The 1945 Trinity test was run to prove the complicated “Fat Man” design would even work. But why were scientists so confident of the “Little Boy” design that the didn’t feel the need to do a full test of the design? The design was simpler, but still, no one had ever done it before. Was it purely for time? Lack of materials? Or was the design and principles so sound, it couldn’t fail?
In: 11
It can’t fail.
If you fire a “particle” at some uranium, it knocks off another instance of the same particle and releases energy.
If you can fashion a shape big enough (a “critical mass”) to maximize the second particle’s chances of hitting another atom of uranium then you get a chain reaction, or “bomb”. Turns out that any seven inch diameter of uranium will destroy your city.
The trick, of course, is to get that globe over Hiroshima without killing yourself in the first place, so Little Boy was a Navy gun with half the globe at the muzzle and the other half in the breech. Drop the bomb, fire the gun at a pre-determined altitude/pressure and bye-bye Hiroshima.
A gun type design (Little Boy) was a more simple theory and design that they were very confident would create the necessary critical mass to function in a gravity weapon and not just a static test.
The implosion type device (Fat Man)was a more complicated theory and design with much more exacting specifications that needed proving before being placed in a gravity weapon and dropped.
The actual fission reaction of Little Boy was guaranteed to happen, so long as the steps leading up worked.
So as long as the conventional explosives detonated, and as long as the slug didn’t get stuck in the barrel, once it reached the other end, you were guaranteed a runaway fission chain reaction, so all of the ways in which the bomb could fail had nothing to do with the actual nuclear explosion, and so those things could be tested and verified without actually needing a live nuclear payload.
The design of fat man was different, it relied on increasing the pressure of the fissile material enough to make it critical, so even *if* the conventional explosives did detonate, you still weren’t guaranteed a fission reaction, because it was impossible to predict exactly how the explosive pressure wave would propagate. Furthermore, the fat man design used a neutron source, a small ball of material embedded in the fissile core, that was theoretically supposed to emmit a flood of neutrons when the bomb went off, and therefore trigger a lot of fission reactions at once, to kick start the bomb. Both of these mechanisms could have failed to go off, even if the convnetial driving explosives detonated, and so the scientists needed to verify that the force from those driving explosives was actually large enough to trigger fission like they wanted, and that required an actual live fissile test.
TBF to OP, even Paul Tibbets (commander of the Hiroshima mission) asked what would happen if the gun-type bomb failed. Deak Parsons (one of the Manhattan Project brass) told him “Then we will just make a nice big dent in the target area and go back to the drawing board.”
So in a bomb you have nuclear material, for this nuclear material to explode, it must achieve a state called “critical mass.” Generally the idea is to get enough nuclear material close enough together to get a nuclear chain reaction, here are a few ways to do this.
First is by mass itself. If you have enough pure enough U-235 in the same place, it will explode if given a few neutrons to start the reaction, or at the very least it will heat up, maybe expand, maybe melt everywhere, etc. It will start producing energy pretty quickly after hitting this critical state.
Second is shape. The shape of the mass matters, usually spheres are the best shapes for these kinds of materials, because its the shape where on average atoms are closest together.
Third is by density. A more dense material will be closer to critical mass.
Fourth is material purity, though this kinda falls under density. The idea
Beyond this we have a few other tricks, throw in some things called neutron moderators, neutron reflectors, etc.
So generally the idea of a nuke is we have sub critical nuclear fuel that we somehow get into a critical state, the deeper into that critical state the bigger and more efficient the boom.
Initial ideas for the nuke were going to be a large ship because initial beliefs were that we either couldnt get enough pure uranium 235 to get critical mass or that we thought we needed a lot more mass for this critical mass. We eventually figured out we could make it small enough.
Little Boy’s design was simpler in the sense that it was based on the first criteria, mass. It has two chunks of uranium and the “guns” job was to get both together, I think theres a common belief that the two chunks of uranium smashed together but they didn’t really, the “bullet” was a cylinder with a hole that was fired at another smaller cylinder that fit into the hole of the bigger cylinder. So when fired the two cylinders would slide on top of each other, that alone was mostly enough to achieve critical mass. They just had to come together fast enough that the thing didn’t explode/destroy itself as the cylinders were coming together, preventing a big enough reaction from happening.
