The Grand Slam (Earthquake) bomb was developed by Barnes Wallis in late 1944. It weighed nearly ten tonnes and was developed primarily in order to destroy large structures against which earlier, smaller bombs proved ineffective.

Most large Allied World War II aircraft bombs had very thin skins to maximise the weight of explosive which a bomber could carry. Barnes Wallis developed the Tallboy bomb as a bunker buster. When it was calculated that Avro Lancaster bombers could be modified (the 'Special B' Lancaster) to carry an even larger bomb, he redesigned it and managed to produced a much larger one. The design was very aerodynamic with a tail which caused it to spin. This allowed it to break the sound barrier as it fell. It had a much thicker skin than the typical World War II bomb so that it would survive the impact of hitting a hardened surface. Its hardened casing was cast in a single piece in a sand mold, using a concrete core. When dropped on compacted earth it would penetrate over 40 meters into the ground. The explosion would leave a 'cavern' which would undermine foundations of structures above causing collapse.

The 'Special B' Lancaster bomber could only carry one at a time and it had to be dropped from 22,000 feet which limited its accuracy. The Grand Slam was first used on March 14, 1945 when the Royal Air Force No. 617 "Dambusters" Squadron, lead by Squadron Leader C.C. Calder, attacked the Bielefeld railway viaduct destroying two spans. In another attack against Valentin submarine pens near Bremen two Grand Slams penetrated 4 to 7 meters of reinforced concrete. The were also used successfully against the Huuge and Brest submarine pens . By the end of the war 41 Grand Slam bombs had been dropped mainly against bridges and viaducts.

Arthur Harris of RAF Bomber Command wrote in 1947 that:

We already had Wallis's 12,000 Ib. [Tallboy] medium capacity bomb, which was capable of breaking through the roof of a railway tunnel or a very thick concrete roof, and when the success of this bomb was proved Wallis designed a yet more powerful weapon, the 22,000 Ib. bomb, the most destructive missile in the history of warfare until the invention of the atom bomb. This 22,000 Ib. 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.

Vital statistics

External links

• 612 attacked the Bielefeld railway viaduct
• Grand Slam bomb from WW2 (J Mullen)
• in 1958 not a dud but live 1944 explosive filling!
• English Bombs of WWII