On 9 August 1945, Major Sweeney commanded "Bockscar", which carried the atomic bomb "Fat Man" from the island of Tinian to Nagasaki. In addition to "Bockscar", the mission included two observation and instrumentation support B-29s, "The Great Artiste" and "The Big Stink", who would rendezvous with "Bockscar" over Yakushima Island. At the mission pre-briefing, the three planes were ordered to make their rendezvous over Yakushima at 30,000 feet due to weather conditions over Iwo Jima (the Hiroshima mission rendezvous). That same morning, on the day of the mission, the ground crew notified Sweeney that a faulty fuel transfer pump made it impossible to utilize some of fuel in the tail, but Sweeney, as aircraft commander, elected to proceed with the mission.