Category JAPANESE SECRET PROJECTS

twin-engine ground attack aircraft

The Hs 129 was a dedicated ground attack platform which, while having problematic engines and poor manoeuvrability, pos­sessed excellent armour and the ability to carry a wide array of armaments. Allied intel­ligence reported that German sources listed two Hs 129 aircraft for delivery to Japan some­time in 1944. Since none reached Japan, it can be presumed that they were never dis­patched and the aircraft were used in Ger­man service.

Henschel Hs 130 high-altitude medium bomber

The Hs 130 was the evolution of the Hs 128, the latter a testbed for pressure cabins and high-altitude flight. Initially developed as a reconnaissance platform, the Hsl30 would later evolve into a bomber. Several prototypes were constructed and tested but the type did not enter service and was cancelled in 1944. It was believed by Allied intelligence that the Japanese obtained complete details on the Hs 130 program in July 1944. Recently pub­lished photographs showing wartime Japan­ese notes on the Hs 130, including sketches of the pressure cabin and associated systems, confirm the intelligence.

Fieseler Fi 103R Reichenberg

A US intelligence report stated that the Japan­ese were aware of the piloted versions of the Fi 103 (V-l) flying bomb and wished to use them. Codenamed ‘Reichenberg’ by the Ger­mans, 175 Fil03Rs were built but the unit

tasked with flying them, 5./KG 200 ‘Leonidas Staffel’, would not see combat. The Fi 103R I, II and III were training versions – the Reichen – berg I was a single-seater without the Argus pulsejet engine, the Reichenberg II a two – seater with no engine and the Reichenberg III a single seat version but with the engine fit­ted. In all cases, ballast simulated the war­head weight. The Reichenberg IV was the operational model. It has been suggested that the Reichenberg was the inspiration for the Kawanishi Baika.

Contemporaries

Blohm und Voss BV 222 Wiking (Germany), Blohm und Voss BV 238 (Germany), Domier Do214 (Germany), Boeing C-98/B-134 (US), Consolidated PB2Y-3R Coronado (US), Martin JRM-1 Mars (US), Martin PBM-3R Mariner (US), Sikorsky VS-44 Excalibur (US), Convair R3Y Tradewind (US), Short Sandringham (UK), Short Solent (UK)

Because the HI 1K1 was not built, the specifications given are estimates made by Kawanishi.

Type Transport Flying Boat

Crew Five

Powerplant

Four Mitsubishi MK4Q Kasei 22 (Ha-32-22) 14-cylinder, air-cooled radial engines developing l,850hp for take-off, l,680hp at 6,886ft and l,550hp at 5,500m/l 8,044ft; each engine drove a 4.3m (14.1ft) diameter, four – bladed, alternating stroke propeller

Armament

Three 13mm Type 2 machine guns with 200 rounds of ammunition per gun

Deployment

None. The HI 1K1 Soku did not advance past the mock-up stage.

There are a few Japanese wartime aircraft such as the Kugisho Tenga jet bomber that remain shrouded in mystery to this day. The Kawanishi K-200 most certainly falls into this category, a design that had it proceeded would have resulted in the first turbojet pow­ered flying boat.

Very little is known of the genesis of the K-200. Kawanishi may have been approached by the UN to initiate the project or Kawanishi may have undertaken the design themselves to see if a flying boat could be constructed using the new jet engines being designed following the success of the Ne20 turbojet. Towards the close of the war, Kawanishi was developing two other large flying boats: the Kawanishi K-60 and the Kawanishi HI IK Soku. Both of these were at the behest of the UN so it may not be unrea­sonable to assume that the UN also asked Kawanishi if they could add a jet powered fly­ing boat to the mix. Exactly when Kawanishi began to study the prospect of the K-200 is not known though 1945 is the likely year.

Depending on the source, the K-200 was either to be the replacement for all UN flying boats in service or the K-200 was to be a car­rier for a Japanese atomic weapon. The for­mer assumption would likely have depended on the performance of the K-200 had it been built. Certainly the prospect of the K-200 did not deter other flying boat projects such as the K-60 nor improvements of the H8K already in use. If the K-200 was to be such a replacement for operational flying boats and proved superior to them, it most likely would not have entered widespread service until 1946. As far as the latter, the K-200 would have needed capabilities that exceeded flying boat designs then in service in order to serve as a means to drop an atomic weapon on the US. It has been suggested that the Nakajima Fugaku was also devised to carry an atomic weapon but there is no support for this notion. The same may be said for the K-200. One can speculate as to whether the K-200 would have been any more successful in pen­etrating US coastal defences than a high flying bomber. As we shall see, the K-200 may have had a flaw that would have made any such use all but impossible.

What the definitive shape of the K-200 was to be is open to conjecture. Certainly Kawan­ishi would have utilised their successes with the H8K and to a lesser degree with the Kawanishi H6K (codenamed Mavis) as a foundation for the K-200. As such, it is likely that the hull design would have followed a similar pattern. One speculative illustration of the K-200 shows a hull not unlike the H6K but deeper, though not to the extent of the H8K. A conventional tail akin to the H8K was used but the horizontal stabilisers were mounted halfway up the vertical stabiliser. The wings appeared very similar to the H8K and were fit­ted to the hull in a like position, this being on the top of the hull and, at least for the K-200, nearly central mounted on the hull. Interest­ingly, the K-200 was illustrated with fixed wing floats, which contrasted with the retractable floats used by the H8K3 as a means to increase speed. Perhaps such a modification would have been considered for the K-200 as well.

