Alan D. Zimm
Page 11
Second, the Japanese were working on an armor-piercing bomb for the Pearl Harbor attack well before Fuchida’s meeting with Genda in late September 1941. Early in 1941, a 20-meter-square armor plate was set up to test a bomb made from a converted 40cm battleship shell. Tests refined the modifications to improve its penetration capability. At that time it was thought that a release altitude of 12,000 feet was required to get the desired penetration. Production of the bomb was started, and by mid-September 150 had been produced.
Genda had indeed almost given up on horizontal bombing in March of 1941 due to the low hit percentage, which was under 10%. Considering that 12 to 16 hits were considered necessary to sink a battleship, at that rate all the carrier attack planes from all six carriers would be required to sink one battleship. This was unacceptable.
In April, Lieutenant Furukawa Izumi arrived aboard Akagi after passing through the bombing course at Yokosuka. In addition, a special team consisting of Chief Petty Officers Watanabe Akira and Aso Yanosuke was formed to work the problem. These men improved the coordination between the pilot and the bombardier. Bombing accuracy improved dramatically, rising in trials to as high as 33% hits. In early June, Genda recommended that the level bombing leaders go to Kagoshima Naval Airfield to train on the techniques developed by Watanabe and Aso. In other words, in June of 1941 Genda put the wheels in motion to include level bombing in the Pearl Harbor attack.19
Another set of tests were in process at the time that Fuchida reported for duty at his new post.
The Navy held experiments beginning around the end of September to test a special missile converted from 16-inch shells…. These tests had been going on for about ten days, with no hits achieved, when [Rear Admiral] Ueno [Keizo] requested Nagumo to send some of his best pilots with their own aircraft to Kashima to see if they could brighten the picture.20
Why was another set of tests being run in September when the earlier tests already approved the bomb for production? Evidently, the planners wanted the level bombers to attack from 3,000 meters instead of 4,000 meters to improve their accuracy, but it was not known if the bomb could penetrate enough armor at that altitude.
Fuchida makes the tests a thing of high drama, starring Fuchida. He claimed that Nagumo sent for him immediately after he was informed of Ueno’s problem, saying that the experiment was vital, and “upon it depends the success or failure of the Pearl Harbor attack.” Fuchida and five others hastened to Kashima, and on the third day, from 3,000 meters, scored a direct hit which smashed through an armored plate replicating the West Virginia’s deck protection.
The September testing was to determine the altitude from which the level bombers would attack, not whether or not they would be included in the attack. Fuchida did not change Genda’s mind about including level bombers in the attack. The decision had already been made, the bombs already manufactured. When Fuchida arrived on the scene it was just a question of refining the details.
Third, by late September the plan had to have progressed well beyond the point where the Japanese considered that the numbers of torpedo planes and level bombers could be juggled. The B5N Kate carrier attack bombers could fulfill four roles: employing torpedoes against capital ships, AP bombs against capital ships, 250kg GP bombs against cruisers or carriers, or 250kg and 60kg land attack bombs against ground targets. There was undoubtedly considerable calculation regarding the allocations, since they constituted the killing power against battleships or the majority of bombs against ground targets. The idea that Genda accepted a new idea to employ B5N Kates with AP bombs at this late date is not credible. It would have required a complete revision of the strike plan, which had to be near its final form.
The training program began just after Fuchida’s arrival. Training as intensive as the Japanese air groups conducted was not something that just happened, but had to be planned. Airfields had to be prepared, aircraft stationed and prepped, fuel and maintenance and weapons staged, maintenance personnel assigned, housed and fed, and other considerations. The outline of the training plan—locations, aircraft allocations, fuel and maintenance and the like—was likely promulgated well before Fuchida’s arrival. The allocations of B5N Kate bombers between torpedo and AP bombs and OCA had to have been made earlier for the training plan to be ready in late September.
Fuchida’s claim that he was responsible for including level bombers with AP bombs in the strike plan cannot be correct.
