Atomic Snapshots: Little Boy Arming Plugs

Arming plugs at the Udvar-Hazy Center of the National Air and Space Museum.

These arming plugs for Little Boy are displayed at the Udvar-Hazy Center of the National Air and Space Museum. They were found in the navigator’s compartment during the restoration of the Enola Gay. It’s not known whether these plugs were from the assembled Little Boy atomic bomb (L-11), or one of the pre-assemblies (without uranium projectiles) used for testing or practice drops (L-1, L-2, L-5, and L-6).

Little Boy in the loading pit on Tinian. The three arming plugs can be seen on the right side. The top secret Yagi antennas have not yet been installed.

Shortly after takeoff from Tinian island, Deak Parsons and Morris Jeppson crawled into the bomb bay of Enola Gay to follow the eleven steps1 to arm the atomic bomb. Step 1 was to check that the three wood-handled green plugs that blocked the firing signal between the fuse and the bomb were installed.

About an hour and a half before the bomb run, Jeppson crawled back into the bomb bay with three wood-handled, five-pin red plugs. He carefully removed each green plug, one-by-one, and replaced them with the red plugs, closing the firing circuits.

At 8:15 a.m. (Hiroshima time), the bomb was released, dislodging the instrument cords that connected it to the airplane and instrument monitoring panels. This immediately transferred power to the 24-volt battery, beginning a 44-second detonation cycle.

First, a timer of eight spring-wound clocks safeguarded that the bomb would not explode until at least 15 seconds after release, about one quarter of the predicted fall time, to ensure the safety of the aircraft.

Second, the firing signal transferred to the barometric pressure switch, designed to close at 7,000 feet. With 9 seconds to detonation, the switch closed and activated the Archie radars, with the Japanese-invented Yagi antennas which began bouncing their signals off the approaching ground.

At close to 1900 feet above the ground, the last relay switch closed. The firing signal jumped across the three red arming plugs, sending the signal to the breech primers and detonating the cordite charges, propelling the uranium 235 projectile down the six-foot barrel toward its uranium target, creating an uncontrolled chain reaction and changing history.

(1) Walker, S. (2005). Shockwave: Countdown to Hiroshima. HarperCollins. p. 192.