By William Pearce
In the late 1930s, designers at Arsenal de l’Aéronautique in France began working on a new fighter powered by two engines installed in tandem. One engine was positioned in front of the cockpit, and the other engine was behind the cockpit. Each engine drove half of a coaxial contra-rotating propeller. This design was eventually developed into the Arsenal VB 10. Takeo Doi was a Japanese designer at Kawasaki and was aware of Arsenal’s tandem-engine design.
Doi was also aware of the evaporative cooling system used on the German Heinkel He 100. Japan had sent a delegation to Germany in December 1938 that successfully negotiated the purchase of three He 100 and two He 119 aircraft. The He 100s were delivered to Japan in the summer of 1940.
In 1939, Doi began to contemplate a high-speed fighter for the Imperial Japanese Army Air Force that used tandem engines and evaporative cooling. At the time, the Japanese aircraft industry was more focused on conventional aircraft, and Kawasaki and Doi were busy with designing the Ki-60 and Ki-61 Hien (Tony) fighters. In October 1940, Kawasaki and Doi received support for the tandem-engine fighter project, which was then designated Ki-64 (Rob). The aircraft’s design was refined, and a single Ki-64 prototype was ordered on 23 January 1941.
The Kawasaki Ki-64 looked very much like a continuation of the Ki-61 design, and while some of its features were inspired by other aircraft, the Ki-64 was an entirely independent design. The single-seat aircraft had a taildragger configuration and was of all-metal construction. Although designed as a fighter, the Ki-64 was primarily a research aircraft intended to test its unique engine installation and evaporative cooling system. Proposed armament included one 20 mm cannon installed in each wing and two 12.7 mm machine guns or 20 mm cannons installed in the upper fuselage in front of the cockpit. The armament was never fitted to the prototype.
The Ki-64 was powered by a Kawasaki Ha-201 engine that was comprised of two Kawasaki Ha-40 inverted V-12 engines coupled to a coaxial contra-rotating propeller. The Ha-40 was a licensed-built Daimler-Benz 601A engine and had a 5.91 in (150 mm) bore, a 6.30 in (160 mm) stroke, and a displacement of 2,070 cu in (33.9 L). As installed in the Ki-64, the shaft for the rear engine extended under the pilot’s seat and through the Vee of the front engine to the propeller gearbox. The rear engine drove the front adjustable-pitch propeller of the contra-rotating unit. The front engine drove the rear fixed-pitch propeller. Each set of propellers had three blades that were 9 ft 10 in (3.0 m) in diameter. The Ha-201 displaced a total of 4,141 cu in (67.9 L) and produced 2,350 hp (1,752 kW) at 2,500 rpm for takeoff and 2,200 hp (1,641 kW) at 2,400 rpm at 12,795 ft (3,900 m). Each engine section could operate independently of the other.
The engine sections had separate evaporative cooling systems. Heated water from the engine at 45 psia (3.1 bar) was pumped to a steam separator, where the water pressure dropped to 25 psia (1.7 bar), and about 2% of the water flashed to steam. The steam was then ducted at 16 psia (1.1 bar) through panels in the wings, where it was cooled and condensed back into water. The water then flowed back into the engine. The evaporative cooling system eliminated the drag of a radiator, and this enabled the aircraft to achieve higher speeds. It was believed that battle damage would not be much of a problem for the cooling system. The low pressure of the steam combined with steam’s low density meant that the amount of coolant lost through a puncture would be minimal, and the separate engines and cooling systems helped minimize the risk of a forced landing if damage did render one system ineffective.
The evaporative cooling system for the front engine was housed in the left wing, and the rear engine’s system was housed in the right wing. Each system consisted of two steam separators, an 18.5-gallon (70 L) tank in the wing’s leading edge near the fuselage, four upper and four lower wing condenser panels, an upper and lower condenser section in the outer flap, and a water tank in the fuselage. Sources disagree regarding the size of each fuselage tank, but combined, the tanks held around 52.8 gallons (200 L). Suspended below the right wing was a scoop that held oil coolers for the engines.
