The Bombing of the Major V-Weapon Sites – The Fortress of Mimoyecques

Blown Periphery, Going Postal
Plan of the one of the Mimoyecques drifts

There were a multitude of Crossbow V-Weapon sites throughout northern France and most were attacked from the air and later overrun by the advancing Allied armies. The light bombers of the Second Tactical Air Force were adept at attacking the V-2 launch sites, the so-called ski-jumps, particularly the Mosquitos that went in at low level. But these launch sites were well defended and losses in aircraft and crews were high. What particularly irked the members of Bomber Command was that operations over France only counted as half-an-op in the tally for a thirty op tour. It would have to be the heavy bombers with their larger, deep penetration bombs that would destroy the fourth and perhaps the most sinister of the major V-Weapon sites, the underground fortress of Mimoyecques.

In May 1943 Hitler’s Armaments Minister, Albert Speer informed the Fuhrer that work was underway to produce a supergun, that would be capable of firing hundreds of projectiles an hour over long distances. The gun was codenamed the Hochdruckpumpe (“High Pressure Pump,” HDP for short) and later designated as the V-3. Long range guns were not new developments, large guns bombarded Paris in the Franco Prussian war, but their disadvantage was excessive barrel wear. In 1942 August Coenders suggested a gradual acceleration of the projectile down the gun’s barrel, by a series of small charges spread along its length. Coenders suggested the charges should be electrically activated to prevent premature ignition of the booster charges, a problem with previous experimental designs.

The HDP’s barrel would be 330 feet long, 5.9 “ in diameter and fire a 214lb shell called the Sprenggranate 448. Hitler was very enthusiastic about the idea and ordered full support for its development. In August 1943, he approved the construction of a battery of HDP guns in the Pas de Calais to supplement the V-1 and V-2s for use against London and the south east of England.

It was found that in order to reach west London the weapon needed barrels 417 feet long, so the weapon could not be mobile. Studies showed the optimum location would be in a hill with a rock core, within which drifts could be tunnelled to support the barrels. German engineers identified a limestone hill near the hamlet of Mimoyecques, which was 103 miles from London. The hill was primarily chalk extending several hundred feet below the surface, to a deep rock layer. The terrain could be mined and tunnelled without using timber supports and the galleries would be reinforced with concrete. The site was in a heavily defended area, protected by the Atlantic Wall

Construction began with a railway line to support the work and tunnelling began in October 1943. The initial layout comprised two parallel complexes approximately 1,000 yards apart, each with five drifts which were to hold a stacked cluster of five HDP gun tubes, for a total of twenty-five guns. The smoothbore design of the HDP would enable a much higher rate of fire than was possible with conventional guns. The entire battery would be able to fire up to ten shots a minute, capable in theory of hitting London with 600 projectiles every hour. Both facilities were to be served by an underground railway tunnel of standard gauge, connected to the Calais-Boulogne main line and underground ammunition storage galleries which were tunnelled at a depth of about 108 feet. The western site was abandoned at an early stage after being disrupted by Allied bombing, and only the eastern complex was built.

The drifts were angled at fifty degrees and reached a depth of 344 feet. The drifts came to the surface and were protected by a concrete slab eighteen feet thick with a small opening, through which the projectile fired. The slabs were further protected by sliding steel plates and the railway tunnel openings were also protected by steel doors. Each drift was orientated on a bearing of 299 degrees, a direct line to Westminster Bridge. The range could be altered by varying the amount of propellant in the booster charges, so that a swathe of destruction could be cut through London.

Blown Periphery, Going Postal
Side elevation and plan of the Mimoyecques complex

5,000 workers, mostly German engineers were used in the site’s construction, with the usual quota of Soviet POWs as slave labour. The intensive Allied bombing campaign caused delays, but construction work continued nonetheless at a high pace underground. The original plans had envisaged having the first battery of five guns ready by March 1944 and the full complement of twenty-five guns by 1st October 1944, but these target dates were not met.