The K-200 is shown as having an armament layout similar to the H8K1. If this was the case, a Type 99 20mm cannon was fitted in a tail turret and in the top mounted turret forward of the wings. On either side of the forward bow was a blister that would have been armed either with the Type 99 cannon or a Type 92 7.7mm machine gun. Finally, a Type 99 cannon would have been fitted in the bow. The K-200 was also probably able to carry a payload of bombs, depth charges or torpe­does.

As far as propulsion, the K-200 was to use six turbojets. They were to be mounted on top of the wings with each turbojet housed in a separate nacelle. Grouped in sets of three, the engines were fitted to each side of the hull on top of the wings. The reason for this was to minimise the amount of sea spray ingested by the engines during use. If the K-200 was of similar dimensions to the H8K, then the tur­bojet engines would have to move something in the region of 24,948kg/55,000 lb of weight when the K-200 was fully loaded. If the Ne 330 turbojet was the engine of choice, all six would produce a combined thrust of 7,800kg/l 7,196 lb. This may have been suffi­cient to give the K-200 a speed superior to the H8K2, which topped out at 467km/h (290mph).

Where the K-200 may have come up short is in terms of its range. Six turbojets would have required a significant amount of fuel in order to give the flying boat a useful opera­tional radius. As an example, the Ne 20 turbo­jet consumed around 740kg (1,6301b) of fuel per hour. The Nakajima Kitsuka, which used two Ne20 engines, carried a maximum of 1,447kg (3,1901b) of fuel (and without drop tanks only 723.5kg/l,595 lb) and therefore, at its cruise speed, could muster a 824km (512 mile) operational range. If the Ne330 con­sumed approximately 2,535kg (5,5881b) of fuel per hour at full thrust, then six would require at least 15,028kg (33,528 lb) of fuel for approximately one hour of operation at max­imum speed. Flying at a cruise speed would, of course, extend the operating range. Options to attempt to save weight may have included removing any armour, stripping the defensive armament and/or constructing the aircraft from wood as was the plan for the HI IK Soku. Even with such measures the K-200 would have been hard pressed to match, let alone exceed, the range of the H8K or more conventional piston engine flying boats.

It is not known how far Kawanishi studied the feasibility of the K-200, if at all. With resources allocated to the H8K, the HI IK Soku and the K-60 among other projects, Kawanishi designers may have put the K-200 to one side pending availability of turbojets sufficient to warrant the effort in developing the flying boat. Aside from anything else, even a reliable turbojet such as the Ne20 could only muster four to five hours of operation before it would suffer from problems. It may have been seen that preliminary perfor­mance estimates fell short of expectations and offered no significant advantage over designs currently in use or projected to enter service. Finally, a lack of materials necessary to construct the K-200 may have played a role in sidelining the design; the engine and con­struction material issue saw the K-60 ground to a halt and this may very well have extended to the K-200. Regardless of the reasons, the K-200 would never be anything more than a concept.

As a side note, following the war it was planned to construct a civilian version for use by Japan Airlines.

Kawanishi K-200 – data

Contemporaries

Beriev R-l (Russia), Martin P6M Seamaster (US)

Specifications

There is no exact information available on the Kawanishi K-200.

Deployment

None. The K-200 existed only as a concept or paper design.

As the war progressed the Japanese came to see the invasion of the home islands as a very real threat and various means of attacking the invasion fleet were investigated. One possi­bility was a bomb capable of homing in on the heat emissions of naval vessels. The plan called for the bombs to be used in night attacks when heat signatures of the ships would not be masked by the sun and other ambient heat sources. The project was undertaken by the IJA and given the name Ke – Go and work commenced in March 1944. The First Military Arsenal’s Omiya Department in Tokyo undertook development of the bolometer that would detect heat in the infrared wavelength and bench testing showed that the mechanism was workable. In fact, the sensor could detect the heat from a man’s face at 100m (328ft)! For the bomb to carry the bolometer, three different types were investigated: the B-l, B-2 and B-3. Of the three, only the first showed potential and the remainder were abandoned. A myriad of men and departments were assigned to the task of realising the B-l. Under the Military Ordnance Administration Board was Major Fujita who oversaw the gyro and airframe, Major Hizuta who also worked on the air­frame research, and Major Sonobe who addressed the amplifier. The Seventh Military Laboratory encompassed Professor Konishi of the Shikan Gakko in Osaka who handled the mathematical work, Professor Sano of the Osaka Imperial University who conducted the electrical design and Dr. Itakawa of the Aeronautics Research Laboratory who undertook the aerodynamic design studies. All wind tunnel testing was held at Rikugun Kokugijutsu Kenkyujo.

The first two bomb versions, the Ke-Go 101 and 102, were built in small numbers: ten and five respectively. The hydraulic and mechan­ical linkages for operating the bomb ailerons, coupled with the use of an electric gyro that interfered with the bolometer (A bolometer is a device used to measure the energy of inci­dent electromagnetic radiation and was invented by Samuel Pierpont Langley in 1878.), saw both of these versions aban­doned. Continued study resulted in the Ke – Go 103, 104 and 105. However, none of these designs left the drawing board although the 103 was to use an air gyro instead of an elec­trical version. The gyro only existed to ensure the bomb did not spin faster than 360° in 50 seconds. The Ke-Go 106 would be the first to be built in numbers, fifty examples in all. Overall length of the 106 was 4.7m (15.5ft) long and the wing span was 2m (6.5ft). Four main fins were fitted 1.7m (5.7ft) down the length of the bomb body with a smaller set of four fins 1,2m (3.9ft) behind the main fins, and the tail of the Ke-Go contained dive brakes. In the nose was the bolometer while the shape charged warhead was contained behind it. Two strikers that protruded out from the nose were tipped with small pro­pellers which spun and armed the bomb after release. The strikers served as the impact fuse, but a delayed fuse was also installed should the bomb strike water, in which case it would explode beneath the waterline of a ship. In short order, the Ke-Go 107 joined the program with a total of 30 bombs. The 107 had similar dimensions to the 106 version and weighed 726kg (1,6001b).