Aircraft Allocations
The aircrews were allocated their roles probably early in September. Genda allocated the numbers assigned to each task and each target area.21 He also had the authority to draw additional aviators into the planning process, so he called in a number of experts to help formulate the tactical orders. In addition, he could bring in select Hikotaicho—translated literally as “air unit commanding officer,” a position similar to the CAG (Commander Air Group) on US carriers. Although normally there would be only one Hikotaicho assigned to a carrier, Genda changed this to as many as three per carrier or as few as none, depending on the plan’s needs.
The B5N Kate units were divided between torpedo bombers and level bombers, two very different skill sets. The late-September start allowed six weeks to resolve problems and train the aircrews for their tasks.
A Two-Wave Attack
Carrier Striking Force Operations Order No. 3 specified 351 aircraft to be launched in two waves.22 One hundred and sixty-two were to be directed at fleet units (battleships, carriers, and cruisers), and 189 against aircraft and airfields.
If four carriers launched 90 carrier attack planes armed against battleships, the other two carriers could contribute 54 more B5N Kates, or alternately 45 to 54 D3A Val dive-bombers to the first wave. The Vals were chosen, and sent against airfields to shut them down and prevent fighters from getting into the air, and to disrupt any counterattack preparations.
1st Wave:
The first wave was to consist of 189 aircraft.
90 aircraft would go against the Pacific Fleet:
• 40 B5N Kates carrying 40 torpedoes, 24 primarily targeted against the battleships and 16 against carriers;
• 50 B5N Kates carrying 50 800kg AP Bombs, targeted against inboard battleships; 99 would be directed at enemy airfields or to attack enemy fighters in the air:
• 54 D3A Vals carrying 54 250kg GP bombs directed at airfields;
• 45 A6M Zero fighters.
2nd Wave:
The second wave was to consist of 171 aircraft:
• 81 D3A Vals carrying 81 250kg GP bombs were to go to Pearl Harbor and attack warships. They were primarily after carriers. Alternate targets were cruisers and battleships;
• 54 B5N Kates, carrying either one 250kg GP and six 60kg GP bombs or two 250kg bombs each, targeted against airfield hangars and facilities;
• 36 A6M Zeros.23
Allocations
The following table shows aircraft allocation across the missions.
The torpedo bombers constituted only 28% of the available B5N Kate carrier attack bombers. Thirty-five percent would carry AP bombs, and the remaining 38% would attack airfields.
Of the 280 bombers, 171 (61%) were sent against fleet targets.
The aircraft assigned to offensive counter-air were allocated as follows:
• 54 first wave D3A Val dive-bombers, against the Ford Island and Wheeler airfields;
• 54 second wave B5N Kate carrier attack planes against the airfields at Kaneohe, Ford Island, Barber’s Point, and Hickam.
The next table shows how the ordnance was directed.
The bombers were carrying a total of 315 bombs. By number, 74% of the bombs were assigned to OCA. By payload, 92,250kg (71%) of the ordnance (bombs and torpedoes) was directed against the fleet and 37,900kg (29%) for OCA. The B5N Kates carried 65% of the total bomb payload of the attack.
The OCA effort was augmented by 81 A6M Zero fighters. Including fighters, 66% of the aircraft were assigned to OCA.
Offensive Counter Air v.
Fighter Cover
Strategically, destruction of aircraft in this early part of the war was of some value to the Japanese because of the overall shortage of Allied aircraft. The Americans were attempting to rebuild their military while simultaneously supplying arms to Britain and Russia in the war against Hitler. There were many demands on an aircraft industry that was still gearing up for full production. New aircraft were doled out among claimants in penny lots. No demands were fully satisfied, a situation that would remain critical for at least a year.
Aircraft destroyed at Pearl Harbor were aircraft that could not be shifted to the Philippines or Java to oppose Japan’s offensive. However, this did not justify attacking Pearl Harbor just to destroy aircraft. The value of destroyed aircraft was transitory. Aircraft could be replaced quickly with the proper prioritization of industrial output—America would produce over 12,000 aircraft in 1941, five times that of Japan. The Japanese could hope to destroy at Oahu only a few hundred at best, six day’s production. Many of the aircraft on Oahu were older, obsolescent types of limited military worth.
