Tallboy (bomb) - Design

Design

Most large Allied World War II aircraft bombs had very thin skins to maximize the weight of explosive a bomber could carry—this was an improvement on the early part of the war, when the explosive content of British bomb designs was low.

To be able to penetrate the earth (or hardened targets) without breaking apart, the casing of the Tallboy had to be strong. Each was cast in one piece of high-tensile steel that would enable it to survive the impact before detonation. At the same time, to achieve the penetration required, Wallis designed the Tallboy to be very aerodynamic, so that when dropped from a great height it would reach a terminal velocity much higher than traditional bomb designs.

In the final design, the tail of the bomb was about half the overall length of the finished weapon—the bomb casing was some 10 feet (3 m) of the overall 21-foot (6 m) length. Initially the bomb had a tendency to tumble, so 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, improved the aerodynamics and improved accuracy.

Tallboy was designed to be dropped from an optimal altitude of 18,000 feet (5,500 m) at a forward speed of 170 miles per hour (270 km/h). Impacting at 750 miles per hour (1,210 km/h), it made a crater 80 feet deep (24 m) and 100 feet (30 m) across and could go through 16 feet (5 m) of concrete.

The weight of the Tallboy (approximately 12,000 lb) 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. Even then the Lancaster was not capable of reaching the bomb's intended dropping height of 40,000 ft (12,200 m) but only around 25,000 (7,700 m).

At the same time No. 617 "Dambusters" Squadron trained in the use of a special bombsight, the Stabilizing Automatic Bomb Sight (SABS). For accuracy, multiple corrections had to be made for temperature, wind speed, and other factors. It was only effective if the target could be identified, and several missions were cancelled or unsuccessful because of difficulty in accurately identifying and marking the targets.

For use on underground targets, the bomb was fitted with three separate inertia 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, a total of three separate 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 fuses failed to function, the third would trigger detonation. Despite this elaborate system, at least one Tallboy failed to explode during the second attack on the Sorpe dam and was found during repairs in late 1958 when the reservoir was emptied.

Although the bomb was aimed at the target during an operation, and proved capable of penetrating deep into hardened reinforced concrete when it hit, this was not the primary intention of Barnes Wallis's design. The bomb was designed to impact close to the target, slide into the soil or rock beneath or around the target, and then detonate, transferring all of its energy into the structure, or creating a camouflet (cavern or crater) into which the target would fall.

This 'earthquake' effect caused more damage than even a direct hit that penetrated the armour of a target, since even a burst inside a bunker would only damage the immediate surroundings, with the blast dissipating rapidly through the air. An earthquake impact shook the whole target, and caused major structural damage to all parts of it, making repair uneconomic. The attack reports below should be considered with this in mind.

The construction of each Tallboy was labour intensive because each was largely hand-made, requiring much manual labour during each separate manufacturing stage. The materials used were costly, with very precise engineering requirements with regard to casting and machining. For example, to increase penetrative power, a large and specially hardened steel plug had to be precisely machined and mated to a recess in the nose of the bomb. The ogive had to be machined into a perfectly symmetrical shape to ensure optimum aerodynamic performance. This was no easy task when manipulating a bomb casing with the size and weight of a Tallboy.

The Torpex filling was poured into the base of the upturned casing by hand, after melting it in "kettles". The final stage of explosive filling required that a one-inch layer of pure TNT be poured over the Torpex filling, followed by sealing the base with a 4-inch layer of woodmeal-wax composite with three cylindrical recesses fitted with the explosive boosters and into which (when the bomb was finally armed), a total of three chemical time-fuses were inserted.

Tallboys were not considered expendable, and if not used on a raid were to be brought back to base rather than safely jettisoned into the sea. The value of the weapon offset the additional risk to the aircrew.

Given their high unit cost, Tallboys were used exclusively against high-value strategic targets that could not be destroyed by other means. When it was found that the Lancaster could be modified to carry a bomb larger than the Tallboy, Wallis produced the even larger Grand Slam bomb.