The Ke-Go was guided in flight by the bolometer. The bolometer was part of a unit that consisted of the bolometer itself, a mir­ror, motor, distributor, amplifier, relay box and battery. Heat sources passing through the bolometer would strike the mirror that, depending on where it struck, would trigger the amplifier which operated the ailerons, steering the bomb towards the heat source. The use of dive brakes slowed the plummet of the bomb, allowing time for the bomb to make adjustments in flight before impact. The ailerons could move up or down 20°. For a warhead, the Ke-Go carried between 200 to 300kg (440 to 661 lb) of shaped charge explo­sive, depending on the model. Perhaps in error, a 1946 US Navy report lists the warhead as ranging from 20 to 30kg (44 to 661b) of explosive which is very light for the size of the Ke-Go.

With enough bombs ready and available, field testing began without the bombs con­taining any payload. The test used a floating raft which measured 10m (32.8ft) by 20m (65.6ft) and was anchored in Lake Hamanako (located in Shizuoka Prefecture). On the raft wood and coal in a 4m (13ft) by 4m (13ft) pile was set alight. Both Ke-Go 106 and 107 bombs were released from altitudes varying between 1,524m (5,000ft) and 3,048m (10,000ft) but the results proved very disap­pointing. In all, around 60 drops were made but only 5-6 of the bombs displayed the zig­zag flight path that indicated the bolometer was sensing the heat from the fire and guid­ing the bomb. The remainded simply veered away from the raft. The dismal results were blamed on faulty equipment although the data collected was not sufficient to properly identify the cause of the failures. It was found that the terminal velocity of the Ke-Go 107 was 539km/h (335mph). In July 1945, further testing ceased, though work continued on improving the bomb.

The Ke-Go 108 was developed but it was with the Ke-Go 109, the last to be designed, in which the Japanese hoped the problems would be resolved and the bomb would finally prove successful. The Ke-Go 109 bomb was 5.5m (18ft long), 5m (1.6ft) in diameter, had a wingspan of 2.9m (9.4ft) and weighed 800kg (1,7641b). It was estimated that the ter­minal velocity for the Ke-Go 109 was 579km/h (360mph). It featured a larger wing span because the smaller span was considered to be one of the main causes of the poor perfor­mance of the Ke-Go 106 and 107. It was antic­ipated that the 108 and 109 bombs would be ready for test drops by September 1945. Of course, the end of the war ensured this would never happen and neither bomb was built.

Had the Ke-Go made it into service, the car­rier was to be the Ki-67 Hiryu (codenamed Peggy by the Allies). The bomb required a special brace that was fitted into the bomb bay, the apex of the brace extending a few inches past the exterior of the doors. The bomb was secured to this and the lower wing could fold to give ground clearance. On take­off, a crewman had to lower the wing and did so by means of a crank that activated a servo in the bomb, which then moved the wing. The bombardier would use bombing tables appropriate for the weight of the weapon and on release the bomb’s arming wires were pulled out along with a wire that released the dive brakes. Typically, the bomb would be dropped from 2,000m (6,562ft). After the bomb was away, the Ki-67 saw a 64km/h (40mph) increase in speed.

All of the bombs were built from wood. The only metal components consisted of the nose cap containing the bolometer and the dive brakes. In all cases, the bomb bodies were built by the Atsu Department of the Nagoya Arsenal, the gyroscope by Hitachi Co., the spring/gear parts for the timing mechanism by the Hattori Jewellery Co. and the electrical contacts for the timing mechanism by Sumit­omo Communications Branch.

Interestingly, the US would develop a simi­lar weapon, the VB-6 Felix. This was a 454kg (1,000 lb) bomb fitted with an infrared seeker in the nose that controlled the fins and guided the bomb. It was meant for night warfare, notably against ships at sea. The VB-6 was put into production in 1945 but would not see ser­vice by the close of hostilities.

Both the IJA and the UN developed spe­cialised bomb munitions during the war, some of which remained projects while oth­ers saw service. Several were dedicated to the practice of air-to-air bombing in which bombs were used to attack enemy bomber forma­tions. Although not a complete list, the follow­ing are some of the more interesting examples.

Ko-Dan

The Japanese expended some effort in devel­oping the Ко bomb. This was developed in response to observations made by one Kobayashi of high-speed photographs of det­onations of various experimental shaped charges. The basis of his theory was that the energy produced by an explosion is projected along horizontal lines with the impact sur­face. By extension, the amount of energy cre­ated is proportional to the quantity of the explosive used. To achieve a wider impact area against a target, the Ко bomb had a 2mm thick rubber nose that was mated to a 2mm thick steel case. On impact the nose of the bomb would flatten and expand, thereby enlarging the surface area and increasing the effects of the explosive pellets contained within. The Ко would be used against hard targets such as concrete emplacements. In tests, a Ко bomb with 50kg (1101b) of TNT and ultropine in an even split mixture could penetrate a lm (3.2ft) slab of concrete and utterly destroy it. It would take a regular 250kg (551 lb) bomb to accomplish the same effect. A part of this concept is known as the Misz- nay-Schardin Effect and is in practice today with modern HESH (High Explosive Squash Head) munitions and EFP (Explosively Formed Penetrator) weapons.

