The Bombing of the Major V-Weapon Sites – The Siracourt V1 Bunker

Blown Periphery, Going Postal
Contemporary aerial view of the Siracourt Bunker

The Allies could finally claim air superiority over France and Belgium by 1943. It had been a hard-fought battle that started after the Battle of Britain in the Circus and Rhubarb sweeps over France, the Dieppe landings and constant attacks on German airfields and by now the Allies were joining battle over the enemy’s territory. By June of 1944, they would be able to claim air supremacy over the Germans under certain conditions. Allied fighter bombers roamed over occupied Europe, looking for trouble like feral children in a sink estate.

The departments of the Luftwaffe, which had responsibility for the V-1 Flying Bomb programme were faced with the problem of how best to deploy their new weapons, to avoid destruction by Allied aircraft. The Luftwaffe Flak Division wanted to disperse the V-1s to multiple, well-camouflaged launch sites. General Erhard Milch, head of the production programme backed by Hitler, wanted the construction of a massive bunker where the V-1s could be assembled and launched. Hermann Göring made a compromise where it was proposed that ten heavy launch bunkers and ninety-six light installations would be built.

The heavy bunkers were all intended to be built to a standard design, codenamed Wasserwerk (waterworks) to conceal their true purpose. The first two would be built in the Pas-de-Calais at Desvres near Lottinghen and Siracourt near Saint-Pol-sur-Ternoise. The two sites are about 110 miles and 130 miles from London respectively. Two more would be built on the Cotentin Peninsula near Cherbourg. It was intended that all four would be operational by December 1943, with further bunkers to be built subsequently.

The Siracourt bunker is 705 feet long, 118 feet wide and thirty-three feet high and used some 55,000 cubic metres of concrete in its construction. Lessons had been learned from the Allied destruction of the Watten Bunker while it was still under construction. It was built on high ground north of the Hesdin/Saint Pol road north of the village of Siracourt. It was built into a loamy soil, twenty-five feet deep and rested on a layer of chalk bedrock. The German engineers first built the roof flat on the ground and then excavated beneath it to create the rest of the facility.

The bunker was to be connected to the Saint Pol to Abbeville railway line, to enable trains to bring in V-1s and supplies into the structure. The bunker was basically a reinforced concrete railway tunnel, with storage for 150 flying bombs and eventually, two openings from which they could be launched on parallel ramps.

Blown Periphery, Going Postal
Site plan

Discovery and Destruction

Allied photographic reconnaissance sorties spotted the construction of the Siracourt bunker as soon as building work started in September 1943. Heavy bombing hindered the construction but work continued until the end of June 1944. The site was wrecked by Tallboy and standard high capacity bombs dropped by the RAF. The concrete was about 90% completed and it was though by the Germans that the site was by then bomb proof. The bunker was unable to withstand the six-ton Tallboys. One bomb fully penetrated the roof and exploded underneath, while another caused substantial damage when it exploded next to one of the walls. The ground around the site was churned up by over 5,000 tons of bombs. By the time the site was abandoned in August 1944, the exterior had practically been completed but the excavation of the interior had only just begun.

