Mark 11
The Mark 11 was an improved version of the Mark 8, slightly heavier, and according to the National Atomic Museum, "able to penetrate up to 22 feet of reinforced concrete, 90 feet of hard sand, 120 feet of clay, or five inches of armor plate." Precisely this capability is also attributed [possibly in error] to the Mark 8 ["Weapon Design - We’ve Done a Lot but We Can’t Say Much," by Carson Mark, Raymond E. Hunter, and Jacob J. Wechsler LOS ALAMOS SCIENCE Winter/Spring 1983].
The bomb contained the same gun-type fuse as the Mark 1 "Little Boy," in which two subcritical masses (each less than what is required to cause a chain reaction) of nuclear material are placed at opposite ends of a long tube. To trigger detonation, an explosive charge at one end of the tube propels one of the nuclear masses down the pipe into the other subcritical mass, hence creating a supercritical mass (a quantity sufficient to cause a nuclear chain reaction) and resulting in a massive release of energy. This type of trigger makes for a simpler and more reliable type of bomb.
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