Good point, not covered in
http://en.wikipedia.org/wiki/Tallboy_bomb
But I note this from this article;
- 854 were manufactured by Vickers.
- When dropped from 20,000 ft (6,100 m) 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 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 N0. 617 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, etc. However it was only effective if the target could be identified and several missions were canceled 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. 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. In order 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 fuzes failed to function, the third would trigger detonation. Despite this elaborate fusing system however, 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.
- Tallboys were not considered expendable and if not used on a raid were to be brought back to base rather than safely dropped in 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 which could not be destroyed by other means.
Comments and questions;
- Tallboys were used on the Sorpe dam, so it was definitely approved ordnance for earthdam-busting. So one uncertainty is removed.
- Did 617 Squadron have a monopoly? They used a special sight and had specially modified Lancs, so that suggests they were the only approved squadron.
- But perhaps although a special sight was necessary for hitting buildings and ships like the Tirpitz, a dam is such a large structure that it did not need such accuracy. Perhaps the earthquake effect of one Tallboy falling anywhere on or near the Urft dam would have shattered it?
- On December 3, according to the BC War Diary, 190 Lancasters dropped markers but no bombs on the Urft Dam'. What was that about? Perhaps BC was after all trying to find a way of using Tallboys on the Urft Dam.
Does anyone know of an RAF source for any contemporary or memoir discussion about destroying the Urft Dam?
Tony