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What Was Fat Man?

Mary McMahon
Updated May 23, 2024
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Fat Man was an atomic bomb manufactured in the United States and dropped on the Japanese city of Nagasaki on 9 August, 1945. Six days later, Japan surrendered to the United States, effectively ending the Second World War. The use of atomic weapons in the World War II continues to be a topic of controversy, with some people arguing that the deployment of such weapons was necessary, while others feel that it violated the rules of common decency. Whatever one's feelings about the use of atomic weapons to end the war might be, they certainly reshaped human history.

The nuclear explosion generated by Fat Man was only the third man-made nuclear explosion in history, and the second use of a nuclear weapon in warfare, preceded by the bombing of Hiroshima three days earlier. The bomb released the equivalent of 21 kilotons of TNT, a paltry amount when compared with modern nuclear weapons, but it managed to be quite devastating.

The origins of the name “Fat Man” have been debated. The bomb's creators have suggested that it was named for its distinctive squat shape, which did sort of resemble a fat man sitting on an armchair. Others have said that it was named for British Prime Minister Winston Churchill, one of the more notable large men involved in the Second World War. Whatever the origins of the name were, it was originally intended simply as a code name, so that people could discuss the bomb in secrecy.

The bomb was deployed from a B-29 bomber known as Bockscar from 1,800 feet (550 meters) above the city. Fat Man was an implosion-type device, meaning that the nuclear reaction was generated by a shaped charge which exploded inward, compressing the plutonium core of the bomb to create a nuclear explosion. The design was rather innovative, and some people weren't even certain that Fat Man would work when deployed in action.

These fears proved groundless; within seconds, Fat Man exploded, killing an estimated 45,000 in the city instantly, and causing thousands more to die in the following weeks due to injuries sustained as a result of the blast and the accompanying fires, which ravaged Nagasaki. Within a year, the death toll had risen to 80,000 people. In the decades following, survivors of the bomb, known as Hibakusha or “bomb-affected people,” also experienced a variety of health problems as a result of radiation exposure, ranging from fertility issues to a high incidence of cancer.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a Historical Index researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By stevenb — On Jun 15, 2015

My comment is more a question. My grandfather worked on the two bombs. He was an applied mechanical engineer. I would like to honor his memory. He lived to age 101 and I miss him. Anyway, I was hopping you could help me with this in finding more info on his role in the making of these.

By Izzy78 — On Dec 03, 2011

If I remember correctly, Nagasaki wasn't even supposed to be the original location of the bomb drop. I don't remember what the planned second location was, but it was going to be a more residential city. I think the weather or flight conditions for something that day weren't very favorable, so they abandoned the plan. I want to say maybe the plane was even en route to the city when they aborted.

Instead, they chose Nagasaki, which was an important industrial town that had an important port for the Japanese navy.

I believe the book was about Hiroshima, but "Black Rain" by Masuji Ibuse is an interesting book following someone affected by the fallout for several weeks after the bomb was dropped. I don't believe it is based on a specific person, but it uses historically accurate information.

By jmc88 — On Dec 03, 2011

@turkay1 - I believe I have also heard that all of the plutonium didn't react. I would have to guess it produced more than 1 percent of its potential, though.

For anyone who is interested in these types of things, you can actually see Bockscar on display at the United States Air Force Museum in Dayton, Ohio. I was passing through on a trip a couple of years ago and saw it. The whole place is very interesting. Next to the plane, they actually have a replica of the Fat Man and Little Boy bombs, too. They are actually smaller than what I expected compared to the bombs of today.

By candyquilt — On Dec 03, 2011

@fify-- Yea, but wasn't Fat Man much more powerful than Little Boy?

I understand it didn't kill as many people but I remember reading that the explosion Fat Man created was not even 1% of its capacity. I don't know if scientists had planned it this way or if this was actually a failure on their side.

Forget about missing target, if Fat Man worked at full capacity, maybe there wouldn't even be a Japan today. Thank goodness that didn't happen!

I've also seen pictures of Fat Man and it does resemble one, so I'm pretty sure it was named after it's shape.

It doesn't look as extravagant as I expected it to be. No one could guess from its appearance that it has the potential of creating so much destruction so quickly.

By serenesurface — On Dec 02, 2011

I'm of the view that Little Boy atomic bomb and Fat Man atomic bomb were not necessary.

I tend to think of politics and war as temporary situations. There is no friendship or animosity in politics. Countries act on what their interests are at that time, which can easily change in a short period of time.

But a weapon like the atomic bomb leaves close to permanent effects. It doesn't end with the immediate deaths. Even today, there are many Japanese suffering from cancer and other illnesses that were passed down to them from their grandparents who experienced Hiroshima and Nagasaki.

Yes, we can't change the past, but we can make sure that Little Boy and Fat Man are not made again.

By fify — On Dec 02, 2011

@jonrss-- I think Hiroshima gets more attention because the atomic bomb that was dropped on Hiroshima (called 'Little Boy') killed more people than Fat Man did. If I remember correctly, Little Boy killed at least 20,000 more people than Fat Man and injured way more too.

But we also tend to forget that Fat Man and Little Boy were different kinds of atomic bombs. Fat Man was a plutonium bomb but Little Boy was made of uranium.

Something else that few people know is that both Fat Man and Little Boy missed their targets. They were both a little off and still killed so many people.

Can you imagine how many more deaths they would have caused if they had been right on target?

By nextcorrea — On Dec 01, 2011

The atomic bomb is one of the most dangerous things that mankind has ever invented. But it is also one of the most clever. The atomic bombs used in WW@ were incredible feats of engineering. This fact often gets lost when we think about what the bombs were used for.

The atomic bomb represents a huge step forward in man's understanding of physics, chemistry and engineering. As the article points out, the fat man used experimental technology that was largely unproven. But clearly it worked. Many of the men who worked on the Manhattan project came to regret it but we must still acknowledge all that they accomplished.

By jonrss — On Dec 01, 2011

People often forget that the US dropped 2 atomic bombs during WW2. Hiroshima gets most of the attention but Nagasaki was an event just as horrific and consequential.

I actually visited Nagasaki a few years back and it was a haunting experience. Little if any evidence of the bomb drop remains but there is still a strange energy in the city that I think much come from its tragic past.

Obviously we cannot change what has been done but I hope that people will remember all the lives that were lost or changed forever by what happened in Nagasaki.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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