Aircraft in the 1920s were largely used in reconnaissance, scouting, and gunfire-spotting roles. Their use for fleet air defense was fairly limited. Furthermore, the offensive potential of naval aircraft was limited by the short range of planes at the time. By the 1930s, the increased performance characteristics of newer aircraft allowed the Imperial Japanese Navy (IJN) to begin looking into the possibility of using bombers to attack enemy ships (Evans & Peattie, 1997, p. 326). In the late 1930s, the IJN further developed carrier air power tactics in anticipation of hostilities with the U.S. Navy. By 1940-41, they were further honing their tactics into a cohesive offensive system (Evans & Peattie, 1997, p. 343). This post will examine the development of the IJN’s aerial attack profiles. The three attack profiles to be examined are horizontal bombing, dive bombing, and aerial torpedo bombing.
Anti-ship Aerial Attack Profiles
Why is it difficult to hit a moving target with horizontal bombing?
As they say with bombing, there’s a 99% chance the ordnance will hit something; even if that’s the ground. The above illustration shows why hitting a moving target with horizontal bombing is so hard. Giuseppe Fioravanzo (1979) provides a simple mathematical examination of why horizontal bombing was so ineffective at hitting moving targets. As shown in the above illustrations, assume that the aircraft are targeting the ship below (at Point N), so they need to maneuver to drop their bombs on Point O. Point O is ahead of the ship at a distance equal to the speed of the target vessel multiplied by the time it takes for the bomb(s) to fall. That’s a fancy way of saying that any bombs will need to be dropped before the aircraft reaches Point O, and at such a time so as to intercept the target’s track when the target itself will be at Point O. If the ship were stationary, then Point N and O would coincide with each other. Thus, the aircraft is headed towards a theoretical position above Point O and the bombing sights can calculate the appropriate time of bomb release to hit Point O (Fioravanzo, 1979, p. 176 – 177).
The problem is that success lies in the aircraft determining the course and speed of the target, which is often easier said than done. Should the target ship turn and maneuver evasively before the bombing aircraft can correct its course, and as long as the evasive maneuver takes it out of the dispersion of the bombs, then it won’t be hit. Thus, turning greatly reduces the probability of being hit. This shows why zigzagging was just as effective at disrupting the attacks of aircraft as long as the zigzags were short and unpredictable (Fioravanzo, 1979, p. 177).
Horizontal Bombing Tactics
The early development of horizontal bombing techniques was fairly rudimentary. Bombing operations were conducted at Tsingtao in 1914 against German defenses, but the results were insignificant due to the lack of training and bomb sights. The British Semphill Mission in 1921 allowed Japan to acquire both training and bomb sights, along with other advancements (Peattie, 2001, p. 18-20). Following the massive publicity of the American bombing tests against ships in 1921 off the Virginia Capes (famously promoted by General Billy Mitchell), the IJN began looking into the tactic of horizontal bombing (suihei bakugeki 水平爆撃) (Evans & Peattie, 1997, p. 327). In 1924, the IJN conducted similar experiments against the hulk of the Iwami (ex-Russian battleship Orel) off Yokosuka using bombs dropped from an average height of 1,000 meters (3,300 feet). While the Iwami was eventually sunk, it took approximately four hours to do so, and only about 20% of the bombs hit the target (Peattie, 2001, p. 34). For the remainder of the 1920s, annual tests with horizontal bombing, by either single aircraft or groups of two to three, demonstrated good accuracy at a horizontal bombing altitude of 2,000 – 5,000 meters (6,500 – 16,400 feet) against stationary targets. Further exercises conducted after 1930 against moving targets, however, showed poorer results. Bombing tests conducted at lower altitudes of 100 – 1,000 meters (300 – 3,300 feet) and speeds of roughly 50 knots produced better hit ratios, but it was recognized that such parameters would make the planes easy targets for anti-aircraft fire. By the mid-1930, the altitude for bomb deployment was again increased, but with larger aircraft formations to create a larger bombing pattern. Still, a lack of accurate bomb sights and poor aircraft coordination between pilots and bombardiers hindered performance against moving targets. It was only right on the eve of the Pacific War that improved team coordination and training produced more accurate results (Peattie, 2001, p. 34-35).
The Second Sino-Japanese War in China allowed the IJN to extensively test its horizontal bombing techniques. However, their high-altitude horizontal bombing missions over China were shown to be inaccurate. By 1941, with the upcoming Pearl Harbor strike being planned, aerial torpedoes and dive bombers were deemed insufficient in guaranteeing a successful attack on ships, so horizontal bombing was to be included (Evans & Peattie, 1997, p. 343-344). To increase accuracy, tacticians reduced the bomb deployment altitude from 4,000 meters (13,100 feet) to 3,000 meters (9,800 feet) which was the minimum height for an armor-piercing bomb to carry enough velocity to penetrate the deck armor. Additionally, rigorous training for bombardiers and tighter bombing formations was implemented which increased accuracy from 10% to 33%. The results paid off for high-level bombers when the USS Arizona was sunk by an armor-piercing bomb. However, the navy never again achieved such success and no major vessel was sunk while underway with high-altitude level bombing during the Pacific War (Peattie, 2001, p. 139-140).
