The Battle of the Ruhr (5 March – 31 July 1943) was a strategic bombing campaign against the Ruhr Area in Nazi Germany carried out by RAF Bomber Command during the Second World War. The Ruhr was the main centre of German heavy industry with coke plants, steelworks, armaments factories and ten synthetic oil plants. The British attacked 26 targets identified in the Combined Bomber Offensive. Targets included the Krupp armament works (Essen), the Nordstern synthetic oil plant at Gelsenkirchen and the Rheinmetall–Borsig plant in Düsseldorf, which was evacuated during the battle. The battle included cities such as Cologne not in the Ruhr proper but which were in the larger Rhine-Ruhr region and considered part of the Ruhr industrial complex. Some targets were not sites of heavy industry but part of the production and movement of materiel.
The Ruhr had been attacked by Bomber Command from 1940; its defences and the amounts of industrial pollutants produced a semi-permanent smog that hampered bomb aiming. Along with anti-aircraft guns, searchlights and night fighters, the Germans built large decoys such as the Krupp decoy site (Kruppsche Nachtscheinanlage) near Essen to spoof the bombers into hitting open country. Cities outside the Ruhr were attacked to prevent the Germans from concentrating their defences and before the end of the battle, Operation Gomorrah on 24 July 1943, began the Battle of Hamburg. After the turn to Hamburg, Bomber Command continued to raid the Ruhr to keep the German defences dispersed.
Background
Bomber Command, 1942
In 1942 some answers to the chronic problems of night navigation and target finding began to emerge but the number of bombers had stagnated. In November 1941 Bomber Command had a daily average of 506 bombers available and in January 1943 the average was 515. To carry out the Thousand-bomber raids Bomber Command drew on crews and aircraft from the Operational Training Units, which could only be exceptional. Navigation had been helped by the introduction of Gee but this device lacked accuracy for bombing through the dark and smog of the Ruhr, lacked range and from 4 August 1942 the Germans began to jam the device.
The Pathfinder Force (PFF) was established on 15 August 1942 but with Gee jammed and no target indicator bombs to mark the aiming point for the rest of the bombers (the Main Force), the task of the PFF varied from thankless to impossible. Despite its problems, Bomber Command had been able to achieve some spectacular results but these had been isolated events and due to favourable circumstances as well as judgement. The loss of 1,404 aircraft and 2,724 damaged to German night defences of increasing quantity and quality, especially German night fighters (Nachtjäger) had become a serious threat to the viability of the command and of strategic bombing as a theory of war.
In 1942 Bomber Command had created 19 new squadrons but 13 had been transferred to other commands. The quantity of aircraft had barely increased but a big improvement in quality had been achieved. Bristol Blenheim light bombers and Armstrong Whitworth Whitley medium bombers had been retired from the command in mid-1942, followed by the Handley Page Hampden medium bomber in September. The disappointments of the Short Stirling and the early Handley Page Halifax variants and the fiasco of the Avro Manchester, withdrawn in June 1942, was balanced by the Avro Lancaster, which made its operational début in March and demonstrated its superiority over all other bombers. Re-equipment with new types of aircraft led to an average of 16.36 per cent of Bomber Command squadrons withdrawn from operations for conversion onto new aircraft in 1942, against 3.3 per cent in 1943. On 1 January 1942 the command had 48 squadrons, 9 with heavy bombers, 34 with medium and five with light bombers (Blenheims). On 1 January 1943, there were 49 squadrons, 32 heavy, 11 medium and six light (de Havilland Mosquito). The command had flown 30,508 operational sorties in 1941 and dropped 31,646 long tons (32,154 t) of bombs, in 1942 it dropped 45,501 long tons (46,231 t) from 29,929 sorties.
Gee
Gee worked by wireless signals transmitted from three ground stations in England, on a line about 200 mi (320 km) long, being displayed on a cathode ray tube to the navigator and placed on a Gee chart, giving a fix of the aircraft's position in less than a minute. Accuracy varied from 0.5 to 5 mi (0.80 to 8.05 km) and Gee had a range of 300–400 mi (480–640 km), accuracy falling with distance. Gee worked well as a homing device but early hopes of it being accurate enough for blind bombing were not realised. Crews appreciated the value of the apparatus for navigation on the return journey, removing the fear of flying into hills and other obstructions. By August 1942, 80 per cent of the bomber force was equipped and 100 per cent by January 1943.
Coverage of the Ruhr was known as the eastern chain and later northern and southern chains were added. Gee was usually ineffective east of the Ruhr and was easy to jam, which began on 4 August 1942, from when Gee fixes were only obtainable over the North Sea and parts of France. Gees loss of accuracy with distance made it a better target-finding device for Luftwaffe raiders over Britain. The signals were encoded to prevent German use but this made it harder for Bomber Command navigators to get Gee fixes. Anti-jamming devices were short-lived in effectiveness as the Germans quickly overcame them but Gee Mk II was easier for navigators to use.
Oboe
Oboe was a blind-bombing device controlled by two ground stations in England which measured the distance of an aircraft from them with radar pulses. Cat tracked the aircraft over the target and Mouse calculated the point on the track where the aircraft should bomb. Oboe transmissions did not follow the curvature of the Earth, making the altitude of the aircraft the determinant of range. An aircraft flying at 28,000 ft (8,500 m) could receive Oboe transmissions at about 270 mi (430 km), enough to mark targets in the Ruhr, which led to the device being installed in fast, high-flying Mosquito bombers, which usually navigated with the usual aids until beginning an Oboe run about 10 mi (16 km) from the target. Use of the Mosquito made an Oboe run safer, even when no evasive action could be taken before bombing. Accuracy was measured in hundreds of yards which increased with greater experience of the aircrews and ground operators.
