Following on from the bombing of Peenemunde in August 1943, the Allied bomber offensive continued to attack precision targets in France with heavy bombers. The “Crossbow” offensive lasted from June until September of 1944, much to ACM Harris’ chagrin as his Command was diverted from what he considered to be the essential task of destroying German cities. Nevertheless, Bomber Command carried out their directive with due persistence and considerable sacrifice during a number of attacks. Specifically the objectives were the destruction of the “Crossbow” sites of the V weapons and of these there were three specific types of targets:
- The launch ramps for the V1 flying bombs, widely dispersed, well defended and well camouflaged.
- The V weapon logistics train such as branch lines and assembly plants.
- The so-called “Large Sites,” massive constructions whose purpose the Allies didn’t fully understand at that time.
These “Large Sites” were major engineering projects and were mainly located in the Pas de Calais, which by June of 1944 was one of the most comprehensively defended areas in the world. Around four infantry divisions occupied the sector between Abbeville to Dunkirk, protected by the most complete section of the Atlantic Wall.
The Blockhaus d’Éperlecques or Watten Bunker
The A4 rocket or V-2 programme required huge amounts of cryogenic liquid oxygen (LOX), to act as an oxidiser to fuel the missiles. Because LOX evaporates rapidly, a large source was required close to the launch sites. There was insufficient manufacturing capability of LOX throughout Europe, to mount a sustained V2 campaign. Each V2 launch required 15 tons of LOX and production in 1942 throughout Europe was 215 tons per day. The Kriegsmarine, Luftwaffe and German industry were also heavily dependent on LOX.
The missiles were intended for use against London and the South Coast Channel Ports, so the launch sites had to be located near the Channel coast, which was well within range of Allied air attacks. There were two concepts of the location and nature of the V2 launch sites: Walter Dornberger was the head of the A-4 development project at the Peenemünde Army Research Center. He suggested that the missiles should be based in heavily defended fixed sites of a bunker-style design similar to the massive submarine pens then under construction in occupied France and Norway. The rockets could be stored in such sites, armed, fuelled from an on-site LOX production plant, and launched. This offered significant technical advantages; not only would the LOX loss be minimised, but the complex process of pre-launch testing would be simplified. A high rate of fire could be sustained as the facility could effectively operate like a production line, sending a steady flow of missiles to the launch pads. The German Army preferred an alternative approach which would use trailer-style mobile launch platforms called Meillerwagen accompanied by testing and fuelling equipment mounted on railway cars or trucks. Although this configuration was far less efficient and would have a much lower rate of fire, it would have the great advantage of presenting a much smaller and difficult to find targets for the Allied air forces. The Army was not convinced that fixed bunkers could resist repeated air attacks and was particularly concerned about the vulnerability of the launch sites’ road and rail links, which were essential for resupplying them with missiles and fuel.
Hitler preferred the bunker option, a grandiose statement and two designs were submitted, one where the rockets were assembled within the bunker and transported on a rail outside for firing. The second elevated the missile from inside the bunker to be fired from the roof. In October 1942 Speer’s Organisation Todt (Todt had died in an aircraft crash at Rastenberg on 8th February 1942), scoped the constructed two bunkers to a “special fortification standard,” with a reinforced ceiling of sixteen feet and walls of eleven foot thickness. One would be on the Côte d’Opale near Boulogne-sur-Mer and the other on the Cotentin Peninsula near Cherbourg. Each would be capable of launching 36 missiles a day, would hold sufficient supplies of missiles and fuel to last three days, and would be manned by 250 troops.
However, in December 1942, Speer ordered Peenemünde officers to tour the Artois region in north-west France and locate a suitable site for an A-4 launch facility. The site chosen was just to the west of the small town of Watten, in the Forest of Éperlecques, near Saint-Omer in the Pas-de-Calais department. It was given the cover name of Kraftwerk Nord West (Northwest Power Plant). The location was close to the Calais and Saint-Omer railway, the canalised River Aa and main roads and electric grid lines. Additionally there were sand, gravel and cement works nearby, because the bunker would require 200,000 tons of concrete and 20,000 tons of steel. The location was sufficiently inland to be safe from naval gunfire and was sheltered by a ridge rising 300 feet above the site. At Saint-Omer a major Luftwaffe base could provide air defence for the area both aircraft and flak batteries. This bunker was much larger than those proposed by the Organisation Todt, and it would have its own LOX production plant on site, within the bunker.
