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The plan had been outlined in detail two days before, on the morning of 22 July, when the operation was first ordered. 3In theory it was fairly simple. Every available aircraft from bomber squadrons across the country would take off between about ten o’clock and ten thirty that night. They would fly to specific points along the coast, and merge together into one huge stream of bombers flying across the North Sea. About eighty miles from the German coast they would converge on a single point, where they would turn in a tight flow, and fly down towards Hamburg. (The bomber stream never flew directly towards the target, for fear of giving away their destination to German radar stations.) At exactly one o’clock on Sunday morning, the Pathfinder aircraft would drop red and yellow target marker flares over Hamburg to indicate the aiming point. Two minutes later the first crews would start dropping their bombs.
Because of the sheer number of planes taking part in the raid, they were to attack in six waves of 100 or 120 bombers each. 4Each wave would have an average of about eight minutes to clear the target, which meant that there would be fifteen or sixteen bombers passing over the aiming point every minute. The most important thing was to achieve as much concentration as possible, so that the whole of the area around the aiming point was saturated with bombs. Then the fire services would be overwhelmed, and unable to prevent massive conflagrations springing up. To avoid the bombing becoming unfocused, more Pathfinders would continue to mark the target – this time with green target indicators – after the attack had begun. If the bombers could not spot the red markers, they were to aim at the greens. Having released their loads, they were all to return home on a roughly parallel course.
Such plans were easier ordered than executed. Even an undefended target could be difficult to find in the darkness of night, and in the past the RAF had often bombed the wrong parts of a city, or even missed it altogether. To prevent such disastrous wastefulness, British scientists had developed a range of electronic navigational aids. The most important of these for medium to long-range targets, like Hamburg, was called H2S. It worked a little like an airborne radar device, except that instead of transmitting high-frequency pulses into the surrounding sky it would direct them at the ground. By plotting the echoes on the screen of a cathode-ray tube, it was possible to get a rough picture of the ground below, even through heavy cloud. The system was still in its infancy, and the picture it gave was sometimes so fuzzy as to be useless, but it was particularly good at picking up built-up areas surrounded by water. Hamburg would therefore be relatively easy to identify: the wide river Elbe, and the distinctive lake in the centre of the city would provide an unmistakable outline.
The second major difficulty was the strength of the defences, both on the way to Hamburg and over the city. The whole coast of Europe was guarded by squadrons of German night fighters. As soon as a formation of bombers came within a hundred miles of mainland Europe, the German long-range ‘Freya’ radar would pick them up and the defences would get ready for action. Once the bombers came within thirty miles or so, a second, short-range, radar system called ‘Würzburg’ would be able to direct night fighters towards the bomber stream. The ‘Würzburg’ system was extraordinarily effective for its time. Using one radar set to pick up an individual plane, a second set could guide a night fighter to within a few hundred yards of his quarry. The pilot would then be able to engage his own ‘Lichtenstein’ radar, and home in for the kill. The only drawback of the system was that each radar station could direct only one interception at a time. That was why the British had evolved the tactic of concentrating all their bombers into a tight stream: if they could push as many planes as possible through a single point, the German defences would intercept only a handful before the majority had gone past unscathed.
It was not only German night fighters that were directed by radar. The Reich had flak defences that stretched all the way from the sea to Berlin and beyond, and they, too, were radar-controlled. As soon as RAF bombers appeared over a city like Hamburg, radar-controlled searchlights that were a slightly different colour from all the others – usually an intense bluish beam – would begin to hunt them down. Once the blue master-beam had locked on to a British bomber, all the other searchlights would join it, creating a huge cone of lights with the hapless aeroplane at its apex. Thus lit up, the plane would have to dive violently to escape the force of German flak batteries intent on blowing it out of the sky.
Another problem was predictive flak. Using their radar screens, the German defenders could plot the height, speed and direction of flight of any one of the British bombers. They could then predict exactly where the aeroplane would be in the time it took the flak shells to fly 20,000 feet into the air, and direct the flak batteries accordingly. The only way for a British pilot to avoid this was to zigzag and corkscrew across the sky – which, when the sky was full of other aeroplanes, greatly increased the chances of a collision. When a crew were about to release their bombs even this course of evasive action was denied them: if they were to hit the target they were obliged to fly straight and level for a full minute before the bombs were released and they could think about escaping. Only when the photoflash had gone, marking the place they had bombed on an intelligence photograph, could they turn tail and head away from the hail of flak.
Hamburg had some of the most formidable flak defences in Germany. Not only was there a ring of batteries on the outskirts of the city, but four massive gun towers stood in the centre and the port. The heaviest guns were capable of firing a pair of 128mm shells twice a minute, each weighing 26 kilograms, to a range of 45,000 feet vertically into the sky. 5If an aircraft received a direct hit from a shell like this it spelled disaster for the crew inside – if not immediately, then during the long flight over the North Sea on the way home. Even if an aircraft was not hit, the threat of the incredible firepower exploding around them could seriously unnerve a crew, and cause them to drop their bombs early and off-target.
Given the importance of the target and the strength of Hamburg’s defences, Harris was determined to use every advantage he could to make sure the operation succeeded. By the summer of 1943 Germany’s radar-controlled defences were causing intolerable casualties, so Harris began to press the Prime Minister to authorize the use of a new secret weapon that would jam German radar. Codenamed ‘Window’, it consisted of bundles of paper strips coated with metal foil on one side. When the bundles were dropped down the flare chute during the flight over Germany they would disperse, and as the strips floated to earth they created a false ‘blip’ on German radar screens. With thousands of false readings it would become impossible for the operators to tell where the real bombers were – at a stroke, all of their defences would be rendered useless.
Until now the device had never been used because the Ministry for Home Security was terrified that Germany would copy it and use it against Britain. However, by 1943 the threat of a new Blitz by the Luftwaffe was unlikely, so on 15 July Churchill gave Harris the go-ahead. Ironically, the Germans already knew of the principle behind Window – their own version was called Düppel – but the German Chief of Air Signals, General Martini, had prevented its use because, he, too, was afraid of the consequences if the British ever copied the idea. 6Over the coming week Martini’s worst fears would come true.
Since Window only worked on the Würzburg and Lichtenstein frequencies, the bombers would also use two other devices: ‘Mandrel’, which interfered with the German long-range Freya radar, and ‘Tinsel’, which jammed German radio frequencies by transmitting the sound of the aircraft engines to drown the voices of the pilots and their radar controllers.
This, then, was the British plan of attack for what would become known as the battle of Hamburg. As Harris made his way back to his office, his deputy, Air Vice Marshal Sir Robert Saundby, remained in the Operations Room to organize the practicalities of the raid. First he telephoned Air Vice Marshal Don Bennett, head of the Pathfinder Force, which would lead the operation. After discussing the precise route to and from the target with him, Saundby set about organizing the other details of the attack: bomb loads, take-off times, aiming points and so on. Only after every detail had been precisely established would he and the rest of the staff retire to their own offices in the base at High Wycombe. For the rest of the day, department by department, the devastation of Hamburg was carefully prepared.