Post-WWII Fat Man Bomb Designs

Artisan Bombs to Production Bombs

Even before the end of World War II development of post-war bombs was underway. The Manhattan Project created a new engineering and field testing site in July of 1945, and the development of the Mark IV started two weeks before the atomic bombings of Hiroshima and Nagasaki. Z-Division--a new site focused on the engineering and assembly of atomic bombs, located adjacent to Kirkland Air base, near Albuquerque, New Mexico--greatly improved the ability of engineers to test and improve these weapons. One of the many improvements, as you can see in this image, made outside the China Lake Museum in Ridgecrest, California, is the completely redesigned tail section. The boxy, poorly designed shape of the stabilizer has been replaced now with fins that improved the bomb's accuracy from high altitudes. This unit appears to be the only Mark IV on public display anywhere in the world (the one at the Diefenbunker Cold War Museum is a model). A small sign next to the display indicates the bomb is on loan from the Naval History and Heritage Command. — Even before the end of World War II development of post-war bombs was underway. The Manhattan Project created a new engineering and field testing site in July of 1945, and the development of the Mark IV started two weeks before the atomic bombings of Hiroshima and Nagasaki. Z-Division–a new site focused on the engineering and assembly of atomic bombs, located adjacent to Kirkland Air base, near Albuquerque, New Mexico–greatly improved the ability of engineers to test and improve these weapons. One of the many improvements, as you can see in this image, made outside the China Lake Museum in Ridgecrest, California, is the completely redesigned tail section. The boxy, poorly designed shape of the stabilizer has been replaced now with fins that improved the bomb’s accuracy from high altitudes. This unit appears to be the only Mark IV on public display anywhere in the world (the one at the Diefenbunker Cold War Museum is a model). A small sign next to the display indicates the bomb is on loan from the Naval History and Heritage Command.

Another improvement in the Mark IV's design that is visible on the outside is the nose plate, which was removable. As a safety measure the Mark IV was designed so that the nuclear core could be installed by a technician while in flight (an approximately half-hour process), thus protecting the air base, the airplane, and preventing an accidental nuclear explosion in the event that the plane would crash. (The Fat Man's nuclear core was installed prior to takeoff but it did have several safeguards in place as well, just not safeguards as robust as having the core separated from the bomb). This removable core (or "pit," as it is called) also allowed the weapon to be armed for a variety of explosive yields, from somewhat larger than that of Fat Man at 31 kilotons all the way down to one kiloton, in order to make the weapon useful against a variety of potential targets. — Another improvement in the Mark IV’s design that is visible on the outside is the nose plate, which was removable. As a safety measure the Mark IV was designed so that the nuclear core could be installed by a technician while in flight (an approximately half-hour process), thus protecting the air base, the airplane, and preventing an accidental nuclear explosion in the event that the plane would crash. (The Fat Man’s nuclear core was installed prior to takeoff but it did have several safeguards in place as well, just not safeguards as robust as having the core separated from the bomb). This removable core (or “pit,” as it is called) also allowed the weapon to be armed for a variety of explosive yields, from somewhat larger than that of Fat Man at 31 kilotons all the way down to one kiloton, in order to make the weapon useful against a variety of potential targets.

The smaller size of the Mark V (which actually entered service a year after the Mark VI and three years after the Mark IV, despite the numerical sequence of their names) is obvious when you see the more streamlined shape of this bomb as compared to the others. Located at the National Museum of the US Air Force near Dayton, Ohio.

The Mark V bomb, as you can see, has door panels that can be opened to facilitate the removal of the nuclear "pit" (to either safeguard the weapon from a nuclear explosion or to allow the use of pits with various degrees of explosive power). The part that goes in and out of the weapon with the pit is called the "birdcage" and the insertion process could be automated. An example of a birdcage is sitting right there in the clear case next to the bomb. — The Mark V bomb, as you can see, has door panels that can be opened to facilitate the removal of the nuclear “pit” (to either safeguard the weapon from a nuclear explosion or to allow the use of pits with various degrees of explosive power). The part that goes in and out of the weapon with the pit is called the “birdcage” and the insertion process could be automated. An example of a birdcage is sitting right there in the clear case next to the bomb.

