Grand Slam (bomb)

Medium Capacity (M.C.) bombs were designed to address the shortcomings of General Purpose (G.P.) bombs, which had a charge-to-weight ratio of about 27 per cent (contemporary German bombs had a ratio of fifty per cent). M.C. bombs were to have a charge-to-weight ratio of at least forty per cent and use explosives of greater power, although shortages led often to inferior explosive types being used. High Capacity (H.C.) bombs had a charge-to-weight ratio of up to 75 per cent. M.C. bombs had greater blast effect than G.P. bombs but had casings which were robust enough to confer a considerable capacity to penetrate.[7]

Grand Slam bomb casings awaiting delivery

On 18 July 1943, work started on a larger version of the Tallboy bomb, which became the Grand Slam.[8] As with the earlier Tallboy, the fins of the Grand Slam generated a stabilising spin.[9] The bomb had a thicker case than a G.P. bomb, which allowed deeper penetration and the Grand Slam had a charge-to-weight ratio of nearly fifty per cent.[7] The Grand Slam was so heavy that in the air, the wing tips of the Lancaster bent upwards by 6–8 in (150–200 mm) until the bomb was released; the aircraft then leapt 200–300 ft (61–91 m).[10] After release, the Grand Slam would reach a near-supersonic speed of 1,049 ft/s (320 m/s) and would penetrate deep underground before detonating.[11] The explosion could form a camouflet (cavern) and shift the ground, undermining the foundations of the target.[12] The first Grand Slam was tested at the Ashley Range in the New Forest, on 13 March 1945 and left a hole 30 ft (9.1 m) deep with a diameter of 124 ft (38 m).[2] The first successful Grand Slam operation was flown the next day.[13]

A modified Avro Lancaster B.Mk 1 (Special) bomber was designed for the Grand Slam, equipped with Merlin 24 engines, minus front and mid-upper turrets and the crew reduced to five; the bomb bay doors were removed and a stronger undercarriage installed.[14] Like the Tallboy, after hot molten Torpex was poured into the casing, the explosive took a month to cool and set. Aircrews were told to land with an unused bomb on board rather than jettison them into the sea if a sortie was aborted.[15] The commander of 617 Squadron, John "Johnny" Fauquier gave orders that if returning with a bomb, an aircraft would divert from RAF Woodhall Spa in Lincolnshire, and use the longer runway at RAF Carnaby near the coast at Bridlington in East Yorkshire.[16]

By mid-March 1945, over 3,500 long tons (3,600 t) had been dropped on the Bielefeld viaduct in 54 attacks and damage from 17 hits in one raid was repaired in 24 hours. After abortive attempts on 9 and 13 March, on the next day, fifteen Lancasters of 617 Squadron carrying 14 Tallboys and a Grand Slam fuzed for an 11-second delay, returned. A Mosquito of 627 Squadron was present to film the attack along with four Oboe Mosquitoes of 8 (Pathfinder Force) Group to mark the target, escorted by eight squadrons of P-51 Mustangs. The cloud base was at 13,000 ft (4,000 m) and the air was quite hazy; at the viaduct, cloud cover at the north end precluded a north-to-south bombing run. The Lancaster carrying the Grand Slam, flown by Squadron Leader C. C. Calder, made a run from the south and dropped the Grand Slam bomb from 11,965 ft (3,647 m), a height far from ideal, at 4:28 p.m.. The Lancaster rose suddenly 500 ft (150 m) as the bomb left the aircraft. The pilot of the filming aircraft, which recorded the attack from 4:15 to 4:35 p.m., called that the bomb had destroyed the viaduct. The effect of the Grand Slam could not be distinguished from the eleven Tallboys but photographic reconnaissance later showed that 200 ft (61 m) of the north viaduct and 260 ft (79 m) of the south viaduct had been demolished.[17][lower-alpha 2]

A 617 Squadron Lancaster dropping a Grand Slam bomb on the Arnsberg viaduct, March 1945.