Japan could not afford a battle of attrition with the United States; destroying aircraft alone was not worth the risk of a trans-oceanic attack. Every bomb targeted against aircraft or airfield facilities was one less weapon directed against the destruction of the Pacific Fleet battleships. Battleships took years to replace. Hangars could be rebuilt in weeks, aircraft in days.
Trained aviators took longest to produce and were the most limiting factor. Destroying an aircraft on the ground did not mean that the aircrew was lost, so squadrons could be quickly rebuilt with new (and more modern) aircraft.
Perversely, a Japanese success against the American aircraft on Oahu would serve as a powerful stimulus to allocate more production to the Pacific. The Pacific would get more modern aircraft as replacements diverted from aircraft intended for Europe, so the intermediate term benefits would be seen more by the Germans than the Japanese.
The Japanese had reason to limit the number of munitions expended against aircraft. An (overly simplistic) calculation can illustrate the point. Consider the choice of having a D3A Val dive-bomber attack either an eight-ton medium bomber or a 10,000-ton cruiser. It might take two Vals to destroy the bomber, for a return of four tons of industrial production per bomb. Against the stationary cruiser, four hits might be needed requiring eight dive-bombers, for a return of 1,250 tons of industrial production per bomb. That is over a 300-times better return for attacking the cruiser. And, while the Americans had thousands of aircraft, the Pacific Fleet had only 21 cruisers, and were much harder to replace.
This simplistic comparison does not take into account a host of factors, including relative availability of the different construction materials, cost per ton, production rates, complexity of construction, replacement production rates, labor requirements, and the different capabilities of requirements for the different platforms. But it does illustrate how a planner considers allocations on the basis of “return per weapon expended.” Given eight D3A Vals to allocate, the potential return was either four enemy bombers or one cruiser.
Most naval officers of the period—even aviators—would have agreed that every one of the Japanese weapons could have been profitably employed against fleet targets. But strategic, operational, and tactical needs can conflict. All the weapons could not be concentrated against the fleet because there were other needs to consider. The aim of the OCA effort was short-term, tactical, and transitory: to suppress the American aircraft long enough to allow the Japanese to deliver their attack, recover their aircraft, and move beyond retaliation range. Their wargames had illuminated the potential damage that could be inflicted by an aerial counterattack. Losing two or three carriers at the outset of the war could have significant ramifications.
The Japanese knew that carriers were fragile creatures, and believed that ground-based bombers were effective against them. This belief echoed their own concept of operations for the defense of their home waters, a layered defense with land-based air operating from island airfields, concentrating in successive barriers to wear down the advancing U.S fleet. Their confidence that their own land-based air could sink carriers would naturally translate into a fear that the enemy’s could do the same, another manifestation of mirror imaging.
The US Army Air Corps (AAC) had 57 medium and heavy bombers on Oahu. Few were cutting edge in aviation technology, but they still represented a creditable threat. In addition to AAC aircraft, any US carriers in port would have flown off their air groups to various Oahu airfields, and were a threat. The Japanese carriers might be hit by as many as 165 AAC and Navy bombers carrying as many as 300 bombs and 36 torpedoes, escorted by F4F Wildcat and F2-A Buffalo fighters.
The Japanese would have to divide their fighters, some to accompany the strike and some to remain with the carriers. This allocation represented a difficult decision. If surprise was achieved, more fighters with the strike would mean more enemy aircraft destroyed on the ground by strafing, and less chance of a counterattack. In that case the ships’ CAP would not contribute any damage to the enemy and would effectively be wasted. If surprise was not achieved, more fighters with the CAP would provide a better chance that the carriers would survive any counterattacks.
Forty-eight A6M Zeros, about one-third, were retained for fleet defense. Of those, one-third to one-half could be expected to be airborne and available to oppose a counterattack at a given time. With the Japanese predilection for the offensive, retaining even those was probably a struggle.
To place this in context, using the NWC wargaming rules, 24 fighters engaging 57 medium bombers would shoot down eight bombers prior to dropping their bombs, leaving 49 bombers to deliver their ordnance. With three bombs apiece and 5% hits there would be seven hits, enough to cripple or sink two carriers. Using Japanese expectations for level bombers, 16% accuracy would give 24 hits, sufficient for a Japanese disaster.