The text on the sign reads: "M102 Birdcage. For the first 15 years of the nuclear weapons era, all nuclear components were stored separately from the rest of the weapon. The M102 was the first pressurized storage and shipping container, often called the 'birdcage.' The nickname 'birdcage' came from the metal frame that surrounds the central storage tube. Used with bombs and artillery rounds, the frame kept nuclear components separated. The neutron absorbing material in the central storage tube prevented nuclear components that were stored close together from causing a nuclear reaction." Located at the Nuclear Museum in Albuquerque. — The text on the sign reads: “M102 Birdcage. For the first 15 years of the nuclear weapons era, all nuclear components were stored separately from the rest of the weapon. The M102 was the first pressurized storage and shipping container, often called the ‘birdcage.’ The nickname ‘birdcage’ came from the metal frame that surrounds the central storage tube. Used with bombs and artillery rounds, the frame kept nuclear components separated. The neutron absorbing material in the central storage tube prevented nuclear components that were stored close together from causing a nuclear reaction.” Located at the Nuclear Museum in Albuquerque.

This is the nose of a Mark VI bomb at the National Museum of the US Air Force in Dayton, Ohio. It is painted bright yellow. The cover is removable so that the nuclear pit can be inserted. I do not understand why there are only four bolts, arranged asymmetrically, holding it to the body of the bomb.

A wider view of the same bomb at the National Museum of the US Air Force.

Another view of the National Museum of the US Air Force's Mark VI bomb. Behind the bomb you can see the large wing of a Boeing B-50D Superfortress, an updated B-29 (although this particular aircraft was, in the last years of the plane's service, converted to a weather reconnaissance role). — Another view of the National Museum of the US Air Force’s Mark VI bomb. Behind the bomb you can see the large wing of a Boeing B-50D Superfortress, an updated B-29 (although this particular aircraft was, in the last years of the plane’s service, converted to a weather reconnaissance role).

A Mark VI at the Museum of Aviation located at the Robbins AFB in Warner-Robbins, south of Macon, Georgia. The bomb sits under a B-29, the same general model of aircraft that dropped the atomic bombs on Hiroshima and Nagasaki.

A rear view of the Mark VI at the Museum of Aviation. Note the thickness of the tail fins when seen from this view.

I do not know why the Hill Aerospace's unit lacks the rounded nose cover seen at the Museum of the US Air Force, nor why, lacking that cover, it also lacks the bolt holes that would have secured the cover to the main body. — I do not know why the Hill Aerospace’s unit lacks the rounded nose cover seen at the Museum of the US Air Force, nor why, lacking that cover, it also lacks the bolt holes that would have secured the cover to the main body.

Weapon Specifications

Note that the relationship between explosive power and destruction is not linear—a weapon’s destructive effects grow far more slowly than its explosive power.

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

NukeMap

Simulated destruction of the Mark VI bomb as if detonated at Albuquerque, New Mexico. Click on the map to change parameters.

Videos

These curated videos provide additional context for this weapon — showing test footage, deployment scenes, technical explanations, interviews, or other historical material, allowing viewers to go deeper into the weapon’s design, use, and place in nuclear history.

Rebecca Ullrich: Spinning Off from the Manhattan Project, 1 hour and ten minutes

Los Alamos Historical Society: The Origins of Sandia National Laboratories, February 9, 2021. This presentation will detail Sandia’s origins and its first decade of growth, during which it defined its ordnance engineering mission, developed a significant environmental testing capability, and created a distinct community.

Project Crossroads – Nuclear Test Film (1946)

Nuclear Vault: Project Crossroads was a series of two nuclear tests, Able and Baker, the first tests conducted at the Pacific Proving Ground, near the Bikini Atoll in the Marshall Islands. The two bombs used were of the Fat Man configuration, very similar to the implosion-type weapons tested at Trinity and dropped on Nagasaki, Japan.

Mark 4 America’s First Widely Produced Nuclear Bomb, 1 minute

Topaz: This bomb was not only easier to manufacture (than that Fat Man/Mark III) but also much simpler to use in combat situations. The Mark 4 was deployed in 1949, and the United States produced a total of around 550 nuclear bombs with yields ranging from 1 to 31 kilotons

Mark-5 (Mk 5) Nuclear Weapon, 3 minutes

Cold War Films: The United States first light weight nuclear weapon was the Mark-5. It was an atomic (Fission) type weapon.

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].