Mk. 7 (IJN)

Conceived around 1936 by Dai-Ichi Kaigun Koku Gijutsu-sho, the Mk. 7 chemical bomb was not given priority since at the time there was little need for such a weapon and it was thought the work needed to bring it to fruition would be too great. The idea surfaced near the end of the war, the Mk. 7 then being denied a second time in favour of guided missiles. The design was based on the 1kg (2.21b) prac­tice bomb. It had a rubber nose and within a special tail was a glass bottle of Bacillus bac­teria, most likely B. anthracis, which causes anthrax. It had no fuse or explosive charge, the impact being enough to break the bottle and release the bacteria. The planned colour for the bomb was a purple nose with a purple tail.

Mk.19 (IJN)

This was an air-to-air rocket-propelled bomb design intended for use against bomber for­mations. It weighed 7.5kg (16.5 lb). Although it was never built, the work eventually resulted in the Mk. 28 rocket bomb.

‘Silver Paper Scattering Bomb’ (IJN)

This was the Japanese version of the German Diippel, better known as ‘Window’. A 2kg (4.41b) sheet steel cylinder was packed with 200 pieces of metal coated paper measuring.3m x 8.4m (1 ft x 27.5ft). The bomb used a pull fuse that the crewman yanked to arm it and was then thrown out of the aircraft, the sub­sequent charge expelling the papers into the air to act as false radar targets. This anti-radar weapon was used heavily from 1944-1945.

In June 1941, Japan received one example of the Fi 156 for evaluation. The IJA was impressed with the ‘Stork’ and desired to have its own version. The result was the Kokusai Ki-76, codenamed Stella by the Allies. The Ki-76, despite the obvious similar­ity to the Fi 156, was not a direct copy as the prototype was completed and flying before the Japanese received their Fi 156. In com­parison testing, the Ki-76 was actually found to be superior in all areas except landing distance.

Focke-Achgelis Fa 330 Bachstelze rotorkite

This little engine-less rotorkite (the German name means ‘wagtail’) was used by a small number of German U-boats as a means to provide increased visibility to observers while the submarines were on the surface. The Fa 330, after being unpacked and assembled, was tethered to the submarine and the wind would turn the three-bladed rotor as the sub­marine moved forward. Once airborne, the observer could see up to 25 miles instead of the few miles afforded to an observer in the conning tower. One intelligence report sug­gested that the Japanese may have shown an interest in the Fa 330 had they been made aware of it.

Focke-Wulf Fwl90A-5 fighter

In 1943, the Japanese imported one example of this aircraft to pit against Japanese fighter designs and compare their performance. Figuring that Japan would use the Fw 190 in combat, the Allies assigned the codename Fred to the aircraft but none encountered.

Focke-Wulf Та 152 high-altitude fighter

In April 1945, the Japanese purchased the specifications for the Та 152 as a means to rapidly acquire a high-altitude fighter. How­ever, by this time, there was simply no way for the Japanese to act on the material obtained.

Gotha Go 242 transport glider/

Go 244 transport

In a letter dated 7 February 1944 discovered at Bad Eilsen in May 1945, the Deutsche Mitsui Bussan requested from the Germans design plans for both the Go 242 and the Go 244, the latter being a twin-engine version of the glider. No evidence has surfaced to suggest that either of these craft were sent to Japan. However, it is suggested that they were the inspiration for the Kokusai Ku-7 Manazuru (meaning ‘Crane’) glider and the powered version of the Ku-7, the Kokusai Ki-105 Otori (‘Phoenix’). Both the Ku-7 and the Ki-105 were codenamed Buzzard by the Allies.

Every aircraft creator seeks to reduce drag in their designs. The definition of drag is the force that resists movement through a fluid, which, of course, includes air. The more drag, the slower the aircraft moves through the air due to the resistance. Drag cannot be com­pletely removed from a design, but even in the early years of aviation various methods for minimising drag were investigated and many different solutions were tried. Not surpris­ingly, such applications were valued by those providing the military with aircraft and in Japan, prior to the outbreak of hostilities with the US, the Dai-Ichi Kaigun Kokti Gijutsu-sho would study such efforts in an attempt to pro­duce fast flying aircraft.

Form drag is the component caused by the shape of the body moving through the air. Therefore, when designing an aircraft the form and shape of the plane is one of the most important factors a designer has to con­sider. The wider the cross section, the more drag is produced. Having significant form drag results in lower speeds because the faster the aircraft moves through the air, the more drag force is applied to the aircraft.

Therefore, in order to realise higher air speeds, the designer must take steps to reduce drag and thereby lower the amount of drag force slowing the aircraft down.

Before World War 1 some aircraft design­ers appreciated the need to reduce drag. This often took the form of fuselages that had clean lines in an attempt to remove protru­sions and also to streamline propeller hubs to help them cut through the air more efficiently. The best example would be the 1912 Deper – dussin that won the Gordon Bennett race in Chicago, Illinois, which became the first air­plane to exceed 161 km/h (1 OOmph) (in 1913 a later model of the plane would achieve 205km/h (127mph).