Problems with horizontal bombing accuracy were not limited to the IJN. During the interwar years, U.S. Navy fleet exercises (Fleet Problems) originally assessed horizontal bombing accuracy to be 20% in 1934 but reduced it to 8% in 1938 (Nofi, 2010, p. 306-307). High-altitude horizontal bombing against maneuvering targets during wartime was so inaccurate that the only warship to be arguably “sunk” by such a method was the destroyer Mutsuki on August 25, 1942. While slowly maneuvering to rescue survivors from a disabled troop transport, Mutsuki was hit by one bomb from a B-17. Reportedly, Mutsuki’s captain said during an inquiry that “even B-17s can get a hit once in a while.” Later on, following the First Naval Battle of Guadalcanal, three B-17s attacked the battleship Hiei as she limped away from the previous night’s engagement. Although she had stopped to pump out her steering compartment, she quickly built up speed to 15 knots and, while making an evasive turn to starboard, was hit by roughly one of the twenty-eight 500-pound bombs dropped. Later on, she was attacked by fourteen B-17s that dropped fifty-six 500-pound bombs and claimed one hit. However, overall the two bomb hits did little damage and she sank later as a result of cumulative damage. From the seventeen B-17s that dropped a total of 84 bombs and scored 2 hits, their accuracy was an appalling 2.4% (Nofi, 2010, p. 34-35).
Dive Bombing Tactics & Attack Profile
In contrast to horizontal bombing, dive bombing benefited from improved accuracy due to the lower height of bomb release and the ability of the pilot to more easily adjust their attacks against evasive targets (Evans & Peattie, 1997, p. 328).
Mark Peattie (2001) notes that U.S. air show demonstrations and reports from Japanese naval attaches aroused interest in dive bombing techniques. In 1929, Vice Admiral Ando Masataka, chief of the Naval Aviation Department, began collecting data for the training and development of dive bombing in the IJN. In 1930, naval fighters of the First Carrier Division dive-bombed the old cruiser Akashi in Tokyo Bay using 4 kg (8.8 lb) training bombs. The next two years saw further development with larger training bombs used against target vessels, with some bombing runs scoring 100% accuracy (p. 40).
Okumiya Masatake, one of the first aviators trained in dive bombing tactics, described attacks on land targets in dives made at a 30-degree angle from a height of 1,000 meters (3,300 feet) and 2,000 meters (6,500 feet) from the target. A large white cloth was placed in a field and next to it was optical equipment that measured their dive angles. He says that most pilots initially had trouble correctly diving at the proper angle, but eventually they could dive at 60 degrees. The biggest problem was compensating for wind, which threw off the pilot’s aim. Further development of techniques would see dives begin at 3,000 meters (9,800 feet) and pilots adjusting their dive angle until approximately 1,000 meters above the target. At this point, they would be doing about 250-270 knots. Once at 1,000 meters, they adjusted their aim until they reached the release point at 500 meters (1,600 feet). (Peattie, 2001, p. 40-41).
Tactical development in the mid-1930s by both American and Japanese naval airmen saw the aircraft carrier as the main target for offensive air operations. The rationale was not to sink the carriers, but rather, to destroy the flight deck in order to cripple air operations. Aerial torpedo bombers could sink the ship, but the Japanese navy believed that an aerial torpedo attack would take too long and allow the enemy carrier to launch aircraft. Hence, dive bombing was seen as the best tactic for disrupting a carrier’s capabilities (Peattie, 2001, p. 40). In 1935, dive bombing tests were conducted at the Kashima range off the coast of Ibaraki Prefecture using a mock-up of the USS Saratoga. By 1937, the IJN had a corps of extremely skilled aircrews; among them was Lt. Egusa Takashige who would become one of Japan’s dive bombing aces during the Pacific War (Peattie, 2001, p. 40-41).
