Kayaba Katsuodori


The Katsuodori depicted here is shown in the colours and markings of the 71st Sentai. It is intercepting Tokyo-bound Northrop B-35 bombers of the 44th Bomb Squadron, 40th Bomb Group operating from Tinian.



Kayaba envisioned that his design for a fast, point defence interceptor would sweep through the Allied bombers like the kat­suodori bird hunts for fish. Impressed with the prowess of the katsuodori, Kayaba named his design after the bird. But as we will see, his vision was to meet with a harsh reality.

The genesis of the Kayaba Katsuodori began as far back as 1937 with the Kayaba Ramjet Study Group, a collection of engineers and scientists who sought to investigate ram­jet propulsion in Japan. The concept of the ramjet was actually patented in 1908 by French engineer Rene Lorin, but it was the Russian I. A. Merkulov who first built and tested one, the GIRD-04 in 1933. A ramjet is a very basic engine with few moving parts. In simple terms, it uses the high pressure air generated by the aircraft’s forward motion and forces it through the inlet. The air is then mixed with combusted fuel – this heats the air and is forced out of the rear of the engine, pro­viding forward propulsion. Unlike pulsejets (which were to be used on the Kawanishi Baika, see Page 61), the fuel flow is continu­ous. Without getting into the specifics of a ramjet, adjustments in the design of the inlet (to maximise the intake of air), combustor (to ensure effective operation during flight move­ments) and the outlet nozzle (to effect accel­eration increases) all come into play on
designing such an engine. The main draw­back with a ramjet engine is that at subsonic speeds its performance is poor. Below 612km/h (380mph), a ramjet suffers signifi­cant loss in speed and becomes highly ineffi­cient in terms of fuel consumption. The ramjet typically requires another power source to bring the aircraft up to the speed at which the ramjet can operate efficiently. Typ­ically, this speed is at least 966km/h (600mph). Once the ramjet reaches that speed the engine is self-sufficient and, with­out fuel injection moderation, would propel the plane to speeds far in excess of the design’s ability to handle the high tempera­tures and Mach number.

The Kayaba Ramjet Study Group saw the benefits of high speed with a relatively easy to manufacture engine. The group produced two test models before the final product, the Kayaba Model 1 ramjet, was realised. The Model 1 was projected to be able to offer speeds of 900km/h (559mph). With the engine complete, all that was needed was the aircraft to fit it into.

The airframe design began with Kumazo Hino. Hino was an officer in the IJA and had been the first Japanese to unofficially make a flight on 14 December 1910 when he acci­dentally took to the air in a Hans Grade mono­plane while he was taxiing. This aircraft had
been purchased from Germany. His interest in aviation saw him produce four aircraft designs: the Hino No. 1, No. 2 and the No. 3 and No. 4 Kamikaze-go airplanes. However, each of these designs was a failure. Pressure from his military superiors saw Hino give up on aviation by 1912.

However, in 1937, Hino was inspired to cre­ate a tailless glider. The project was taken over by the Kayaba Seisakusho (Kayaba Man­ufacturing Works) and then by Dr. Hidemasa Kimura who worked for the Aeronautical Research Institute of the Tokyo Imperial Uni­versity under Dr. Taichiro Ogawa. The result was the HK-1. The HK-1 (standing for Hino Kumazo) was built by the Ito Нікбкі K. K. and was completed in February 1938. It was purely a research glider to test the tailless concept. Testing commenced in December 1938 with ground towing at Kashima in Ibaraki Prefecture and the first air released flights began in September 1939 at Tsu – danuma in Chiba Prefecture. Because it showed positive results, the IJA took an inter­est in the concept. The HK-1 was purchased by the Rikugun Kokugijutsu Kenkyujo in April

1940 for continued testing. However, a subse­quent test flight on 16 April by an IJA officer pilot resulted in a hard landing that damaged the HK-1 beyond repair. In all, 182 flights had been made in the HK-1.

