Grand Slam bomb exploding near Arnsberg viaduct 1945.
Lancasters distinguished themselves in the evening skies over Europe by delivering 608,612 tons of bombs in 156,000 sorties. However, they are best remembered for two very special attacks. The first, launched against the Mohne and Eder dams on May 17, 1943, utilized the famous Barnes Wallis "skipping bomb" that demolished its targets. The second fell upon the German battleship Tirpitz in Norway. On November 12, 1944, 31 Lancasters armed with 12,000-pound "Tallboy" bombs finally sank the dreaded raider in a fjord. By war's end, Lancasters had been modified to carry the 22,000- pound "Grand Slam" bomb.
Barnes Wallis scaled down his proposals for his gravity-assisted penetrating bomb, and in 1944 designed instead the 12,0001b (5,400kg) Tallboy bomb, which could be carried by the current bombers. Later in the war, the Avro Lancaster improved to such an extent that it could just support a 10-ton payload and so, as we shall see, the 22,0001b (10,000kg) Grand Slam bomb was finally put into production. It was a secret weapon of unprecedented power. As in the case of the Tallboy bomb, the Grand Slam was spin-stabilized by its fins and was built with a thick, heavy steel case to allow it to penetrate deep layers of the ground unscathed. Dropped from high altitude, it would impact at nearly the speed of sound. During manufacture, hot liquid Torpex explosive was poured in to fill the casing and this took a month to cool down and solidify. Torpex (named because it had been developed as a TORpedo EXplosive) had more than 150 per cent the force of TNT The finished bomb was so valuable that aircraft that could not drop their weapon in an abortive mission were ordered to return to base and land with the bomb intact, instead of jettisoning it over the open sea.
Barnes Wallis had planned to create a 10-ton weapon in 1941, but it was not until June 1944 that the bomb was ready for use. It was first dropped on the Saumur rail tunnel from Lancaster bombers of 617 Squadron. No aircraft were lost on the raid, and one of the bombs bored 60ft (18m) through the rock into the tunnel, blocking it completely. These massive 'earthquake' bombs were also used on the great concrete structures that the Germans were building to protect their rocket storage bunkers and submarine pens, and caused considerable damage. The Valentin submarine pens at Bremen, Germany, were made with reinforced concrete roofs some 23ft (7m) thick yet they were penetrated by two Grand Slam bombs in March 1945.