While dive bombing was used to some effect during the war in China, no targets called for hitting an underway vessel. Thus, further development of techniques was facilitated by more training and the introduction of the Aichi D3A “Val” dive bomber. By 1939, during exercises, the dive bombers were achieving a 53.7% hit rate which increased to 55% two years later. By 1940, dive bombing tactics pioneered by Lt. Takahashi Sadamu, called for an approach of a target on the opposite course at an altitude of 3,000 meters (9,800 feet) at 20-30 nautical miles from the target. The aircraft would then go into an echelon formation and begin a shallow 10-degree dive at full throttle. Then, the aircraft would, in succession, dive at 65 degrees and aim for a point ahead of the target. Bomb release would occur at an altitude of about 600 meters (2,000 feet). Once they released their payload, the aircraft would then retract their dive brakes and pull up sharply. The navy estimated that 18 planes would be required to make 5-6 direct hits on a target, out of which 8 planes would be lost due to anti-aircraft fire (Peattie, 2001, p. 140-141).
At the time of the Pacific War, Japanese dive bombing doctrine stipulated that the bombers approach a target either bow-on or stern-on if the wind was negligible. Bow-on attacks were preferred. However, if the wind exceeded 30 knots, then the attack would be made with the wind at the tail of the plane to minimize drift. With several formations attacking simultaneously, they would attempt to attack from different directions. These were the standard dive bombing tactics of the IJN and were used successfully in the sinking of the HMS Hermes, HMS Cornwall, and HMS Dorsetshire (Peattie, 2001, p. 141).
Torpedo Bombing Tactics & Attack Profile
Aerial torpedo attacks were actually the first method studied by Imperial Japanese naval aviation and emerged in 1912 (Peattie, 2001, p. 35). Simulated night attacks by aircraft against fleet units in Tateyama Bay were conducted as early as 1916 which inspired early naval aviators of the tremendous potential for using aircraft as attack platforms, but the problem was that aircraft and torpedo technology had not advanced far enough to make it feasible (Evans & Peattie, 1997, p. 327). In fact, most aerial torpedo practice was conducted with singular aircraft up to 1928. The technological lag also led the IJN to experiment with some “unique” tactics during their fleet maneuvers. During exercises off of Amami Oshima in 1928, the Hosho‘s torpedo attack squadrons practiced coordinating their attacks with cruiser and destroyer torpedo attacks. Their tactics called for the torpedo planes to get on station and orbit the target; waiting for the cruisers and destroyers to get into attack position. The torpedo planes would then circle around and attack the targets from the opposite flank (Peattie, 2001, p. 37).
Obviously, there were some problems with these tactics, and they were not lost upon the tacticians at the time. First of all, the circumstances for such a tactic would take hours to develop and the window for the attack might only last for a few minutes. The degree of fine coordination required was ridiculous. Secondly, the poor air-to-air and ship-to-air communications at the time were woefully inadequate to ensure such coordination. Arguably, it says more to the Japanese inexperience in using naval aircraft that such tactics were even accepted into doctrine (Peattie, 2001, p. 37). In many ways, this tactical complexity is a common feature of Japanese doctrine in that it expects the enemy to behave in a very prescribed manner as if going step-by-step through a playbook. The Japanese were extremely skilled at offensive tactics, and their intense training had honed their skills down to a precision edge (as was shown in their successes in the first few years of the Pacific War). Theoretically, such a hammer and anvil-style tactic using coordinated air and surface attacks from opposite directions is feasible, and the Japanese would have trained rigorously in such a tactic, but it ignores the realities of actual combat. This particular tactic seems tailored to a very specific situation which requires all the moving parts to be in the right place at the right time. It’s one thing to execute the tactic perfectly in the relative safety of a training exercise, but try doing it when the enemy ships and aircraft are firing live ammunition. As history has shown, tactics developed in peacetime tend to be overly complex. They are then simplified or done away with when the shooting starts for real. The best tactics are often the simplest, most adaptable, and left for the individual commanders to decide and implement.
Technological advancements improved hit rates with torpedoes. By 1932, the probability of hits against moving targets had increased to 60%, and then to 88.4% the following year. In 1935, the torpedo bombers were regularly achieving hit rates of 60% in poor daylight conditions and up to 100% under good conditions. However, one reason for their high hit rates could be explained by the weak anti-aircraft defenses at the time which were judged to have downed a few attacking aircraft. That being said, the high hit rates are a testament to the skill of the aviators given that the calculations required to pull off a successful aerial torpedo attack against an evasive target were done by eye (Peattie, 2001, p. 39). By 1937, improved technology had allowed for torpedoes to be dropped from heights of up to 200 meters (660 feet) and speeds of up to 120 knots. Furthermore, continued training in night attack methods had torpedo bombers follow behind flare-dropping aircraft (Peattie, 2001, p. 143).
Various experiments in massed attack, coordination of carrier and land-based torpedo squadrons, and night torpedo bombing allowed the IJN to further develop its tactics and institute rigorous training for its aircrews. However, this raised the issue of how to avoid midair collisions with multiple planes attacking a target and taking evasive maneuvers against anti-aircraft fire. In order to minimize the risk of midair collisions the navy further refined the rules for torpedo launches at greater heights and distances during training, but it was not until the adoption of the Type 91 aerial torpedo that some of these dangers were alleviated and improved hit performance was seen. Still, the issues remained (Peattie, 2001, p. 143-144). An examination of the Type 91 aerial torpedo, its development, and modifications is for another time.