At the beginning of World War 1, few of the major combat aircraft utilised significant drag reducing methods. Exhaust stacks, radiators, protruding machine guns, wire bracing, struts and engines only partially cowled predomi­nated. One of the few exceptions was the Morane-Saulnier N ‘Bullet’. Nevertheless, the rapid pace of combat aircraft development during World War 1 saw designers looking for ways to increase speed as a means to get the
edge over the enemy. The Albatross D series and the Roland ‘Walfisch’ would epitomise those efforts.

Following World War 1, the resurgence in air racing such as the Schneider Trophy in Europe and the National Air Races in America saw rapid advances in aerodynamics and drag reduction to produce fast flying racing aircraft for competition. Aircraft such as the Curtiss R2C-1 Navy Racer, the Adolphe Bernard ‘Ferbois’ (capturing the world speed record of 451 km/h (280mph) in 1924), Gloster III, Supermarine S.5, Kirkham-Williams Racer (which, unofficially, flew to a speed of 519km/h (322mph) in 1927) and the Savoia – Marchetti S.65 typified high performance race aircraft. The benefits of these innovations were not lost on military aircraft designers.

With the war clouds looming on the hori­zon, the seeds planted by the air racers of the 1920s and early 1930s were germinating in the aircraft used by the air forces of the major powers. Designs by Curtiss for the US Army Air Force were influenced by the Curtiss rac­ers while the retractable landing gear of the 1920 Dayton Wright RB racer would become

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a hallmark of Grumman aircraft such as the F2F. In Great Britain, R. J. Mitchell would draw heavily from his experience designing Schneider Trophy racers to build the Super­marine Type 300 which would eventually evolve into the Spitfire. In Italy, Mario Castoldi, lead designer for Macci, would turn his skills in constructing racing aircraft to producing fighters for the Regia Aeronautica with types such as the Macci C.202 Folgore.

Japan, like other countries, sought to pro­duce racing aircraft and planes designed to beat world speed records. An early example was the Emi 16 Fuji-go built by Itoh Hikoki Kenyusho (Itoh Aeroplane Research Studio), which from 1920 was used in Japanese com­petitions, and the contemporary racing air­craft from Shirato Hikoki Kenkyusho. Kawanishi was not far behind with the K-2 speed racer which, despite extreme mea­sures to minimise drag, suffered from a drag – inducing radiator mounted on top of the fuselage. The K-2 achieved an unofficial speed of 258km/h (160mph) in a flight made on31 July 1921. Other refinements in aerody­namics could be seen in the Kawasaki Ki-28 of 1935 which, despite its advantages in speed, climb and acceleration, was not suc­cessful in attracting IJA contracts.

In 1938, a group of designers sought to pro­duce a high-speed aircraft to challenge the world air speed record. Once war had broken out this aircraft, called the Ken III, was soon taken over by the IJA. Redesignated the Ki-78, its development was continued under Kawasaki. During this time, it may have been the UN who decided to conduct its own stud­ies of high speed aircraft with Ktigisho assigned the task of doing so. Whether the studies were initiated in response to the IJA’s own high-speed aircraft project is unknown but the prevalent aircraft design philosophy of both the UN and the IJA prior to the war was of speed, agility and range at the expense of firepower, durability and protection.

Kugisho examined over half a dozen aspects of aerodynamics in order to produce data on what would be needed to realise an aircraft capable of significant speed. One area of research was the main wings. The shape of a wing is one of the more critical aspects of aircraft design. Factors such as wing loading, expected air speeds, angles of attack and the intended use of the aircraft all influence how the wing is shaped. For high speeds, a low aspect ratio wing is often con­sidered. Typically, these are short span wings with the benefits of higher manoeuvrability and less drag. In addition, having a backward sweep to the wing also lowers drag. The drag most associated with wings is termed induced drag, which is caused by wing tip vortices that change how the air flows over the wings. This change results in less and less lift which then requires a higher and higher angle of attack to compensate and, from this, induced drag results. Elliptical wings offer less induced drag than more conventional straight wings. However, low aspect ratio wings are more prone to larger vortices because they cannot be spread out across a longer wing. Kugisho’s study on wing shapes was the likely result of testing various airfoils in a wind tunnel to determine their effective­ness and record the results.

Another aspect Kugisho engineers reviewed were the merits and flaws of using either an inline or a radial engine and how each type reduced the form drag. In both cases the engi­neers drew up two concept aircraft and each made use of streamlining. Streamlining is the process of shaping an object, in this case, a fuselage, to increase its speed by reducing the sources of drag. One concept used the 1,159hp Daimler-Benz DB601A, a 12-cylinder, inverted-V, liquid-cooled, inline engine. This engine would be licence built for the UN as the Aichi AE1 Atsuta (the ‘A’ stood for Aichi, ‘E’ for liquid-cooled and T for first liquid – cooled engine; Atsuta was a holy shrine in Aichi Prefecture) and for the IJA as the Ha-40 before it was renamed the [Ha-60] 22. The second concept aircraft used a 1,000hp Naka – jima NK1В Sakae 11 which was a 14-cylinder, air-cooled, radial engine. This engine was a licence version of the Gnome-Rhone 14K Mis­tral Major (in engine nomenclature, the ‘N’ was for Nakajima, ‘K’ for air-cooled, T as the first air-cooled engine, while the ‘B’ was for the second version of the NK1; Sakae means prosperity in Japanese).