Discovery and Destruction

Blown Periphery, Going Postal
Halifax over the Mimoyecques complex, while bombs burst below

In 1943 French agents reported that the Germans were about to mount an offensive, using secret weapons that resembled giant mortars, sunk into the ground and served by a railway network. This was quite specific intelligence and reconnaissance photographs showed railway activity, including loop lines in the Mimoyecques area. An analyst named André Kenny discovered a series of shafts when he saw from a reconnaissance photograph that a haystack concealing one of them had disintegrated, perhaps through the effects of a gale, revealing the entrance, a windlass and pulley. The purpose of the site was unclear, but it was thought to be some kind of shelter for launching rockets or flying bombs. An MI6 agent reported that “a concrete chamber was to be built near one of the tunnels for the installation of a tube, 150 feet long, which he referred to as a ‘rocket launching cannon.” The shafts were interpreted as “air holes to allow for the expansion of the gases released by the explosion of the launching charge.”

The Allies being unaware of the HDP gun, thought the site was for launching V-2s through “projector tubes.” The Crossbow Committee was frustrated by the lack of intelligence coming from these major engineering sites, probably because the workforce was largely German or Soviet slave labourers. The Committee’s head, Duncan Sandys, pressed for greater efforts and proposed that the Special Operations Executive be tasked to kidnap a German technician who could be interrogated for information. The suggestion was approved, but was never put into effect. In the end the Chiefs of Staff instructed General Eisenhower to begin intensive attacks on the so-called “Heavy Crossbow” sites, including Mimoyecques, which was still believed to be intended for use as a rocket-launching site

Several bombing raids were carried out on the Mimoyecques site between November 1943 and June 1944, but these caused very little significant damage. It raids did cause some disruption and delayed work by about a month. On 6th July 1944 the RAF began bombing the site with ground-penetrating Tallboy bombs. One Tallboy hit the concrete slab on top of Drift IV, collapsing the drift. Three others penetrated the tunnels below and substantially damaged the facility, causing several of the galleries to collapse in places. By any measure, this was an outstandingly accurate example of precision bombing. Group Captain Cheshire circled the top of the drift in a Mustang fighter at low level and instructed the bomb aimers to aim at the wing tip of his aircraft.

Around 300 Germans and forced labourers were buried alive by the collapses. Adding to the Germans’ difficulties, major technical problems were discovered with the HDP gun projectiles. They had been designed to exit the barrels at a speed of about 4,900 feet per second, but the Germans found that a design fault caused the projectiles to begin “tumbling” in flight at speeds above 3,300 feet per second, causing them to fall well short of the target. This was not discovered until over 20,000 projectiles had already been manufactured.

Following the RAF’s devastating raid, Hitler ordered major changes to the site’s development, reducing the number of HDP guns to five, mounted in a single drift. The two others would mount conventional Krupp K5 artillery pieces with a 12” bore. Unaware of this the Allies mounted several more air attacks on the site as part of the USAAF experimental Operation Aphrodite, involving radio-controlled B-24 Liberators packed with explosives. Two such attacks were mounted but failed; in the second such attack, on 12 August, Lt Joseph P. Kennedy, Jr. – the elder brother of future US President John F. Kennedy – was killed when the drone aircraft exploded prematurely. By the end of the bombing campaign, over 4,100 tons of bombs had been dropped on Mimoyecques, more than on any other V-weapons site. The site was captured by the Canadian 3rd Infantry Division on 5th September 1944.

