The Little Man bomb
The Fat Man Man Bomb
There is more than one way to skin a cat and I think the little boy was a much more elegant design. here is a diagram of the fusion reactor that I did not see on Popular Mechanics: 
The Little Man bomb
The Fat Man Man Bomb
There is more than one way to skin a cat and I think the little boy was a much more elegant design. here is a diagram of the fusion reactor that I did not see on Popular Mechanics:
You may think that but you’re wrong. Little Boy was crude and inefficient - and the design was never used again after an implosion style device was shown to work.
U-235 proved too time-consuming to create. Not only was the process slow, the K-25 plant at Oak Ridge had so many centrifuges separating out the 235 from the 238 that it was consuming 1/7th of all the electricity created in the entire country. So they had to switch to Plutonium.
P-239 was even more unstable than U-235 so they knew they could get the same yield from a much smaller mass of plutonium, and the plutonium promised to be faster to produce. The hitch was that you had to have a functioning nuclear reactor to create it. Use a reactive pile to bombard U-238 with neutrons released from a fission reaction and, Bada-Bing, you got P-239.
So they sent Leslie Groves looking for real estate. He spent a couple of weeks in Washington scouting for locations, found what he wanted, and within six months they had two new fission reactors up and running, producing P-239.
Then the numbers guys at Los Alamos discovered a problem. The plutonium they were producing wasn't pure enough to use in the gun-design bomb. Too many U-238 atoms were picking up a second extra neutron, making P-240. And the impurity made the material unsuitable for the much simpler design. The U-235 was too slow coming so there was no alternative. They had to switch to the implosion design for the plutonium bombs.
But that design was fraught with technical problems. It used 32 separate detonators and all 32 had to go off within one one-millionth of a second of each other or the implosion wouldn't be sufficiently uniform to achieve fission.
In fact there were so many uncertainties that they decided they had to have a test detonation to prove that it would work. Which is why the Trinity bomb was a plutonium-implosion design. Because there was never any doubt that the gun design U-235 bomb would work. Plutonium-implosion ... not so much.
When they conducted the Trinity test, the USS Indianapolis was waiting in San Diego harbor with the bomb assembly and installation team and the components of Little Boy already on board. Because they didn't want to use the first bomb unless the second one was at the ready, just in case the Japanese were really that pig-headed. So there the Indy sat, without knowing why, waiting on the Trinity test.
Within hours of when Trinity went Ka-Boom!, Indianapolis was making flank speed for Tinnian.