Attack on Pearl Harbor (12 page)

A Two-Wave Attack

Carrier Striking Force Operations Order No. 3 specified 351 aircraft to be launched in two waves.
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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:

2nd Wave:

The second wave was to consist of 171 aircraft:

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:

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.
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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),
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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.