Little Boy at Hiroshima
Little boy was U²³⁵- fuelled gun type bomb which was dropped on Hiroshima on August 6, 1945, from the ‘Enola Gay’, a B-29 bomber flown by Colonel Paul. W. Tibbets.
“There in front of our eyes was without a doubt the greatest explosion man has ever witnessed.” Captain Robert. A. Lewis (Co pilot of the Enola Gay.)
About The Bomb
The core of this bomb comprised two pieces of U²³⁵ that were assembled into a supercritical whole by propelling a hollow cylindrical projectile piece of U²³⁵ initially located in the tail of the bomb forward to seat with a solid cylindrical target piece of U²³⁵ located in the nose.
The target and projectile pieces were contained within what was essentially the barrel of an artillery canon, and upon assembly were surrounded by a tungsten carbide. It was referred to as Long Boy, Long Man and Thin Man as the designs changed and was finally named as “Little Boy”.
In effect Little Boy was simply a big gun consisting of both a U²³⁵ bullet and target with an average enrichment of about 80%. Present day standard is 93.5% enrichment.
Little Boy though incredibly inefficient (only a reported 1.38% of U²³⁵ fissioned) still produced an explosion with a force of about 16±3 kilotons of TNT.
The Little Boy was 304.8 cm long, 71.12 cm in diameter and approximately weighed 4400 kg. Outer ballistic case center and tail sections were made from 0.952 cm hardened homogeneous armour steel.
The nose section, consisting mainly of the target case, U²³⁵ target discs and tamper, represented more than half the weight of the bomb. It was the heart of the weapon.
PC-Atomic Heritage Foundation.
Such was the confidence of everyone in the success of the Little Boy design that no full-scale proof test of the nuclear components was ever conducted.
Operation Centerboard I
The Little Boy mission was flawless. It was referred to as a ‘textbook’ operation.
On Saturday August 4, 1945, an afternoon briefing was held on Tinian. It was attended by 7 crews that would be going on the mission to drop Little Boy. They were
- Enola Gay (Strike Aircraft) led by Colonel Paul. W. Tibbets.
- The Great Artiste (Instrument Aircraft) led by Major Charles. W. Sweeney.
- Necessary Evil (Photograph Aircraft) led by Captain George. W. Marquardt.
- Straight Flush (Hiroshima Weather Plane) led by Major Claude. R. Eatherly
- Full House (Nagasaki Weather Plane) led by Major Ralph. R. Taylor Jr.
- Jabbit III (Kokura Weather Plane) led by Major John. A. Wilson.
- Big Stink (Stand-by Plane to Iwo Jima) led by Captain Charles. F. McKnight.
The crew were told the three target cities would be Hiroshima, Kokura and Nagasaki. They were shown the pictures of the Trinity test taken place in New Mexico.
There would be no fighter escort on the way in as it is going to be a surprise attack. On the way back there would be rescue planes and submarines to help them in case trouble developed.
By Sunday evening Little Boy was loaded in Enola Gay and was ready to fly. At 2:45 AM, August 6, 1945, Enola Gay took off from Tinian followed a few minutes later by ‘The Great Artiste’, ‘Necessary Evil’ and ‘Big Stink’. The three weather planes had taken off an hour earlier.
About 6:00 AM the four planes rendezvoused over Iwo Jima. The ‘Big Stink’ stayed behind and the rest climbed to 29,566 meters and headed towards Japan. At 8:09 AM Radio Hiroshima announced air raid alert as ‘Straight Flush’ was sighted.
At 8:25 AM radio operator of ‘Enola Gay’ Richard Nelson received message from weather plane, “Y-3, Q-3, B-2, C-1” which was translated as “Cloud cover less than 3/10ths at all altitude. Advise: bomb primary”
At 9:00 AM Enola Gay spotted their target. The three planes headed straight with ‘Enola Gay’ in lead with ‘The Great Artiste’ to the left and ‘Necessary Evil’ on the right. Five minutes before the bomb was dropped ‘Necessary Evil’ made a turn so that it was in position 10 miles away to take photos when the bomb was off.
They reached the IP (Initial Point) at 9:12 AM. Ninety seconds before the bomb’s release, Tibbets ceded final command to Enola Gay’s bombardier Major Thomas. W. Ferebee, “It’s all yours.” while releasing his grip on the control.
Ferebee after last minute maneuvering correction started the sixty second signal tone at 9:14:17 AM indicating the beginning of automatic release sequence. The AP (Aiming Point) was T-shaped Aiui Bridge over the Ota River that runs through the center of the city.
At 9:15:17 AM (8:15:17 AM Hiroshima time) the tone stopped, and the bomb was released from 9631.68 meters.
When Little Boy was clear of the plane, Tibbets immediately put Enola Gay into safe distance from the explosion. In a few short moments, the course of human history would be irrevocably changed. The first atomic bomb has been used.
As the atom bomb fell, the bombardier of The Great Artiste Kermit Beahan dropped three parachuted instrument packages from his plane. These instruments transmitted primary and reflected acoustical pressure wave signals back to the scientists in The Great Artiste. This data was used to help determine the blast yield.
Exactly 44.4 seconds later Little Boy exploded at an altitude of 600±15 meters.
During an interview Ray Gallagher stated that:
“You know, at that time there was a monster that was loose, and that monster was War. He was killing everybody. He was killing people in your country, our country, and all over the world. Something had to happen. We did it, we had it, and we used it. Do you think for one minute that if you had it that you wouldn’t have used it?
God was with me that day. You know, a lot of people were killed. A lot of people were injured. There was a lot of heart break… but we did it.”
A city of almost 300,000 was destroyed. Since the end of war, the Japanese government stated that 14000 people were presumed to be dead but later downgraded the confirmed deaths to 87,833 in 1995 after four-year extensive survey.
Emperor Hirohito responded to journalists, “I feel it is very regrettable that nuclear bombs were dropped, and I feel sorry for the citizens of Hiroshima. But it couldn’t be helped because it happened in wartime.”
It was assumed that Japan would surrender but they did not which meant there would be second mission i.e., the dropping of ‘Fat Man’ the second atomic bomb at Nagasaki.
Sources
- The Physics of Manhattan Project — Bruce Cameron Reed
- Atom Bombs — John Coster-Mullen
- American Prometheus — Kai Bird and Martin. J. Sherwin
- Atomic Heritage Foundation