The Japanese needed an OCA effort to limit, disrupt, or prevent a counterattack on the carriers. Consequently, the first-wave dive-bombers were primarily assigned to airfield attack, followed by level bombers in the second wave.
Another reason to send some first-wave aircraft against airfields was to prevent congestion over the harbor. Dive-bombers, level bombers and torpedo bombers attacking the fleet simultaneously held the potential for mutual interference. At best, bomb runs would be aborted or disrupted, reducing the accuracy of weapons delivery; at worst, there could be midair collisions and weapons jettisoned.
A little over half the 250kg bombs and all the 60kg bombs allocated to OCA were carried by the B5N Kate carrier attack planes of the second wave. These bombers were trained to fly in formation and release their bombs when the lead plane released so the bombs would strike in a pattern over the footprint of the formation. This type of bombing was effective against area targets such as hangars, fuel tanks, and maintenance and administrative areas, but would be largely wasted against point targets such as individual aircraft in revetments.
The 250kg bombs on the D3A Val dive-bombers would best go against point targets, such as aircraft in revetments and on parking pads. The D3A Val was the “precision” delivery system and the B5N Kate the “area” system, with many more “precision” targets than there were Vals. The aircraft targets on the ground were expected to be widely dispersed. Those not destroyed by bombs would be the target of strafing fighters.
Why attack hangars and administrative buildings? The reason centered on the number of bombers versus the number of aircraft targets. It was unlikely that the dive-bombers would be able to eliminate all the American bombers. The Army at Wheeler had 85 revetments capable of protecting 109 aircraft. With only 54 D3A Vals assigned, there weren’t enough OCA dive-bombers to be able to put one on each revetment.24
Revetments greatly reduce aircraft vulnerability by blocking the spray of fragments from ground burst bombs (air burst bombs had not yet been developed), requiring the bomb to hit inside the revetment or in line
with the entrance to be effective. Revetments also provided partial protection from strafing attacks. The fighter revetments were mostly sized to contain only one aircraft and were not much wider than the P-40 fighter’s wingspan of about 38 feet. The Japanese would expect that any aircraft parked outside of revetments would be dispersed. The metric for proper dispersal was to ensure that a single bomb would not be able to destroy more than one aircraft.
Considering that dive-bomber accuracy historically was on the order of 50 to 210 feet circular error probability (CEP),25 a good performance by the dive bombers would destroy or critically damage one aircraft for every three bombs against aircraft in revetments, or one aircraft per one or two bombs for targets in the open. The 54 dive-bombers might be expected to eliminate 18 to 54 aircraft, out of the estimated 550 on Oahu, depending upon the mix of targets attacked. Something else had to be done to disrupt the enemy’s response.
Consequently, level bombers had to reinforce the OCA effort. Fifty-four B5N Kate carrier attack planes were armed with GP bombs and assigned to airfield attack. However, every Kate so assigned was one less delivering battleship-killing ordnance.
One could argue with the wisdom of hindsight that the Japanese made good decisions in their allocation of the B5N Kates between warship and OCA targets, since the attack met their overall mission objective. But there were so many more fleet targets than battleships alone. The Japanese could have used additional torpedoes profitably, for example, against the four cruisers outside the Navy Yard, the nests of destroyers, and the fleet’s tenders and auxiliaries; or, the B5N Kates with 250kg bombs could have profitably hit the shipyard piers, where four modern 10,000-ton cruisers were tied up.
The carrier attack plane aircrews on the two newest Japanese carriers were judged by the planners as insufficiently skilled to deliver torpedo attacks. According to Japanese sources, most training for these aviators was centered on basic flying—things like take-offs, landings, carrier qualifications, and flying in formation. Hangars and aircraft parking area targets did not require the precision flying required for a torpedo attack and were more tolerant of sloppy flying. So, all the bombers on Shokaku and Zuikaku, dive-bombers and carrier attack planes alike, were assigned to OCA. The aviators on the senior carriers would attack the fleet.