The construction plans were given to Hitler who agreed for the production to go ahead immediately. Work began on the site in February 1943 and 6,000 workers from Construction Battalion 434 began the huge project. It was expected to be operational by the November of 1943. The workforce consisted of a mixture of German specialists and forcibly conscripted Frenchmen from the Service du Travail Obligatoire (STO). They were supplemented by Belgian, Dutch, French, Polish, Czech and Soviet prisoners of war and civilian conscripts, who were used as slave labour. The labour force also included many French political prisoners and Spanish Republicans who had fled to France after General Franco’s victory in the Spanish Civil War but had then been interned by the invading Germans
The camps for the workers and German overseers were guarded by the French civil police, plus POWs from Belgium and the Netherlands, who had volunteered for guard duty. Over 35,000 foreign workers passed through the camps and they worked in shifts of 3,000 to 4,000 for twelve hours, seven days a week. Any attempts of escape were punished by summary execution and conditions were exceptionally harsh for the Russian and Eastern European prisoners, who in many cases were worked to death. A German commission that inspected the labour camps in the area in late 1943 commented: “The Eastern [European] worker is very tough. He works at his job until he falls flat on his face in the mire, and all that is left for the doctor to do is to issue the death certificate.”
A large supply dump was established at Watten next to the River Aa and this site was eventually used to store material required for all the V-weapon sites in the Saint-Omer area. Building materials were brought there by barges and trains where they were unloaded onto a Decauville narrow-gauge railway for transportation to the construction site, where concrete mixers operated day and night. A 90 kV power line running to a transformer at Holque north of Watten provided electricity. An old quarry at Wizernes codenamed Schotterwerk Nordwest (Gravel Quarry Northwest), some 12 kilometres (7.5 mi) south of Watten, was also converted into a storage dump to supply the Watten facility.
Discovery of the Construction Work
In early April 1943, an Allied agent reported “enormous trenches” being excavated at the Watten site, and on 16 May 1943 an RAF reconnaissance mission led to Allied photographic interpreters noticing unidentified activity there. Hundreds of thousands of air reconnaissance photographs were taken of France in preparation for the Overlord Landings in Normandy and other large facilities were observed to be under construction elsewhere in the Pas-de-Calais. The purpose of the construction works was unclear; Lord Cherwell, Winston Churchill’s scientific adviser, admitted that he had little idea what “these very large structures similar to gun emplacements” were but he believed that “if it is worth the enemy’s while to go to all the trouble of building them it would seem worth ours to destroy them.”
At the end of May, the British Chiefs of Staff ordered that aerial attacks be carried out against the so-called “heavy sites” being built by the Germans. On 6th August, Duncan Sandys, who headed a high-level Cabinet committee to coordinate the British defence against the German V-weapons, recommended that the Watten site should also be attacked because of the progress being made in its construction. The British Chiefs of Staff noted that a daylight attack by US bombers was under consideration but they raised objections to the proposal, as the Air Staff thought that Watten had nothing to do with rockets, suggesting that instead it might be merely a “protected operations room.”
Sir Robert McAlpine, the chairman of the famous construction company, advised that the site should be attacked while the concrete was still setting. On 27th August 1943, 187 B17 bombers of the US 8th Air Force made the first successful bombing of the site. The bombing was effective and it was noted that after the raid the site was:
“…a desolate heap of concrete, steel, props and planking. The concrete hardened. After a few days the shelter was beyond saving. All we could do was roof in a part and use it for other work.”
The bombing killed and injured hundreds of the slave workers on site; although the Allies had sought to avoid casualties by timing the raid with what they thought was a change of shifts, the shift pattern had been changed by the Germans at the last minute to achieve the day’s work quota.
