This is from pages 344-346 of Groves' Now it can be told:
"...One very serious problem came up just before take-off, which
placed Farrell in the difficult position of having to make a decision
of vital importance without the benefit of time for thought or con-
sultation. Despite all the care that had been taken with the planes,
the carrying plane was found at the last moment to have a defective
fuel pump, so that some eight hundred gallons of gasoline could not
be pumped to the engines from a bomb bay tank. This meant that
not only would the plane have to take off with a short supply of fuel,
but it would have to carry the extra weight of those eight hundred
gallons all the way from Tinian to Japan and back. The weather was
not good, in fact it was far from satisfactory; but it was good enough
in LeMay's opinion, and in view of the importance of dropping the
second bomb as quickly as possible, and the prediction that the
weather would worsen, Farrell decided that the flight should not be
held up. Just before take-off Purnell said to Sweeney, "Young man, do
you know how much that bomb cost?" Sweeney replied, "About $25
million." Purnell then cautioned, "See that we get our money's worth."
Because of the weather, instead of flying in formation, the planes
flew separately. To save fuel, they did not fly over Iwo Jima but went
directly to the coast of Japan. Their plan was to rendezvous over the
island of Yokushima, but this did not work out. The planes were not
in sight of each other during their overwater flight and only one of the
observation planes arrived at the rendezvous point. The missing plane
apparently circled the entire island instead of one end of it, as it was
supposed to do according to Sweeney's plans. Although Sweeney had
identified the one plane that did arrive he did not tell Ashworth. Un-
fortunately, because it did not come close enough, Ashworth was
unable to determine whether it was the instrument-carrying plane,
which was essential to the full completion of the mission, or the other,
which was not. Sweeney's orders were to proceed after a short delay
of fifteen minutes but he kept waiting hopefully beyond the deadline,
The result was a delay of over half an hour before they decided to go
on to Kokura, anyway.
At Kokura, they found that visual bombing was not possible, al-
though the weather plane had reported that it should be. Whether this
unexpected condition was due to the time lag, or to the difference
between an observer looking straight down and a bombardier look-
ing at the target on a slant, was never determined.
After making at least three runs over the city and using up about
forty-five minutes, they finally headed for the secondary target,
Nagasaki, On the way they computed the gasoline supply very care-
fully, Ashworth confirmed Sweeney's determination that it would be
possible to make only one bombing run over Nagasaki if they were
to reach Okinawa, their alternate landing field. If more than one run
had to be made they would have to ditch the plane they hoped near
a rescue submarine.
At Nagasaki, there was a thick overcast and conditions at first
seemed no better for visual bombing than at Kokura. Considering
the poor visibility and the shortage of gasoline, Ashworth and Sweeney
decided that despite their positive orders to the contrary, they had
no choice but to attempt radar bombing. Almost the entire bombing
run was made by radar; then, at the last moment, a hole in the clouds
appeared, permitting visual bombing. Beahan, the bombardier, syn-
chronized on a race track in the valley and released the bomb. Instead
of being directed at the original aiming point, however, the bomb was
aimed at a point a mile and a half away to the north, up the valley
of the Urakami River, where it fell between two large Mitsubishi
armament plants and effectively destroyed them both as producers of
war materials.
On the way to Okinawa warning ditching orders were announced;
but the plane made it with almost no gas left. Sweeney reported there
wasn't enough left to taxi in off the runway.
The Nagasaki bomb was dropped from an altitude of 29,000 feet.
Because of the configuration of the terrain around ground zero, the
crew felt five distinct shock waves.
The missing observation plane, which fortunately was the one
without the instruments, saw the smoke column from a point about
a hundred miles away and flew over within observing distance after
the explosion. Because of the bad weather conditions at the target, we
could not get good photo reconnaissance pictures until almost a week
later. They showed 44 per cent of the city destroyed. The difference
between the results obtained there and at Hiroshima was due to the
unfavorable terrain at Nagasaki, where the ridges and valleys limited
the area of greatest destruction to 2.3 miles (north-south axis) by
1.9 miles (east-west axis). The United States Strategic Bombing Sur-
vey later estimated the casualties at 35,000 killed and 60,000 injured.
While the blast and the resulting fire inflicted heavy destruction on
Nagasaki and its population, the damage was not nearly so heavy as
it would have been if the correct aiming point had been used. I was
considerably relieved when I got the bombing report, which indicated
a smaller number of casualties than we had expected, for by that
time I was certain that Japan was through and that the war could
not continue for more than a few days."
So Sweeny was truly between a rock and a hard place, and was forced by circumstances to make on the spot decisions