With the IJA still interested, the Rikugun Kokugijutsu Kenkyujo set aside 200,000 yen to continue the project. Kimura, along with Kayaba’s chief development designer Dr. Shigeki Naito, set about the task of producing a new tailless aircraft, this time with a possi­ble military application. The result was the Ku-2. The Ku-2 had no tail but rudders were fixed to the wing tips and the design was tested extensively from November 1940 through to May 1941 making 270 flights in all before it was damaged in a crash on 10 May. To further test the concept, Kimura (with the aid of Joji Washimi) produced the Ku-3 which had no vertical control surfaces at all and fea­tured a cranked delta wing form to test vari­ous angles of sweep. The only control came from the flaps arranged along the wings. 65 flights were carried out with the single Ku-3 before a crash in 1941 wrecked the glider.

The last design put forward by Kimura was the Ku-4. At the request of the Rikugun Kokugijutsu Kenkyujo, the Ku-4 was to be powered and a rear mounted 120hp de Hav – illand Gipsy 4-cylinder, air-cooled, inline engine was selected, turning a two-bladed propeller. Unfortunately for Kimura, the IJA lost interest in the entire concept. With the loss of the Ku-2 and the Ku-3, the IJA can­celled the Ku-4 before it could be finished. With no backing, Kayaba could not afford the 100,000 yen to finish the Ku-4 alone. The IJA had paid Kimura and Kayaba 17,000 yen out of the 200,000 yen project money for costs associated with the Ku-2, Ku-3 and what was already paid into for the Ku-4. The remaining funding was not released. Shiro Kayaba, how­ever, still had hopes that the concept could be a potent weapon and from this came the Kat- suodori.

The roots of the Katsuodori come from the Ku-2. Unlike the Ku-2, the wings were moved higher on the fuselage and the wing form had a rearward sweep. The Katsuodori retained the vertical wingtip rudders used on the Ku-2. The ramjet filled most of the fuselage which meant there was no room for landing gear. Instead, a main skid was incorporated on the underside of the fuselage along with a small wheel mounted at the rear of the aircraft. Without integral landing gear, the Katsuodori was to be fitted with a simple, sprung set of landing gear that could be jettisoned when the aircraft took to the air. The pilot sat towards the front of the fuselage and was pro­vided with a one piece canopy that offered respectable visibility to the front and sides.

In order to get the Katsuodori off the ground, Kayaba envisioned the use of four rocket booster units. Secured to each side of the fuselage under the wings were two rocket units and together all four could provide an estimated 7,200kg (15,8731b) of thrust. The planned procedure for using the rockets was to have one on each side being fired first, and when these had burned out the next pair would be fired. Each rocket contained pro­pellant for five seconds of thrust and, all told, the scheme would give the Katsuodori a total of ten seconds of thrust with which to get the plane off the ground and the ramjet function­ing. Kayaba estimated that the Katsuodori would need to achieve 367km/h (228mph) before the ramjet would operate and cer­tainly the speed provided by the rocket units would have been sufficient for this to happen. The rockets, once used, may or may not have been releasable but the latter is likely in order to minimise drag.

With the ramjet operating, the estimated performance of the Katsuodori was a speed of 900km/h (559mph) and a climb rate of three minutes to reach an altitude of 10,000 (32,808ft). Fuel load was 1,500kg (3,3061b) and with a fuel consumption of 50kg (1101b) per minute would grant a combat endurance of thirty minutes. Once the fuel was exhausted, the Katsuodori would use its glid­ing properties to return to base.

For weapons, Kayaba planned on mount­ing two 30mm cannons externally, one under each wing near the wing root. Kayaba did not wish to use existing 30mm cannon designs such as the Ho-155 preferring to produce a 30mm version of the 40mm Ho-301 cannon which his manufacturing facilities were con­structing for use in the Ki-44-П Hei Shoki fighter. The Ho-301 used caseless ammuni­tion with each round being, in effect, a rocket. The propellant cavity was partially lined with a thin aluminium cap. When the primer was struck, the propellant was ignited and the pressure would build up until the cap burst, the exhaust gas being vented out the back of the round to move the projectile forward. The main advantage of the weapon was its light weight for such a heavy calibre.

