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Pearl Harbor:  The Bombs of the Second Wave

On December 7, 1941, Japanese torpedoes and bombs devastated Hawaii’s warships and air fields. The “Weapons Wall” at Pacific Aviation Museum has full-size models of three of these weapons—the ones used in the first wave of the attack. The wall also shows the aircraft that carried different types of ordnance during the first wave.

This article goes beyond the information shown on the Weapons Wall, to look at the bombs used in the second wave. The Japanese Kates and Vals of the second wave delivered different types of bombs than they delivered in the first wave—including two types of bombs not used in the first wave. Table 1 summarizes basic data about the torpedoes and four types of bombs used during the attack, and about the aircraft that delivered each weapon during the two waves.

Table 1: Japanese Bombs and Torpedoes at Pearl Harbor

Weapon Weight First Wave Second Wave
Type 91 Model 2 torpedo 838 kg
205 kg warhead
1,847 lb
452 lb warhead
B5N2 Kates  
Type 99 Model 5 ordinary (anti-ship) bomb 800 kg 1,763 lb B5N2 Kates  
Type 98 land bomb 250 kg 551 lb D3A1 Val B5N2 Kates
Type 97 land bomb 60 kg 132 lb   B5N2 Kates
Type 99 Model 1 ordinary (anti-ship) bomb: 250 kg 551 lb   D3A1 Val

Note: In Imperial Japanese Navy terminology, land bombs were general-purpose bombs used to attack land targets, while ordinary bombs were anti-ship bombs.

Two Waves

It is important to understand that the Japanese had two different types of targets during the attack. Most obviously, their main targets were battleships, carriers, and cruisers in Pearl Harbor. However, they also attacked air fields throughout Oahu to destroy the fighters at Wheeler Air Force Base and Bellows AFB and to destroy the bombers and patrol bombers at Hickam AFB, Naval Air Station Kaneohe and NAS Pearl Harbor. Fighters could intercept the attackers, and big planes could find and destroy the Japanese carriers.

The First Wave

In the first wave, B5N2 bombers, which the U.S. called “Kates,” attacked the ships. The Kate was the largest aircraft on Japanese carriers. It had a crew of three, including a pilot, an observer/bombardier, and a gunner/radio operator. It carry carry either a single torpedo or several bombs, and the rear gunner would strafed targets with his single 30 caliber machine gun.In the first wave, 40 Kates carried the Type 91 Model 2 torpedo. Although only about half the size of Japan’s potent Long Lance torpedo launched from surface ships, the Type 91 still had a big 205 kg (452 lb) warhead that that exploded below the water line, doing immense damage. Most of the ships that were sunk in the attack were sunk by these torpedoes. By the way, the wooden tail fins kept the torpedo from sinking into the mud in the shallow harbor. The fins, which were pioneered by the British in their attack on the Italian fleet at Taranto a year earlier, distinguished the Type 91 Model 2 from the original Type 91.

Another 49 Kates of the first wave carried converted naval shells. These Type 91 Model 5 bombs weighted 800 kg (1,763 lb). Dropped from high altitude, these streamlined bombs hit ships with tremendous force, penetrating several decks before exploding. It was a Type 91 bomb that destroyed the Arizona, but that was its only capital ship fatality from the Type 91 bomb.

For land targets in the first wave, Japan turned primarily to its Aichi D3A1 dive bomber, which Americans called the “Val.” Each Val carried a single 250 kg (551 pound) Type 98 “land bomb.” In Japanese terminology, a land bomb was a general purpose bomb rather than an anti-ship penetration bomb. During the first wave, 51 Vals used these bombs to shred aircraft on the ground at Wheeler, Hickam, and the seaplane base on Ford Island. Kaneohe was left to strafing A6M Model 21 Zeroes. After dropping their Type 98 land bombs, the Vals and Zeroes strafed the air fields with machine guns, and Zeros jointed in with machine guns and 20 mm cannon.

The Second Wave

In the second wave, the Vals and Kates switched roles. The Vals went after ships in the harbor, while the Kates went after air bases.

With all of the smoke in the harbor, the Japanese needed the pin-point accuracy of dive bombing to attack the remaining ships. To attack ships, the Vals used a penetrating bomb, the Type 99 “ordinary” bomb. Here, “ordinary” means that the bomb was designed for the role that carrier dive bombers were created to carry out—destroying ships.

Vals with Type 99 ordinary bombs were able to hit ships in dry dock, including the battleship Pennsylvania. However, a large fraction of the Vals went after the Nevada, which had managed to get underway. Overall, the Val attacks of the second wave did far less ship damage than the Kates of the first wave.

This left the Kates free to attack the airfields. Dropping bombs from high altitudes would not give high accuracy, but Kates could carry much heavier bomb loads than Vals. Many of the Kates in the second wave carried two of the 250 kg (551 pound) Type 98 land bombs that Vals dropped singly in the first wave. Others carried another type of bomb, the smaller 60 kg (132 pound) Type 97 land bomb, usually along with one or two Type 98s.

Vals could carry also two Type 97 bombs--one under each wing. However, there is no evidence that they did so in either wave of the Pearl Harbor attack. Pictures of “stuff” under Val wings during the attack only showed the Val’s unique dive brakes.

Bomb Damage Confusion

The fact that the Japanese used four types of bombs complicates bomb damage assessment. Many American after-action accounts stated that a particular ship was hit by a certain number of bombs without describing the types of bombs. If the timing of a bomb hit was recorded, it would be possible to estimate the type of bomb that made the hit, but timing was often unreported or was reported only with confusion.

Sources 

 

For Type 91 Model 2 torpedoes, data comes from Tagaya, Osamu, Imperial Japanese Naval Aviator 1937-1945, Osprey Publishing, Oxford, UK, 1988, p. 29.

Data on the bombs was collected by David Aiken, as was information on how many Japanese aircraft of each type were involved in each aspect of the attack. Mr. Aiken conducted an extensive study of Japanese records as well as American records about the attack.

Post by Ray Panko

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