Attack on Pearl Harbor (8 page)

The most modern carrier aircraft, the A6M Zero, joined the fleet in 1940. The aircraft and qualified pilots were in short supply. Aircraft were produced in a factory and towed by oxen up a winding trail through the main street of a small town to the nearest airfield. Production was slow. Some Japanese carriers would not get the A6M Zero until well into 1942.

Weapon—Target Pairings

The A6M Zero, besides being a superb dogfighter, carried a step up in armament over the previous generation of fighters. They had two 7.7mm nose-mounted machine guns, and two low-velocity 20mm cannon in the wings. The 7.7mm machine gun bullets did not have much penetrating power, nor did the 20mm, as its shell was fuzed to detonate on contact with the lightest structures. It was an effective strafing platform.

Besides the A6M Zero fighter, the Japanese carrier air wings consisted of two types of bombers. The “heavy hitter” was the B5N Kate, an aircraft with a crew of three that could be used either as a high-altitude level bomber or a torpedo bomber; payload options were one 250 kilogram (kg) high explosive general-purpose (GP) bomb and six 60kg GP bombs, or two 250kg bombs,
²¹
one 800kg AP bomb, or one 800kg 18-inch torpedo.

The D3A Val dive bomber was a two-seater aircraft similar in aspect to the German Ju-87 Stuka and capable of carrying a single 250kg GP bomb.

A weapons-target table is a means of summarizing which weapons are appropriate for use against each type of target. Weapons-target matching was critical in determining the roles the aircraft could fulfill. The table below summarizes the alternatives available to the Japanese planners in accordance with their doctrine. An “X” denotes an appropriate match, but should not be read to mean an optimal match—only that there was a reasonable capability with that weapon system (aircraft + ordnance) against that target. “SEAD” stands for Suppression of Enemy Air Defenses, accomplished by strafing or bombs. This is a modern term, but the tactic was available to the planners at the time.

Torpedoes v. Capital Ships (Battleships and Carriers)

The weapon of choice against capital ships
²²
was the torpedo, and the Japanese had a good aerial torpedo in their Type 91 Mod 2 weapon.

Between 1924 and 1936 the Japanese carried out extensive experiments to determine the resistance of various underwater protection designs. Charges were placed against the hull of the incomplete battleship
Tosa
(to be sunk in compliance with provisions of the Washington Naval Treaty), and against a full-size model thought to be similar to the underwater protection scheme of the
Colorado
class battleships, as well as against many scale models. A complex formula was derived to predict the performance of underwater protection schemes against different combinations of air- and liquid-filled voids of various depths and bulkhead thicknesses.

In the tests against the
Colorado
model, a 350-pound warhead from a Type 91 Mod 1 torpedo penetrated all the torpedo defense compartments and broke through the innermost holding bulkhead.

By 1941, the Mod 2 torpedo carried a 452-pound warhead using an improved Type 97 explosive (60% TNT and 40% Hexyl). The Japanese had every expectation that their aerial torpedoes would be able to defeat the anti-torpedo protection of the American battleships.
²³

The Japanese underestimated the quality of the American anti-torpedo protection. The Type 91 Mod 2 proved to be only marginally capable against the oldest battleships, while the more advanced protection in the later Treaty battleships held. Against
Nevada
(BB-36), a hit between turrets one and two at frame 41 did not penetrate the torpedo defense system’s innermost holding bulkhead, but did split seams and cause leaks.
California
’s (BB-44) holding bulkhead was deflected inward but was essentially undamaged.
²⁴

The Japanese believed that four or five torpedo hits would likely sink a battleship,
²⁵
three or four a carrier.

A significant impediment to a torpedo attack was the shallow water in Pearl Harbor, 40 to 45 feet deep. Aircraft torpedoes would typically dive below 100 feet, and could go as deep as 150 to 300 feet, before rising to their intended running depth. Genda resolved to use torpedoes in the attack even before this technical problem was solved.

(4) US Technical Mission to Japan Ordnance Data Page for 800-kg AP bomb
²⁶

AP Bombs v. Battleships

The Japanese Type 99 No. 80 Mark 5 armor-piercing bomb was converted from a 41cm (16.14-inch) gun shell, a Mark 5 or Mark 6/Type 88 APC projectile (circa 1921) used on
Nagato
-class battleships. These shells were replaced after 1931, making them available for conversion.
²⁷

To retain the capability to penetrate armor, most of the bomb was solid metal. The AP cap and the windscreen were removed. To reduce the weight further the body was machined down and tapered toward the base, and the thick projectile base plug was eliminated. The inside of the lower cavity was machined to accommodate a thinner base plug with two fuzes. A threaded area below the new base plug was added to attach the tail.

The old explosive was replaced by Type 91 (trinitroanisol), a more effective explosive than the older Shimose (picric acid). The explosive filler weight, 50 pounds, was less than in the original shell due to the volume of the second fuze and the room needed for connection threads for the new tail section.

This is a small explosive charge for such a heavy bomb—in contrast, the American 1,600-pound (725.7kg) AP bomb carried 240 pounds of explosive.
²⁸
The Japanese used their Type 91 explosive due to its ability to withstand the shock of impact.
²⁹
It was more powerful than Shimose, but less than an equivalent weight of TNT. Detonation was initiated by two independent long-delay fuzes similar to those on AP projectiles, requiring a heavy impact to set them off. If the bomb hit several thin plates before striking armor it might be slowed to the point where there was insufficient deceleration to initiate the primer.

