Little Boy, 10 feet long and 28 inches in diameter, was similar to a gun in which a “bullet” made of uranium 235 was fired into a target also of uranium 235. When the two collided, a supercritical mass was attained, and a chain reaction and explosion would occur. No preliminary firing tests were made.

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Fat Man measured 10 feet 8 inches long and 5 feet in diameter. It contained a sphere of plutonium. Conventional explosives surrounding the plutonium were fired so that the plutonium was compressed into a supercritical mass, producing a chain reaction and an explosion. Fat Man was tested in the New Mexico desert, near Alamogordo, on
July 16, 1945. A blinding explosion, the world’s first nuclear blast, was equivalent to 18,600 tons of TNT. By the time the more complicated Fat Man had been tested, most of Little Boy’s elements were already en route to Tinian.

After Tibbets returned from Hiroshima, Sweeney’s crews watched as Fat Man was loaded on August 8. Sweeney’s greatest fear, he said later, was of “goofing up.” He said, “I’d rather face the Japanese than Tibbets in shame if I made a stupid mistake.”

Sweeney did not make any “stupid mistakes,” but the second atomic mission seemed jinxed from the start. When queried recently, General Tibbets called the second mission a “fiasco” through no fault of Sweeney’s.

The two target cities had been carefully selected. They had purposely not been bombed heavily by LeMay’s B-29s so that, as the after-action report noted, “The assessment of the atomic bomb damage would not be confused by having to eliminate previous incendiary or high explosive damage.”

Kokura, on the northeast corner of Kyushu, was chosen as the primary target for Fat Man because it was the enemy’s principal production source for automatic weapons. It was also the site of the Mitsubishi Steel and Arms Works and was one of the largest shipbuilding and naval centers in Japan.

Nagasaki, the secondary target, was the third largest city on Kyushu. It was also one of Japan’s leading shipbuilding and repair centers. It was not considered a completely “virgin” target, however, because it had been bombed many weeks before by Twentieth Air Force bombers. Niigata was originally considered as a third target, but it was too far away from the other two cities.

The crews were given their final briefing during the early morning hours of August 9. They would cruise-climb to the bombing altitude of 31,000 feet. Meanwhile, the two weather planes would report the conditions over both targets. Radio silence between the bombers was to be absolute. If any of the planes had to ditch, rescue ships and submarines were in position; also, aircraft were on alert, to be dispatched to locate a downed plane or its crew.

With his airplane stripped of all armament except two .50-caliber tail guns, Sweeney lifted Bockscar off at 3:49 a.m., Tinian time. The flight route to Kokura was originally planned to proceed via Iwo Jima, but bad weather forced a change to Yaku-Shima in the Ryukus. En route, Commander Ashworth armed Fat Man.

When Bockscar arrived at the rendezvous point, only The Great Artiste was there. Due to poor visibility, Hopkins, in Full House, had lost contact with the other planes.

It had been agreed that Sweeney would not linger more than 15 minutes over the rendezvous point, but he circled for 45 minutes looking for Hopkins. Meanwhile, Hopkins was circling at another point many miles to the south. Breaking radio silence, Hopkins called out, “Chuck, where in the hell are you?”

Sweeney did not answer. Frustrated, he told the crew, “We can’t wait any longer,” and turned toward Kokura with the single B-29 escort. He wanted the mission to be a complete success, but
it would be difficult to call it that if
the explosion were not properly documented by the photography that the equipment on Hopkins’ plane would produce. Meanwhile, in the bomb bay, something had gone wrong. The black box containing the electrical switches that armed the bomb had a red light. As long as the light blinked in a regular rhythm, it meant that the bomb was properly armed. If it blinked irregularly, something was malfunctioning.

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