Unternehmen Donnerkeil (Operation Thunderbolt) was the codename for a German military operation during the Second World War. Donnerkeil was an air superiority operation designed to support Operation Cerberus, the Kriegsmarine (German Navy) plan to evacuate capital ships from France to Germany (an operation known to the British as the Channel Dash).
In 1941, Kriegsmarine surface vessels had conducted commerce raiding operations in support of German U-boats in the Battle of the Atlantic. Operation Berlin was launched in January 1941, followed by Operation Rheinübung in May. The Royal Navy's dominance prevented the German units from returning to ports in the Baltic Sea or Germany. The surviving ships, the battleships Scharnhorst and Gneisenau and the cruiser Prinz Eugen, docked at the port of Brest, France. Throughout 1941, RAF Bomber Command repeatedly attacked the ships. The proximity of the port to Royal Air Force (RAF) airfields allowed for frequent sorties against the vessels. The Oberkommando der Marine (Naval High Command) and Adolf Hitler desired to move the ships out of range of air raids.
In December 1941, the Oberkommando der Luftwaffe (High Command of the Air Force, OKL) was ordered to devise a plan to protect the three capital ships during their escape from Brest to Germany through the English Channel. General der Jagdflieger (General of the Fighter Force) Adolf Galland prepared the Luftwaffe's aircraft for the mission. Cerberus and its supporting air operation, Donnerkeil, began on 11 February 1942. The Germans achieved surprise during the initial phase of the operation; their ships reached Germany on 13 February 1942, two days after the operations began.
The Luftwaffe defeated British air attacks during the Channel Dash, enabling the naval contingent to reach German waters. In the air battles over the Channel, the British had significant losses. German losses were modest, and the operation achieved its objective.
Background
The first German capital ships to dock at Brest were the Scharnhorst and the Gneisenau on 22 March 1941, following their eight-week participation in Operation Berlin against Allied shipping in the Atlantic. The heavy cruiser Prinz Eugen also sought refuge in the harbour on 1 June 1941, after the failed Operation Rheinübung. The ships were within easy range of British air power in port, and were subjected to frequent attacks and some damage. A ten-month offensive by the RAF beginning on 29 March 1941 consisted of 2,928 sorties, 171 of them in daylight. A raid on 24 July lost 12 per cent of its attacking force. Night bombing was safer for the British; the Germans lacked dedicated night-fighters, and the 18 losses to enemy action were attributed to anti-aircraft guns. During their campaign, the RAF employed more sophisticated navigation aids; on the night of 7/8 December 1941, the Oboe navigation system was used for the first time.
Concerned after the loss of the Bismarck, Hitler ordered the Kriegsmarine to move the ships to Germany for overhauling in preparation for their deployment to Norway; they would serve as a fleet in being and as the primary naval defence of German-occupied Norway. After lengthy discussions, the Oberkommando der Marine opted for the shorter (but, arguably, more dangerous) route through the English Channel.
On 12 January 1942, Hitler met the commanders of the operation at his headquarters in East Prussia (the Wolf's Lair). Present were Wilhelm Keitel (Commander-in-Chief of the Wehrmacht), Hans Jeschonnek (Chief of the Luftwaffe General Staff), Alfred Jodl (Chief of Staff for Military Operations), Adolf Galland (General der Jagdflieger), Erich Raeder (Commander-in-Chief of the Navy) and Vice-Admiral Otto Ciliax, who was to lead the battle group (Kampfgruppe). During the meeting, Hitler likened the German fleet to "a patient with cancer who is doomed unless they submit to an operation. An operation, on the other hand, even though it may have to be drastic, will at least offer some hope that the patient's life may yet be saved. The passage of our ships is such an operation. It must be attempted". Few operational details were discussed, but the Luftwaffe was ordered to provide air cover and diversionary raids against British targets. Jeschonnek promised about 250 aircraft.
