Post-WWII Fat Man Bomb Designs
Artisan Bombs to Production Bombs
Explosive Power
IV: 1-31 kt., V: 6-120 kt., VI: 8-160 kt.
Hiroshima Equivalent Factor
IV: 2x , V: 8x, VI: 10.6x
Dimensions
IV: 10.6 x 5ft, V: 10.75 x 3.6 ft , VI: 10.6 x 5 ft.
Weight
IV: 5.4 tons, V: 3000+ lbs, VI: Approx. 4 tons
Year(s)
IV: 1949-53, V: 1952-63, VI: 1951-62
Purpose
Expand and begin differentiating nuclear arsenal
About the POST-WWII Fat Man Bomb Designs
Little Boy, though the most famous bomb of all, had no future. Smashing two pieces of uranium together produced a nuclear explosion alright, but it took a lot of uranium for each bomb–and producing the uranium was the hard part of producing nuclear bombs. Plutonium (created from uranium) was the better answer but in the Thin Man bomb design—which mimicked Little Boy, now slamming two pieces of plutonium together to create an explosion—the plutonium proved too active and too likely to prematurely reach high levels of fission, breaking up the bomb before a proper explosion. Thin Man was abandoned and Fat Man, so much more complicated than Little Boy and Thin Man, became the new bomb.
The designers of Fat Man used punch cards and computation, the punch card the heir, the heirloom of the loom, a thing of the Industrial Revolution, and soon to be the instructions and memory of the electronic computing machines and their own revolution. In the years that followed WWII the scientists at Los Alamos would refine Fat Man’s design, using these old and new computational technologies, not seeking some perfection but seeking to make the atomic bomb more useful.
The one hundred and twenty Mark IIIs were custom-built bombs, science lab bombs, each probably a little different than the one before, craftsman-made weapons, hard to build in quantity and hard to maintain once built.
New bomb designs were slow to come in the first years after Japan’s surrender. Work went on but there was no focus, no sense of urgency. The armed forces were demobilizing, the US Air Force was breaking off from the Army to form its own service, and there was a sense amongst the employees of the Manhattan Project that their futures lay elsewhere. There was testing, two detonations at the Bikini Atoll, which consumed much of the remnants of the Manhattan Project’s attention, but those weapons were a variety of the Mark III bomb, not something new.
But new bombs were designed, new bombs were built.
The Mark IV, a version of the Mark III was a more professional bomb, one with removable cores, its shape wind-tunnel tested, its design process commencing a week before Hiroshima but its production meandering along for four years, as the United States, distracted, struggled to formulate a policy to govern these new weapons. The Mark IV entered service the same year as RDS-1, the first Soviet bomb, whose test detonation surprisingly surprised so many. The Mark VI was a modernization of the two-year-old Mark IV, a more flexible, more powerful bomb designed for assembly-line production. And a year later the Mark V, smaller and able to be carried by many more aircraft types than the fat Fat Man-sized bombs, its warhead versatile enough to be repurposed for the first atomic rockets.
The United States was focused now, and it was earnestly building a nuclear arsenal. And the Russians were, too.
Gallery
Nukemap
NUKEMAP is a web-based mapping program that attempts to give the user a sense of the destructive power of nuclear weapons. It was created by Alex Wellerstein, a historian specializing in nuclear weapons (see his book on nuclear secrecy and his blog on nuclear weapons). The screenshot below shows the NUKEMAP output for this particular weapon. Click on the map to customize settings.
Videos
Click on the Play button and then the Full screen button on the lower right (the brackets on a mobile device) to view each video. Click on the Exit full screen button (the “X” on a mobile device) to return.
Further Reading
- Wikipedia (IV, V, VI), Hidden History (Mk. V).
- A discussion of Z-Division and its role in post-WWII atomic weapons development (framed as a history of one of its buildings) can be found here. Note especially the uncertain nature of knowing what went on where and the “The legend of Building 828” appendix (starting on page 23) that attempts to clarify the author’s best guess of the division of labor of the actual “assembly” of the bombs.
- The first tests of nuclear weapons after Trinity and the bombings in Japan was Operation Crossroads at the Bikini Atoll in 1946. Three detonations were planned. One (Able) was detonated at 520 feet above a pretend fleet of ships arranged below (the ships were real–aged US units and captured Japanese vessels). Crossroads seems to have originated in an effort to prove that the development of the atomic bomb didn’t render naval ships obsolete.)The B-29 crew, despite training and two practice runs at the target (not to mention a lack of hostile forces) missed the target by a half mile. Onlookers, expecting a show, were unimpressed. Baker, detonated at 90 feet under water, however, did impress. You’ve seen the footage: A great mushroom cloud rising up out of the ocean and a spreading white disc engulfing silhouetted ship after silhouetted ship. It is probably the most famous movie clip in nuclear weapons history. The radioactivity brought by the water splashing all over the ships (which otherwise survived) proved far more problematic than anyone had anticipated and the third detonation, Charlie, a deep water test, was canceled. All of these tests used variations of the Fat Man bomb.
- Artist Bruce Conner made several interesting films including one entitled Crossroads, an extended reel of Crossroads footage, played at different speeds, set to music. It works better than you might think (all of the links to meaningful excepts on the web have been removed, sadly, but it true that the film is much better in person).
- The Mark IV had been in use for less than a year before the first nuclear weapons accident occurred (the first in history). In early 1950 a bomber flying from Eielson AFB (near Fairbanks, Alaska) with the intension of simulating a bomb run on the Soviet Union (San Francisco apparently standing in for Moscow) developed serious engine trouble and jettisoned the bomb (following proper procedure) before the aircraft crashed in a remote part of Canada. The crew detonated the bomb mid-air (the bomb did contain the high-explosives that would have been used to compress the plutonium core). The US Air Force did not admit to the accident for decades and there are many theories about the accident.
- The Mark IV has been in other accidents as well, over the St. Lawrence seaway in Canada (Wikipedia) and near Tracy, California (and see this contemporary account).
- The Mark V featured a mechanism where the pilot could, by pressing a button, insert the core of the bomb, this allowing the weapon to be used on aircraft with limited crew or limited movement of crew. You can see a video of this mechanism in action at Nuclear Compendium (that clip is also part of the Mark V video, above) along with many other interesting photographs and links.
- The military is serious about keeping an accurate historical record (though they will often keep it a secret), and this 1967 “History of the Mark 5 Bomb” shares the background to the bomb’s development. this document was collected by Martin Pfeiffer.
- Another collection of documents collected by Martin Pfeiffer, this 1967 “History of the MK 6 Bomb (Including the TX/XW-13, Mk 18 and TX-20)” appears to be from the same series as the Mark 5 history (see above) and runs 114 pages.
- The warhead version of the Mk. 5 bomb, designated W5, was used in other weapons, including Regulus I, Matador, and Rascal [links to AmericanNukes.com forthcoming].