Kugisho would use the same basic air­frame for the engine study. It consisted of a well streamlined fuselage with the pilot mounted in a cockpit set behind the wing and just forward of the vertical stabiliser. This style was found in a number of racing aircraft such as the American GeeBee R1 and Geebee Z. Both used a standard tail-sitter configuration for the landing gear. The concept equipped with the DB 601A engine had a fuselage shape that was not unlike the Kawasaki Ki-61 Hien (meaning Swallow; codenamed Tony by the Allies) which would appear in prototype form in December 1941. The wings were mounted low on the fuselage. The fuselage appear­ance was due to the inverted-V engine which, by design, offered lower height, weight and length when compared to more conventional motors. By contrast, the concept using the NK1B had a more ovoid fuselage shape, the result of the height of the radial engine. To maintain the aerodynamic streamlining a large spinner was used. Also, in contrast to the DB 601A equipped design, the wings were mounted mid-fuselage.

Kdgisho would not produce any direct pro­totype aircraft from either concept. Instead, the results of the various studies were likely kept available as reference for engineers to access as a means of obtaining data on the aerodynamic problem. Perhaps Kugisho in hindsight considered themselves fortunate to not have expended additional expense and effort in producing working prototypes given the failure of the IJA’s Ki-78, a program that lingered on into 1944 and never met its design goals.

The DB 601A engine aircraft is shown in the colours originally used on a Mitsubishi A6M3, serial 3032, tail code V-190 of the Tainan Kokutai. It was found on Buna Airfield on 27 December 1942 in disrepair. It was a presen­tation aircraft donated by Sadahei, a civilian volunteer group. The Hukuko number was 874. The NK1В engine design is painted in the standard training orange used on prototypes and trainer aircraft.

The profile shown is based on one of the MXY6 gliders found at Atsugi in September 1945.

The paint is training orange as normally used on experimental and training aircraft.

The development of the Kyushu J7W Shinden was an ambitious undertaking. Captain Masaoki Tsuruno, the man behind the Shin­den, needed to confirm the handling charac­teristics of a canard aircraft before proceeding further with the plans and con­struction of the J7W itself. To do this, he com­missioned Kugisho to design and build three gliders that were based on his J7W1 aircraft plans. The result was the MXY6.

KQgisho drew up the design of the MXY6 with the assistance of Captain Tsuruno. Con­structed entirely of wood, the MXY6 featured a slightly swept wing, vertical stabilisers fitted inside of the wing ailerons and canards mounted along the nose of the fuselage. The braced tricycle landing gear was fixed and provided with suspension. Once the MXY6 was finalised, construction was entrusted to Chigasaki Seizo K. K. and they had completed the three gliders by the fall of 1943. Flight tri­als got under way soon thereafter and the MXY6 was found to have good handling char­acteristics which provided verification to the concept of the J7W.

For further testing, one of the three gliders was modified by having a small engine installed in the rear of the fuselage in the same pusher configuration as the proposed J7W. The engine, a Nihon Hainenki Semi 11 ([Ha-90] 11), allowed the handling under power to be studied as opposed to unpow­ered flight only. Following the conclusion of the testing of both the unpowered and pow­
ered MXY6, the validation of the canard design provided the needed proof of concept and as such the UN instructed Kyushu to pro­ceed with the J7W Shinden.

Kugisho MXY6 – data

Contemporaries

Hamburger Ha 141-0 (Germany), FGP 227 (Germany), Goppingen Go 9 (Germany), Horton Ho IIIB and Но IV (Germany), Berlin В 9 (Germany), Junkers Ju 49 (Germany), Lippisch DM-1 (Germany), DFS194 (Germany)

Type Proof of Concept Glider

Crew One

‘No longer can we hope to sink the numeri­cally superior enemy carriers through ordi­nary attack methods. I urge the use of special attack units to crash dive their aircraft and I ask to be placed in command of them.’

These words by 1JN Captain Eiichiro Jyo, commander of the carrier Chiyoda, reflected a mood he had observed in some of his pilots and men. Their feelings were that to carry on with conventional tactics was doomed to fail­ure. While death in combat was worthy, a death that did no good was shameful and would not serve the Emperor or Japan. Jyo’s words, written in a memo to Rear Admiral Soemu Obayashi and Vice Admiral Jisaburo Ozawa, would be the catalyst for the forma­tion of special attack units and from this a new weapon would arise that would become the only purpose-built special attack aircraft to see operational combat service during World War 2: the Kugisho MXY7 Oka.

Vice Admiral Takijiro Onishi is most often credited with officially forming and organis­ing the special attack units, the first of which became operational in October 1944. A piv­otal man in the formation of the UN’s Rikusentai (airborne troops), Onishi was also eccentric which did not always endear him to his superiors and so, prior to his assuming command of the UN land air forces in the Philippines, he served as a supply officer. Speaking to the officers of the 201 st Air Group, Onishi stated that because of the limited resources only a Mitsubishi A6M Reisen with a 250kg (551 lb) bomb that was crashed into enemy ships would suffice in slowing the US fleet. From this began the rise of the UN spe­cial attack force, the Shimpti Tokubetsu

Kogekitai. Their story, as well as that of the IJA’s Shimbu Tokubetsu Kogekitai, is beyond the scope of this book (however, for those interested there is a wealth of material avail­able on the subject such as David Brown’s Kamikaze and Earl Rice’s Kamikazes).