The Ki-64 had a 44 ft 3 in (13.50 m) wingspan and was 26 ft 2 in (11.03 m) long. The aircraft had a top speed of 435 mph (700 km/h) at 13,123 ft (4,000 m) and 429 mph (690 km/h) at 16,404 ft (5,000 m). The Ki-64 could climb to 16,404 ft (5,000 m) in 5.5 minutes and had a service ceiling of 39,370 ft (12,000 m). Since the wings housed the cooling system, little room was left for fuel tanks. Each wing had a 22-gallon (85 L) fuel tank, and an 82-gallon (310 L) tank was housed in the fuselage; this gave the Ki-64 a 621 mile (1,000 km) range. The aircraft weighed 8,929 lb (4,050 kg) empty and 11,244 lb (5,100 kg) loaded.
While the Ki-64 was being built, a Ki-61 was modified to test the evaporative cooling system. With its radiator removed and evaporative panels added to its wings, the modified Ki-61 first flew in October 1942. Around 35 flights were made before the end of 1943, and they served to develop and refine the cooling system. The aircraft proved the validity of the evaporative cooling system and achieved a speed 25 mph (40 km/h) in excess of a standard Ki-61. However, the evaporative cooling system did require much more maintenance than a conventional system.
The Ki-64 was completed at Kawasaki’s plant at Gifu Air Field in November 1943. The aircraft underwent ground tests that revealed a number of issues. By December, the issues were resolved enough for flight testing to commence. The aircraft made four successful flights, but the rear engine caught fire on the fifth flight. The pilot was able to make an emergency landing at Kakamigahara, but the rear engine and parts of the rear fuselage and cooling system had been damaged. The Ha-201 engine was sent to Kawasaki’s engine plant in Akashi for overhaul, and the Ki-64 airframe was sent back to Gifu for repairs.
The short flying career of the Ki-64 had shown that its cooling system was insufficient. The system worked well for level flight, but it was inadequate for ground running, takeoff, and climb. When the system was overloaded, steam was not condensed back to water and was subsequently vented overboard via a 16 psi (1.1 bar) relief valve. The cooling system lost about 12 gallons (45 L) of water during a rapid climb from takeoff to 18,000 ft (5,500 m). Water freezing within the system, either while in flight or on the ground during cold temperatures, was another concern. Adding an alcohol mixture to the water coolant was a possible solution, but the Ki-64 never underwent any cold weather testing.
While undergoing repairs, the Ki-64 was to be modified and redesignated Ki-64 Kai. The existing propellers would be replaced with fully adjustable and feathering contra-rotating propellers, which would make it easier for one engine to be shut down in flight. The engines were to be replaced with more powerful Ha-140s that provided an output of 2,800 hp (2,088 kW) for the coupled unit. With the changes, it was estimated that the Ki-64 Kai would have a top speed of 497 mph (800 km/h). However, the propeller and engines were delayed with more pressing war-time work, and the Ki-64 program was cancelled in mid-1944.
The Ki-64 airframe remained at Gifu where it was captured by American forces in 1945. Various parts of the cooling system were removed from the aircraft and shipped to Wright Field in Dayton, Ohio for further analysis and testing. The remainder of the Ki-64 was eventually scrapped.
Japanese Army Fighters Part 1 by William Green and Gordon Swanborough (1977)
Japanese KI-64 Single Fighter with Two Engines in Tandem and Vapor-Phase Cooling, Air Technical Intelligence Review Report No. F-IR-100-RE by Petaja and Gilmore (31 July 1946)
Japanese Secret Projects by Edwin M. Dyer III (2009)
Japanese Aircraft of the Pacific War by René J. Francillon (1979/2000)
Encyclopedia of Japanese Aircraft 1900–1945 Vol. 4: Kawasaki by Tadashi Nozawa (1966)
The Xplanes of Imperial Japanese Army & Navy 1924–45 by Shigeru Nohara (1999)
Heinkel He 100 by Erwin Hood (2007)