The final assembly weighed 1,760 pounds.

US battleships carried deck armor distributed over several decks with an aggregate thickness of 5 to 6.5 inches. Testing consisted of dropping bombs against armor plate designed to replicate the protection of
West Virginia
with a combined deck thickness of 5.75 inches. In a test drop from 3,000 meters altitude, the bomb smashed through the plate.
³⁰

This test was an illuminating demonstration of the state of the accuracy of level bombing and the cobbled-together character of Japanese weapons testing. The Japanese dropped bomb after bomb from 3,000 meters, trying for several weeks to hit the plate without success. They were on the verge of giving up when, at the last moment, a successful hit was achieved. There is an almost comic-opera atmosphere to this incident. Other nations tested bomb penetration and fuzing by firing their bombs from a howitzer at close range, regulating the powder charge to obtain the desired impact velocity.
³¹

The Japanese may not have tested fusing and detonation during these drops.

Their conclusion was that, if dropped from high enough, the 800kg bomb had the capability to penetrate into the armored citadel of a battleship. In comparison, the US 1,600 pound armor-piercing AN Mark 1 bomb was rated to penetrate 6-1/4 inches of armor when dropped from 10,000 feet (3,048 meters), with an uncertainty of plus or minus 15%.
³²

The Type 99 No. 80 Mark 5 bomb carried two base fuzes for reliability. There was a 0.2 second delay to allow the bomb to penetrate deep into the hull. The fuze was rather insensitive, requiring the bomb to hit something heavy and substantial, such as armor plate, for fuze initiation. This made the weapon unsuitable for use against carriers, cruisers, or smaller ships, where the fuze would likely not initiate. During the attack on Pearl Harbor, an 800kg AP bomb passed entirely through a ship, the
Vestal
, exploding underneath it.

According to the US Naval War College Maneuver Rules and Fire Effect Tables of the period,
³³
it would take seven (using the bomb effects tables) to fourteen (using the 16-inch shell tables) penetrating hits to sink a battleship. The Japanese believed that twelve to sixteen direct hits from big guns would sink a ship in a surface battle, and that those results would likely carry over to shells converted into bombs.
³⁴

Later US Navy analysis determined that American AP bombs, containing three to five times the amount of explosive filler as the 800kg Type 99, had insufficient explosive power to cause extensive flooding in a battleship. AP bombs could sink a battleship only if they induced an explosion in a main or secondary magazine. Since magazine areas were 23% of a battleship’s target area, six hits would give a 79% chance of sinking the ship.
³⁵

A challenge associated with using AP bombs is battle damage assessment (BDA). From a 10,000-foot altitude it would be difficult for bombardiers to visually follow the path of their bombs all the way to the target. Aviators would have to look for the signature of the bomb as it hit. For a miss, the signature would be an explosion on the land or in the water, visible from the aircraft unless the bomb buried itself in the ground, failed to detonate, or had a low-order explosion.

If the bomb hit and penetrated into the ship before exploding, the signature would be difficult to detect. A bomb exploding in an engineering space would mostly be marked by a cloud of white steam escaping from ventilators or open hatches, if the ship had steam up; otherwise, the smoke of the explosion might not be seen. For hits in boiler rooms, a cloud of soot and smoke forced up the ship’s stacks would be the most prominent sign. Bombs could also hit the ship but not defeat the deck armor, exploding outside the citadel.

In general, prominent explosions denoted failure. Consequently, hits were estimated not by counting successes but by counting those that missed, and assuming that anything that did not miss must have exploded deep inside the target. Duds or low-order detonations would throw off the count.

It was difficult to determine if an AP bomb hit, or if a hit achieved its objective. This is a significant problem strategically. Not knowing the extent of the damage meant not knowing if the Pacific Fleet battle force was immobilized. This would influence whether Japanese forces could be dispatched south to the front lines of the advance, or had to be retained to counter possible American counterattacks from the east. If the Japanese had no assurance the Pacific Fleet was immobilized, there would have been significant impact on their operations. For example, it is unlikely that the Indian Ocean raid employing all the available carriers of
Kido Butai
would have been conducted had the Japanese thought the American fleet capable of an offensive.

GP Bombs v. Capital Ships

The Type 99 No. 25 model 1 Ordinary 250kg general purpose (GP) bomb carried by the D3A Val dive-bomber was designed to attack unarmored ships. In contrast to the 800kg AP bomb with 50 pounds of explosive, the 250kg GP bomb packed about 136 pounds of explosive, almost three times as much as the AP bomb.

The GP bomb had limited utility against battleships. According the Japanese experts, “Nor can we expect too much from dive bombing because the [bombs] are too light to penetrate the heavy armor of a United States battleship.”
³⁶
Topside and superstructure damage was relatively easy to repair, and would not keep a battleship out of the war for the requisite six months. In the context of the Japanese objectives for the Pearl Harbor raid, GP bombs would largely be wasted against battleships.

(5) US Technical Mission to Japan Ordnance Data Page for 250-kg GP bomb
³⁷