A Legend is Born

Japan's Mitsubishi A6M2 Zero

by Randy Wilson

Copyright © 1993 by the Confederate Air Force and Randy Wilson. All rights reserved.

In 1937, Japan had been at war with the Chinese for six years, and her army and naval air forces had been in combat with a variety of European, American and Russian designed aircraft. Eventual expansion of the war into the Pacific was obviously going to require a carrier based fighter capable of out-performing any enemy land or carrier based aircraft.

When Jiro Horikoshi, chief aircraft designer for Mitsubishi Heavy Industries, saw the Imperial Japanese Navy (IJN) requirements for the 12-shi carrier fighter (the experimental designation for the Zero design), they must have seemed impossible — a maximum speed of over 310 mph at 13,000 feet, the ability to climb to 9,840 feet (4,000 meters) in 3.5 minutes, 1.2 to 1.5 hours endurance at full power, an armament of two 20 mm cannon and two machine guns, greater maneuverability than existing fighters and able to operate from an aircraft carrier.

Horikoshi had toured the world’s greatest aircraft factories as part of his postgraduate studies for Mitsubishi. He had designed the previous IJN carrier fighter, the Mitsubishi A5M Type 96 which had a maximum speed of 270 mph at best, and knew that Japanese aircraft engine designs lagged behind those of Western countries. Thus, the 12-shi design requirements would have to be met with an engine of 1,000 horsepower or even less.

Three engine designs were available, all twin-row, 14-cylinder air-cooled radials, two from his own company Mitsubishi, the 875 hp Zuisei 13 and the 1,070 hp Kinsei 64, and rival Nakajima’s 950 hp Sakae 12.

Given a limit on the power available, Horikoshi decided to review the normally accepted stress limits, structures and even materials used in aircraft designs. He decided that careful tuning of a plane’s individual structural elements, especially the wing spar and its attachment to the fuselage, could significantly lower the design’s weight with little loss of strength. In addition, he became aware of the development of a new lighter-weight but higher-strength aluminum by Sumitomo Metals, called Extra Super Duralumin (ESD), and incorporated it into the design.

The result was an advanced all metal semi-monocoque, flush riveted design of extremely light weight, with a fully retracting landing gear and enclosed cockpit. The design was without armor protection for the pilot or self-sealing fuel tanks, since neither were required in the specifications, nor did such defensive structures fit in with Japanese military traditions honoring only the attack.

On April 1,1939 the prototype A6M1 Type 0 carrier fighter made its first flight. The prototype was powered by a Mitsubishi MK2 Zuisei 13 ("Auspicious Star") engine of 780 horsepower and fitted with a two bladed propeller. In Japan, the design quickly became known as the Rei-sen or "Zero fighter", rei meaning "zero" and sen short for Sentoki or "fighter".

Although given the Allied code name "Zeke," the A6M’s true designation was known fairly early to Allied intelligence, and the planes were referred to as "Zeros."

The third prototype was powered by a Nakajima NK1C Sakae 12 ("Prosperity") engine of 950 horsepower and fitted with a constant speed three bladed propeller due to minor vibrations. Designated the A6M2, the Nakajima powered version first flew on Jan.18, 1940, and with the higher power and greater possible fuel economy of the Sakae engine, it exceeded all of the original IJN 12-shi specifications.

Word of the new fighter quickly reached combat units in China, and 15 preproduction Rei-sens were sent to the 12th Kokuwi (Naval Air Corps) at Hankow on July 21 —10 days before the IJN officially accepted the A6M2 into service. On Sept. 13, the new fighters of the 12^th Kokuwi saw their first combat, shooting down 27 Russian built Chinese Air Force Polikarpov I-15 and I-16 fighters, without loss to the Rei-sens.

In China, the A6M2 bested every fighter flown by the Chinese, In early 1941, General Claire Chennault and other American observers in China reported on the Japanese navy’s new fighter and some U.S. intelligence services passed on the warnings, but U.S. commanders repeatedly failed to recognize the new design’s significance.

Production of the A6M2 Type 0 Model 11 began at Mitsubishi’s Nagoya plant in December 1939. To prevent damage to the new fighter’s longer span wings while being moved up and down on the Japanese carriers’ 11 meter (36 feet) wide elevators, a modification was introduced allowing the outer 20 inches of each wing tip to be folded manually.

Placed into production, the new model became the A6M2 Model 21, the model Rei-sen which was to equip the Japanese carrier forces from the beginning of the Pacific War until their effective destruction at Midway and Guadalcanal. By Dec. 7,1941, A6M2s had replaced the earlier fixed-gear A5Ms on all but three of the smaller carriers, and the Rei-sen’s phenomenal range was to give the Japanese naval air forces the ability to strike over greater distances than could the American fighters. Thus, Japan began World War II with the most advanced carrier based fighter in the world.

The A6M3 Model 32 was an attempt to increase the Rei-sen’s high-altitude performance, fitting a 1,130 hp Sakae 21 engine with a two-speed supercharger and squared wing tips. Total fuel capacity had to be reduced to fit the larger engine. Most of these were relegated to a training role in Japan, after the new version’s shorter range caused a number of operational losses.

The most produced Rei-sen version was the A6M5 Model 52, with over 6,000 manufactured. Featuring a new wing with heavier gauge skin and non-folding rounded wing tips, the A6M5 used the same Sakae 21 engine as the A6M3 but with individual exhaust stacks. Although over 400 pounds heavier than the A6M3, the Model 52 could reach 351 mph at 20,000 feet and could safely exceed 410 mph in a dive. Production of A6M5s began reaching Japanese Naval Air Force units in the Solomons in March of 1944, where they were found to almost equal the new American F6F Hellcat, if flown by an experienced pilot. However by 1944, very few experienced Rei-sen pilots were left.

May of 1945 saw production begin of the last operational Rei-sen model, the A6M7 Model 63. Fitted with a Sakae 31 engine with water-methanol injection which proved unreliable, not many A6M7s saw service.

Orders were placed for 6,300 of the final variant, the A6M8 Model 64, fitted with a much larger 1,560 hp Mitsubishi MKSP Kinsei 62 engine. However none were completed before the end of the war, due to the disruption of the Japanese aircraft industry caused by American long-range strategic bombing.

Like the Grumman F4F Wildcat, the Zero’s primary opponent in the first two years of the Pacific War, the Mitsubishi A6M Rei-sen soldiered on throughout the entire war. However, unlike the Wildcat, which was replaced in front line carrier units by the F6F Hellcat in late 1943, the Japanese navy never succeeded in replacing the Rei-sen. In the end, Horikoshi’s genius in meeting the original specifications by minimizing the design’s structural weight meant that by 1943, the Rei-sen was obsolete.

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