The majority of the shimpti missions were flown using types already in service. In addi­tion to the Reisen, the Kugisho D4Y Suisei (meaning ‘Comet’ but known to the Allies as Judy), Kawasaki Ki-48 {Lily), Nakajima Ki-49 Donryu (meaning ‘Storm Dragon’ but called Helen by the Allies), Aichi D3A {Val) and many others were modified, sometimes heavily, and used against the Allies, but none were specifically built from the ground up for shimpu (suicide) operations. It would be UN Ensign Mitsuo Ota, a transport pilot flying with the 405th Kokutai, who put forward a design for a piloted glide bomb.

Ota’s concept was not the only one that called for a dedicated shimpti aircraft. Other ideas were considered such as the Showa Тока (see elsewhere in this book on the Nakajima Ki-115 Tsurugi for more informa­tion), but what set Ota’s idea apart was that he wanted to have the explosive payload car­ried internally as opposed to fitting an exter­nal bomb. Also, the aircraft had to be carried and released by a parent plane and rocket boosters would be used to speed the approach and terminal dive onto the target.

Ota did not have any aeronautical engineer­ing experience and would not have been able to present a definitive plan for his aircraft. In order to help his cause, Ota sought and received assistance from the Aeronautical

Research Institute of the University of Tokyo. Professor Taichiro Ogawa headed the study of Ota’s concept while Hidemasa Kimura pro­vided the basic design of the aircraft and even produced models that were wind tunnel tested. Within weeks, the proposal for Ota’s design was drafted, the design illustrated and performance estimates presented along with the data obtained from the wind tunnel testing.

In August 1944, Ota brought his proposal to the attention of Lieutenant Commander Tadanao Miki. Miki was the department head of the aircraft design section of the Dai-Ichi Kaigun Kokh Gijutsu-sho. It is said that when Miki reviewed Ota’s concept he was taken aback and shocked at the idea of putting men into piloted bombs. However, by this time the policy of shimpu tactics had been approved and regardless of how Miki felt personally he could not deny the submission. Miki placed the design before the Naval General Staff on 5 August 1944. Air Staff Officer Minuro Genda, after looking over Ota’s plan, approved the concept and instructed chief of staff Admiral Koshiro Oikawa to set the wheels in motion for turning the design into reality. Perhaps it was ironic that the task of starting the devel­opment of the aircraft fell to Miki. Kugisho was the organisation that would develop the aircraft, which was given the initial designa­tion MXY7. Miki assembled a team of engi­neers led by three men, Masao Yamana, Tadanao Mitsugi and Rokuro Hattori, and they began drafting and refining the MXY7 design.

The MXY7 was essentially a glider bomb with a pilot providing the guidance. There were several specific factors involved in the MXY7, most of which were out of necessity. In order to conserve war materials, the MXY7 was to be constructed using wood as well as non-critical metals such as aluminium, if nec­essary. It was expected that pilots with mini­mal skill would be required to fly the machine and therefore the aircraft had to possess good handling and manoeuvrability to ensure a successful strike. Not surprisingly, instrumen­tation for the MXY7 was kept to the bare min­imum. The aircraft also had to be simple to construct so as to allow rapid mass produc­tion by semi-skilled and unskilled labour.

The MXY7’s primary mission was anti-ship. The flight profile began with the MXY7 being carried aloft by a modified Mitsubishi G4M bomber. At the point where it was within range of the target, the G4M would release the MXY7 which would then glide towards the intended victim. During the approach the pilot would ignite the rocket motors in the rear of the plane to increase its speed and close in to the target as quickly as possible. This would minimise the chances of inter­ception and present a fast moving target to defending anti-aircraft gunners.

Miki and his team completed the design of the MXY7 in weeks and by the end of Sep­tember 1944 ten MXY7 had been completed and were ready for testing. The aircraft was then renamed the Oka Model 11, Oka mean­ing ‘Cherry Blossom’. A 1,200kg (2,6461b) explosive charge was fitted into the nose and five fuses were installed, one in the nose and the remaining four on the rear plate of the charge. The fuses were armed by the pilot from inside the cockpit and they could be set to explode on impact or the detonation could be delayed by up to 1.5 seconds to allow the Oka to penetrate the target (such as a ship hull) and explode inside. The carrier for the Oka was the Mitsubishi G4M, known to the Allies as Betty. A number of G4M2a Model 24B and 24C bombers were modified by having their bomb bay doors removed to be replaced by the required shackles to hold the Oka. These modified carriers were redesignated G4M2e Model 24J. However, the Oka’s loaded weight of 2,140kg (4,7181b) far exceeded the bomber’s standard load of 1,000kg (2,205 lb) and as a consequence the G4M2e suffered from poor handling and performance.

As the Oka did not take-off on its own nor was it anticipated that it would fly at speeds under 322km/h (200mph), the wings were kept very short. For propulsion, three Type 4 Mark 1 Model 20 solid fuel rockets were installed in the tail of the fuselage. Each rocket could produce up to 267kg (588 lb) of thrust for a total of 801kg (1,7641b). The pilot could activate them as he saw fit and could fire them one by one or all three at once. Total burn time for each rocket was 8-10 seconds. Given that the Oka would have to fly through significant anti-aircraft fire as it approached its target as well as the possible aerial inter­ception by Allied fighter cover, the pilot was afforded protection through armour plate. A 19mm strip of plating was fitted along the underside of the fuselage near to the pilot’s feet while his bucket seat had between 8mm and 15mm of armour, the majority protecting his back.

As discussed above, the instrumentation was kept to a minimum. The instrument panel contained the altimeter, compass, atti­tude indicator (artificial horizon), airspeed indicator, arming handle for the fuses and the rocket motor ignition switches.

