Alan D. Zimm
Page 7
The first priority targets were the capital ships of the fleet, both the battleships that would provide the headlines and the aircraft carriers that could threaten the Japanese fleet with counterattack. Part of the strike would be directed against American air bases in what is known in modern terms as offensive counter-air (OCA), to keep fighters from interfering with the Japanese bombers and to prevent bombers from counterattacking.
The Japanese feared land-based air. Yamamoto originally considered a one-way attack employing only torpedo bombers. If this was not feasible, he suggested launching a full strike from 500 to 600 miles off Oahu in a one-way attack (katamichi kogeki). The aviators would ditch in the vicinity of Pearl Harbor and be recovered by submarines. Yamamoto estimated that when the American people saw this form of attack they would think the Japanese “such a unique and fearless race that it would be useless to fight them.”
Genda rejected Yamamoto’s concepts. “A one-way attack would have a bad psychological effect on the airmen if they knew their only means of survival would be the slim chance of being picked up at sea…. Ditching in enemy territory would be a needless waste of planes and highly trained airmen.”7
With two or even four carriers, Genda’s estimate was that the total aircraft available could not haul sufficient bombs and torpedoes to destroy everything. Damage estimates showed that more carriers were needed. The Japanese would have a total of six fleet carriers available in November 1941 when Shokaku and Zuikaku joined the fleet, albeit with air groups that would be “really green.”8
However, there was competition for fleet carriers: the Operations Section of the Naval General Staff9 felt they needed at least two fleet carriers to support the Southern Advance, especially the invasion of the Philippines, which would be opposed by a significant American air component.
Genda’s conclusion was that the effort should not be made unless all available fleet carriers were thrown into the attack. The Operations Section knew that fighter support would be necessary to support the attacks on the Philippines. The conflict seemed insurmountable.
Eventually, trials with the A6M Zero naval fighter developed fuel conservation techniques that gave the fighter sufficient range to escort bombers launched from Formosa against the main Philippine military facilities at Subic, Clark Field, and Cavite. By lowering engine RPM and leaning out the fuel supply, the A6M Zero’s cruising speed was reduced to 115 knots, but fuel consumption was cut from 35 gallons per hour to 17. This gave the Zero a range of as much as 1,250 nm with an endurance of 11 hours.10 This freed the fleet carriers to strike Pearl Harbor.11
It was the weapon of the samurai that provided the inspiration for the attack. In the repertoire of the sword there is the “one swift stroke,” Kinshicho-Oken, where the samurai in one motion pulls his sword from its scabbard and decapitates his opponent, then returns to his original position.12
Japanese Weapons, Naval Air Power, Carriers, and the China Experience
The Japanese in 1941 were leaders in carrier aviation. Their technical development was advanced relative to the other navies and seasoned by combat experience in the China War, a war which was to see the introduction of a new level of performance in naval aviation with the introduction of the monoplane, an all-metal aircraft of unprecedented performance.13
During the war with China, Japanese aircraft carriers ranged along the Chinese coast, launching strikes. When the war moved further inland, the air groups from Ryujo and Hosho were based inland at Kunda outside Shanghai.14 With Japan’s new high-performance fighter, the A5M Claude, and the development of a special looping turn maneuver (a displacement roll) that allowed the Claude to cut inside the turns of enemy fighters, eventually the Japanese gained control of the air.
They also gained an opportunity to test out doctrine and equipment. For example, they learned that long-range, high-altitude and high-speed medium bombers required fighter escort, and that high-altitude bombing was not always devastating, contradicting the beliefs of many Western air power theorists.
Operational experience also revealed weaknesses. Japanese aero engine technology was behind the West in making reliable high-powered engines. As a consequence, and due to a philosophy that enshrined maneuverability and eschewed “defensive” protective features such as armor plate and self-sealing fuel tanks, their aircraft were lightly built. This gave them superior maneuverability and speed and allowed them to carry heavier payloads over longer distances than other nations’ aircraft with comparable engine horsepower.
However, this also made the aircraft susceptible to enemy fire. Japanese aircraft fuel tanks did not include self-sealing inner liners, making them susceptible to leaks and compression vapor explosions when hit, earning the Japanese aircraft the sobriquet of “Ronsons,” the name of a popular cigarette lighter of the era. One aircraft type that was to see service at Pearl Harbor, the D3A Val dive bomber, had a fuel tank under the pilot’s seat.
The Japanese viewed these as acceptable trade-offs. Their emphasis was on offensive capabilities, with defensive capabilities scorned as “not Japanese.” For example, the instructions to the designers for the 12Si carrier fighter competition (that which resulted in the A6M Zero fighter), formulated after the initial Chinese combats in 1937, required no armor protection. None could be provided considering specifications that called for extreme maneuverability, speed, and rate of climb—the Japanese were just not concerned with attributes that might be considered “defensive.”15
The Japanese tradition of personal combat, dating back to the middle ages, moved Japanese fighter pilots to chase enemy fighters and engage in dogfights, forgoing the mundane “defensive” task of escorting bombers. Bombers took severe losses in the air war as the fighters pursued personal glory.16
These material and doctrinal weaknesses were not considered deficiencies, but just the cost of waging war the Japanese way. Japanese society considered it a great honor to die in combat. Fallen aviators—any fallen warrior—were worshipped as deities. Death in battle was to discard what Buddhism calls the small self so as to serve the greater cause, to live in the great Imperial Virtue, resulting in a readiness for self-sacrifice that was clearly manifested in the Japanese people. Coupled with the Japanese belief that war was an act of will, and that “an iron will can accomplish anything,” these concepts led almost inevitably to aircraft that emphasized offensive capabilities. Defensive characteristics were eschewed almost as if they were an insult to the aircrews’ fighting spirit, Yamato damashii.17
The Imperial Army took a different direction. Early reports from the war in Europe indicated that armor protection and self-sealing fuel tanks were indispensable. Those features were included in the design of the Ki-61 Tony fighter that began production in August of 1942. It was considered a success, and over 3,000 were produced during the war.18
By late 1941 the Japanese had four fleet carriers available, along with one escort and two light carriers, with a total capacity of 378 operational aircraft.19 She had five fleet carriers, two light carriers and two escort carriers either under construction or conversion, or undergoing final workups, which meant an additional capacity of 406 aircraft.20 The light and escort carriers did not have the range to accompany an attack against Pearl Harbor.
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 pl
atform.
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 ships22 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 bomb26
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.