Editing Little Boy (section)

===Fuze system===
[[File:Little Boy arming plugs in October 2009.JPG|thumb|Arming plugs for a Little Boy type atomic bomb on display at the [[National Air and Space Museum]]'s [[Steven F. Udvar-Hazy Center]]]]

The [[fuze|fuzing]] system was designed to trigger at the most destructive altitude, which calculations suggested was {{convert|1900|ft|m|sp=us}}. It employed a three-stage interlock system:{{sfn|Hansen|1995a|pp=2–5}}
* A timer ensured that the bomb would not explode until at least fifteen seconds after release, one-quarter of the predicted fall time, to ensure the safety of the aircraft. The timer was activated when the electrical [[arming plug|pull-out plugs]] connecting it to the airplane pulled loose as the bomb fell, switching it to its internal 24-volt battery and starting the timer. At the end of the 15 seconds, the bomb would be {{convert|3600|ft}} from the aircraft, and the radar altimeters were powered up and responsibility was passed to the [[barometric]] stage.{{sfn|Hansen|1995a|pp=2–5}}
* The purpose of the barometric stage was to delay activating the radar altimeter firing command circuit until near detonation altitude. A thin metallic membrane enclosing a vacuum chamber (a similar design is still used today in old-fashioned wall barometers) gradually deformed as ambient air pressure increased during descent. The barometric fuze was not considered accurate enough to detonate the bomb at the precise ignition height, because air pressure varies with local conditions. When the bomb reached the design height for this stage (reportedly {{convert|2000|m|disp=or|sp=us|order=flip}}), the membrane closed a circuit, activating the radar altimeters. The barometric stage was added because of a worry that external radar signals might detonate the bomb too early.{{sfn|Hansen|1995a|pp=2–5}}
* Two or more [[redundancy (engineering)|redundant]] [[radar altimeter]]s were used to reliably detect final altitude. When the altimeters sensed the correct height, the firing switch closed, igniting the three BuOrd Mk15, Mod 1 Navy gun primers in the breech plug, which set off the charge consisting of four silk powder bags each containing {{convert|2|lb|g}} of WM slotted-tube [[cordite]]. This launched the uranium projectile towards the opposite end of the gun barrel at an eventual [[muzzle velocity]] of {{convert|300|m/s|ft/s|order=flip|sp=us|sigfig=1}}. Approximately 10 milliseconds later the chain reaction occurred, lasting less than 1 microsecond. The radar altimeters used were modified U.S. Army Air Corps [[Monica (radar)|APS-13]] [[tail warning radar]]s, nicknamed "Archie", normally used to warn a fighter pilot of another plane approaching from behind.{{sfn|Hansen|1995a|pp=2–5}}