Preparations
Luftwaffe plan
The OKL was apprehensive about Cerberus; Jeschonnek said to Galland that if the operation failed, the Luftwaffe would be made a scapegoat. During the 12 January meeting, the Navy demanded maximum fighter cover and won Hitler's support. Jeschonnek stood his ground against Galland, refusing to guarantee reinforcements to the Western fighter forces from other theatres. Galland was given executive authority for the air operation, which was codenamed Unternehmen Donnerkeil (Operation Thunderbolt). The operation was so secret that Jeschonnek and Galland had to sign secrecy pledges when they left Hitler's headquarters.
The plan's details were developed with Oberst (Colonel) Karl Koller, the Luftflotte 3 chief of staff (Air Fleet 3, under Generalfeldmarschall Hugo Sperrle). Some training units had to be mobilised to assemble sufficient strength, since most of the Jagdwaffe (fighter force) was deployed in the Soviet Union for Operation Barbarossa.
The route was divided into three sectors, based on Jafü (Fighter Sector) boundaries. To ensure local control, former Jagdgeschwader 27 Geschwaderkommodore (Wing Commander) Max Ibel was appointed Jagdfliegerführer Schiff (Fighter Controller Ship, abbreviated Jafü Schiff. He embarked on the Scharnhorst as a signals officer to communicate with Luftwaffe units. Eight dummy operations, involving about 450 sorties, were flown between 22 January and 10 February to train for the mission; it is unclear whether the British were aware of these exercises.
To disrupt British radio transmissions, the Funkhorchdienst (Radio Monitoring Service, a signals intelligence unit commanded by Wolfgang Martini) was tasked with jamming radio-telephone frequencies. They developed a subtle jamming technique that increased atmospheric interference, degrading the performance of British coastal radar. Dornier Do 217s from Kampfgeschwader 2 were ordered to fly electronic deception missions over the western channel to divert enemy aircraft. Fliegerkorps IX, commanded by Joachim Coeler, prepared to strike RAF bases in south-western England and to engage and slow any British naval forces that might attempt an interception. Fernaufklärungsgruppe 123 was responsible for maintaining reconnaissance at the east and west ends of the channel and was to support Fliegerkorps IX.
To ensure constant air support, Jagdgeschwader (Fighter Wings) and Nachtjagdgeschwader 1 (Night Fighter Wing 1) were ordered to achieve a frantic pace in servicing and preparing aircraft for their next missions. To maintain an aerial vigil over the task force, ground crews ("black men") had to rearm and refuel aircraft in 30 minutes or less. Galland insisted that the aircraft be split between high and low altitudes to provide robust cover; the low-altitude groups would be able to evade detection by British coastal radar. He demanded an umbrella of at least 16 fighters over the ships at all times along the length of the channel. The fighters were to be split into formations of eight aircraft for their respective patrol altitudes, with each formation further divided into two Schwärme of four aircraft. The Schwärme tactics involved one formation flying out to sea and another towards land in a zigzag pattern. All Schwärme were ordered to fly back and forth along the line of ships in wide figures-of-eight, maintaining radio silence. Each sortie was meticulously timed to allow fighters 30 minutes over the ships, enough time to maintain cover and allow the relieved units to refuel, rearm, and return. The relieving sorties arrived after only 20 minutes during Donnerkeil, however, increasing cover to 32 fighters for half of the operation.
Galland made it clear to the fighter pilots that the ships must be protected at all costs. They were expected to fly at least four sorties on the day of the operation, whose success would not be measured by the number of enemy aircraft shot down. RAF aircraft leaving the target area would be ignored, but attacking aircraft were to be engaged at all costs—with ramming, if necessary.
British preparations
Operation Fuller was the codename for the RAF plan to counter a German break-out. The RAF anticipated that the Germans might opt for the English Channel as their route. Captain Norman Denning, head of the Admiralty's Operational Intelligence Centre, was uncertain the Germans would attempt it but regarded it as a possibility. At the end of January, he warned that German warships were preparing to put to sea and a major operation should be expected. Denning sent a message to First Sea Lord Dudley Pound:
The short cut of the German ships is via the English Channel. It is 240 miles from Brest to Cherbourg and another 120 miles from Cherbourg to the Dover straits. While ships could make the passage from Brest to Cherbourg or from Cherbourg to the Dover straits in the same dark period, they could not make the complete passage from Brest to Dover in one dark period. At first sight this passage up the Channel seems hazardous for the Germans. It is probable, however, that as their heavy ships are not fully efficient, they would prefer such a passage, relying for their security on the destroyers and aircraft which are efficient, and knowing full well that we have no heavy ships to oppose them in the Channel ... Taking all factors into consideration, it appears that the Germans can pass east up the Channel with much less risk than they will incur if they attempt an ocean passage.
