Grand Slam bomb
The last of Barnes Wallis’ giant bombs was the Grand Slam 22,000 lb weapon, originally known as the Tallboy (Large). At 26.5 feet in length and 46 inches in diameter, the bomb’s dimensions were not dramatically larger than those of Tallboy, but the later bomb had an explosive charge almost twice that of its predecessor.
The job of carrying Grand Slam was allocated to the Lancaster B 1 (Special), just over thirty of which had been supplied to the RAF’s 617 Squadron. The refit process for the airframe was fairly extensive, given the bomb’s dimensions and the fact that it weighed nearly as much as the Lancaster itself. The only defensive armament retained was the tail turret, and the bomb bay doors had to be removed. Since a loaded B1 (Special) was considerably heavier than a standard Lancaster, use of the stronger landing gear designed for the Avro Lincoln was necessary.
Grand Slam was not available until the very last stages of the war in Europe, but even then suitable targets were still to be had. The Bielefeld Viaduct was one example; this massive structure carried rail lines over the River Werre, and had proven to be virtually untouchable by conventional bombing; even a raid on February 22, 1945 using Tallboys had not been able to knock it out. Finally, on March 14, 617 dealt Bielefeld a crippling blow during the course of the first mission with the new bomb. Most of the Lancasters involved were again armed with Tallboys, but two had Grand Slams. One aborted before making to the target, but the other successfully dropped its weapon, which helped destroy hundreds of feet of the viaduct. By the end of the war, 41 Grand Slams had been dropped on targets like the massively reinforced U-Boat pens near Bremen.
The US adopted the bomb as the T-14 / Mk. 110, and conducted trials of the weapon from B-29s. The Superfortress could carry a single weapon in the bomb bay, although the lack of ground clearance mandated the use of a loading pit. A pair of T-14s could be carried externally on underwing pylons. But even the Grand Slam would be dwarfed by the US T12 Cloudmaker, a massive 43,000 lb weapon derived from Wallis’ work and intended for use from the B-36 Peacemaker. A test B-29 dropped the first Cloudmaker in March 1948, and in January of the following year a B-36 had demonstrated its enormous lifting potential by dropping a pair of T12s.
Scale Aviation Modeller August 2007 Review of Hasegawa’s 1/72 Lancaster B Mk I with Tallboy kit