For transport to Trinity one set of castings was temporarily left out, replaced by a trapdoor plug through which the core assembly could be positioned in the tamper. The reserved castings — an inner of solid Composition B, an outer lensed — were boxed separately with one spare of each type. The men completed preparing the HE assembly for the slow drive down to Trinity by bagging it in waterproof Butvar plastic, boxing it in a braced shipping crate of knotty pine and lashing the resulting package securely to the bed of a five-ton Army truck. A tarpaulin then muffled its secrets in inconclusive drape.
The plutonium core left the Hill first, at three that Thursday afternoon, shock-mounted in a field carrying case studded with rubber bumpers with a strong wire bail. It rode with Philip Morrison in the backseat of an Army sedan like a distinguished visitor, a carload of armed guards clearing the way ahead and another of pit-assembly specialists bringing up the rear. Morrison also delivered a real and a simulated initiator. At about six o'clock a sunburned young sergeant in a white T-shirt and summer uniform pants carried the plutonium core in its field case into the room at McDonald Ranch where it would spend the night. Guards surrounded the ranchhouse to keep vigil.
For security and to encounter less road traffic the HE assembly would make the trip by night; Kistiakowsky deliberately scheduled that more conspicuous convoy to leave at one minute after midnight on Friday, July 13, to put reverse English on the day's unlucky reputation. He rode in the lead car with the security guards. He soon dozed off and was then startled awake by the scream of the car's siren as the convoy ran through Santa Fe; the Army wanted no late-night drunken drivers rolling out of sidestreets to collide with its truckload of handmade high explosives. Beyond Santa Fe the convoy slowed again to below thirty miles an hour; the haul to Trinity took eight hours and Kistiakowsky got some sleep.
On Friday morning at nine the pit-assembly team gathered in white lab coats at McDonald Ranch to begin the final phase of its work. Brigadier General Thomas Farrell was on hand as Groves' deputy, Robert Bacher as the team's senior adviser. Bainbridge looked in; so did Oppenheimer. The ranchhouse room where the core had spent the night had been thoroughly vacuumed in preparation and its windows sealed against dust with black electrical tape to convert it to a makeshift clean room. On a table there the assemblers spread crisp brown wrapping paper and laid out the pieces of their puzzle: two gold-faced, nickel-plated hemispheres of plutonium, a shiny beryllium initiator hot with polonium alphas and, to confine these crucial elements, the several pieces of plum-colored natural uranium that formed the cylindrical 80-pound plug of tamper. Before assembly began Bacher asked for a receipt from the Army for the material it would soon explode. Los Alamos was officially an extension of the University of California working for the Army under contract and Bacher wanted to document the university's release from responsibility for some millions of dollars' worth of plutonium that would soon be vaporized. Bainbridge thought the ceremony a waste of time but Farrell saw its point and agreed. To relieve the tension Farrell insisted on hefting the hemispheres first to confirm that he was getting good weight. Like polonium but much less intensely, plutonium is an alpha emitter; “when you hold a lump of it in your hand,” says Leona Marshall, “it feels warm, like a live rabbit.” That gave Farrell pause; he set the hemispheres down and signed the receipt.
The parts were few but the men worked carefully. They nested the initiator between the two plutonium hemispheres; they nested the nickel ball in turn in its hollowed plug of tamper. That required the morning and half the afternoon. Two men lugged the heavy boxed assembly on a barrow out to the car. It arrived in its lethal dignity at Zero at 3:18 p.m.
There Norris Bradbury's crew had been busy with the five-foot sphere of high explosives Kistiakowsky had delivered that morning. At 1 p.m. the truck driver had backed his load under the tower. The men had used a jib winch to lift off the wooden packing crate, had swung it aside and lowered around the sphere a massive set of steel tongs suspended from the main winch anchored one hundred feet up at the top of the tower. With the tongs securing the sphere its two tons were winched up off the truck bed; the driver pulled the truck away and the winch lowered the preassembled unit to a skid set on the asphalt-paved ground. “We were scared to death that we would drop it,” Bradbury recalls, “because we didn't trust the hoist and it was the only bomb immediately available. It wasn't that we were afraid of setting it off, but we might damage it in some way.” Before they opened the upper polar cap to expose the trapdoor plug they erected a white tent over the assembly area; thereafter a diffused glow of sunlight illuminated their work.