Oboe could be jammed and suffer interference from Monica and other Bomber Command devices. Oboe Mk I operated on a wavelength of 1.5 metres, K Oboe was in general use from mid-June 1943 and was free from jamming. Centimetric Oboe Mk II and Mk III retained the effectiveness of Oboe until the end of the war but are beyond the scope of this article; rushing Oboe Mk I into service delayed Mk II but jamming did not begin until August 1943. Cat and Mouse stations could handle only one aircraft at a time and a marking run took ten minutes, allowing six bomb- or marker-runs per hour. The illumination from a Target Indicator bomb usually lasted for six minutes, guaranteeing four-minute gaps in marking. A failed marking run increased the gap to fourteen minutes. The introduction of multi-channel control and more ground stations eventually increased the concentration of Oboe marking. From the introduction of Oboe in December 1942 until the end of the war, Oboe aircraft made 9,624 sorties on 1,797 raids.
1943
At the beginning of 1943, the political resolve of the British government to support the bomber offensive remained and more resources had been provided. A plan to create a 4,000-strong bomber force had been abandoned as unrealisable but the diversion of Bomber Command squadrons to RAF Coastal Command and overseas commands was constrained and its expansion continued. A blind bombing device, Oboe, had been introduced on the night of 20/21 December 1942, H2S, a ground-scanning radar, on 30/31 January and a new target indicator bomb had come into service on 16/17 January. The new devices and the increase in the number of heavy bombers promised a large improvement in the quantity of bombs dropped and in accuracy of aim.
For the first time since day bombing was abandoned in 1940, Bomber Command was released from the constraints occasioned by the adoption of night bombing. The tactical use of the new devices was developed quickly but the new equipment had limitations. Oboe had a range which was little beyond the Ruhr and research on repeater aircraft to extend its range was stopped because H2S was expected to be a better system and only a few aircraft could use the device simultaneously; although Oboe had the potential for a vast improvement in target finding, it was not of pinpoint accuracy. H2S could be installed on any aircraft but was complicated, difficult to use and emitted radiation which could be detected, paradoxically exposing the aircraft to interception. Gee remained useful as a means of navigation on return journeys but required development to overcome German jamming.
Target Indicator bomb
The Target Indicator bomb (TI) was an aerodynamic metal case which ejected coloured pyrotechnic candles at a set height by a barometric fuze. If set to ignite immediately they made a cascade. When sky marking, the candles were on parachutes and if fuzed for ground burst, they created a pool of coloured fire. The usual 250 lb (110 kg) TI covered an area on the ground of about 100 yd (91 m) and a ground burst TI contained some candles which were explosive, to deter attempts to extinguish them. The TI was introduced on the night of 16/17 January 1943 and was a great success, making pathfinding a practical operation of war.
H2S
H2S was a device for navigation and blind bombing by radar. The downward emissions of the radar were reflected and received as echoes which were distinctive of the ground. Built-up areas returned echoes different from fields and forests, land echoes could be distinguished from the sea and sea echoes from those of a ship. The radar had a scanner which swept vertically and the echoes were detected by a receiver and displayed on a cathode-ray tube, with a sweep rotating at the same speed as the scanner, giving an impression of the ground. The contrast between water and land made coasts, lakes and rivers particularly recognisable. Towns also stood out and sometimes railway lines, from which the navigator could determine his position. When closer to the target, if it was recognisable, an H2S bombing run could be made; if the target could not be distinguished the bomber could make a timed run from a landmark in the vicinity. Since the device was airborne its range was limited only to that of its aircraft. The apparatus was limited by echoes from towns and cities, which were harder to distinguish than those from countryside and town.
H2S was inferior to Oboe for blind bombing except on coastal targets like Hamburg. Being a transmitter, H2S disclosed itself to the Germans as soon as enough parts had been recovered from shot-down bombers to analyse its characteristics. German detectors could find a bomber stream and direct night-fighters into it. Once Bomber Command began to use centimetric H2S in January 1943, it was inevitable that the Germans would retrieve one from a crashed bomber and realise that it was similar to air-to-surface-vessel radar (ASV) used by Coastal Command to detect surfaced submarines. In October 1943, beyond the scope of this article, the Germans introduced the Naxos radar detector in night-fighters and U-boats. By mid-January 1943, only 10 Halifax bombers and 13 Stirlings carried H2S; the rate of production was slow and by May 1943, no more than 18 H2S-equipped bombers had participated in one raid; by August, 840 H2S sets had been manufactured.
Prelude
German air defences
The threat of Anglo-American strategic bombing had been a concern to German strategists since 1940 at the latest. After the Fall of France, a belt of Freya radar stations was built to give early warning of aircraft entering German-controlled airspace, from Denmark south to Switzerland. Freya lacked the accuracy needed for ground-controlled interception of aircraft and was supplemented later in 1940 by Würzburg radar stations which were accurate enough to guide Flak and night fighters, a Würzburg-guided Flak battery shooting down a bomber in September 1940. In October Oberst (Colonel) Josef Kammhuber established three night-fighter zones on the approaches to the Ruhr. The zones were 56 mi (90 km) long and 12 mi (19 km) wide, with a battalion of searchlights and two Würzburg radars in each.