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Known officially as the Bomb, Medium Capacity, 22,000 lb[b], it was a scaled-up version of the Tallboy bomb and closer to the original size that the bombs' inventor, Barnes Wallis, had envisaged when he first developed his earthquake bomb idea. It was also nicknamed "Ten ton Tess".
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. Unlike the Tallboy, the Grand Slam was originally designed to penetrate concrete roofs. Consequently, it was more effective against hardened targets than any existing bomb.
Grand Slam bomb exploding near Arnsberg viaduct 1945
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. The Grand Slam was so heavy that the Lancaster's wingtips bent upwards by six to eight inches (150 to 200 mm). When the bomb was dropped, the plane leapt up 200 to 300 feet (61 to 91 m).
Like the Tallboy, after the hot molten Torpex was poured into the casing, the explosive took a month to cool and set. Therefore, the Grand Slam had a low rate of production and consequent high value for each bomb. As a result, aircrews were told to land with their unused bombs on board rather than jettison them into the sea if a sortie was aborted. If returning with an undropped bomb, the bomber had to divert from Woodhall to Carnaby which had a longer runway.
Grand Slam combat operations
A 617 Sqn Lancaster dropping a Grand Slam bomb on the Arnsberg viaduct, March 1945.
The damage caused by one of the two Grand Slams that exploded in the roof of the Valentin submarine pen, 27 March 1945; a figure stands at the edge of the rubble pile, providing a sense of the scale of the damage
An RAF officer inspects the hole left by a 'Grand Slam' bomb which exploded in the reinforced concrete roof of the German submarine pens at Farge, north of Bremen, Germany. This was the result of a daylight raid by 18 Avro Lancasters of No. 617 Squadron RAF on 27 March 1945. Two direct hits by 'Grand Slams' caused sections of the partially-completed roof to collapse
By the end of the war, 42 Grand Slams had been dropped in active service:
The No. 617 Squadron RAFAvro Lancaster of Squadron Leader CC Calder dropped the first Grand Slam bomb from 11,965 ft (3,647 m) on the Schildesche (or Bielefeld) viaduct.  A large section of the viaduct collapsed  from the earthquake bomb effect of the Grand Slam and Tallboy bombs of No. 617 Squadron. No aircraft were lost. Previously by mid-March over 3,500 tons had been dropped on the viaduct in 54 attacks, but the viaduct was still standing. Damage from 17 hits in one raid was repaired in 24 hours. After this raid, seven arches incorporating 200 feet (61 m) of the northern span and 260 feet (79 m) of the southern span had been obliterated.
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
Nineteen 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. There was a whole school under the arches, and those in it were all suffocated.
Arbergen, 21 March 1945
Twenty 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; two Tallboy hits caused sufficient damage to the approaches to the bridge to put it out of use. One 617 Lancaster was lost.
Twenty Lancasters of No. 617 Squadron, six carrying Grand Slams, the remainder Tallboys, attacked the railway bridge at Nienburg, between Bremen and Hanover. One Grand Slam and two Tallboys made direct hits and the bridge was destroyed. The remaining five Grand Slams were brought home by the squadron.
Twenty 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.
Twenty 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 hit the pen, failing to penetrate a 14 ft 5 inches (4.5 m) thick roof but causing large holes by exploding within the concrete. No aircraft were lost.
Seventeen 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.
Twenty 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.
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 near Bremen, 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 including the rocket-assisted Disney bomb.
^Torpex is 50% more powerful than TNT. Truman described the Little boy bomb (yield > 13 kilotons) against Hiroshima in terms of the Grand Slam: "...more than two thousand times the blast power of the British Grand Slam" (Truman 2008).
^"Medium capacity" refers to the ratio of bomb case to explosive filling; in the case of the Grand Slam, this was less than 50 percent explosive by weight, in contrast to "high capacity" bombs like the Blockbuster bombs, which contained up to three-quarters of their weight in explosive.