<P> The Little Boy pre-assemblies were designated L - 1, L - 2, L - 3, L - 4, L - 5, L - 6, L - 7, and L - 11 . L - 1, L - 2, L - 5, and L - 6 were expended in test drops . The first drop test was conducted with L - 1 on 23 July 1945 . It was dropped over the sea near Tinian in order to test the radar altimeter by the B - 29 later known as Big Stink, piloted by Colonel Paul W. Tibbets, the commander of the 509th Composite Group . Two more drop tests over the sea were made on 24 and 25 July, using the L - 2 and L - 5 units in order to test all components . Tibbets was the pilot for both missions, but this time the bomber used was the one subsequently known as Jabit . L - 6 was used as a dress rehearsal on 29 July . The B - 29 Next Objective, piloted by Major Charles W. Sweeney, flew to Iwo Jima, where emergency procedures for loading the bomb onto a standby aircraft were practiced . This rehearsal was repeated on 31 July, but this time L - 6 was reloaded onto a different B - 29, Enola Gay, piloted by Tibbets, and the bomb was test dropped near Tinian . L - 11 was the assembly used for the Hiroshima bomb . </P> <P> Parsons, the Enola Gay's weaponeer, was concerned about the possibility of an accidental detonation if the plane crashed on takeoff, so he decided not to load the four cordite powder bags into the gun breech until the aircraft was in flight . Parsons and his assistant, Second Lieutenant Morris R. Jeppson, made their way into the bomb bay along the narrow catwalk on the port side . Jeppson held a flashlight while Parsons disconnected the primer wires, removed the breech plug, inserted the powder bags, replaced the breech plug, and reconnected the wires . Before climbing to altitude on approach to the target, Jeppson switched the three safety plugs between the electrical connectors of the internal battery and the firing mechanism from green to red . The bomb was then fully armed . Jeppson monitored the bomb's circuits . </P> <P> The bomb was dropped at approximately 08: 15 (JST) 6 August 1945 . After falling for 44.4 seconds, the time and barometric triggers started the firing mechanism . The detonation happened at an altitude of 1,968 ± 50 feet (600 ± 15 m). It was less powerful than the Fat Man, which was dropped on Nagasaki, but the damage and the number of victims at Hiroshima were much higher, as Hiroshima was on flat terrain, while the hypocenter of Nagasaki lay in a small valley . According to figures published in 1945, 66,000 people were killed as a direct result of the Hiroshima blast, and 69,000 were injured to varying degrees . Of those deaths, 20,000 were members of the Imperial Japanese Army . </P> <P> The exact measurement of the yield was problematic since the weapon had never been tested . President Harry S. Truman officially announced that the yield was 20 kilotons of TNT (84 TJ). This was based on Parsons's visual assessment that the blast was greater than what he had seen at the Trinity nuclear test . Since that had been estimated at 18 kilotons of TNT (75 TJ), speech writers rounded up to 20 kilotons . Further discussion was then suppressed, for fear of lessening the impact of the bomb on the Japanese . Data had been collected by Luis Alvarez, Harold Agnew, and Lawrence H. Johnston on the instrument plane, The Great Artiste, but this was not used to calculate the yield at the time . </P>

How much mass was converted to energy in the hiroshima bomb