The design of the Katsuodori was nearly complete by 1943 and Kayaba anticipated that he could have had a flying prototype by

1944. By this time, however, the IJA was already involved with the rocket powered Ki-200 (the IJA version of the UN’s Mitsubishi J8M1 Syusui – see Page 96) and so paid no attention to the Katsuodori. Kayaba, in trying to salvage the design, stated that he could adapt the Katsuodori to accept the Kugisho Ne20 turbojet or the KR10 rocket motor as used in the Ki-200. And since his design was

nearly complete a prototype Katsuodori could be ready for testing before the Ki-200.

The advantages of the Katsuodori included a ramjet that was far less complex to con­struct than a turbojet. This would have been a critical asset in a Japanese war industry that was devastated by US bombing. It could also use standard aviation fuel, unlike the Ki-200 that required special fuels, and by extension, could operate from any airfield without the need of special fuelling apparatus and proce­dures. While the speed of the Katsuodori was on par with the Ki-200, the Katsuodori’s com­bat endurance was far superior to the Ki-200 and the IJA’s own planned rocket intercep­tor, the Rikugun Ki-202 Sytisui-Kai (see Page 40).

However, the Katsuodori had several draw­backs. The first was the use of the rocket boosters to get the plane up to speed. The Japanese did not have a successful track
record for using such units. Improper place­ment of rocket boosters was the reason behind the aborted second flight of the Naka – jima Kitsuka (Page 114), heavily damaging the aircraft. Attempts to use rocket boosters on the Mitsubishi Ki-109 to boost take-off and climb met with such poor results that the rockets were removed from the Ki-109 devel­opment all together. A misfire or variation in the thrust output might result in the plane careering out of control. Like the Ki-200, once fuel was exhausted the Katsuodori lost its speed advantage, and on the ground its recovery would take longer since the Kat­suodori could not move on its own without means of wheeled apparatus. This made it vulnerable to intruder aircraft dedicated to airfield interdiction missions. Kayaba’s elec­tion to use a 30mm version of the Ho-301 can­non would have been a recipe for disaster. The 40mm version, as used in combat by the

Japanese, had an incredibly short range – only 149.5m (490ft) since it had a muzzle velocity of 241m/sec (790ft/sec). Coupling the very short range of such a weapon with a high closure rate due to the speed of the Kat­suodori against a slow bomber, the pilot would have had mere seconds or less to line up the target, fire, and then bank to avoid col­lision. Since Kayaba did not proceed with a 30mm variant of the Ho-301, the muzzle velocity for the round is unknown but it can­not have been substantially more than the Ho-301.

Despite the potential advantages over the Ki-200, the Katsuodori would never see life outside plans on Kayaba’s design board. The IJA was looking to the Ki-200 and their own Ki-202 for their interceptor needs and thus ended Kayaba’s dream of seeing his Kat­suodori taking to the skies to defend Japan.

Kayaba Katsuodori – data




Handley Page H. P.75 Manx (UK), BOK-5 (Russia), Blohm und Voss P.210/P.215 (Germany), Heinkel P.1078 (Germany), Northrop XP-56 Black Bullet (US), Northrop XP-79 Flying Ram (US), Skoda-Kauba SK P.14.01 (Germany), Lippisch Li Р. ІЗа (Germany), Messerschmitt Me P. l 101L (Germany), Heinkel He P.1080 (Germany), Stockel Rammschussjager (Germany), Leduc Model 010 (France), Kostikov 302 (Russia)


Point Interceptor




One Kayaba Model 1 (or possibly later) ramjet producing 300kg (661 lb) of thrust at 367km/h (228mph), 420kg (925 lb) of thrust at 490km/h (304mph), 550kg (1,2121b) of thrust at 612km/h (380mph) and 750kg (1,6531b) of thmst at 734km/h through 1,103km/h (456mph through












Wing area



Wing sweep










Max speed



Landing speed





248 miles


3 min to 10,000m (32,808ft)





Two 30mm cannon


None. The Katsuodori did not advance beyond the drawing board.