During WWI the English developed the General Purpose series of bombs, that came in standard sizes of 25, 40, 125, 250 and 500 lbs. They were generally thin skinned bombs that frequently failed to explode. When they did explode it was usually on the surface of whatever they hit, as they were not strong enough to penetrate much before they split apart. The fuses were typically instantaneous as the bombs would not penetrate very much they didn't need timed fuses. A great many of these bombs were still in armouries when WWII started. They proved to be nearly useless due to their design inadequacies and because they had deteriorated over the years. Never-the-less they were all the English bombers had for some time, so the Whitleys and Hampdens dropped them without much effect.
It became quickly evident that this war would require much better bombs. Two families of, using the modern parlance, "iron" or "dumb" bombs were developed. There was the relatively thin skinned, cylindrical High Capacity blast bombs weighing 2,000, 4,000 or 8,000 lbs. These looked for all the world like large hot-water heaters, filled with high explosive and fitted with fuses. They were completely un-aerodynamic, were not fitted with fins and were not aimable. Accuracy was poor so they were used primarily for area bombing where accuracy was measured in hundreds of yards. They were used for relatively high altitude, strategic raids as they had a high blast effect and were terribly destructive in built-up areas of cities. They were combined with large numbers of incendiary bombs weighing either 4 or 25 lbs. The cookies tore apart the buildings, and the incendiaries lit the rubble on fire. These were, as the English found in Rostock, Cologne, and Dresden, and the Americans in Tokyo, terribly effective in producing fire storms in cities built largely of wood.
The other family were the Medium Capacity bombs. These were thick skinned, slightly tapered bombs fitted with fins on the rear. These were much more aerodynamic and could penetrate buildings, streets, dirt, concrete, etc. to provide a penetrating destructive potential. These bombs were used primarily for tactical targets.
|A typical area load of a single 4,000 lb "Cookie" and incendiaries.||A typical tactical load in a Lancaster bomber was a 4,000 lb "Cookie" and 20 500 lb MCs.|
Then there were the specialty bombs used by Nos 9 and 617 Sqdns to destroy high value targets. There were three designed by Barnes Wallis, who also designed the Wellington bomber. His first was the rolling bomb that would skip across water like a stone. It was used on only one mission to destroy the Moehne and Esser dams. Next came Tall Boy that weighed 10,000 lbs and was cast in one piece of high tensile steel. It was designed to penetrate hard targets like battle ships, railway tunnels and bridges. His final bomb was Grand Slam, it weighed 22,000 lbs and was constructed of specially designed steel. It had to be cast in specially built molds of very tough steel. Only one machinery company in England had the equipment to cut the steel to 1/10,000 in. tolerances. It was 30 ft long, X ft in diameter and was sharply pointed at both ends. The tail was built with curved fins to stabilize the bomb into a nose-down drop and to start it rotating so it would remain stable when it broke through the sound barrier. It was the heaviest bomb dropped in WWII. The concept behind Grand Slam was to provide a huge explosive power in a steel casing that could penetrate 30 ft of reinforced concrete, bedrock or 100 ft of dirt. Wallis, being a structural engineer, understood how structures were built, their strengths and their underlying weaknesses. Many of the structures built or used by the Germans, such as the Bielefeld Viaduct, were so massive that even Tall Boy bombs were not effective in toppling them. His idea was to create a massive subterranean explosion that would blow a huge hole in the earth, called a "camouflet", immediately beside the structure. This hole would undermine the supports of the viaduct, they would topple into the hole and bring down the rest of the viaduct. The Grand Slams and Tall Boys also created destruction by causing small earthquakes in the immediate vicinity of buildings and bridges, thus either weakening them to the point where they couldn't be used, or destroying them totally. The Grand Slam was also used to penetrate the massive 30 ft thick reinforced concrete ceilings of the submarine pens at Huuge, and Brest.
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