteries instead, since the batteries without radar were extremely unlikely to hit anything and thus were already leaving gaping holes in the perimeter. A further calculation he made established that the whole idea of “complete coverage” had been an illusion anyway. Though the spacing between each of the 30 batteries in the original
deployment scheme corresponded to the effective range of the guns under daytime conditions against slow-flying targets, the effective coverage of the guns using radar against modern fast bombers at night was far more limited: unless a bomber was coming almost directly toward the battery, its change in bearing took place so rapidly that the paper-and-pencil methods necessary for handling the radar data could not keep up. In fact, even the original 30-battery deployment was riddled with gaps between each battery’s effective field of fire. Concentrating the guns into 15 batteries hardly changed the size of the holes in the defensive screen, while greatly increasing the odds that any given gun would successfully engage a target by giving them all access to the radar data.

The scientists calculated that it was taking 20,000 shells to bring down one German bomber when they started their work. By the following summer, Blackett’s methods had cut the number of “rounds per bird,” as they termed it, to 4,000. One anomaly in the data which caused much head scratching was that AA batteries deployed along the coasts were bringing down enemy bombers with half the number of rounds per bird as inland sites. “All kinds of far-fetched hypotheses were considered as possible explanations of this strange result,” Blackett recalled. Perhaps the radar worked better over water; perhaps the enemy bombers flew straighter or lower while coming in across the sea than they did over land. “Then suddenly the true explanation flashed into mind.” It was simply that the inevitably exaggerated claims of planes shot down by each battery could be verified on land by locating the crash site, but not over sea. “This explanation should have been thought of at once,” Blackett observed, “as there is plenty of evidence to show that unchecked combat claims, made in absolute good faith, are generally much too high.”
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In the absence of Blackett’s explanation, disastrous changes in the deployment of the guns and radars might have been ordered on the mistaken belief that units ought to be concentrated on the coasts for greatest effectiveness.

Another bit of statistical reassurance that Blackett was able to provide was an even more elementary yet more crucial one. Pile, as Blackett would describe him in a postwar lecture to the Royal Statistical Society, was “an extremely intelligent general but he was a little flighty in his emotions and if one night he brought down six aircraft and the next night only two he would think something was wrong with the method and want to alter the methods and reprimand the crews.… I circulated a table of distributions
round all the staff at the Command during the blitz showing that if they expected five aircraft there would be many times they would shoot down two [or] three.”