Little Boy and Fat Man

A museum case you can walk round. Little Boy, the uranium gun-type bomb dropped on Hiroshima on 6 August 1945, and Fat Man, the plutonium implosion bomb dropped on Nagasaki three days later, are modelled in 3-D from their published outside dimensions. You can turn them, pull them apart, cut them open, click any part to find out what it was, and follow each one being put together on Tinian, step by step — including Captain Parsons loading Little Boy’s propellant in the bomb bay after take-off and Fat Man’s core going in last through its trap door.

This is an exhibit, not a blueprint. The level of detail is what you would find in the National Museum of Nuclear Science & History, the Smithsonian or an encyclopaedia: the casings, weights and reported yields are the published figures, and every internal part is a simple labelled block at illustrative proportions, not to scale and without engineering detail.

How to use it

Pick a bomb — Little Boy, Fat Man, or Side by side. Drag to orbit, scroll or pinch to zoom, right-drag (or two-finger drag) to pan; Reset view brings it back.
Look inside. The Cutaway slider slices the near half away; at full travel you see a half section down the middle. The Exploded slider pulls the parts apart along their assembly directions — Fat Man’s 32 explosive blocks and their detonators move outwards like a burst football.
Click a part (or its label, or its name in the Parts list) for an information card: what it was, what it did, and at which assembly step it went in. Internal parts are marked Schematic — simplified, not to scale.
Step through the assembly with Next and Prev (or the numbered dots). Each step’s parts fly into place, with the date and the story: where the uranium came from, why Little Boy flew with its propellant out, the green safing plugs swapped for red arming plugs in flight, Kokura hidden by haze and Bockscar turning for Nagasaki.
See the idea. Fire the gun cuts Little Boy open and shoots the projectile down the barrel onto the target; Show the implosion squeezes Fat Man’s core with an inward push from every side. Tick One block late and run it again to see why symmetry was the hard part: an uneven push squirts the core sideways instead of compressing it. Both are conceptual — not the real timing or proportions.
Side by side puts both bombs on a 1 m grid next to a person 1.75 m (5 ft 9 in) tall, with a comparison table.
Below the exhibit: a timeline from the Einstein–Szilárd letter to the first Soviet test, the crews and aircraft (Enola Gay and Bockscar, both now in museums), the human cost, the arms-control treaties that followed, and the sources.

The two designs, briefly

Little Boy (gun-type, uranium): 3.0 m (10 ft) long, 71 cm (28 in) across, about 4,400 kg (9,700 lb). A gun inside the casing fired one piece of enriched uranium (the projectile) onto another (the target) in the nose. It was simple enough that it was used without a test. About 15 kilotons over Hiroshima.
Fat Man (implosion, plutonium): 3.25 m (10 ft 8 in) long, 1.52 m (60 in) across, about 4,670 kg (10,300 lb). A sphere of 32 high-explosive blocks, fired together, squeezed a plutonium core. Making the push even was so hard that the design was tested first, at Trinity on 16 July 1945. About 21 kilotons over Nagasaki.

The human cost

The estimates of the dead by the end of 1945 are about 90,000–166,000 in Hiroshima and 60,000–80,000 in Nagasaki, most of them civilians; the survivors, the hibakusha, lived with the consequences for the rest of their lives. The exhibit’s Human cost tab links to the Hiroshima Peace Memorial Museum and the Nagasaki Atomic Bomb Museum, which tell that story properly.

Notes and sources

External dimensions, weights, yields, times and altitudes are the published figures, chiefly from the Atomic Heritage Foundation, the National Museum of Nuclear Science & History, the Smithsonian and the Wikipedia articles on Little Boy and Fat Man. Yields are the commonly quoted estimates (Little Boy 15 kt, with published estimates of 13–18 kt; Fat Man 21 kt; Trinity about 25 kt after a 2021 re-analysis).
The assembly sequences follow the historical accounts of Project Alberta on Tinian — Richard Rhodes, The Making of the Atomic Bomb (1986), and the Atomic Heritage Foundation’s histories — at the level of who did what, and when.
The casing shapes are simplified (Little Boy a cylinder with a blunt nose and box tail; Fat Man an ellipsoid with a box tail and its California Parachute). The internal parts are labelled blocks, deliberately not to scale; the 32-block shell is drawn as the familiar football pattern of 12 pentagons and 20 hexagons seen in museum replicas, with nothing of the blocks' inner make-up.
Casualty figures are ranges, from the Radiation Effects Research Foundation and the two cities; the true numbers will never be known.

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