Getting the A-bomb there

Silverplate

76 years ago yesterday, on August 9, 1945, the modified B-29 Superfortress Bockscar dropped the Fat Man plutonium bomb on Nagasaki, killing or wounding around 64,000 men and making the Japanese city the second, and so far last, target of atomic weapons dropped on an actual enemy. Today's article is about United States' quest for an airplane that could actually fly to Japan and drop the two atomic bombs. By the spring of 1943, physicists and engineers working on the Manhattan Project were starting to have an idea of what weight, size and shape a nuclear bomb is going to have. They were looking at two specific types, which differed in how the nuclear warhead was actually going to be detonated.
 
The simpler one was the "gun-type" bomb. Imagine a cylindrical tube: one half of the nuclear material (plutonium in the original plans) resides in one end of the tube, the other half, along with conventional explosives, in the other. Once the explosives detonate, the pressure pushes the second, moving half of the plutonium toward the far end of the tube – very much like how gunpowder propelled round shot inside a cannon in age-of-sail era warships. Only in the gun-type bomb, there's no hole in the far end where the moving "shot" could fly out. Instead, it slams against the other half of the warhead. While the two halves are individually below critical mass, the collision will send their total mass past critical level, causing a nuclear explosion.

The other type being considered was the "implosion-type" weapon. Simplified, it's a spherical nuclear warhead which has almost critical mass. This is surrounded on all side by a complex arrangement of conventional explosives. If these explosives are detonated in the same moment with the same force, their pressure would squeeze the warhead (like you squeezing something between your hands), making it slightly denser, which makes it go critical. As you can probably imagine, this type of bomb is much more complicated than the gun-type. The implosion bomb didn't have to be as long as the gun-type, but it was going to have a thicker, oval shape.

Ordnance designers calculated that the barrel of a gun-type bomb would need to be at least 17 feet long – this was a problem, since no American bomber at the time had a bomb bay that could fit a payload of that size. Two men were placed in charge of managing the delivery of the atomic bomb: Captain (later Rear Admiral) William 'Deke' Parsons of the U.S. Navy, who had previously developed the proximity fuze, and Dr. Norman Ramsey, a physicist who would win the Nobel Prize in an unrelated field (an invention important to the construction of atomic clocks) in 1989.

Ramsey first thought of modifying a Consolidated B-24 Liberator to carry the bomb, but he learned that the Navy already tried and failed to convert Liberators to carry a torpedo in its bomb bay. The only other American candidate was the B-29 Superfortress, but it had its own problem. The B-29's two 24-feet-long bomb bays, one located behind the other, could be combined into one, but the bomb would need to be not more than 2 feet thick. This was because even if the bays were opened into each other, the wing spar carry-through box would still be in the way, and a thicker bomb wouldn't have fit between that and the fuselage.

A third option was the British Avro Lancaster heavy bomber with its cavernous bomb bay and excellent heavy lifting capability (which, at a later date in the war, would allow it to carry the famous “earthquake bombs”). As luck would have it, the Lancaster's chief designer, Roy Chadwick, was in Canada in October 1943. Ramsey visited Chadwick, and while he couldn't talk to him about the atomic bomb, he did show him early sketches of both the gun- and implosion-type bomb casings. Chadwick assured him that the Lancaster could easily carry bombs of that size and shape.

The reason the Lancaster never ended up dropping the atomic bomb was far from technological. When they learned about this plan, both Chief of USAAF Gen. Henry "Hap" Arnold and Manhattan Project director Major General Leslie Groves Jr. protested vigorously, insisting that the American atomic bomb should be carried and dropped by an American plane, if it was at all possible. The only option left was to modify the Superfortress.
 
The project to do so was designated as Silver Plated Project, but was later shortened to Silverplate. In order to further secrecy, a false background story was invented for the project. According to the story, British Prime Minister Winston Churchill (codenamed “Fat Man” for obvious reasons) and President F. D. Roosevelt (codenamed “Thin Man”) were going to make a tour of the United States, visiting defense industries. Their trip required a specially modified ("silverplated") Pullman railway carriage. This was, of course, all false. Fat Man and Thin Man were the implosion- and gun-type bombs, and the Pullman was the first airplane to be modified to carry them.

The project was so secret that the shortened name "Silverplate" was not even registered. This became a problem when the War Department innocently allocated the name to an unrelated project. Arnold's office had to order that project to stop using that name.
 
