Friday, December 02, 2005

Window Breaks the Kammhuber Line

In a nine-day campaign in late July and early August 1943, nearly 1,000 night bombers of the British RAF Bomber Command, supported by US daylight bombers of 8th Air Force, struck at targets in the north German port city of Hamburg. The intensity of the attacks coincided with the near-paralysis of German radar-directed defenses engineered through anti-radar countermeasures. Nearly 50,000 Germans perished in the bombings and ensuing firestorm that consumed most of the city.

The widespread devastation and shocking death toll of the RAF-led campaign was the result of meticulous planning, the orchestration of heavy bombers, and the application of heretofore top-secret technology, released for the purpose. It was also the mindspring of Air Marshal Sir Arthur Harris, Commander-in-Chief of Bomber Command, who burned to bring the population centers and heavy industry of Germany under the wings of his four-engine heavy bombers. "They have sown the wind, and so they shall reap the whirlwind," Harris said while watching fires rage out of control during the Luftwaffe's "Blitz" raids against London in 1940.

Spiders in the Web
The success against Hamburg broke a period of heavy losses for Bomber Command at the hands of night fighters of the Luftwaffe's Nachtjagdgruppen that held the so-called "Kammhuber Line." Named for its chief architect, Generalleutnant Josef Kammhuber, a former bomber commander, the Kammhuber Line involved an extensive network of searchlights, radar, and night fighters based in occupied France, Belgium, and Holland, covering the approach routes of the British bombers. Early on, searchlights illuminated each bomber as a Messerschmidt Bf-110 or Junkers Ju-88 night fighter assigned to that area closed in for the kill. During 1941, a radar-controlled master searchlight introduced in 1941 made the Kammhuber Line even more effective by locking onto bombers automatically, illuminating the target with a pale blue guide beam that manually directed searchlights could pick up.

Radar-directed searchlights gave way to a more elaborate system of search and tracking ground radar and radio stations, known collectively as the "Himmelbett" system. A Himmelbett station consisted of a Freya radar for early warning with a range of 60 to 150 km, a Würzburg radar for plotting bombers, and a second Würzburg radar for guiding the night fighter. Each Himmelbett zone or "box" had a radius equal to the range of the Würzburg tracking radar (about 43 km wide and 34 km deep). These boxes were the building blocks of the improved Kammhuber Line. Target range, altitude, speed, and bearing data were sent to a ground control station that directed night fighters toward the enemy bomber "stream." Thus, each night fighter was like a spider at the center of an invisible web of beams.

The British employed a succession of radar jammers in an effort to blind the German Himmelbett network. The Germans responded with modifications to existing equipment and new systems that operated in different frequency ranges. This deadly game of one-upmanship spiraled on, and still bombers fell to the predations of the night fighters. In the weeks prior to the Hamburg raids, Bomber Command lost 872 bombers in the Battle of the Ruhr. However, the British had a radar countermeasure up their sleeves that they knew would be completely effective, but they had been reluctant to use it.

In the late 1930s, British electronic-warfare pioneer Reginald V. Jones demonstrated that strips of metal foil produced radar echoes. Subsequent tests with bundles of metal strips released from aircraft showed that in sufficient numbers the echoes took on the appearance of bombers as far as radars were concerned. Squadrons of bombers releasing foil bundles would effectively counter any radar operating between 200 and 600 MHz. Interestingly, similar tests were proceeding in parallel in Britain and Germany under the strictest secrecy. Both sides were worried that their respective radar networks would be compromised were knowledge of the technique to get out. The fear prompted Reichmarschall Hermann Göring to ban all further development of the German project, code named "Dueppel." The British proceeded with their project; code named "Window," but Prime Minister Winston Churchill would not allow it to be used operationally -- yet.

Operation Gomorrah
Churchill decided to "open the Window" for Air Marshal Harris' proposed air offensive against Hamburg. This campaign, dubbed Operation Gomorrah, was designed to utterly destroy the city in true biblical fashion. On the night of July 24-25, RAF Mosquito pathfinder aircraft equipped with H2S terrain-mapping radar sets guided nearly 800 Lancaster, Halifax, and Stirling four-engine bombers toward their targets. The aircrews hurled prodigious amounts of Window bundles from their airplanes through chutes cut into the fuselages.

The Window strips were 30 centimeters long and 1.5 centimeters wide and were packaged in packets containing 2,000 strips each. 46,000 packets were dropped, 92 million strips in all. Window completely disrupted German air defenses as the "heavies" plodded relentlessly toward their targets. German night fighter radar operators reported phantom bombers appearing and disappearing abruptly and repeatedly. Only 12 British bombers were lost during the raid.

The RAF would revisit Hamburg three times more in the days ahead, adding more incendiary bombs to the inferno. US B-17 Flying Fortresses made modest contributions twice during daylight raids. In those nine days in late July and early August nearly as many German civilians would die as all British civilians killed by German bombs in the entire war. In Germany, the firebombing of Hamburg became known as "Die Katastrophe."

For more on the German night fighter command and control system in WWII, see "Who'll Stop the Rain?" by Colonel Michael Svejgaard, Royal Danish Air Force.

For more on the destruction of Hamburg, see "Window on Gomorrah" by V. A. Pheasant, MBE.


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