Mk2

“Little Boy” was the code name of the first uranium gun-type weapon. Originally called the “Thin Man“, when plutonium-239 was used as the nuclear material until it was revealed that the pre-initiation rate of this particular isotope of plutonium would have needed higher velocities within the weapon that could be attained._4.3

Earlier models of the “Little Boy” included the 1418 and 1491, these were the “Thin Man” prototypes, then the models 1791, 1792, and 1793. The models 1418 and 1491 were “dummies” with the former having no gun or breech block and the latter did. The 1721 was a modification of the 1491 and the 1791 was a modification of the 1792. The models 1792 and 1793 were identical to the 1791 except for different target assemblies in their noses._12.1

Was given the code-name “Target” for use in correspondence prior to its drop on Japan. It was also known as “the gadget”, “the device”, “the gimmick” (an expression used by Tibbets), “the beast” (used by scientists critical of the project), “S-1” (preferred by Stimson), and “it” (Used by the 509th, still mystified about what, exactly, the weapon was)._13.1

Gun-type weapon, where two subcritical masses were brought together without compression. Initiation was by four polonium-beryllium neutron sources called “Squab“_10.1

For added security for the weapon, construction of the gun was assigned to three independent plants: the Naval Gun Factory made the gun and the breech; the target case was produced at the Naval Ordnance Plant at Center Line, Michigan; and the Expert Tool and Die Company of Detroit made the bomb tail._14.1

The L-11 combat unit that was dropped on Hiroshima was most likely painted FS 24079, closest to olive-drab primer_8.1

Responsibility for the fusing and detonating system remained with the Fuse Group of the Ordnance Division. The Gun Group and the Fuse Group collaborated in the drop tests carried out as part of Project Alberta._5.1

Had a diameter of 28″ and weighed 8,900lbs_4.2

3 Mk2 bombs were maintained in the stockpile from 1945 to 1948._6.1

In 1949 the Mk2 was still being maintained in stand-by status._4.2

Also used the code-name “LB”_4.1

Little-Boy was untested before it was dropped on Hiroshima August 6, 1945._1.1 The Mk2 had a yield of approximately 12-15Kt._7.1

Controlled by a precursor to the Aircraft Monitor and Control (AMAC) system known as a Flight Test Box (FTB). The FTB could measure battery voltages, turn on radars, and could verify certain components had not failed. Two manually inserted arming plugs were used to arm the weapons prior to pressurization of the cockpit at 8,000′._2.1

Used a lead acid battery with a charge of 2.0v._2.2

Dropped on the city of Hiroshima Japan on August 6th, 1945 by the Army Air Corps 509th Composite Group, 393rd Bombardment Squadron._3.1

Test assemblies of the Mk2 “Little-Boy” bomb participated in Operation EASTWIND_9.1, an exercise to test bomb assemblies on a modified Midway-class aircraft carrier with strengthened decks to accommodate larger aircraft capable of carrying atomic weapons. The test assemblies were dropped by the Armed Forces Special Weapons Project (AFWSP) Navy Special Weapons Unit No. 471._3.2

On December 25th 1948 a Navy XAJ-1 aircraft arrived at Kirtland AFB for wiring and test loading of the Mk2 “Little-Boy”, Mk3 “Fat-Man”, and Mk4 atomic bombs._3.3

During the last year of the war the gun-type weapon development effort was consolidated into a single group, the Gun Group of the Ordnance Division. During the earlier period of the laboratory and implosion bomb composed entirely of U235 was not completely ruled out. With receiving more accurate cross-section data it appeared as though an implosion device using only U235 would not be as efficient, talks of a composite core of U235 and Pu239 had been discussed._5.2 The use of U235 was far more attractive in both weapons due to the relative ease it was at attaining this particular isotope in comparison to the complex and slow process of generating Pu239. In February of 1945 all laboratory programs had been “frozen” in their developmental decisions to maintain deadlines, the decision was final to use U235 only in the gun-type weapon._5.1