Shallow water torpedo attacks were already being practiced by the IJN in 1939 against ships in Saeki Bay in Kyushu, before the British air raid on the Italian fleet at Taranto in 1940. The navy was studying problems of potential attacks with five Asian and Pacific ports in mind, including, but not solely limited to, Pearl Harbor. In addition to modifications to the Type 91 torpedo for shallow waters, relentless training, and experimentation were conducted by the Yokosuka Naval Air Group in Kagoshima Bay in the autumn of 1941. Leading the air group was Lt. Cmdr. Murata Shigeharu, the leading torpedo expert, who managed to perfect the required tactics used to great effect during the surprise attack on Pearl Harbor (Evans & Peattie, 1997, p. 345-346).
Even land-based air groups took part in the development of aerial torpedo tactics. When it wasn’t operating over China in the summertime, the Kanoya Air Group’s G3M “Nell” twin-engined bombers participated in the annual Combined Fleet exercises from December through April and practiced torpedo runs under fighter cover. By late-1941, the new G4M “Betty” bombers gave them improved performance (Peattie, 2001, p. 146)
At the start of the Pacific War, the standard IJN aerial torpedo bombing tactic was for the torpedo bombers to fly at an altitude of 1,000 – 3,000 meters (3,300 – 9,800 feet), depending on atmospheric and target conditions. On making their approach, they would fly on an opposite course to the target and split up approximately 10 – 12 miles away in order to attack from two sides. As with dive-bombing tactics, they would attack in either a loose string or line abreast if facing AA fire. The parameters for dropping their torpedoes were:
- Altitude: 50 – 100 meters (160 – 330 feet)
- Airspeed: 140 – 162 knots
- Range: 800 – 1,200 meters (2,600 – 3,900 feet) from the target
The idea was to catch the target ship in a converging attack from several directions so that no matter which way the target ship turned, it would always present a broadside to one group of attackers (Peattie, 2001, p. 144).
Evans and Peattie (1997) note that IJN aerial torpedo tactics, in keeping in line with their offensive mindset, were related to surface torpedo tactics (p. 327). Indeed, a cursory comparison between the previously discussed Otsu Tactic and the aerial torpedo tactics does appear to show similarities. However, whether the Otsu Tactic directly inspired the latter is unknown.
Coordination of Massed Air Attacks
Imperial Japanese Navy research into coordinating a massed air attack using various types of aircraft concluded that an attack would be launched from carriers 200 miles from the enemy. Generally speaking, the strike would have vanguard and rear elements with bombers converging on their targets from predetermined altitudes. Some fighter planes would deal with the enemy’s combat air patrol while others would strafe the superstructures and decks of the enemy carriers. Immediately following the fighters, the horizontal bombers would attack at 3,000-4,000 meters, and then the dive bombers from 3,000 meters in a 50-70 degree dive, releasing their bombs at 500 meters. While this was occurring, the torpedo bombers would simultaneously attack at an altitude of 50-100 meters. Obviously, such an operation would require rigorous planning, training, and organization to both prevent mishaps and coordinate forces. Additionally, the navy would need to develop the appropriate doctrine for massing its carriers. All of this would have been essential for the upcoming attack on Pearl Harbor (Peattie, 2001, p. 146-147). A discussion of the advantages and disadvantages of carrier dispersion versus massing is for another time.
As we see, throughout the interwar years, the development of tactics benefited from some foreign assistance, improved aircraft, and weapons technology. Experimentation in naval exercises with different types of bombing was further supplemented by experience in bombing operations in China. Additionally, tactical innovation was spearheaded by aviation pioneers who perfected the tactics in training and relentlessly drilled aircrews to high standards. All of these elements were brought together by the IJN into a developed and cohesive air offensive doctrine which it put to use in the early stages of the Pacific War.
Evans, D.C. & Peattie, M.R. (1997). Kaigun: Strategy, Tactics, and Technology in the Imperial Japanese Navy, 1887-1941. Annapolis, MD: Naval Institute Press.
Fioravanzo, G. (1979). A History of Naval Tactical Thought (A.W. Holst, Trans.). Naval Institute Press.
Nofi, A.A. (2010). To Train the Fleet for War: The U.S. Navy Fleet Problems 1923-1940. Newport, RI: Naval War College Press.
Peattie, M.R. (2001). Sunburst: The Rise of Japanese Naval Air Power 1909-1941. Annapolis, MD: Naval Institute Press.