Trials with dummy bombs began in August 1943, first with Grumman TBF Avengers dropping small-scale Thin Man models that fared atrociously: they started spinning sideways in the air, and broke apart on impact. The Fat Man also had problems with a systemic strong wobble due to its round shape. After much testing, the addition of the right configuration of stabilizing boxes and fins fixed these problems.

Another problem was releasing the bombs from the bomb bay. The first approach was based on the mechanism used to detach glider from their towplanes, and used two lugs that would come loose at the correct moment to let the bomb fall. Several tests of the system failed. In March 1944, a bomb released prematurely and dropped onto the closed bomb bay door, causing serious damage. Afterwards, the two-point attachment was replaced with the one-point attachment the British were using for their Tallboy bombs. This way, even though Lancasters never carried the bombs, a small component of theirs became an important part of the project.

Since the atomic bombs were relatively complicated weapons, a new crew member was also needed, just to handle the special payload. The "weaponeer" sat in the cockpit in front of a special panel, and had to monitor the release and detonation of the bomb. The Hiroshima and Nagasaki missions also had an assistant weaponeer onboard. On Enola Gay above Hiroshima, the weaponeer was none other than Captain Parsons, who had been working with the bombs since the inception of Silverplate.
 
While the kinks were being worked out with the B-29s, bomb development at Los Alamos took an unpleasant turn. Plutonium made at the Hanford site had impurities in it, which increased its fissionability. This was a show-stopper: the two parts of the warhead would react to each other before properly meeting and go off early, before critical mass could be achieved. In theory, this could have been fixed by launching the moving part of the warhead at the stationary one at a higher speed, but this in turn would have needed more explosives and a much longer barrel – and that wouldn't have fit inside the modified B-29s.

This was a blessing in disguise. The plutonium in the Thin Man was changed to uranium, which was less efficient but quicker to produce. Also, uranium did not run the risk of predetonation, and in fact allowed for a shorter barrel, one that didn't actually need the modified bomb bay of a B-29, since it could fit inside a standard bomb bay just fine. With these changes, the Little Boy was born.
 
This meant that the bomb bays no longer had to be opened into one. The original Pullman plane, the first B-29 to be modified, was modified back to its original bay configuration, and further Silverplate Superfortresses were delivered with the standard bay. The aft bay, no longer needed for the bomb, could hold additional fuel tanks or conventional bombs.

Of course, the bomb bay was not the only part of the B-29 that had to be modified for the mission. Everything unnecessary was stripped to compensate for Little Boy's and Fat Man's mass (both weighed about 10,000 pounds). The fire-control system and most of the armor was removed, and so were all turrets except for the one in the tail. The final batch of Silverplate Superfortresses were also equipped with improved, fuel-injected engines. These planes could fly higher and faster than any other version of the B-29, carry heavier loads and have better fuel economy. Ramsey wrote that "they were without doubt the finest B-29s in the theater." After much work and several problems to overcome, the U.S. finally had the plane to take Little Boy and Fat Man on their historic missions.

A total of 46 Silverplate B-29s were built during the war, and an additional 19 afterwards. The planes used for testing saw heavy service and suffered much wear and tear, so new ones had to be built to replace them and to fly on the actual missions over Japan. Also, both missions involved more than just one plane (7 for the Hiroshima mission and 6 for Nagasaki, with some overlap): beside the plane carrying the actual bomb, there was also one for observation and photography, one carrying instruments to measure the blasts, and several weather reconnaissance planes.

The USAAF shrunk rapidly after the war. So much so, that in 1947, Strategic Air Command realized they no longer knew where the Silverplate B-29s actually were and what condition they were in. They had to send out inspectors to track down and find the planes. The project name itself was also compromised in the meantime, and thus had to be discarded. A new codename, Saddletree, was introduced in May 1947 to replace Silverplate. This time, the potential enemy was the Soviet Union, and thus Saddletree modifications included "winterization" so the new generation of special nuke-carrying planes could take off from and land at Arctic bases.

Two of the Silverplate B-29s that participated in the historical bombings can still be seen by the public today: Enola Gay is on display at the National Air and Space Museum's Steven F. Udvar-Hazy Center in Chantily, Virginia, and Bockscar is at the National Museum of the United States Air Force in Dayton, Ohio. The other Silverplates have all been scrapped, though another B-29, Hagarty's Hag,was repainted to resemble the Silverplate Straight Flush and is displayed at the Hill Aerospace Museum in Roy, Utah.