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Getting ready to drop the big one:

Top Secret Combat operations

After ground training for the combat crews, the 509th began operations on 30 June 1945, with a calibration flight involving nine of the B-29s on hand. During the month of July and the first eight days of August the thirteen bombers of the 393d Bombardment Squadron flew an intensive training and mission rehearsal program that consisted of:

17 individual training sorties without ordnance,
15 practice bombing missions between 1 and 22 July against airfields on Japanese-held Truk, Marcus, Rota, and Guguan in which 90 B-29 sorties dropped 500- and 1000-pound bombs to practice radar and visual bombing procedures,[49]
12 combat missions between 20 and 29 July against targets in Japan dropping high-explosive pumpkin bombs, in which 37 B-29 sorties delivered conventional-bomb replications of the Fat Man:[50] four on 20 July, three on 24 July, two on 26 July, and three on 29 July. Some 27 sorties were made visually and 10 by radar, striking 17 primary targets, 15 secondary targets, and five targets of opportunity. Two other aircraft did not drop their bombs: one jettisoned its pumpkin bomb into the sea near Iwo Jima, and the Strange Cargo's bomb came loose from the bomb rack and plunged through the closed bomb bay doors while the bomber was still on the ground. One B-29 incurred minor battle damage in the attacks.[51] Flying at 30,000 feet (9,100 m) put them above the effective range of flak.[52] Each pumpkin bomb mission was conducted by a formation of three aircraft in the hope of convincing the Japanese military that small groups of B-29s did not justify a strong response. This strategy proved successful, and Japanese fighters only occasionally attempted to intercept the 509th Composite Group's aircraft.[53]
7 component-tests between 23 July and 8 August involving rehearsal drops of four inert Little Boy gun-type fission weapons and three Fat Man assemblies,[54] and
a practice mission on 29 July to Iwo Jima in which an inert Little Boy was unloaded and then reloaded to rehearse the contingency plan for using a back-up bomber in an emergency.[54][55]

While this training was taking place, the components of the first two atomic bombs were shipped to Tinian by various means. For the uranium bomb code-named "Little Boy", fissile components consisted of a cylindrical target and nine washer-like rings that made up the hollow cylinder projectile. When the bomb detonated, these would be brought together to create a cylindrical core.[56] The uranium-235 projectile and bomb pre-assemblies (partly assembled bombs without the fissile components) left Hunters Point Naval Shipyard, California, on 16 July aboard the cruiser USS Indianapolis, arriving 26 July.[57] 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. L-6 was used in the Iwo Jima dress rehearsal on 29 July. This was repeated on 31 July, but this time L-6 was test dropped near Tinian by Enola Gay. L-11 was the assembly used for the Hiroshima bomb.[58][59] On 26 July three C-54s of the 320th Troop Carrier Squadron left Kirtland Army Air Field, each with three of the uranium-235 target rings, and landed at North Field on 28 July.[60][61]

The components for the bomb code-named the Fat Man arrived by air the same day. The bomb's plutonium core (encased in its insertion capsule) and the beryllium-polonium initiator were transported from Kirtland to Tinian by C-54 in the custody of Project Alberta couriers. Three Fat Man high explosive pre-assemblies designated F31, F32, and F33 were picked up at Kirtland on 28 July by three B-29s, two from the 509th and one from the 216th AAF Base Unit, and transported to North Field, arriving 2 August.[62] The B-29s were Luke the Spook and Laggin' Dragon of the 509th, and 42-65386, a phase 3 Silverplate of the 216th AAF Base Unit. F33 was expended during the final rehearsal on 8 August, and F31 was the bomb dropped on Nagasaki. F32 presumably would have been used for a third attack or its rehearsal.[60]

The final item of preparation for the operation came on 29 July 1945. Orders for the attack were issued to General Carl Spaatz on 25 July under the signature of General Thomas T. Handy, the acting Chief of Staff of the United States Army, since General of the Army George C. Marshall was at the Potsdam Conferenc