Air raids on the Siracourt site

January 31, 1944 Mission 203: 74 of 74 B-24’s hit V-weapon site construction at St. Pol/Siracourt, France; 2 aircraft are damaged beyond repair; no losses. The B-24’s are escorted by 114 P-47’s)
February 2, 1944 Mission 205: 95 of 110 B-24s hit V-weapon construction sites at St Pol/Siracourt and Watten, France; 2 B-24s are lost, 1 is damaged beyond repair and 2 damaged; casualties are 10 KIA and 19 MIA. 183 P-47s escort the B-24s without loss.
February 6, 1944 Mission 212: 150 B-24s are dispatched to St Pol/Siracourt V-weapon site but 37 hit Chateaudun Airfield
February 8, 1944 Mission 214: 53 of 54 B-24s hit the V-weapon site at Siracourt
February 11, 1944 Mission 218: 94 of 201 B-24s, including the HEAVEN CAN WAIT II of the 68th Squadron, bomb the Siracourt V-weapon site in France with PFF equipment
February 12, 1944 Mission 220: 97 of 99 B-24s hit the V-weapon site at St Pol/Siracourt, France; 29 B-24s are damaged (the HEAVEN CAN WAIT II crash-landed); no losses or casualties; escort is provided by 84 Eighth and Ninth Air Force P-47’s and 41 P-51s; no claims, losses or casualties.
February 13, 1944 Mission 221 included the 453rd Bombardment Group
February 15, 1944 Mission 223: 52 of 54 B-24s hit V-weapon sites at St Pol/Siracourt, France; 29 B-24s are damaged; no losses or casualties.
March 12, 1944 Mission 256: 46 of 52 B-24s dispatched hit a V-weapon site at St Pol/Siracourt, France and 6 hit targets of opportunity, all using blind-bombing techniques; 1 B-24 is lost and 26 damaged; casualties are 1 WIA.
February 11, 1944 Mission 218: 94 of 201 B-24s, including the HEAVEN CAN WAIT II of the 68th Squadron, bomb the Siracourt V-weapon site in France with PFF equipment
February 12, 1944 Mission 220: 97 of 99 B-24s hit the V-weapon site at St Pol/Siracourt, France; 29 B-24s are damaged (the HEAVEN CAN WAIT II crash-landed); no losses or casualties; escort is provided by 84 Eighth and Ninth Air Force P-47’s and 41 P-51s; no claims, losses or casualties.
February 13, 1944 Mission 221 included the 453rd Bombardment Group
February 15, 1944 Mission 223: 52 of 54 B-24s hit V-weapon sites at St Pol/Siracourt, France; 29 B-24s are damaged; no losses or casualties.
March 12, 1944 Mission 256: 46 of 52 B-24s dispatched hit a V-weapon site at St Pol/Siracourt, France and 6 hit targets of opportunity, all using blind-bombing techniques; 1 B-24 is lost and 26 damaged; casualties are 1 WIA.
April 5, 1944 Mission 288: 21 of 50 B-24s dispatched hit V-weapon sites at St Pol/Siracourt, France without loss; heavy clouds and the failure of blind-bombing equipment cause other B-24s to return to base without bombing. 50 P-47s escort the B-24s without loss.
April 20, 1944 Mission 309 included the 466th Bombardment Group
April 22, 1944 B-17 42-95928 Shot down by flak on a mission to Siracourt V-1 launch site. MACR#4093.
April 27, 1944 Mission 322 included the 466th Bombardment Group
April 30, 1944 Mission 329: 52 of 55 B-24s bomb V-weapon sites at Siracourt; 3 B-24s are damaged; 1 airman is WIA. Escort is provided by 128 P-38s, 268 P-47s and 248 Eighth and Ninth Air Force P-51s
May 1, 1944 A mission included the 401st Bombardment Group
May 2, 1944 Mission 335 included the 453rd Bombardment Group
May 6, 1944 Mission 340: 168 bombers and 185 fighters are dispatched to hit NOBALL (V-weapon) targets in France; 90 B-17s dispatched to the Pas de Calais area return to base with bombs due to cloud cover over the target; 70 of 78 B-24s hit Siracourt; 48 B-17s are damaged. Escort is provided by 57 Ninth Air Force P-38s, 47 P-47s and 81 P-51s without loss.
May 15, 1944 Mission 356: 166 bombers and 104 fighters hit V-weapon sites in France with 1 fighter lost; 38 of 58 B-17s bomb Marquise/Mimoyecques with 5 B-17s damaged; 90 of 108 B-24s bomb Siracourt with 8 B-24s damaged; escort is provided by 104 P-51s with 1 lost (pilot is MIA). One B-24 received a direct hit by an AAA shell. At least one aircraft aborted.
May 21, 1944 Mission 360: 150 bombers and 48 fighters hit V-weapon sites in France without loss; 25 of 40 B-17s hit Marquise/Mimoyecques and 13 B-17s are damaged; 99 of 110 B-24s hit Siracourt and 1 B-24 is damaged. Escort is provided by 48 P-47s without loss.[5] At least one aircraft aborted.
May 22, 1944 [19] Mission 361: 94 of 96 B-24s hit V-weapon sites at Siracourt, France; 1 B-24 is damaged. Escort is provided by 145 P-38s, 95 P-47s and 328 P-51s; P-38s claim 8-1-5 Luftwaffe aircraft, P-47s claim 12-1-2 and P-51s claim 2-2-1; 3 P-38s, 3 P-47s and a P-51 are lost; 1 P-38 and 2 P-47s are damaged beyond repair; 1 P-38, 2 P-47s and a P-51 are damaged; 6 pilots are MIA.
May 30, 1944 Mission 380 included the 447th Bombardment Group
June 21, 1944 Mission 429: In the late afternoon, 31 B-24s bomb CROSSBOW (V-weapon) supply sites at Oisemont/Neuville and Saint-Martin-L’Hortier and 39 bomb a rocket site at Siracourt, France. AA fire shoots down 1 B-24; escort is provided by 99 P-47s, meeting no enemy aircraft, but 1 group strafes railroad and canal targets.
June 22, 1944 234 aircraft – 119 Lancasters, 102 Halifaxes, 13 Mosquitos – of Nos 1, 4, 5 and 8 Groups to special V-weapon sites and stores. The sites at Mimoyecques and Siracourt were accurately bombed by 1 and No 4 Group forces with Pathfinder marking but the No 617 Squadron force attacking Wizernes failed to find its target because of cloud and returned without dropping its bombs. 1 Halifax lost from the Siracourt raid.
June 25, 1944 323 aircraft – 202 Halifaxes, 106 Lancasters, 15 Mosquitos – of Nos 1, 4, 6 and No 8 Group attacked 3 flying bomb sites. The weather was clear and it was believed that all 3 raids were accurate. 2 Halifaxes of No 4 Group were lost from the raid on the Montorgueil site. No 617 Squadron sent 17 Lancasters, 2 Mosquitos and 1 Mustang to bomb the Siracourt flying-bomb store.
June 29, 1944 286 Lancasters and 19 Mosquitos of Nos 1, 5 and 8 Groups attacked 2 flying-bomb launching sites and a store. There was partial cloud cover over all the targets; some bombing was accurate but some was scattered. 5 aircraft – 3 Lancasters and 2 Mosquitos – lost, including the aircraft of the Master Bomber on the raid to the Siracourt site.
July 6, 1944 551 aircraft – 314 Halifaxes, 210 Lancasters, 26 Mosquitos, 1 Mustang (Group-Captain Leonard Cheshire’s marker aircraft) – attacked 5 V-weapon targets. Only 1 aircraft was lost, a No 6 Group Halifax from an on Siracourt flying-bomb store. Four of the targets were clear of cloud and were believed to have been bombed accurately but no results were seen at the Forêt de Croc launching site. Three Tallboy bomb hits are claimed, but the post-war Sanders Report indicates no direct hits.
August 1, 1944 No. 617 Squadron RAF.  No details.
August 4, 1944 Mission 515: The first Operation Aphrodite mission is flown using 4 radio-controlled war weary B-17s as flying bombs. The B-17G 42-39835 Wantta Spa pilot was killed, and the Siracourt drone had control problems and crashed in a wood at Sudbourne.

