The Ad Hoc Lashup That Resulted in Japan's Best Fighter of WW2

Japan's finest fighter of World War 2.

Unlike Germany and the Allies during the Second World War, the vast majority of Japanese fighter aircraft were powered by radial engines while other nations had a mix of both radial and inline engines for their fighter units. Only one production-standard Japanese fighter entered service with an inline engine, the Kawasaki Ki-61 "Hien" (Allied code name Tony). Over 2,500 Ki-61s were produced and saw service with the Imperial Japanese Army Air Force (IJAAF) from 1943 to the end of hostilities in 1945. The Hien was designed by Kawasaki's chief designer, Takeo Doi and his deputy, Shin Owada (Doi would later go on to design the postwar YS-11 airliner) in response to an IJAAF requirement for a multirole fighter that would use a license-produced version of the German Daimler-Benz DB601 liquid-cooled inline engine that also powered the Messerschmitt Bf 109. Proving itself to be a promising design, the Ki-61 turned out to have a major Achilles heel in its Kawasaki-built engine which was designated the Ha-140. Though the DB601 would be one of the Second World War's finest engines, by the time the Ki-61 entered service it was an old design that was already being outclassed by more powerful engines of both radial and inline design. Worse yet, the DB601 had very tight manufacturing tolerances and in Japanese license production, getting Ha-140 engines built as well as the DB601 proved elusive given the increasing toll being taken on Japanese industry which found itself constantly short of quality metals and lubricants given the tightening US Navy submarine blockade of the home islands. 

Ki-61 to Ki-100.

Worse would come as the war progressed into 1944 with Kawasaki's Akashi factory constantly being plagued by supplier shortages and properly manufactured crankcases and cylinder blocks. By the spring of 1944, more than half of the Ha-140 engines produced at Akashi that did leave the assembly line failed to pass acceptance testing! As a result, at Kawasaki's facility in Kagamigahara that produced the Ki-61 fighter found itself with increasing numbers of finished fighters sitting in open storage waiting for their Ha-140 engines. Matters worsened on 19 January 1945 when B-29 Superfortresses completely destroyed the Akashi engine plant, leaving 275 Ki-61 fighters without any engines. With the B-29s ranging with near impunity over the home islands, the IJAAF headquarters, the Koko Hombu, suggested that the engineless Ki-61s be adapted to take the most readily available and reliable engine- IJAAF engineers suggested that Kawasaki find a way to fit the engineless fighters with the Mitsubishi Ha-112 radial engine, a 14-cylinder twin row radial engine comparable in power to the Pratt & Whitney R-1830 radial used on the Consolidated B-24 Liberator and the Douglas C-47 Skytrain/Dakota. 

Takeo Doi was initially not pleased with the Koku Hombu's directive to fit the Ha-112 to the Ki-61. The radial engine was 48 inches in diameter and the fuselage of the Ki-61 was only 36 inches in diameter. But seeing as to how there was little alternatives and the need for fighters great, Doi had his engineering staff work around the clock to make the Ha-112 work with the Ki-61 Hien. Fortunately Kawasaki was in possession of a Focke Wulf Fw 190 which had been delivered to the Japanese several years earlier for engineering study. Using the cowl and engine mount of the Fw 190 as a starting point, Doi and his team even replicated the horizontal alignment and collection of the exhaust pipes of the Fw 190 which in the Kawasaki design actually added between 6 to 9 mph of thrust augmentation to the aircraft's speed. Designated the Ki-100, the prototype aircraft made its first flight a mere three months after start of design work- a figure all the more incredible in light of the tremendous toll the B-29 attacks were taking on Japanese industry. By the time the third Ki-100 prototype took flight, the Koko Hombu was sufficiently impressed that all the engineless Ki-61s were ordered to be converted to Ki-100 standard. 

On many occasions, Ki-100s bested Hellcats and Corsairs.

"Sufficiently impressed" would be an understatement- the Ha-112 radial engine actually made the Ki-100 superior in every performance parameter to the Ki-61, much to the surprise of Takeo Doi. Although just slightly slower in cruise than the Hien, the Ki-100 was more maneuverable and had a faster climb rate. The IJAAF even took the Ki-100 prototypes into mock air combat against a captured P-51C Mustang. With the flight test program only lasting three weeks, the first Ki-100s went into combat with the 18th Sentai in Chiba on the same night in March that B-29s had laid waste to sixteen square miles of Tokyo in a firebombing attack. Less than six weeks elapsed since first flight to first combat! Production of the Ki-61 was terminated immediately and in less than three months all 275 engineless Ki-61s were rebuilt as Ki-100s AND the Kagamigahara factory also managed to build nearly 75 new-build Ki-100s that featured a cut back rear fuselage to improve aft vision for the pilot. As fast as Ki-100s could be built they were funneled straight to home defense units who found the Ki-100 a formidable fighter. Numerous large dogfights against Grumman F6F Hellcats and Vought F4U Corsairs found the Ki-100 on the winning side with more American fighters shot down than Ki-100s. By May 1945 the Ki-100 finally had met its match as long range P-51D Mustangs from Iwo Jima were now escorting the B-29s all the way to Japan and back. Dogfights with P-51Ds ended up swinging in favor of pilot skill and numerical advantage rather than any deficiency on the part of either aircraft. 

The one weakness of the Ki-100 was its high altitude performance as it lacked a turbocharger and as a result, few B-29s fell to the Ki-100. Work was underway for a supercharged high altitude Ki-100 when the war came to an abrupt end with the atomic bombing of Hiroshima and Nagasaki. Unfortunately for Japan, only about 500 Ki-100s were built before the war ended, but compared to other Japanese fighter designs, the Ki-100 proved to be Japan's finest fighter aircraft as a swansong for the once-feared IJAAF. From inception to the end of the war, the lifespan of the Ki-100 was barely ten months with an ad hoc marriage of engine and airframe out of necessity! The only surviving Ki-100 can be seen today at the RAF Museum. 

Source: Air International, October 1976, Volume 11, Number 4. "The Last Swallow of Summer: The Extraordinary Story of the Ki-100", p185-191.