La Coupole (), also known as the Coupole d'Helfaut-Wizernes and originally codenamed Bauvorhaben 21 ('Building Project 21') or Schotterwerk Nordwest (Northwest Gravel Works), is a Second World War bunker complex in the Pas-de-Calais department of northern France, about from Saint-Omer, and some 14.4 kilometers (8.9 miles) south-southeast from the less developed Blockhaus d'Éperlecques V-2 launch installation in the same area. It was built by the forces of Nazi Germany between 1943 and 1944 to serve as a launch base for V-2 rockets directed against London and southern England and is the earliest known precursor to modern underground missile silos still in existence.
Constructed in the side of a disused chalk quarry, the most prominent feature of the complex is an immense concrete dome, to which its modern name refers. It was built above a network of tunnels housing storage areas, launch facilities and crew quarters. The facility was designed to store a large stockpile of V-2s, warheads and fuel and was intended to launch V-2s on an industrial scale. Dozens of missiles a day were to be fuelled, prepared and launched in rapid sequence against London and southern England.
Following repeated heavy bombing by Allied forces during Operation Crossbow, the Germans were unable to complete the construction works and the complex never entered service. It was captured by the Allies in September 1944, partially demolished on the orders of Winston Churchill to prevent its reuse as a military base, and then abandoned. It remained derelict until the mid-1990s, when efforts to preserve the site commenced. In 1997 it opened to the public for the first time, as a museum. Exhibits in the tunnels and under the dome tell the story of the German occupation of France during World War II, the V-weapons and the history of space exploration.
Background The V-2 rocket was one of several innovative long-range weapons developed by the Germans after the failure of the Luftwaffe to strike a decisive blow against Britain. It was a revolutionary weapon—the world's first operational SRBM—that had been developed in a secret programme begun in 1936. The German leadership hoped that a barrage of rockets unleashed against London would force Britain out of the war. Although Adolf Hitler was at first ambivalent, he eventually became an enthusiastic supporter of the V-2 programme as Allied air forces carried out increasingly devastating attacks on German cities.
The 12.5-ton missile, standing high on its launch pad, was fuelled primarily by liquid oxygen (LOX) and ethanol. Deploying the V-2 on a large scale required far more LOX than was available from existing production sites in Germany and the occupied countries. New sources of LOX were required, situated close to the missile launching sites to reduce as far as possible the loss of propellant through evaporation. The missile's operational range of meant that the launch sites had to be fairly close to the English Channel or southern North Sea coasts, in northern France, Belgium or the western Netherlands.
Because of the complexity of the missile and the need for extensive testing prior to launch, the V-2's designers at the Peenemünde Army Research Center favoured using heavily defended fixed sites where the missiles could be stored, armed, and fuelled from an on-site LOX production plant before launching. The German Army and the V-2 project's head, Major-General Walter Dornberger, were concerned that the sites would be vulnerable to aerial attack by the Allies. The Army's preferred option was to use Meillerwagen, mobile firing batteries, which presented a much smaller target for the Allied air forces.
The Army was nonetheless overruled by Hitler, who had a long-standing preference for huge, grandiose constructions. He preferred fixed installations along the lines of the virtually impregnable U-boat pens that had been built to protect Germany's U-boat fleet. In March 1943, he ordered the construction of a massive bunker (now known as the Blockhaus d'Éperlecques) in the Forest of Éperlecques near Watten, north of Saint-Omer. The bunker was soon spotted by Allied reconnaissance, and on 27 August 1943, a raid by 187 Boeing B-17 Flying Fortress bombers wrecked the construction site before it could be completed. A surviving portion was reused by the Germans as a LOX production facility.
Design and location
The successful attack against the Watten bunker forced the German Army to find an alternative location for a launch site nearby. They had already taken possession of an old quarry between the villages of Helfaut and Wizernes, south-west of Saint-Omer and some south of the Watten bunker, near the river Aa alongside the Boulogne–Saint-Omer railway line, about from Wizernes station. The quarry had been designated for use as a missile storage depot where V-2s would be housed in tunnels bored into the chalk hillside before being transported for launching. The Germans undertook major work in August 1943 to lay extensive railway sidings to connect the quarry to the main line.
On 30 September 1943, Hitler met with Albert Speer, the Minister of Armaments and War Production, and Franz Xaver Dorsch, the chief engineer of the Todt Organisation, to discuss plans for a replacement for the out-of-commission Watten facility. Dorsch proposed to transform the Wizernes depot into a vast bomb-proof underground complex that would require a million tons of concrete to build. It would be constructed within a network of tunnels to be dug inside the hillside at the edge of the quarry. A concrete dome, thick, in diameter and weighing 55,000 tons, would be built over the top of the central part of the facility to protect it from Allied bombing. Beneath it, about of tunnels were to be dug into the chalk hillside to accommodate workshops, storerooms, fuel supplies, a LOX manufacturing plant, generators, barracks and a hospital.
