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National Museum of Nuclear Science & History

Little Boy and Fat Man

Date:
Wednesday, July 23, 2014
Little Boy ready to be lifted into the Enola Gay

An explosive nuclear chain reaction occurs when a sufficient quantity of nuclear fuel, such as uranium or plutonium, is brought together to form a critical mass.  This is the minimum amount of fissionable material needed to start a chain reaction. The chain reaction starts when neutrons strike the heavy uranium or plutonium nucleus which splits releasing a tremendous amount of energy along with two or more neutrons which, in turn split more nuclei, and so on. For more information, see The Science Behind the Atom Bomb.

Little Boy

In this gun-type device, the critical mass is achieved when a uranium projectile which is sub-critical is fired through a gun barrel at a uranium target which is also sub-critical.  The resulting uranium mass comprised of both projectile and target becomes critical and the chain reaction begins. Dropped on the Japanese city of Hiroshima on August 6, 1945, it was the first nuclear weapon used in a war.

  • Weight: 9,700 lbs
  • Length: 10 ft.; Diameter: 28 in.
  • Fuel:  Highly enriched uranium; “Oralloy”
  • Uranium Fuel: approx. 140 lbs; target – 85 lbs and projectile – 55 lbs
  • Target case, barrel, uranium projectile, and other main parts ferried to Tinian Island via USS Indianapolis
  • Uranium target component ferried to Tinian via C-54 aircraft of the 509th Composite Group
  • Efficiency of weapon: poor
  • Approx. 1.38% of the uranium fuel actually fissioned
  • Explosive force: 15,000 tons of TNT equivalent
  • Use:  Dropped on Japanese city of Hiroshima; August 6, 1945
  • Delivery:  B-29 Enola Gay piloted by Col. Paul Tibbets

Fat Man

“Fat Man” was the second plutonium, implosion-type bomb.  The first was the “Gadget” detonated at the Trinity site on July 16, 1945.  In the implosion-type device, a core of sub-critical plutonium is surrounded by several thousand pounds of high-explosive designed in such a way that the explosive force of the HE is directed inwards thereby crushing the plutonium core into a super-critical state. Dropped on the Japanese city of Nagasaki on August 9, 1945, it was the second nuclear weapon used in a war.

  • Fat Man being prepared for an airfield tripWeight: 10,800 lbs
  • Length: 10 ft 8 in.; Diameter: 60 in.
  • Fuel:  Highly enriched plutonium 239
  • Plutonium Fuel: approx. 13.6 lbs; approx. size of a softball
  • Plutonium core surrounded by 5,300 lbs of high explosives; plutonium core reduced to size of tennis ball
  • Bomb Initiator:  Beryllium – Polonium
  • All components of Fat Man ferried to Tinian Island aboard B-29’s of the 509th Composite Group
  • Efficiency of weapon: 10 times that of Little Boy
  • Approximately 1 kilogram of plutonium fissioned
  • Explosive force: 21,000 tons of TNT equivalent
  • Use:  Dropped on Japanese city of Nagasaki; August 9, 1945
  • Nuclear Weaponeer: Cdr. Frederick Ashworth
  • Delivery:  B-29 Bockscar piloted by Maj. Charles Sweeney

Photos courtesy the National Archives.

Little Boy, covered by a protective tarp for security reasons

Manhattan Project scientists and military personnel gather around the bomb pit, ready to watch the Little Boy bomb being loaded into the Enola Gay

Little Boy ready to be lifted into the Enola Gay

Little Boy ready to be loaded into the Enola Gay

The Enola Gay backs up over the bomb pit

Little Boy is lifted via a hydraulic lift into the Enola Gay

Little Boy is lifted via a hydraulic lift into the Enola Gay

A view from underneath the hydraulic lift, in the bomb bay

Little Boy has been successfully lifted into the bomb bay and is being attached to sway brackets that will keep it secure

Little Boy in the Enola Gay bomb bay

Bomb pit on Tinian

Little Boy, covered by a protective tarp for security reasons

The assembled implosion sphere for Fat Man ready to be placed in the casing

Sealant is applied via spray gun

Putty is applied to the forward plate

Norman Ramsey signing his name on Fat Man

The Fat Man bomb being towed toward the airfield with an escort

The sealant has been applied. Note the signatures on the tail assembly and the "bomb" on the worker's shirt

Fat Man is lowered on to a transport dolly for the trip to the airfield. Note the signatures on the tail assembly

“A Second Kiss for Hirohito!” Signed by Rear Admiral W.R. Purnell, USN, on the side of Fat Man

Signatures on Fat Man's tail assembly. You can see the small signature of "W. S. Parsons, USN" or Enola Gay weaponeer Deak Parsons, to the far left

Preliminary pit alignment

Fat Man is manually pushed to the bomb pit

Fat Man's alignment with the bomb pit is checked

The towing cable is disconnected. Note the MPs keeping guard

Fat Man is raised on a hydraulic lift

Once alignment is complete, Fat Man is lowered into the pit

Fat Man is lowered into the pit

Related Video:
This silent footage, in both color and black and white, shows the preparation of the “Little Boy” and “Fat Man” atomic bombs on Tinian Island. It includes the takeoff and return of the Enola Gay, which dropped "Little Boy" on Hiroshima on August 6, 1945. The footage also depicts the mushroom cloud above Nagasaki.