Part 6-2. Confound and Destroy II: The Arrival of Window
As British jamming was countered by the Germans, waiting in the wings was another British radio weapon: 'Window', which the Americans were later to codename 'Chaff'. By the spring of 1942 it had been determined that metallic foil strips one half-wavelength in length would create a significant radar echo on a scope. If bundles of these, cut to 27cm (or half the wavelength of Würzburg
) could be dropped out of a bomber at 10,000 feet, a cloud of echoes would appear, obscuring any aircraft within it for around 15 minutes. Precise Würzburg
control of searchlights, flak or fighters would become impossible. What made the scheme even more useful was that the Würzburg
ground radar and Lichtenstein
AI worked on the same frequency and were both vulnerable to the strips. Plans for the bomber stream to lay carpets of these strips were made.
Suddenly, the plans were called off. For more than a year, Bomber Command was forced to sit on their hands and keep Window in storage. The fear was that if the system was used the Luftwaffe would reverse-engineer it and retaliate in kind. Lord Cherwell, Churchill's scientific advisor, lobbied against its use until a countermeasure could be found. Arguments raged back and forth, but it was not until the new generation of Allied radars, such as the American-made SCR-720 and the British Type 11 GCI, had proven that they could distinguish Window from targets, was the okay given.
Curiously, the Germans had been working on their own version of Window, named Düppel
('Dipole'). In 1942 the head of the Luftwaffe's signals service, Wolfgang Martini, presented a report to Göring, emphasizing the danger that a countermeasure like Düppel
presented to the Reich Air Defence. Göring was horrified. He ordered all copies of Martini's report destroyed and research on Düppel
and countermeasures to be halted, lest the secret leak out. It was yet another of the Fat Man's grand errors and the lack of research on countermasures was to greatly set back the night defences.
The first use of Window could not have been more dramatic. Sir Arthur Harris launched Operation Gomorrah against the city of Hamburg on the night of 24/25 July 1943. Almost 800 bombers streamed towards Hamburg, and on the approach each bomber began to drop small bundles of Window strips out of its flare chute at the rate of one a minute. A carpet of blips formed, filling radar scopes with hundreds of false targets. Suddenly searchlights and flak, all controlled by Würzburg
, could not find any bombers amongst the clutter while nightfighters began to hunt down false echoes. The night defences fell to pieces and just 1.5% of the attackers were lost. The RAF was to return twice in the following week and, almost unmolested by the defences, stirred up a firestorm that burned out the heart of the city. Though the loss rate climbed, thanks to the intervention of the Wilde Sau
fighters (see Part 5), Window was a catastrophe for the Luftwaffe. PHOTO: A cloud of Window (the spray of sparkles to the right of the picture) is released into the bomber stream. Initially the Window bundles were quite large, with 2,000 strips making up a package of about 27 ounces. As each bomber popped out a bundle at the rate of one a minute, this meant vast quantities of aluminium foil were used. Fortunately, American industrial know-how came to the rescue, with the development of a rotary cutter that that sliced the foil and bent it into a 'V' shape for rigidity. Seventy-five of these cutters were flown to Britain as priority cargo. The strips they produced were thinner and more efficient than the original Window. Type 'C' Window made an 800-strip bundle of 6 ounces, while the cellophane-backed Type 'E' Window reduced the weight to a manageable 2.6 ounces.
An answer to Window was not long forthcoming. The Germans were to refer to their countermeasures as 'de-lousing', and within a week of the first Hamburg raid a modification of the Würzburg
radar code-named Würzlaus
was demonstrated. This used the Doppler effect, the shift in frequency based on whether an object was moving toward or away from the transmitter, similar to the way the sound of a car changes as it moves towards and away from a listener. Because Window lost all forward speed very quickly once released, it was possible to separate the foil strips from the fast-moving bombers. Würzlaus
was not without problems--for example, it only worked when the relative speed difference between Window and the bomber was high, or in other words when the bomber was coming towards or away from the radar. Another de-lousing device named Nürnburg
permitted the operator to tell the difference between a Window cloud and the modulated signal that returned from an aircraft's propellors. Nürnburg
was a tricky device that only a handful of skilled operators mastered, but by November 1943 both it and Würzlaus
had been added as modifications to 1,500 Würzburg
One of the most important responses to Window was in the field of AI radar. The Lichtenstein BC
radars could not be modified to reduce the effects of Window, so the Luftwaffe turned to Telefunken's Lichtenstein SN-2
radar, which had previously been rejected because of its poor minimum range. Based on a ship search radar, the SN-2 had slightly improved range and much wider search coverage (see Part 5), but what made it so valuable was that it was immune to Window, as it operated on a much lower frequency. A crash programme was launched to get the SN-2 into production. At first it had to be paired with the Lichtenstein C-1
in order to have the necessary minimum range performance, a clumsy installation that degraded aircraft performance, but by spring of 1944 the minimum range issue had been solved. Critically, the British radio intelligence efforts failed to detect the deployment of the SN-2. As casualty rates climbed they were unaware that jam-proof nightfighters were now hunting in the night skies.
Mandrel, Tinsel and Window, pitched against Würzlaus
and SN-2 were just the opening shots in a titanic technical struggle. In the coming months the balance of power would swing back and forth between the British and Germans as an array of clever new devices were deployed. The RAF would tie together their RCM efforts under the umbrella of 100 Group (Bomber Support) in an all-out effort to degrade the Luftwaffe defences. Next:
Confound and Destroy III: The Bomber Support Campaign.
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