We were pretty sure at this point that some kind of big explosion would happen, the exact size though was unknown. It was also pretty inefficient for the amount of nuclear fuel we would use.
The implosion design was a bit more interesting. The idea was we could use less fuel and get more efficient use of fuel if we increased the density by triggering an explosion that compressed the nuclear fuel. The design would be a sphere, which is the best shape you can use, achieving more bang for the amount of fuel.
So whats difficult about it. Amusingly its not the nuclear part, its the conventional explosives part.
The explosives had to press on this sphere of plutonium evenly and with enough force to increase its density. So firstly, you can’t just light the explosive to blow up all at once, the explosion has to travel through the TNT or whatever to trigger more explosion. This can introduce places and times where the explosion doesn’t press on the plutonium evenly from all angles, it might press it more from some directions, deforming its shape more than compressing its density. So we had the explosion trigger from multiple points along a sphere around the plutonium, but this also has issues in that we could accidentally trigger some explosive earlier than others causing it to fail, even if just by a few microseconds. Then this explosion takes time to travel, it could be that in the time it takes to trigger all the explosives, some of the explosion has already exhausted its energy and can’t press on the sphere anymore.
If everything went well, we had a few microsecond window to introduce neutrons into the compressed sphere. If we mistimed this, the bomb would fizzle out and just blow itself apart from the conventional explosives. The gun design didn’t have this issue, once the cylinders were together they were together.
Even if we fully understood how regular TNT worked and would have acted, the math for calculating that is hard. Its not too bad if everything is spherically symmetrical, we can assume a lot of convenient assumptions and the math works out, but when its not things get complicated fast and theres an unreasonable amount of number crunching that has to be done, and back in these days this had to be done by pen and paper. Nowadays with computers our nukes arent symmetrical, they are usually more egg shaped for reasons and have all sorts of imperfections and non symmetries.
There were just a lot of factors, we were pretty sure it could work but if we misstimed anything or made any mistake it wouldn’t have worked.
Another big factor to point out was **trinity was a test of many aspects of the gun design as well.** As a more complicated bomb, it also tested many principles of the less complicated one, like whether this reaction would go through in the first place. It was also a cheaper variant, it used like a 10th the amount of fuel the gun one used, if it didn’t go, oh well, not too much fuel wasted.
A lot of answers here have good points, but what they fail to consider is material availability. They simply didn’t have enough uranium for a test. The “Little Boy” bomb uses U-235 to explode. The “Fat Man” uses Plutonium. U-235 is a lot harder to make than Plutonium. In fact, you make Plutonium using the waste material of U-235 extraction. As such, in July 1945, they had enough material for 3 bombs – two “Fat Man” and one “Little Boy”. Then the logic become obvious as to why they tested “Fat Man” – it was the only spare they had to work with
Actual ELI5
To make a nuclear bomb you need to have lots of neutrons in one area, let’s say it’s 10 neutrons in a 1 meter circle.
With uranium it works out that a one meter circle of uranium has 11 neutrons. So if we split it in half and have two semi circles of 5.5 neutrons each and then bring them together, we can be pretty sure it will work
Plutonium on the other hand is too big you can’t fit 10 neutrons into a 1 meter circle if they are connected to plutonium, but you can fit 50 neutrons in a 1.5 meter circle….
The question is how do you shrink the circle so that at least 10 neutrons are in that 1 meter circle.
The answer- explosives
Because once you have enriched uranium making nuclear bomb explode is completely trivial. You could literally detonate it with a big hammer or a screw press. I’m not kidding.
There were early plans to build a plutonium cannon bomb called Thin Man, but they came to nothing. The bomb would use plutonium 239, but the metal coming out of the Hanford reactors contained a sizeable amount of the Pu240 isotope. Plutonium 240 has a much higher spontaneous fission rate than Pu239 which means there are more available neutrons.
In the time a uranium cannon took to assemble a critical mass, it was found highly likely that contaminated Pu240 would have triggered a premature chain reaction; the bomb would ‘fizzle’ – it’d be perhaps a few tonnes equivalent.
The way it could have worked would have been to have a much higher muzzle velocity for the cannon which required a longer barrel. Whilst the required velocity was *just* about possible, the weapon would be too long for any existing or planned bomber and Thin Man was abandoned in July 1944.
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