Only 35% of the bunker had been completed at the time of the first bombing raid and it was clear it could no longer be used as a launch site. But it was still useful for the production of LOX to supply the other site and mobile launch platforms. The Organisation Todt engineers determined that the northern part of the facility was irretrievably damaged but decided to focus on completing the southern part to serve as a LOX factory. One of the OT’s engineers, Werner Flos, came up with an idea to protect the bunker from bombardment by building it up from the roof first. This was done by initially constructing a concrete plate, flat on the ground, which was 6 feet thick and weighed 37,000 tons. It was incrementally raised by hydraulic jacks and then supported by walls which were built underneath it as it was raised, becoming the roof. The resulting concrete cavern was intended to be used by the Germans as a bombproof liquid oxygen factory. The thickness of the roof was chosen on the assumption that Allied bombs were incapable of penetrating such a depth of concrete; the Germans, however, were unaware of the British development by Barnes Wallis of the 12,000 lb Tallboy earthquake bombs and later the 22,000 lb Grand-Slam.
The Germans switched their main focus to the construction of the La Coupole bunker at Schotterwerk Nordwest, the former quarry at nearby Wizernes, where work had been ongoing to build a bombproof V-2 storage facility. This project was expanded to turn the quarry into a fixed launch facility. Plans were put into effect to build a huge concrete dome. The Allies carried out further heavy bombing against both the Watten and Wizernes sites with little initial effect on the buildings themselves, although the rail and road network around them was systematically destroyed. On 3rd July 1944, Oberkommando West gave permission to stop construction at both sites, which had been so disrupted by bombing that work could no longer proceed. Three days later an Allied raid succeeded in wrecking the interior of the Watten bunker with a Tallboy bomb that brought down part of the roof. Finally, on 18 July 1944, Hitler decreed that plans for launching missiles from bunkers need no longer be pursued. Dornberger’s staff subsequently decided to continue minor construction at Watten “for deception purposes”. The site itself was now useless, as the Germans recognised when they wryly codenamed it Concrete Lump, and the liquid oxygen generators and machinery were transferred to the Mittelwerk V-2 factory in the central German Harz Mountains, well protected in mines hewn out of rock and enlarged by slave labour.
Allied Bombing of the Watten Bunker and Site
|27 August 1943||VIII Bomber Command Mission 87/11 Group RAF Ramrod S.8: 187 Boeing B-17 Flying Fortresses bombed Watten at 1846–1941 hours, dropping 368 2,000 lb (910 kg) bombs. The site was thought to be a V-1 flying bomb facility at the time and crews were briefed on an ‘aeronautical facilities’ mission with instructions to bomb, from low level, the freshly poured concrete beginning to harden.
The bombing caused the still-wet cement to solidify into a mess that was beyond repair. Allied losses were two Flying Fortresses lost to flak, one lost to Bf 109 fighters, one damaged by flak crash-landed in the UK. From the escorting force, one USAAF Republic P-47 Thunderbolt failed to return, two pilots from No. 41 Squadron RAF were shot down and captured and two pilots from No. 341 Squadron RAF including René Mouchotte were killed in action.
|30 August 1943||VIII Air Support Command Mission 38/11 Group RAF Ramrod S.14: 24 North American B-25 Mitchell, 18 Lockheed Venturas, and 36 Martin B-26 Marauder medium bombers attacked Watten, described as an “ammunition dump at Éperlecques” at 1859 hours, dropping 49 tons of bombs. One No. 180 Squadron bomber was lost to flak with two of the crew killed. Fourteen other bombers were damaged by flak.|
|7 September 1943||VIII Bomber Command Mission 92: 58 B-17s bombed Watten, dropping 116 tons of bombs between 0820 and 0854 hours.|
|2 February 1944||Mission 205: 95 of 110 Consolidated B-24 Liberators, escorted by 183 Republic P-47 Thunderbolts, hit the V-weapon sites at Siracourt and Watten.|
|8 February 1944||Mission 214: 110 B-24s bombed the V-weapon sites at Siracourt and Watten, dropping 364 tons of bombs. More than 200 B-26s returned during the morning to carry out follow-up attacks.|
