The ‘Tallboy’ and subsequent larger variant the ‘Grand Slam’ were heavy bombs designed specifically for penetration of heavily reinforced targets. Their designer, Barnes Wallis, had proposed the use of so called ‘earthquake bombs’ which could cause massive structural damage to weakened ground targets by blasting an underground cavity upon detonation. The British military were sceptical of the applicability of such large bombs, whose production was heavily labour intensive and expensive, however the Tallboy soon proved highly effective. With a weight of around 12,000 lb (5,444 kg), the Tallboy was larger than any current conventional airborne bomb and from summer 1944 it was used in numerous raids against high priority targets with great success. The reinforced steel shell enabled the Tallboy to break through 16 ft (5 m) of concrete upon initial impact alone, after which a timed fuse would detonate the explosive load. The Grand Slam, which weighed 22,000 lb (9,980 kg), saw its first use in March 1945 and was especially effective against heavily reinforced concrete structures.
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