Air Marshal Philip Joubert de la Ferté, commander-in-chief of RAF Coastal Command, agreed that this was the probable route. He expected the Germans to make an attempt any time after 10 February. Unfortunately for the British, the Air Ministry and the three RAF commands—Coastal Command, RAF Bomber Command, and RAF Fighter Command—believed that the Germans would use darkness for the longest and most dangerous part of the journey through the straits and would have to leave in daylight. They believed that the inadequate forces at their disposal would be best used at night. Most of RAF Bomber Command was ordered to stand down, leaving it unprepared to operate in daylight on 12 February.
Coastal Command had agreed to provide three squadrons of Bristol Beaufort torpedo bombers. The Fleet Air Arm (FAA) contributed one squadron of Fairey Swordfish torpedo bombers. RAF Bomber Command had about 300 bombers on standby. The Swordfish were slow, making escorting them difficult. The Beaufort squadrons were dispersed, making it difficult to bring them together. One squadron was based at Leuchars, Scotland; one at Thorney Island in Portsmouth, and one near St Eval in Cornwall.
Forces
Luftwaffe
The Luftwaffe contributed five wings to the operation. Jagdgeschwader 1 (JG 1), Jagdgeschwader 2 (JG 2), and Jagdgeschwader 26 (JG 26) were equipped with day fighters, primarily Messerschmitt Bf 109s and Focke-Wulf Fw 190s. JG 2 and JG 26 operated the Fw 190, and JG 1 operated the Bf 109. Nachtjagdgeschwader 1 was also pressed into service, its Messerschmitt Bf 110s operating in much smaller numbers. Kampfgeschwader 2 played a supporting role, mainly conducting maritime interdiction and air raids on enemy airfields in southern England to distract the RAF. In total, the Germans had 252 fighters, 30 heavy fighters, and 32 bombers. JG 26 maintained a "monopoly" on the Fw 190 at the time.
RAF and FAA
Confidence in the British bomber force was low; with no anti-shipping-attack training, their ability to inflict damage was poor. The main hope was in the torpedo bomber force, composed mainly of Beaufort and Swordfish aircraft from Coastal Command and the Fleet Air Arm. Beauforts from No. 42, No. 86, and No. 217 Squadron RAF were made available, but they were short of torpedoes. These three squadrons were the only ones available on 12 February 1942; about 57 Beauforts had been diverted to other theatres, leaving a chronic shortage of torpedo bombers in Britain. No. 415 and No. 489 Squadrons had been withdrawn to convert to Handley Page Hampdens, and No. 22 Squadron RAF was in the midst of transferring to the Middle East.
No. 825 Squadron FAA and their Fairey Swordfish were also made available. Lockheed Hudsons from No. 224 Squadron RAF and No. 233 Squadron RAF were committed to reconnaissance operations. No. 22 Squadron RAF was recalled from leave to take part. Hudsons from No. 407 Squadron RCAF were also available, placed on high alert, and took part in the fighting. RAF Bomber Command contributed No. 5 Group RAF, with 242 of its 300 aircraft available for service. Fighter Command committed No. 1, 19, 91, 41, 118, 129, 137, 234, 401, 403, 607, 316, 411, 452, 485, 128, 64, 65, 72, and 11 squadrons.
Channel Dash
On the evening of 10 February 1942, the German flotilla prepared to begin the operation. RAF bombers appeared overhead as the ships slipped anchor, forcing them to return to the dock. The bombers released their payloads but did little damage. Fortunately for the Germans, the British did not notice anything unusual.