A standard gauge railway tunnel, codenamed Ida, was to be built on a curving path that would connect it with both the east- and west-bound main line railway, allowing trains to run straight through the complex without needing to reverse or be turned around. This would serve as the main unloading station, where missiles and supplies would be offloaded onto trolleys that would transport them into the connecting galleries Mathilde and Hugo. Hugo connected in turn with Sophie, a dead-end railway tunnel branching from the main line into Ida. Each of the main tunnels had a number of unnamed side tunnels of the same dimensions as the main tunnels and up to long. The central feature of the complex was a huge octagonal rocket-preparation chamber directly under the dome. It was never completed but would have been in diameter and up to high. A number of intermediate floors, possibly as many as ten, would have been built up the sides of the chamber.
The western side of the chamber opened onto two tall passageways, opening onto two tracks to the outdoor launchpads, with the tracked passages and launchpads named Gustav (the southerly-located pad) and Gretchen (the northerly-located pad), both on the western side of the domed complex. Each was to have been protected by bomb-proof doors made of steel and concrete. The passageways were to be wide and at least high and were angled in a Y-shape, both exiting westwards into the quarry. The outdoor launchpads for the V-2 rockets would have been at the end of each passageway. The two passageways were angled at 64° 50' and 99° 50' west of north respectively—not aligned with any probable target but merely permitting the rockets to be transported to either one or the other, of their pair of sufficiently separated launch pads.
The facility was designed, as was its predecessor at Watten, to receive, process and launch V-2 rockets at a high rate. Trains carrying V-2s would enter the heart of the complex through the Ida rail tunnel, where they would be unloaded. A large number of V-2s could be stored in the side tunnels; liquid oxygen would be produced on-site with five machines and a combined monthly capacity of 1500 tons. When the time came, the rockets would be moved into the octagonal preparation chamber where they would be lifted to a vertical position for fuelling and arming. From there they would be transported on motorised launch carriages, still in a vertical position, through the Gustav and Gretchen passageways. The launch pads were located at the end of the track on the floor of the quarry, from where the missiles would be fired.
The priority target for the V-2s was away: London, which Hitler wanted to see pulverised by the end of 1943. The Allies were alarmed when an analyst found that part of the complex was aligned within half a degree of the Great Circle bearing on New York City, and its equipment was large enough to accommodate a rocket twice the size of the V-2: the "America Rocket", the proposed A10 intercontinental ballistic missile.
Although physically separate, another facility built in nearby Roquetoire was an integral part of the Wizernes complex. Umspannwerk C was built to house a Leitstrahl radio command guidance system which could be used to send course corrections to missiles launched from Wizernes to fine-tune their trajectory during the launch phase.
Construction
The Allies first noticed construction activity at Wizernes in mid-August 1943 when the Germans began building railway track and the offloading stores into the old quarry. After Hitler authorised the decision to turn the depot into a missile launch site, construction was accelerated. Work on the dome began in November 1943 and tunnelling in the cliff face below began in December. At the start of January, Allied reconnaissance aircraft observed an elaborate system of camouflage on the hilltop, installed to conceal the dome. The building works were greatly hindered by the constant air-raid warnings, which stopped work 229 times in May 1944 alone. In response to Hitler's desire to see the site completed the workforce was expanded substantially from 1,100 in April 1944 to nearly 1,400 by June. About 60% of the workers were Germans; skilled workers, such as miners from Westphalia, were recruited to excavate the tunnels and build the dome. The remainder were slave labour: principally Frenchmen conscripted by the Service du travail obligatoire (STO), plus Soviet prisoners of war. The project was overseen by several large German construction companies, with Philipp Holzman A.G. of Frankfurt am Main and the Grossdeutsche Schachtbau and Tiefbohr GmbH serving as the chief contractors.
One of the most difficult challenges faced by the Germans was constructing the great dome while under regular air attack. The dome's designer, Todt Organisation engineer Werner Flos, devised a plan under which the dome would be built first, flat upon the ground, and the soil underneath it would be excavated so that the construction works below would be protected against aerial attacks. A circular trench was excavated on the top of the hill above the quarry to an outside diameter of . The dome was built within this trench and the galleries and octagonal preparation chamber were excavated below."An Engineer Returns ... And A Museum Is Born", After the Battle 57:49–53.