Bockscar is the name of the United States Army Air Forces B-29 bomber that dropped the "Fat Man" nuclear weapon over Nagasaki on August 9, 1945, the second atomic weapon used against Japan. It was assigned to the 393d Bomb Squadron, 509th Composite Group. The name painted on the aircraft after the mission is a pun on "boxcar" after the name of its aircraft commander, Captain Frederick C. Bock. Bockscar had been flown by Sweeney and crew C-15 in three test drop rehearsals of inert "Fat Man" assemblies in the days leading up to the second mission. The Great Artiste, which was the assigned aircraft of the crew with whom Sweeney most often flew, had been designated in preliminary planning to drop the second bomb, but it had been fitted with observation instruments for the Hiroshima mission. Moving the instrumentation from The Great Artiste to Bockscar would be a complex and time-consuming process, and when the second atomic bomb mission was moved up from August 11 to August 9 because of adverse weather forecasts, the crews of The Great Artiste and Bockscar instead exchanged aircraft. The result was that the bomb was carried by Bockscar but flown by the crew C-15 of The Great Artiste.