Grand Slam (bomb)

Development

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.

On 18 July 1943, work started on a larger version of the Tallboy bomb, which became the Grand Slam. As with the original Tallboy, the Grand Slam's fins generated a stabilizing spin and the bomb had a thicker case than a conventional bomb, which allowed deeper penetration. After the hot molten Torpex was poured into the casing, the explosive took a month to cool and set. Like the Tallboy, because of the low rate of production and consequent high value of each bomb, aircrews were told to land with their unused bombs on board rather than jettison them into the sea if a sortie was aborted.

After release from the Avro Lancaster B.Mk 1 (Special) bomber, the Grand Slam would reach near-supersonic speed, approaching 1,049 ft/s (320 m/s), 715 mph (1150 km/h). When it hit, it would penetrate deep underground before detonating. The resulting explosion could cause the formation of a camouflet (cavern) and shift the ground to undermine a target's foundation.

Unlike Tallboy, Grand Slam was originally designed to penetrate concrete roofs. Consequently, it was more effective against hardened targets than any existing bomb.

The first Grand Slam was tested at the Ashley Range in the New Forest, on 13 March 1945.

Grand Slam combat operations

By the end of the war, 42 Grand Slams had been dropped on active service.

Bielefeld, 14 March 1945

The No. 617 Squadron RAF Avro Lancaster of Squadron Leader CC Calder dropped the first Grand Slam bomb from 11,965 ft (3,647 m) on the Schildesche viaduct. A large section of the Bielefeld viaduct collapsed through the earthquake bomb effect of the Grand Slam and Tallboy bombs of No. 617 Squadron. No aircraft were lost.

Arnsberg, 15 March 1945

Two aircraft of No. 617 Squadron RAF each carried a Grand Slam and 14 aircraft of No. 9 Squadron RAF carried Tallboy bombs to attack the railway viaduct in poor weather. One Grand Slam and 10 Tallboys were dropped, while one of the Lancasters was forced to bring its bomb back. The viaduct was not cut and no aircraft were lost.

Arnsberg, 19 March 1945

19 Lancasters of No. 617 Squadron, six carrying Grand Slams, the remainder Tallboys, attacked the railway viaduct at Arnsberg. All Grand Slams were dropped and blew a 40-foot (12 m) gap in the viaduct. The standing structure was severely damaged.

Arbergen, 21 March 1945

20 Lancasters of No. 617 Squadron, two carrying Grand Slams, the remainder Tallboys, attacked the railway bridge at Arbergen. The Grand Slams landed off target due to heavy flak and aiming problems; 2 Tallboy hits caused sufficient damage to the approaches to the bridge to put it out of use. One 617 Lancaster was lost.

Nienburg, 22 March 1945

20 Lancasters of No. 617 Squadron, six carrying Grand Slams, the remainder Tallboys, attacked the railway bridge at Nienburg, between Bremen and Hanover. 5 Grand Slams made direct hits and the bridge was destroyed. Another 5 bombs were brought home by the squadron.

Bremen, 23 March 1945

20 Lancasters of No. 617 Squadron, six carrying Grand Slams, the remainder Tallboys, attacked a railway bridge near Bremen. The Grand Slams appear to have landed too far from the target, which was brought down by a Tallboy. Author Jon Lake claims instead that two Grand Slams struck the bridge.

Farge, 27 March 1945

20 Lancasters of No. 617 Squadron attacked the Valentin submarine pens, a huge, nearly-ready structure with a concrete roof up to 23 ft (7.2 m) thick. Two Grand Slam bombs penetrated in parts of the pen with a 14 ft 5 inches (4.5 m) thick roof, which rendered the shelter unusable. No aircraft were lost.

Hamburg, 9 April 1945

17 aircraft of No. 617 Squadron, two with Grand Slams and the remainder with Tallboy bombs successfully attacked the U-boat shelters. The Grand Slams appear to have missed, but six Tallboy hits caused considerable damage. No aircraft were lost.

Heligoland, 19 April 1945

20 aircraft of No. 617 Squadron, six with Grand Slams and the remainder with Tallboy bombs, along with 16 aircraft from No. 9 Squadron, attacked coastal gun-batteries. No aircraft were lost.

Post–war operations

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 target selected was the Valentin submarine pens, that had been rendered unusable and abandoned since 617 Squadron's attack on 27 March 1945. Grand Slams were carried by Lancasters from No. 15 Squadron RAF and US Boeing B-29 Superfortress. Around 140 sorties were flown, testing a range of different bombs.

Survivors

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.

A T-14 bomb (an American-made variant of the Grand Slam) is displayed at the Air Force Armament Museum in the United States.