Mk1

Man’ weapon being developed first, with the uranium gun-type following thereafter._3.1 The plutonium gun-type design is an early iteration of what would become the “Little Boy” bomb_6.1with the code name of “Thin-Man” originating from Robert Serber, coming up with the name from the title of a Dashiell Hammett detective novel which had recently been made into the William Powell-Myrna Loy movie._13.1This is interesting because many sources cite the naming of “Thin Man” after a relation to Roosevelt._{14.1, 15.1}

During the starting period of the laboratory (Los Alamos; Site Y) the main focus was the plutonium gun weapon. The gun had several unusual features, the assembly velocities required to ensure against the pre-initiation of the fissile material were near the upper limit of standard gun design, 3000 feet per second. The first guns were designed and being produced from the Naval Gun Factory in September 1943, and were received at Los Alamos in March 1944. Proof firing to test the behavior of propellants and to investigate problems of projectile and target design was begun with a 3-inch Naval anti-aircraft gun. The objective of this program was to investigate “blind” target assembly, and to prove the polonium-beryllium gun initiator, then being developed. With these general gun-type weapon experiments taking place it assisted the transition to the low-velocity uranium weapon without meeting new difficulties while the main effort of the laboratory was directed at the mounting difficulties of the implosion program._8.2

In December 1943 with plans for full scale tests starting, a B-29 from the 468th Bombardment Group of the 58th Bombardment Wing located at Smoky Hill Army Air Field was delivered to Wright Field, aircraft serial number B-29-5-BW-42-6259. Initially Wright Field assigned project number MX-469 to the modification efforts of these aircraft and gave it the short title of “Pullman”._17.1 To solidify modifications needed to be done to the B-29s, two external shapes and weights were selected as representative of current plans at Los Alamos. These shapes were of the “thin man” and “fat man”._8.1 The “thin man” bomb needed the most intrusive modification done to the aircraft, requiring the center fuselage structural section to be removed to join the two bomb bays and separate bomb suspension mechanisms be installed. With the abnormal length of the “Thin Man” bomb, a custom release mechanism comprised of two glider tow cable attach-and-release mechanisms were modified and installed in the front and rear bomb bay area’s so as to connect with the two attachment lugs of the bomb. Additionally, the radome normally mounted between the two bomb-bays was removed to make room for provisions to operate the release mechanism. This resulting bomb bay was roughly 33 feet long and used two separate lengthened bomb bay doors._17.1 Later when the “Thin Man” design was dropped, the need for the lengthened bomb-bay was no longer needed and the Silverplate bombers were returned to their original two bomb-bay configuration._17.2

The aerodynamic qualities were first tested at the Dahlgren Proving Grounds in Virginia. A 14/23 scale-model of the weapon was crudely constructed by welding a 14-inch diameter pipe into the middle of a split standard 500-pound bomb. Due to its material construction, it was locally known as the “Sewer Pipe Bomb”. Initial test drops were not good, it fell into a flat spin so required an increase in the fin area and a forward shift of the center of gravity to increase stability._12.1

In the summer of 1944, when the first Clinton Laboratory (now Oak Ridge National Laboratory)-produced plutonium made from its X-10 graphite reactor arrived, Emilio Segre with his group at Los Alamos discovered it was much more heavily irradiated than the previous samples made by cyclotron bombardment causing higher levels of Pu240 to exist within the samples of Pu239  that was planned on being used with the “Thin Man”._18.1 With Pu240 having a higher neutron background (higher rate of spontaneous fissions), to prevent pre-initiation of the critical mass, the assembly velocities would have to be much greater than those possible with the plutonium gun._1.1 The problem of pre-initiation wouldn’t be avoidable until later when tritium-boosting was introduced into weapon design._{9.1, 10.1}

With plutonium being easier and cheaper to produce, it was the first choice in the decision to create an aircraft deliverable atomic weapon. Later, a development in 1944 Los Alamos Laboratory researchers confirmed that, if Pu239 were used in the simple gun design it would spontaneously begin fissioning prematurely._5.1 (This statement goes against officially published documents stating that it was because of the presence of Pu240 that the preinitiation rate was too high_{4.1, 2.2}.)

In the spring and summer of 1944, Emilio Segre’s experimental physics group realized that spontaneous fission in Pu240 made the plutonium gun idea unworkable._7.1

By the end of July 1944, Hirschfelder’s last task was to assess the possibility of building a plutonium gun that could deal with the problems of pre-detonation, which had recently been highlighted by the discovery in the Experimental Physics Division of significant spontaneous fission by pile-produced plutonium. Hirschfelder found such a gun impractical and vetoed any further work._11.1

The design of the plutonium gun-type weapon codenamed ‘Thin Man’ is officially abandoned July 17, 1944._2.1 With the discovery that Pu239 that was contaminated with Pu240 not being feasible to be used in a gun-type weapon, the decision was made to use U235; thus allowing the length of the weapon to be reduced (due to the lower speed of assembly of the critical material) . This change of fissile material marked the movement of the project from ‘Thin Man’ to ‘Little Boy’._{4.1, 2.2}

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._15.2 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_15.3