 Bockscar lifted off at 03 : 47 on the morning of 9 August 1945 , with Kokura as the primary target and Nagasaki the secondary target . The weapon was already armed , but with the green electrical safety plugs still engaged . Ashworth changed them to red after ten minutes so that Sweeney could climb to 17 @,@ 000 feet ( 5 @,@ 200 m ) in order to get above storm clouds . During pre @-@ flight inspection of Bockscar , the flight engineer notified Sweeney that an inoperative fuel transfer pump made it impossible to use 640 US gallons ( 2 @,@ 400 l ) of fuel carried in a reserve tank . This fuel would still have to be carried all the way to Japan and back , consuming still more fuel . Replacing the pump would take hours ; moving the Fat Man to another aircraft might take just as long and was dangerous as well , as the bomb was live . 509th Composite Group Commander Colonel Paul Tibbets and Sweeney therefore elected to have Bockscar continue the mission . 