The Siracourt bunker is nowhere near the scale of the other V-weapon heavy sites, but is an impressive structure nonetheless. It is still visible from the Hesdin/Saint Pol road. It is located on private land and not routinely open to the public.

Notes on precision attacks on bunkers

The Bomb

In order to destroy these huge reinforced concrete and steel bunkers, the Allies needed to meet three requirement. The first was to have a suitable bomb that was able to penetrate the structure and explode inside, or cause catastrophic damage with a near-miss. Barnes Wallis presented his ideas for a ten-ton bomb in 1941 in a paper: A Note on a Method of Attacking the Axis Powers, which showed that a very large bomb exploding deep underground next to a target would transmit the shock into the foundations of the target, particularly since shock waves are transmitted through the ground more strongly than through air.

To be able to penetrate the earth or hardened targets without breaking apart, the casing of the Tallboy had to be strong. Each was cast in one piece of high-tensile steel that would enable it to survive the impact before detonation. At the same time, to achieve the penetration required, Wallis designed the Tallboy to be very aerodynamic so that, when dropped from a great height, it would reach a much higher terminal velocity than traditional bomb designs.

In the final design, the No. 78 Mark I tail of the bomb was about half the overall length of the finished weapon; the bomb casing was some 10 feet of the overall 21-foot length. Initially, the bomb had a tendency to tumble, so the tail was modified: the fins were given a slight twist so that the bomb spun as it fell. The gyroscopic effect thus generated stopped the pitching and yawing, improving both aerodynamics and accuracy.

The ‘earthquake’ effect often caused more damage than even a direct hit that penetrated the armour of a target, since even a burst inside a bunker would only damage the immediate surroundings, with the blast dissipating rapidly through the air. An earthquake impact shook the whole target, and caused major structural damage to all parts of it, making repair uneconomic.

An alternative technique was to arrange detonation depth so that the crater broke the surface which was useful for attacking railway marshalling yards and similar targets. The Tallboy produced an 80 foot crater with depths up to 100 feet, unlike conventional bombs which would produce many shallow craters across a target, each one of which could later be filled in rapidly with earth-moving equipment. Such a huge hole was time-consuming to fill; multiple trucks and bulldozers could not be fitted around the periphery of the hole to speed the process.

Tallboy was designed to be dropped from an optimal altitude of 18,000 feet at a forward speed of 170 mph, hitting at 750 mph. It made a crater 80 foot deep and 100 feet across and could go through 16 feet of reinforced concrete. The design and production of the 12,000 lb Tallboy was done without a contract on the initiative of the Ministry, following Wallis’ 1942 paper Spherical Bomb—Surface Torpedo and the design of the “bouncing bomb” for the Dam Busters of Operation Chastise. The front casing was painted a reflective green so that the fall of the weapon could be spotted all the way to the target.