Inserting the plug courted disaster, team member Boyce McDaniel remembers:
The [high-explosive] shell was incomplete, one of the lenses was missing. It was through this opening that the cylindrical plug containing the plutonium and initiator was to be inserted… In order to maximize the density of the uranium in the total assembly, the clearance between the plug and the spherical shell had been reduced to a few thousandths of an inch. Back at Los Alamos, three sets of these plugs and [tamper spheres] had been made. However, in the haste of last minute production, the various units had not been made interchangeable, so not all of the plugs would fit into all [holes]. Great care had been exercised to make sure, however, that mating pieces had been shipped to [Trinity].
Imagine our consternation when, as we started to assemble the plug in the hole, deep down in the center of the high explosive shell, it would not enter! Dismayed, we halted our efforts in order not to damage the pieces, and stopped to think about it. Could we have made a mistake…?
Bacher saw the cause and calmed them: the plug had warmed and expanded in the hot ranchhouse but the tamper, set deep within the insulation of its shell of high explosives, was still cool from Los Alamos. The men left the two pieces of heavy metal in contact and took a break. When they checked the assembly again the temperatures had equalized. The plug slid smoothly into place.
Then it was the turn of the explosives crew. Oppenheimer watched over them, conspicuous in his pork-pie hat, wasted to 116 pounds by a recent bout of chicken pox and the stress of months of late nights and seven-day weeks. In the motion picture that documents this historic assembly he darts in and out of the frame like a foraging water bird, pecking at the open well of the bomb. Someone hands Bradbury a strip of Scotch tape and his arms disappear into the well to secure a block of explosive. He finished the work in late evening under lights. The detonators were not yet installed. That would be the next day's challenge after the unit had been hauled to the top of the tower.
The following morning, Saturday, around eight, Bradbury supervised raising the test device to its high platform. The openings into the casing where the detonators would be inserted had been covered and taped to keep out dust; as the bulky sphere rose into the air it revealed itself generously bandaged as if against multiple wounds. It stopped at fifteen feet long enough to allow a crew of GI's to stack depths of striped ticking-covered Army mattresses up nearly to its skid, a prayer in cotton batting against a damaging fall. Then it started up again, twisting slowly, seeming on its thin, braided steel cable to levitate, rising the full height of the tower and diminishing slightly with distance as it rose. Two sergeants received it into the tower shack through the open floor, replaced the floor panel and lowered the unit onto its skid, positioning it with its north and south polar caps at left and right rather than above and below as they had been positioned during assembly, the same posture in which its militant armored twin, Fat Man, would ride to war in the bomb bay of a B-29. The delicate work of inserting the detonators then began.
Disaster loomed again that day. The Creutz group at Los Alamos had fired the Chinese copy, measured the simultaneity of its implosion by the magnetic method and called Oppenheimer to report the dismaying news that the Trinity bomb was likely to fail. “So of course,” says Kistiakowsky, “I immediately became the chief villain and everybody lectured me.” Groves flew in to Albuquerque in his official plane with Bush and Conant at noon; they were appalled at the news and added their complaints to Kis-tiakowsky's full burden:
Everybody at headquarters became terribly upset and focused on my presumed guilt. Oppenheimer, General Groves, Vannevar Bush — all had much to say about that incompetent wretch who forever after would be known to the world as the cause of the tragic failure of the Manhattan Project. Jim Conant, a close personal friend, had me on the carpet it seemed for hours, coldly quizzing me about the causes of the impending failure.
Sometime later that day Bacher and I were walking in the desert and as I timidly questioned the results of the magnetic test Bob accused me of challenging no less than Maxwell's equations themselves! At another point Op-penheimer became so emotional that I offered him a month's salary against ten dollars that our implosion charge would work.
