HIROSHIMA	NAGASAKI
Gravity bomb known as "Little Boy", a gun-type fission weapon with 60 kilograms (130 lb) of uranium-235, took 43 seconds to fall from the aircraft flying at 31,060 feet (9,470 m) to the predetermined detonation height about 1,900 feet (580 m) above the city.	"Fat Man" was an implosion-type device using plutonium-239.
The design used the gun method to explosively force a hollow sub-critical mass of uranium-235 and a solid target spike together into a super-critical mass, initiating a nuclear chain reaction. This was accomplished by shooting one piece of the uranium onto the other by means of chemical explosives. It contained 64 kg of uranium, of which less than a kilogram underwent nuclear fission, and of this mass only 0.6 g was transformed into energy.	A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive. Thirty-two pairs of detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the core, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear initiation.