An aircraft capable of carrying the bomb

The two other heavy bombers in the RAF’s inventory had bomb bay arrangements that precluded the carriage of the 21 foot long Tallboy. The Halifax lacked the lifting power of the Lancaster and could not gain the required altitude for delivering the weapon. The Stirling’s load was split between the shallow fuselage bomb bay and six bomb cells in the wings. However the aircrafts’ bomb loads caused severe disruption the infrastructure surrounding the sites and arguably the Halifax was a more versatile aircraft, able to drop paratroops, dispatch agents, support SOE operations, conduct maritime reconnaissance and tow gliders.

Early on, Air Chief Marshal Arthur Harris, head of Bomber Command, was scathing in his criticism of the Halifax’s performance in comparison to the new Avro Lancaster, primarily of its bomb-carrying capability: an average Halifax was calculated to drop 100 tons of bombs in its lifetime, compared to a Lancaster’s 154. Harris continued to have a poor opinion of the Halifax, despite the fact that later Hercules-engined machines had lower loss rates and higher crew survival rates after abandoning the aircraft than Lancasters, and came very close to the Lancaster’s speed and altitude performance. Unlike the Lancaster, the Halifax’s bomb bay could not be adapted to carry the 4,000 pound “Cookie” blast bomb, which was an integral part of Harris’s fire-bombing tactics. The Halifax was progressively outnumbered in front-line service over occupied Europe as more Lancasters became available from 1943 onwards; many squadrons converted to the Lancaster. Only the Lancaster bomber could carry the Tallboy and later the ten-ton Grand Slam. Much of the protective armour was stripped out of the aircraft and in the case of the Grand Slam, the mid-upper turret was removed and the front turret faired over. The Tallboy could be carried some distance by the Lancaster, such as the flight from Scotland to northern Norway to sink the Battleship Tirpitz.

Blown Periphery, Going Postal
A Lancaster of No 617 Squadron and Tallboy.  Note the enlarged bomb-bay doors.  Lancasters that carried the 22,000lb Grand Slam dispensed with the bomb-bay doors, the weapon underslung under the fuselage

An accurate bomb sight

Development had begun before the War as the Automatic Bomb Sight, but early bomber operations proved that systems without stabilization of the bomb-sight crosshairs were extremely difficult to use under operational conditions. A stabilizer for the ABS began development, but to fill the immediate need for a new bomb-sight, the simpler Mark XIV bomb site was introduced. By the time the Stabilised Automatic Bomb Site (SABS) was available, the Mark XIV was in widespread use and proving good enough so there was no pressing need to replace it. But while the Mk XIV may have been all right for bombing cities, a far more accurate device was needed to bomb the Crossbow sites. The US Norden bomb-sight was rejected, primarily because of the reluctance of the Americans to release it to the British, its autopilot function was considered unnecessary, its perceived over-complexity and that the Americans were rather over-egging its accuracy.

During the 1930s, advances in mechanical computers introduced an entirely new way to solve the bombsight problem. These sorts of computers were initially introduced for naval uses around the turn of the 20th century, later examples being the Admiralty Fire Control Table, Range keeper and Torpedo Data Computer. Fed a variety of inputs such as the angle to the target and its estimated speed, these systems calculated the future position of the target, the time that the ordinance would take to reach it, and from this, the angles to aim the guns in order to hit the target based on those numbers. They used a system of iterative improvements for the estimated values to calculate any measure that could not be made directly.

This system of progressive estimation was easily adapted to the bomb-sight role. In this case, the unknown measurement is not the target’s speed or heading, but the bomber’s movement due to the wind. To measure this, the bomb aimer first dials in estimates of the wind speed and direction, which causes the computer to begin moving the bomb-sights to stay pointed at the target as the bomber moved toward it. If the estimates were correct the target would remain still in the sights, but if the sights moved away from the target, or drifted, the estimates for wind speed and direction were updated until the drift was eliminated.

The SABS had briefly seen use with the Pathfinder Force before being turned over to No. 617 Squadron, starting in November 1943. The squadron’s Avro Lancasters were undergoing conversion to dropping the Tallboy bomb as a precision weapon, and required the higher accuracy of the SABS for these missions. In this role the SABS demonstrated superb accuracy, routinely placing bombs within 100 yards of their targets when dropped from about 15,000 feet altitude. Bomber Command’s other specialist precision unit, No 9 Squadron achieved good results with an improved XIV bomb site, due to a high degree of training and achieved a direct hit and near miss on the Tirpitz with Tallboy bombs.

The system remained hand built throughout its history and was produced in small numbers. In the end, the 617 would also be the only squadron to see operational use of the SABS, using it with the Tallboy and the larger 22,000 pounds Grand Slam. Some Avro Lincolns intended for operations over Japan also featured the SABS, but saw no operational use. The SABS was the most accurate bomb site in use during the Second World War.

Blown Periphery, Going Postal
The rather complex layout of the SABS

 

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