|19 March 1944||Mission 266: 117 of 129 B-17s bombed Watten, Wizernes and Mimoyecques. A follow-up attack by 65 Douglas A-20 Havoc light bombers was carried out the same afternoon.|
|21 March 1944||56 B-24s bombed Watten, but bad weather forced the recall of all the B-26s sent to join the raid.|
|26 March 1944||500 heavy bombers of the 8th Air Force attacked a total of 16 V-weapon sites in northern France, including Watten, dropping 1,271 tons of bombs. Allied losses were four B-17s and one B-24; a further 236 bombers were damaged by enemy fire.|
|29 March 1944||77 B-24s were sent to attack Watten but equipment malfunctions and navigational problems meant that only 31 aircraft succeeded in bombing the target.|
|6 April 1944||Five B-24 Liberator groups of the USAAF 2nd Bombardment Division carried out an attack against Watten but bad weather prevented all but 12 aircraft from carrying out their attack.|
|18 April 1944||USAAF heavy bombers attacked Watten.|
|19 April 1944||27 B-24s attacked Watten during the afternoon.|
|1 May 1944||More than 500 USAAF heavy bombers were sent to attack V-weapons sites in the Pas-de-Calais, but bad weather forced most to abort. 129 succeeded in attacking Watten and Mimoyecques.|
|30 May 1944||USAAF heavy bombers attacked Watten and Siracourt.|
|18 June 1944||Mission 421: 58 B-17s bombed Watten.|
|19 June 1944||No. 617 Squadron RAF attacked Watten with 19 Lancasters led by 2 Mosquitos; 9 Pathfinder Mosquitos of 8 Group provided preliminary marking. However, the weather conditions were too difficult for accurate bombing and the nearest Tallboy impact missed the target by 50 yards (46 m).|
|6 July 1944||314 Halifaxes, 210 Lancasters, 26 Mosquitos, with Leonard Cheshire in a Mustang fighter marking, attacked five V-weapon targets in the Pas-de-Calais, including Watten. The bunker was penetrated and severely damaged by a Tallboy bomb.|
|25 July 1944||81 Lancasters and 11 Mosquitos of 5 and 8 Groups, with “Willie” Tait having succeeded Cheshire marking, attacked Watten and two other launch sites with Tallboy bombs.|
|4 August 1944||The first Operation Aphrodite mission: four BQ-7s (remotely controlled B-17s) loaded with explosives targeted Watten and other V-weapon sites in the Pas-de-Calais area but missed their targets.|
|6 August 1944||Two more BQ-7s were launched against Watten but had little effect|
|16/17 June 1944||236 RAF Lancasters, 149 Halifaxes with target marking by 20 Oboe-equipped Mosquitos attacked V-weapon sites in the Pas-de-Calais, including Watten, which was attacked with Tallboy earthquake bombs for the first time.|
|18/19 June 1944||10 Mosquitos attacked Watten in a period of bad weather. 9 dropped bombs, but the results are unclear. No aircraft were lost.|
The Watten bunker was captured on 4th September 1944 by Canadian troops. It had been evacuated and allowed to partially flood, making a vast area of the bunker inaccessible to the Allies. It was inspected on 10 September 1944 by the French atomic scientist Frédéric Joliot-Curie, accompanied by Sandys. Following the visit, Sandys ordered a Technical Inter-Services Mission under Colonel T.R.B. Sanders to investigate the sites at Mimoyecques, Siracourt, Watten, and Wizernes, collectively known to the Allies as the “Heavy Crossbow” sites. Sanders’ report was submitted to the War Cabinet on 19 March 1945.
The Watten bunker was bombed once more in February 1945 to test the 4,500lb CP/RA Disney bomb, a rocket-assisted munition that was designed to double impact velocity to pierce reinforced concrete structures. The bunker is still in the Forest of Éperlecques, near Saint-Omer in the Pas-de-Calais department. It is open to the public at certain times of the year, although the accessible areas give no indication on just how vast the underground complex under the bunker was. The site’s webpage is at:
Henshall, Philip (2002). Hitler’s V-Weapon Sites. Stroud: Strutton Publishing.
Hogg, Ian V. (1999). German Secret Weapons of the Second World War. London: Greenhill Books.
Richards, Denis. (1994). The Hardest victory. RAF Bomber Command in the Second World War. Hodder and Stoughton.
Terraine, John. (Paperback 2012) The Right of the Line. The Role of the RAF in World War Two
Middlebrook, Martin. Everitt, Chris. (Paperback 2014) The Bomber Command War Diaries: An Operational Reference Book
© Blown Periphery 2019