Nakajima built the Sabre specifically to carry out Tokko (Japanese for 'special-attack') missions. These one-way flights consisted of a pilot deliberately crashing his airplane into U. S. Navy ships. Japanese military officials concocted this desperate scheme during the fall of 1944 as the Allies systematically destroyed Japanese forces. The tactic inflicted grievous casualties, notably in April 1945 during the invasion of Okinawa when Japan launched more than 1,500 attacks that cost the U. S. Navy 21 ships sunk and 217 damaged. The overall human loss to kamikaze attacks during the war was horrific. They killed nearly 5,000 Allied military personnel, wounded 4,800 more, sank 47 U. S. naval vessels, and damaged 5 Royal Navy aircraft carriers and two Australian warships. Seven percent of all U. S. Navy crew casualties incurred during the entire Pacific war happened because of the kamikazes.
Such combat effectiveness, and a growing scarcity of conventional aircraft, encouraged Japanese military leaders to consider building squadrons of simple, low-cost, special-attack airplanes. These crude designs had just enough performance to allow pilots with minimum training to takeoff, cruise to the target, and make their final dive.
The Japanese also realized that it was only a matter of time before Allied armies invaded the home islands and the invasion fleet would provide thousands of targets. In January 1945, the Japanese Army told the Nakajima firm to develop a special attacker armed with a single bomb. A maximum speed of 515 kph (320 mph) was also required but most important, the aircraft had to be simple to build. At this time, the available laborers were often semi-skilled students who worked part-time shifts in the factories. Nakajima engineer Aori Kunihiro, helped by personnel from Mitaka Research Institute and Ota Manufacturing Co., Ltd., designed an aircraft built from a variety of materials. The internal fuselage was steel covered with steel skin panels, and a cowling rolled from sheet tin enclosed the engine. The tail consisted of a wooden framework covered with fabric. The wings were semi-monocoque and built entirely of aluminum. The main landing gear consisted of steel tubes but each strut was bolted directly to the wing and no shock absorbers were fitted. Kunihiro designed the gear to drop after takeoff. He also designed the Tsurugi to accept several types of surplus engines but all production machines used the 14-cylinder, air-cooled Nakajima Ha35-23 radial. The pilot sat in an open cockpit above the trailing edge of the wing.
Less than two months after the Army ordered it, Nakajima had the first prototype ready and flight-testing began. Initial results were disappointing. Ground handling was the major problem and led to redesign of the landing gear to accommodate simple shock absorbers. Nakajima also added auxiliary flaps to the inboard wing trailing edges. On production Sabres, the firm installed fittings beneath each wing for two solid-fuel rockets to boost speed during the final plunge. With these changes, speed jumped to a maximum of 550 kph (343 mph) while hauling a standard aerial bomb as large as 800 kg (1,764 lb). The Ki-115 could fly 1,200 km (720 miles) nonstop.
Nakajima built 104 Tsurugis at two factories and Mitaka Kenkyujo, one of the firms that helped design the airplane, built one. No Ki-115 saw combat before war's end. A naval variant and several improved versions did not progress from the drawing board.
We believe that the NASM Tsurugi is the only remaining example of this interesting aircraft. Another purpose-built special-attack airplane, the Kugisho Okha 22, also resides in the NASM collection. Little is known about the Sabre test program or planned deployment of these aircraft because Nakajima destroyed most records before Allied forces could intervene. U. S. Navy intelligence specialists probably picked up the NASM Ki-115a at the factory, moved it to a naval base at Yokosuka, and then shipped the airplane aboard one of the three aircraft carriers that returned Japanese aircraft to the U. S. for evaluation. The National Air Museum (later NASM) absorbed the Sabre into its collection along with about 53 other foreign aircraft in 1949.