On 15 March, in poor weather, two aircraft of 617 Squadron carried Grand Slams with 14 Lancasters of 9 Squadron carrying Tallboys to attack the railway viaduct at Arnsberg. The 425 ft (130 m)-long viaduct, built from brick and stone faced with concrete, crossed the Ruhr in five spans. A Grand Slam was dropped from 13,000 ft (3,962 m) at 4:56 p.m. on the fourth attempt and the second bomb was brought home because of cloud cover. Ten Tallboys were also dropped with no effect and no aircraft were lost.[19] On 19 March, 19 Lancasters of 617 Squadron, six with Grand Slams and 13 with Tallboys, attacked the viaduct again. A photographic aircraft from the Australian 463 Squadron filmed the Grand Slams falling and showed one hitting the west end of the viaduct. A Grand Slam hit the side of the bridge and two dropped slightly short of the aiming point, one Lancaster tried to bomb twice but the release mechanism failed. Two spans of a length of about 100 ft (30 m) were brought down into the water and the embankment was destroyed for about 115 ft (35 m) and the rail lines severely damaged; after the war one Grand Slam casing was found at the site, thought to have landed flat on the road, only the filling at the rear of the casing having exploded. The crews had been briefed to avoid a hospital near the viaduct and managed to do so.[20]

Grand Slam bomb exploding near Arnsberg viaduct, 1945

Twenty Lancasters of 617 Squadron, two carrying Grand Slams and the rest Tallboys, flew in clear weather to Bremen to attack a railway bridge which crossed the Weser near Nienburg. The first Grand Slam was dropped from 13,000 ft (4,000 m), landing 30 yd (27 m) short and the second fell 200 yd (180 m) off target to the north, due to Flak (anti-aircraft fire) and aiming problems. The Tallboys hit the middle of the bridge and the ends were blown off their piers, the pier to the east collapsed onto the ground and the western pier was twisted and sagged to the ground at one place; part of the railway track, above the embankment and the first pier on the west side, was wrecked. One Lancaster was shot down near Okel and left a crater 33 ft (10 m) deep; five Lancasters were damaged by Flak and an attack by a Messerschmitt Me 262 jet fighter.[21]

Twenty Lancasters of 617 Squadron, six carrying Grand Slams fuzed for 25–30 seconds' delay and 14 carried Tallboys with one-hour delay fuzes, attacked the railway bridge at Nienburg, between Bremen and Hanover in atmospheric conditions ideal for bombing. One Lancaster crew claimed a near miss with a Grand Slam and another crew claimed a hit. A third Lancaster crew found that their bomb would not fall off at the first two attempts and the bridge collapsed before the third, the crew bringing back the bomb. A fourth Grand Slam was reported to have hit the east end of the structure. Reconnaissance photographs showed that the bridge had been destroyed.[22]

The Grand Slam Lancasters of 617 Squadron were fitted with Avro Lincoln undercarriages, with tyres at 80 psi (550 kPa), double that of Lancaster tires. Another railway Bridge near Bremen was attacked later in the day by twenty 617 Squadron Lancasters, six carrying Grand Slams and 14 with Tallboys in weather that was as good for the Bremen anti-aircraft defences as it was for bombing. Lancaster NG489 was hit by Flak and the crew jettisoned the Grand Slam to regain control. Several other Lancasters were hit by anti-aircraft fire and four others were attacked by Me 262s, fifteen of which were seen on the flight to the target. Crews reported three hits and two near-misses to the south and the bridge was brought down by a Tallboy.[23][lower-alpha 3]

Damage caused by a Grand Slam on the Valentin submarine pen, 27 March 1945; a figure stands at the edge of the rubble

An RAF officer inspects the hole left by a Grand Slam in the reinforced concrete roof

Twenty Lancasters of 617 Squadron attacked the Valentin submarine pens (Valentin was the code name for Vegesack) at Farge, 5 mi (8 km) north of Bremen-Vegesack. The pen was intended to be the assembly plant for the Type XXI submarine and work had commenced in 1942. The plant was 1,370 ft (418 m) long and 315 ft (96 m) wide, with 96 ft (29 m) of the structure above ground and 41 ft (12 m) below. The roof was made of reinforced concrete with arched trusses on the walls, filled with concrete. Twenty Lancasters, thirteen with Grand Slams, seven with Tallboys fuzed for one hour, were escorted by eight RAF Mustang squadrons to the target. The weather was good over Farge, with slight Flak and no fighter opposition. One Lancaster returned soon after take-off and another turned back over the target with engine-trouble, ditching the Grand Slam in the North sea. Two of the fourteen hits on the pens were by Grand Slams, which landed near the middle of the roof and exploded 5–6 ft (1.5–1.8 m) deep in the concrete, each causing about 800 long tons (810 t) of the roof to fall in and leaving large holes; no aircraft were lost.[25]

