Fuse system

The bomb employed a fusing system that was designed to detonate the bomb at the most destructive altitude. Calculations showed that for the largest destructive effect, the bomb should explode at an altitude of 580 meters (1,900 feet). The resultant fuze design was a three-stage interlock system:[28]

    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 safety of the aircraft. The timer was activated when the electrical pullout plugs connecting it to the airplane were pulled loose as the bomb fell, switching it to internal (24V battery) power and starting the timer. At the end of the 15 seconds the batteries then powered the radar system and passed responsibility to the barometric stage.[28]
    The purpose of the barometric stage was to delay activating the radar altimeter firing command circuit until near detonation altitude. A thin metallic membrane was 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 2,000 meters) the membrane closed a circuit, activating the circuit between the radar altimeters and the projectile gun. The barometric stage was added because of a worry that external radar signals might detonate the bomb too early.[28]
    Two or more redundant radar altimeters were used to reliably detect descent. 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 two pounds of WM slotted-tube cordite. This launched the uranium projectile towards the opposite end of the gun barrel at an eventual muzzle velocity of 1000 feet per second (300 meters per second). Approximately 10 milliseconds later the chain reaction occurred, lasting less than 1 μs. The radar altimeters used were modified U.S. Army Air Corps APS-13 fighter tail warning radars, nicknamed "Archie", originally designed to warn a pilot of another plane approaching from behind.[28][29]