What exactly was it that the Manhattan Project was working on?

My (admittedly limited) understanding is that one of the biggest issues being developed was how to get the critical mass together in the most efficient manner.

Once you get a sufficiently large enough quantity of fissionable material together it will start a chain reaction and begin converting its own matter into a large amount of energy. The more matter that is converted into energy, the bigger the explosion.

The problem is that the explosion that is occurring pushes the fissionable material apart and ends the chain reaction. So the scientists and engineers needed to develop a way to bring the critical mass together that was incredibly precise.

Another short answer: “What @Little_Nemo said.”

The Manhattan Project studied the theory on paper, but the practical work was focused on the engineering, material properties, and material studies involved with safely assembling a critical mass, and keeping it assembled for maximum yield.

Explosively-driven assembly is not a forgiving science. A lot of work went into characterizing the properties of explosives at the time (alongside the nuclear science). When you go to working with military explosives that were “energetic enough to get the job done” (i.e. for blasting, breaching), and try to apply that to performing finely-timed mechanical work with finesse, there’s a large gap in knowledge between the two applications.

Then you have all of the nuclear interactions that happen with an assembled mass. You have to develop ways to characterize performance, assess it, and then circle back to improve it.

Short answers, I know. . .

Tripler
There’s lots of work to be done going from theory to practical invention.

Some of the things that the Manhattan Project figured out for making a nuke do what it do in the real world:

Krytrons weren’t actually developed specifically for nuclear weapons, but once their utility for that purpose was understood, they became export-controlled. Krytrons and exploding-bridgewire detonators are some of the technology needed to assure extremely precise synchrony of the detonation of all of the explosive lenses surrounding the fissile plutonium pit.

Project Y at Los Alamos is where they did the deep science…

Feynman tells us that they had human computers , to calculate the yield , based on the theory and from observations, mushroom cloud size, velocities and stuff

“A large group of women worked on numerical calculations in the T-5 (Computations) Group.”

In fact, the preferred bomb design, thin man, had all the parts built but not used because they calculated, by the Los Alamos team, that it would self-destruct before use, based on the contamination of the plutonium that they could get .

Thin man = gun type plutonium

the gadget (trinity test) and fat boy (nagasaki) was an implosion type plutonium bomb.

They used a gun type uranium bomb at Hiroshima, that worked

So the computer team had a lot of stuff to calculate…

This.

I don’t know what we’d use to measure and analyse the precise timing and pressure sequence of a conventional explosion today, but take that instrumentation and subtract 70 years.

Another factor is that the fissionable material in question was extremely rare. They were making / refining the first grams of the right isotope mixes while they were figuring out how to compress it explosively. And they couldn’t do 50 real practice explosions in the backyard, at least not with the fissionable material itself.

Shaped charges are interesting. I remember a talk by a fellow who worked in an open pit - they saw how much commercial shaped charges cost each, and created their own out of a plastic funnel packed with explosive gel and an detonator in the nozzle. he said it was like a giant sledge hammer, since the blast (mostly) propagated spherically from the apex of the cone. they tried it one time on a giant boulder they couldn’t move. Came back after the blast and the thing was still there - until someone tapped it with a backhoe bucket and it fell to pieces.

Feynman has the story that the calculations involved a series of matrix calculations - when an error was found, they had to go back and recalculate from error. He reasoned that the error could be fixed faster, since it only involved the row and column of the result for next stage calculation. Just recalculate those. Since the results were on punch cards for the next step, he used a different color punch card for the correction - eventually the back calculation would get caught up and the color cards could be removed. he showed the ladies doing the work how to do this.

he came back two days later to find a rainbow of different cards all through the system trays as they fixed multiple errors in parallel.

But basically, the purpose of Manhattan was to translate pure theory into a workable device. Also, IIRC they had enough material for one test and then they shipped two bombs to the Pacific. It would be a month before another one was available (also an important state secret) and for a while, only one a month. So it was imperative to be sure the device worked without wasting fissile material on failures.

[True Story]
Shaped charges are fun to build, shoot, and see the effects of! I built an improvised shaped charge using a Bic pen body, packed it with about an 1-1/2" of C-4, primed it, and mounted it on a deadbolt during a breaching exercise. It poked such a clean hole through the housing of the deadbolt, that the door was unscorched, but the cylinder was thrown 30’ down the field. As the cylinder was being “extracted,” it pulled the bolt out of the jamb with it. T’was a cool thing to see.
[/True Story]

And a lot of deep science still goes on!

Tripler
We have machine-based computers to do the work, now. . . :wink:

ok, ok, but the question was the Manhattan project. I was just reading where it was vonNeuman who determined what altitude to explode the bomb at. He was doing physics as well as digital computing… Its intriguing that they didn’t test the uranium implosion device ?
… Not sure why they didnt get Japanese to watch the gadget trinity test… why couldn’t they get a warning ? Why didnt they show the japanese the other bombs … here’s 10 ready to go (but for the fissionable uranium/plutonium of the correct isotope, they could make a few a year… )… I think it was a political message … “we will hit back and we will use these to murder civilians so don’t tempt us !”.

There were two main reasons they didn’t test the uranium bomb:

  1. It was taking a lot of time to enrich the uranium to the point it could be used in a bomb. Testing it would delay the production of a usable bomb.
  2. The design of the uranium bomb was much simpler and they were more confident that it would work. It was basically using a gun to shoot a slug of uranium into the middle of another mass of uranium. It didn’t require as precise of an explosion method as the plutonium bomb.

There weren’t 10 ready to go.

And timing was critical. How long do you think it would require to contact the Japanese, get agreement to send observers into the remotest part of New Mexico in utter secrecy, and have them go back to Japan since there’s no possibility they’d phone home on something like this?

What do you think would happen if anything went wrong en route and word got out that Roosevelt was smuggling high-ranking Japanese into the country to watch a secret weapons test?

Why do you think that watching the test would get them to surrender? The Supreme War Direction Council refused to surrender after the bomb destroyed Hiroshima.

And, number one, first and foremost, what if the test fizzled, as it very well might have?

There was gigantic downside to such an invitation and virtually no upside from the U.S. pov at the time. Hindsight doesn’t add any.