Number 617 Squadron dispatched 17 Lancasters to bomb the U-boat pens at Hamburg, two with Grand Slams and the rest with Tallboys. The formation was escorted by Spitfire and Mustang fighters. Luftwaffe fighters, including jets, attempted to intercept the formation. Five hits were claimed but the Grand Slams appear to have missed; reconnaissance photographs revealed seven hits, four through the roof. Many buildings to the north and west of the pens were destroyed or damaged. Six Lancasters were hit by Flak but no aircraft were lost.[26]

An attack on coastal gun-batteries on Heligoland and Düne in the Heligoland Bight, the south-eastern extremity of the North Sea, was planned by the RAF. The military installations on the main island comprised a radar installation covering the Elbe and Weser rivers, an airfield and a coastal battery each of 12 in (305 mm) and 6 in (152 mm) guns. To ensure that Allied ships could enter the Elbe and Weser estuaries, Bomber Command intended to attack the island with 1,000 lb (450 kg) bombs but at least forty heavy anti-aircraft guns on the islands had to be silenced. The gun positions had a diameter of about 60 ft (18 m) and only hits or near misses could damage them. On 18 April 953 bombers attacked the anti-aircraft guns and achieved some hits. The next day, twenty Lancasters of 617 Squadron, six carrying Grand Slams and the rest Tallboys, along with 16 aircraft from 9 Squadron, escorted by six squadrons of Spitfires and one of Mustangs, attacked the islands and achieved more hits; no aircraft were lost.[27]

In March 156 day sorties were flown, 31 with Grand Slams and 40 with Tallboys. An analysis of the bombing accuracy of 617 Squadron and 9 Squadron on Tallboy sorties. The squadrons used different bomb sights and bombed from heights between 9,000 and 17,000 ft (2,700 and 5,200 m) and the comparison showed that 1 per cent of the Tallboys dropped by 617 Squadron were gross errors, defined as missing the aiming point by more than 400 yd (370 m) against 10 per cent of the Tallboys dropped by 9 Squadron.[28] Unlike the Tallboy, the Grand Slam was designed to penetrate concrete roofs and was more effective against fortifications than earlier bombs.[29] By the end of the war, 41 Grand Slams had been dropped on operations.[18][lower-alpha 4]

When the success [of the Tallboy bomb] was proved, Wallis designed a yet more powerful weapon… This 22,000 lb bomb did not reach us before the spring of 1945, when we used it with great effect against viaducts or railways leading to the Ruhr and also against several U-boat shelters. If it had been necessary, it would have been used against underground factories, and preparations for attacking some of these were well advanced when the war ended.

— Sir Arthur Harris (1947).[31]

Beginning in March 1946, Project Ruby was a joint Anglo–American project to investigate the use of penetration bombs against heavily protected, concrete targets. The Valentin submarine pens near Bremen were chosen as a target, having become unusable and abandoned since the attack on 27 March 1945. Grand Slams were carried by Lancasters from 15 Squadron and US B-29 bombers. Around 140 sorties were flown, testing bombs including the rocket-assisted Disney bomb.[32]

A Grand Slam bomb at the RAF Museum, London

Five complete Grand Slam bombs are preserved and displayed in the United Kingdom at the RAF Museum, London; Brooklands Museum; RAF Lossiemouth; Dumfries and Galloway Aviation Museum and the Battle of Britain Memorial Flight Visitors' Centre at RAF Coningsby. Main portions of these bombs, without their lightweight tails, can be seen at the Kelham Island Museum in Sheffield and Yorkshire Air Museum at Elvington. The T-12 Cloudmaker is an American-made variant of the Grand Slam; an example is displayed at the Air Force Armament Museum in the United States.[3]