Bockscar's Pumpkins

Back in 1945 on this very date, just 3 days after the USA dropped the first atomic bomb on Hiroshima, they dropped a second one on Nagasaki. The first bomb, dropped by Enola Gay was of the gun design, a cylinder of 2 feet diameter, whose cylindrical aerodynamics were well understood. Now the critical masses of atomic bombs have to be assembled very quickly so there are two basic ways of doing this, shoot one half into a second half (the gun design) as used in the Hiroshima bomb (called Little Boy), or implode from a shell shape (called Fat Man).

The name Fat Man refers to the early design of the Nagasaki bomb because it had a wide, round shape. Fat Man was an implosion-type nuclear weapon with a solid plutonium core. Implosion was the best design for a fission weapon. Its only drawback was its diameter. Fat Man was 1.5 metres (5 ft) wide vs 61 centimetres (2 ft) for Little Boy. The aerodynamics of such fat bombs were NOT well understood.

So the folks on the Manhatten project also built several conventional HE (high explosive) bombs (called pumpkins) with the same ballistic and handling characteristics as a Fat Man, so the crew of Bockscar could practice with its different aerodynamics. Bockscar is the name of the B-29 bomber that dropped a Fat Man nuclear weapon over the Japanese city of Nagasaki during World War II. Bockscar was used in 13 training and practice missions from Tinian Island and three combat missions in which it dropped pumpkin bombs on industrial targets in Japan.

Almost 500 pumpkins were built. The inert versions were filled with concrete and sand to the same weight and C of G as Fat Man. The active pumpkins had HE and fuses in them. The initial tests demonstrated that the Fat Man assembly was unstable in flight, and that its fuzes did not work properly.

After training, 49 active pumpkin bombs were dropped in combat onto 14 targets in Japan. Bockscar dropped three and Enola Gay two. With the aerodynamics better understood, Bockscar then dropped Fat Man onto Nagasaki; 21 kilotons yield, 6 more than Little Boy had had. About 20% of the plutonium was consumed in the fission, the rest distributed as fallout.

This is what a Bockscar's Pumpkin looks like (photo taken at Trinity Site).

Comments (2)
Billions of Versions... wrote " I had no idea about all the testing and bombing that went on with the pumpkin bomb." They needed to make sure Fat Man worked; afaik there were no reserves.
Schorsch asks "So if Fat Man assembly was unstable in flight, how was that fixed?" By adding the square tail fins.

Copyright ^© Ole Phat Stu on August 9, 2022 permalink Comments Email

---