penetration required, Wallis designed the Tallboy to be aerodynamically clean so that, when dropped from a great height, it would reach a much higher terminal velocity than traditional bomb designs. In the final design, the No. 78 Mark I tail of the bomb was about half the overall length of the finished weapon; the bomb casing was some of the overall length. Initially, the bomb had a tendency to tumble and the tail was modified; the fins were given a slight twist so that the bomb spun as it fell. The gyroscopic effect thus generated stopped the pitching and yawing, improving aerodynamics and accuracy. The Tallboy was designed to be dropped from an optimal altitude of at a forward speed of , hitting at . It made a crater deep and across and could go through of concrete. The weight of the Tallboy (approximately ) and the high altitude required of the bombing aircraft meant that the Avro Lancasters used had to be specially adapted. Armour plating and even defensive armament were removed to reduce weight, and the bomb-bay doors had to be adapted. No. 617 Squadron were trained on the Stabilizing Automatic Bomb Sight (SABS). Corrections had to be made for temperature, wind speed and other factors. The sight was effective only if the target could be clearly identified. Several missions were cancelled or unsuccessful because of this limitation. For use on underground targets, the bomb was fitted with three separate inertia No. 58 Mark I Tail Pistols (firing mechanisms). These triggered detonation after a pre-set delay, which gave the bomb sufficient time to penetrate the target before exploding. Depending on mission requirements, the time delay could be set to 30 seconds or 30 minutes after impact. To guarantee detonation, three Type 47 long delay fuzes were fitted inside the rear of the bomb. This dramatically improved reliability of the weapon; even if two of the fuzes failed, the third would trigger detonation. At least 2 Tallboys failed to explode, one during the second attack on the Sorpe dam; it was found during repairs in late 1958 when the reservoir was emptied, and a second was found in Świnoujście in Poland (formerly Swinemünde) in 2020. This second bomb detonated in October 2020 while being remotely defused. The bomb was aimed at the target during an operation and proved capable of penetrating deep into hardened reinforced concrete when it hit.