How Big Was the Atomic Bomb Dropped on Hiroshima?

Harry Atkins

2 mins

20 Jun 2018

“Little Boy” may seem like an odd name to give the first atomic bomb used in warfare. This, after all, was a weapon that inflicted unprecedented devastation on the Japanese city of Hiroshima.

Though its impact was physically and historically enormous, Little Boy was so named because its proportions were relatively diminutive compared to the two atomic bomb designs that preceded it: “Fat Man”, which was detonated over the Japanese city of Nagasaki three days after the Hiroshima attack, and “Thin Man”, the development of which was abandoned.

From Thin Man to Little Boy

Little Boy was a development of the abandoned Thin Man design. It derived its explosive power from the nuclear fission of uranium-235 rather than the isotope originally earmarked for use in Thin Man – plutonium-239.

Dan talks to Hirata San, a survivor of the Hiroshima attacks, and one of the few remaining survivors who speak English, about the Hiroshima bombing.Listen Now

Ultimately, plutonium was deemed to be an impractical fissionable material and so designs were reworked to suit the use of uranium. The bomb’s developers were able to simplify the Thin Man design, creating a smaller weapon that could fit into the bomb bay of a B-29 aircraft.

Little Boy measured 3 metres in length and had a diameter of 71 centimetres, smaller than the Thin Man design, which specified a length of 5.2m and a diameter of 97cm. Fat Man’s appropriately rotund proportions, meanwhile, were 3.3m by 1.5m. While Little Boy weighed 9,700 pounds, Fat Man weighed 10,800.

Fat Man vs Little Boy

Plutonium had been deemed unsuitable for a Thin Man or Little Boy gun-type fission design and so a different mechanism was required for such bombs to be used.

Unlike Fat Man, which utilised an implosion-type weapon with a solid plutonium core, Little Boy’s gun-type design instead shot a hollow cylinder of enriched uranium onto a solid cylinder of the same material.

The mushroom cloud that erupted after Little Boy was dropped.

As a consequence of its design, Fat Man was significantly bigger and heavier than Little Man. Indeed, modification had to be made to the B-29 aircraft in order to accommodate it.

Fat Man’s blast yield was also greater than Little Boy’s: while the former’s measured 21 kiloton, the latter’s measured about 15 kiloton.

Damage inflicted

Despite Little Boy’s inferior power, however, Hiroshima’s estimated death toll of 90,000 to 146,000 was considerably higher than that of Nagasaki (39,000-80,000).

This was due to the fact that the hypocentre of the Little Man blast occurred on flat terrain, while Fat Man was dropped off-target in the Urakami Valley. This meant that large portions of Nagasaki city were shielded from the blast by hills and mountains.