One plane. One bomb. One drop. Then, get out of there as fast as you can. It sounds simple, but in August of 1945 there was a lot more to it than that.

The two single-bomb attacks on Hiroshima and Nagasaki were the culmination of billions of dollars and years of extraordinary effort, breaking the limits of known science and changing our world forever. But because of the extraordinary nature of the bomb, even simple acts had to be orchestrated with precision. Even simple acts like the departure.

Why 155 degrees?

The answer comes from geometry and a basic knowledge of flight, with a focus on the turning radius of a B-29 aircraft.

First, the dropped bomb had forward velocity. This meant that it would land several miles ahead of the drop point. This also meant that if the aircraft kept flying in the same direction it would be very close to the bomb blast. The aircraft had to turn away from the bomb in order to get as far away from it as possible.

Second, the B-29 is a big aircraft, and it needs a lot of space to make a turn. Even a tight turn can have a radius of a few miles.

Although it would normally make sense to turn 180 degrees, the wide turning radius changed the geometry of the scene. Thus, by the time the aircraft had straighted out at 180 degrees, it would not have had its tail to the bomb; the bomb would have been aft and to the side!

The bomb target was directly behind the aircraft when the aircraft had turned only 155 degrees. This meant that the aircraft was able to straighten out from a turn earlier, accelerate sooner, and still head straight away from the nuclear explosion.

As the chart on your left shows, the turning radius of the B-29 aircraft was the key to finding the best direction away from the bomb blast.

The Manhattan Project engineers calculated that the shock wave would destroy the B-29 as far as 8 miles from the target. Since the aircraft flew at 6 miles over the target, the crew had to get as far away as possible.

Once the bomb was released, the aircraft needed lots of real estate to turn around. But, a 180 degree turn would have brought it too far around and partly towards the bomb target!

The pilot only had to turn to 155 degrees, and then straighten out and accelerate. At that orientation the aircraft would have had its tail directly towards the blast, and would have been accelerating directly away.

In Earl Johnson's words, "He had a radius of turn which put him at a 'new angle' to the explosion point which they knew. So they were "escaping" from a new position caused by the radius of their own turn plus the known geographical point of the explosion and this turned out to be a turn of approximately 155 degrees."

A simple departure is not always as simple as it looks. When dealing with a new weapon of such power, one of the major objectives of the mission was to be as far away from the blast as possible.

That's why even the departure was planned with precision. Nothing could be left to chance, not even the strange 155 degree turn at departure!

Many thanks to Earl Johnson, who dropped the question in our e-mail forum and gave us the answer when most of us flailed about and a few of us "sort of" got it right. Earl also deserves credit for putting it in the right words!

Also thanks, Rod Drake, for the info on the 8 mile safe distance.