Atmospheric Nuclear Detonation - Little Boy
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Little Boy
Mushroom Cloud Gallery - Little Boy
Test:Little Boy; Date:August 6, 1945;
Operation:?; Site:Hiroshima,Japan;
Detonation:Air Delivered; Yield:15.00kt; Type:Fission,U235;

   Significant and historic as the first combat use of an atomic weapon ever, on August 6, 1945 the United States of America detonated an atomic device code-named Little Boy over Hiroshima, Japan, exactly 8:16:02 a.m., at an altitude of 1900 ft, and at a position of 34°23'44''N, 132°27'13''E, approximately 150m from the Hiroshima Prefectural Industrial Promotion Hall. Upon detonation, the bomb released the equivalent of 15,000 tons of TNT (15kT), and created a mushroom cloud that rose to 45,000 feet.

In an instant 80,000 to 140,000 people were killed and 100,000 more were seriously injured, mortally burned, or fatally irradiated. Of the 90,000 buildings in Hiroshima 60,000 were totally destroyed. The blast height wasn't optimum for this yield, however, but perversely it prevented any significant radioactive fallout over the rest of Japan.

Little Boy was based upon simpler, but less efficient gun-type design than the Gadget device used in Trinity test, which was an implosion bomb. Originally the gun-type scheme has been proposed for the Plutonium bomb, but later when the more efficient, but risky implosion design has been chosen for the Plutonium bomb, serious attention returned to uranium one.

The uranium gun program, lead by A. Francis Birch faced a difficult task. They had to build the bomb, without testing, yet be absolutely sure that it would work. There simply was not enough Uranium to both test and a weapon. Sufficient amount of enriched Uranium was only available by mid 1945. The actual device has been ready for combat use by May 1945.

Little boy used 64.1kg Uranium. 50kg 89% enriched and the rest was 50% enriched. U235 has been separated into two parts, the bullet which weighted approx. 26kg and the target weighting 38kg. The barrel has been borrowed from the anti-aircraft gun. The bore was modified to accommodated the new Uranium bullet. Conventional artillery smokeless powder would drive the bullet at 300 m/sec velocity once ignited.

USS Indianapolis (CA-35) delivered Little boy components to Tinian base on July 26, apart from the target assembly, which was delivered two days later. Originally the plan was to deliver the bomb on Aug. 1, but due to the weather conditions the operation plan has been altered, and on Aug. 6. at 00:00 Col. Tibbets received final debriefing. Hiroshima was the primary target, with Kokura and Nagasaki being alternative targets.

Due to very high secrecy of the operation, Tibbets was given "suicide pills" - cyanide capsules to distribute amongst the crew. After the briefing, he did tell the crew about the cyanide, and Parsons, who was a weapons specialist and knew the most about the device, requested the pill.

The original plan called for loading the fully armed bomb with 3 safety pins installed on the plane. However, a day earlier another B29 plane crashed on the runway, and that caused weapons specialist William Parsons to violate the order. Fearing that in case of a crash during the takeoff the bomb might go off wiping out the whole island, Parsons removed gunpowder charges form the bomb, practicing all day their installation procedure. Charges were inserted once Enola Gay was on its way to the target. After the weather reconnaissance plane confirmed good weather conditions over the primary target, Hiroshima, safety pins were removed and arming pins were installed. Those armed firing mechanism and barometric pressure sensors.

When the reconnaissance plane overflew Hiroshima, air raid alarm was triggered, but because it was a single plane and it left very quickly, alarm was cancelled in 30 minutes, and everyone left the shelters. Appearance of the lone Enola Gay didn't cause an alarm, so when the little boy detonated, no one was in the shelters, causing very high casualty rate.

Little Boy - delivered by B29 Bomber Enola Gay, crew:
Colonel Paul Tibbets - Commander;
Captain Robert Lewis - Co-Pilot;
Captain Theodore Van Kirk - Navigator;
Major Thomas Ferebee - Bombardier;
Lieutenant Jacob Beser - Electronic Countermeasures;
Sergeant Joseph Stiborik - Radar Operator;
Private Richard Nelson - Radar Operator;
Staff Sergeant Wyatt Duzenbury - Flight Engineer;
Staff Sergeant Robert Caron - Tail Gunner;
Captain William "Deke" Parsons - Weaponeer;
2nd Lieutenant Morris Jeppson - Ordinance Expert;

Few minutes after the explosion, seeing what has happened, Robert Lewis wrote in his journal : My God, what have we done?