Air raids on the Mimoyecques site

5 November 1943 More than 150 B-26 Marauders of the USAAF Ninth Air Force bombed “construction works” at Mimoyecques, but poor visibility and bad weather caused one group to miss the primary target and numerous other aircraft to abort their attacks.
8 November 1943 No. 2 Group of the RAF Second Tactical Air Force attacked Mimoyecques with three waves each of 24 Douglas Boston medium bombers. The first two waves (24 aircraft of Nos. 88 and 342 Squadrons and 24 of Nos. 98 and 180 Squadrons) reported bursts on the railway, in the target area, and on the tunnel entrance. The third wave (19 aircraft of Nos. 226 and 305 Squadrons) reported bomb bursts near the north aiming point. Six North American Mitchells of No. 320 Squadron also attacked. In the course of the raid one Mitchell was shot down and 12 bombers were damaged by flak.
19 March 1944 173 B-17 Flying Fortresses of the Ninth Air Force 1st and 3d Bombardment Divisions, escorted by P-47 Thunderbolts, bombed V-weapons sites including Mimoyecques.
26 March 1944 500 heavy bombers of the USAAF Eighth Air Force attacked a total of 16 V-weapon sites in northern France, including Mimoyecques, dropping 1,271 tons of bombs. Allied losses were four B-17s and one B-24 Liberator; a further 236 bombers were damaged by enemy fire.
10 April 1944 Bombers of the USAAF Eighth Air Force attacked Mimoyecques.
26 March 1944 500 heavy bombers of the USAAF Eighth Air Force attacked a total of 16 V-weapon sites in northern France, including Mimoyecques, dropping 1,271 tons of bombs. Allied losses were four B-17s and one B-24 Liberator; a further 236 bombers were damaged by enemy fire.
10 April 1944 Bombers of the USAAF Eighth Air Force attacked Mimoyecques.
22 June 1944 234 aircraft – 119 Avro Lancasters, 102 Handley Page Halifaxes and 13 de Havilland Mosquitos – of Nos. 1, 4, 5 and 8 Groups attacked a number of V-weapon sites and stores, bombing the sites at Mimoyecques and Siracourt with the aid of Pathfinder marking.
27 June 1944 104 Halifaxes from No. 4 Group with 5 Mosquitos and 2 Lancasters of the Pathfinders attacked Mimoyecques in good weather without sustaining any losses.
6 July 1944 No. 617 Squadron RAF attacked with Tallboys. The target was marked by the squadron’s commanding officer, Leonard Cheshire, from his Mustang III fighter.  The bombers hit one of the shafts with a Tallboy that bored into the earth and exploded underground, leaving an enormous crater.  This was Cheshire’s last operation leading No. 617 Squadron, completing his 100th mission.
4 August 1944 The first Operation Aphrodite mission was flown using four radio-controlled B-17s as flying bombs. None of the targets, including Mimoyecques, were hit.
12 August 1944 An Operation Anvil drone targeted on Mimoyecques detonated prematurely over the Blyth Estuary, killing the US Navy crew, Lieutenants Joseph P. Kennedy, Jr. and Wilford J. Willy.
27 August 1944 176 Halifaxes, 40 Lancasters and 10 Mosquitos attacked Mimoyecques without loss.

 

In September 1944, Duncan Sandys ordered the constitution of a Technical Inter-Services Mission under Colonel T.R.B. Sanders. It was given the task of investigating the V-weapons 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 Allies were still unclear as to the site’s use, but Sanders’ investigation brought to light the V-3 project for the first time, to the alarm of the British government. He concluded that although the site had been damaged it “could be completed or adapted for offensive action against this country at some future date, and [its] destruction is a matter of importance.” Sandys brought the matter to the attention of Churchill and advised: “Since this installation constitutes a potential threat to London, it would be wise to ensure that it is demolished whilst our forces are still in France.” Churchill later commented that the V-3 installation at Mimoyecques “might well have launched the most devastating attack of all on London.”

The Royal Engineers were tasked with the destruction of the site and stacked ten tons of explosive in the drifts and detonated them on 9th May 1945. This attempt failed and it required a further 25 tons of explosives to bring down the tunnelled drifts, but a subsequent investigation by the British Bombing Research Mission concluded that the entrances had been heavily blocked and that it would be a very difficult and lengthy engineering task to reinstate them. The British action was taken without informing the French beforehand and infuriated Charles de Gaulle, who considered it a violation of France’s national sovereignty. Oh dear.

The site was a museum but it closed at the end of the 2008 season when the owner retired. Subsequently, the non-profit organisation Conservatoire d’espaces naturels du Nord et Pas-de-Calais (Conservatory of natural sites of the Nord and Pas-de-Calais) purchased it at a cost of €330,000 and it is open at certain times of the year.

THE MIMOYECQUES FORTRESS SECRET BASE V3

Blown Periphery, Going Postal
Mock-up of a Hochdruckpumpe barrel in a surviving drift

 

© Blown Periphery 2019
 

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