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Tallboy bomb and Trolley

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Detailed Tallboy British bomb with heavy load trolley

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Tallboy or Bomb, Medium Capacity, 12,000 lb was an earthquake bomb developed by the British aeronautical engineer Barnes Wallis and used by the Royal Air Force (RAF) during the Second World War.

At 5 long tons (5.1 t), it could only be carried by a modified model of the Avro Lancaster heavy bomber. It proved to be effective against large, fortified structures against which conventional bombing had proved ineffective.

Most large Allied, particularly British, Second World War aircraft bombs (blockbuster bombs) had very thin skins to maximize the weight of explosive that a bomber could carry. This was an improvement on the early part of the war when the explosive content of British bombs was low.

To be able to penetrate the earth (or fortified 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 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 10 ft (3.0 m) of the overall 21 ft (6.4 m) 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 18,000 ft (5,500 m) at a forward speed of 170 mph (270 km/h), hitting at 750 mph (1,210 km/h). It made a crater 80 ft (24 m) deep and 100 ft (30 m) across and could go through 16 ft (4.9 m) of concrete.

The weight of the Tallboy (approximately 12,000 lb or 5,400 kg) 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. 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 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. While being remotely defused, this second bomb detonated in October 2020.

The bomb was aimed at the target during an operation and proved capable of penetrating deep into hardened reinforced concrete when it hit. This, however, was not the primary intention of Barnes Wallis's design. The bomb was designed to make impact close to the target, penetrate 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 surroundings, with the blast dissipating rapidly through the air. An earthquake impact shook the whole target and caused structural damage to all parts of it, making repair uneconomic. The attack reports below should be considered with this in mind.

An alternative technique was to arrange detonation depth so that the crater broke the surface—useful for attacking railway marshalling yards and similar targets. The Tallboy produced an 80 ft (24 m) crater with depths up to 100 ft (30 m), unlike conventional bombs which would produce many shallow craters across a target—each one of which could later be filled in rapidly with earth-moving equipment. Such a huge hole was time-consuming to fill; multiple trucks and bulldozers could not be fitted around the periphery of the hole to speed the process.

(Source: Wikipedia)

We realize the most accurate reproduction of this terrific weapon, ready to be displayed under your Avro Lancaster model kit!

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