With the ten available MXY7 prototypes, flight testing was to commence in October 1944. However, the IJN did not want to wait for the results of the tests and in September, Rear Admiral Jiro Saba, director of the Kugisho Naval Aeronautical Research Labo­ratory, went to Lieutenant Commander Yokei

Oka Model 22 – data

Type Special Attacker

Crew One

Type Special Attacker

Crew One

Oka Model 43A – data (estimated)

Type Special Attacker

Crew One

Matsurra at the Munitions Ministry to sort out the arrangements for opening production of the Oka. Matsurra, who shared a similar dis­taste of the suicide concept to Miki, saw to it that much of the production was handled by military contractors to maintain secrecy and not by the private aviation industry. As such, Kugisho would build the Oka at Dai-Ichi Kai – gun Коксі Gijutsu-sho as well as at Dai-Ichi Kaigun Kokusho, and two sub-contractors, Nippon Нікбкі K. K. in Yokohama and Fuji Hikoki K. K. in Kanegawa, would provide wing and tail assemblies. It was expected that 100 Oka aircraft would be ready by November 1944.

The first unpowered flight tests of the Oka began at the Sagami Arsenal located in Sagamihara in Kanagawa Prefecture. To begin with, unmanned, unpowered flights were conducted to assess the Oka’s flight characteristics and these were followed soon afterwards by unmanned, powered flight tests. All of the Oka drops were made from the G4M2e bombers with the Okas being directed out into Sagami Bay. Flight testing was then moved to Kashimi in Saga Prefec­ture which was near the UN base in Sasebo in Nagasaki Prefecture. At Kashimi, the first manned flight of an Oka took place on 31 October 1944 with Lieutenant Kazutoshi Nagano (other sources have his last name as Nagoro) at the controls. The particular Oka that Nagano was to fly was the prototype for the Oka K-l trainer. In place of the warhead and the three rocket motors were tanks hold­ing water as ballast that simulated the com­bat weight of the Oka. Since there was no room for a conventional landing gear, a cen­tral landing skid was fitted to the underside of the fuselage and under each wing tip were rounded skids to protect the wings and pre­vent them from digging into the ground on landing. Prior to landing, the water was to be jettisoned which slowed the landing speed to 223km/h (138mph). For Nagano’s flight, a rocket booster was fitted to the underside of each wing. At 3,505m (11,500ft) Nagano was released from the G4M2e bomber and entered a good, stable glide. A few minutes into the flight, Nagano activated the booster rockets and almost immediately the Oka began to yaw. Nagano quickly jettisoned the rockets and the problem disappeared. The remainder of the flight went perfectly, Nagano bringing the Oka down without mishap after releasing the water ballast. Sub­sequent investigation showed that uneven thrust from the rockets caused the yawing and Nagano is said to have stated that the Oka handled better than a Reisen.

As flight testing and production of the Oka got underway, 721st Kokutai was formed at

Hyakurigahara Airfield on 1 October 1944 under the command of Commander Moto – haru Okamura with Lieutenant Commander Goro Nonaka and Lieutenant Commander Kunihiro Iwaki as his operations officers. The unit was nicknamed the Jinrai Butai, translat­ing as ‘Thunder God Corps’. Through October the unit received hundreds of volunteers. Those who were too old, married or were only sons, or those with significant family responsibilities, were rejected for the Jinrai Butai, leaving 600 pilots to be accepted into the unit. The 721st Kikotai consisted of the 708th Hikotai and the 711th Hikotai, each with 18 G4M2e bombers. The 306th Hikotai and the 308th Hikotai were assigned the task of escorting the Oka carrying bombers, each squadron maintaining 36 Mitsubishi A6M Reisen fighters. The unit’s initial 10 Oka air­craft were supplemented by some 40 Mit­subishi A6M5 Reisens fitted with 250kg (551 lb) bombs.

Flight testing of the Oka continued through­out November. These tests showed that when dropped from 5,944m (19,500ft) at a downward glide angle of 5.5° the Oka could achieve a range of 60km (37 miles) at a speed of 317km/h (230mph). In a nearly vertical dive it was clocked at over 966km/h (600mph). However, under combat conditions the Oka could manage 25 to 29km (15 to 18 miles). Based on the tests and flight experience, a mission profile was developed for the Oka’s deployment. Flying at a height between 6,096m and 8,230m (20,000ft and 27,000ft), the G4M2e would release the Oka when it was within 17 to 33km (10 to 20 miles) of the target. The pilot would then enter a shallow glide with an airspeed of between 371 km/h and 451 km/h (230mph and 280mph). At a point about 8 to 12km (5 to 7 miles) from the target, and from an altitude of approximately 3,505m (11,500ft), the pilot would activate the rocket boosters increasing the speed to 649km/h (403mph). Prior to striking the tar­get, he would put the Oka into a 50° dive that would take the speed up to nearly 934km/h (580mph). At the last moment, the pilot would pull up the nose to strike the ship at the waterline.

Oka pilot training was soon underway. Typ­ically, the pilot would use a Reisen to practice the Oka attack routine flying the fighter with the engine switched off. For many, they only had the opportunity to become familiar with the Oka while it sat on the ground. A few were fortunate to make an unpowered flight using one of the MXY7 trainer prototypes. As expected, accidents occurred and on 13 November 1944, the Oka claimed its first casualty. Lieutenant Tsutomu Kariya exe­cuted a perfect drop from 2,987m (9,800ft)

and was bringing the Oka down for a landing. He inadvertently released the water ballast from the nose tank, leaving the rear tank full. This immediately caused the nose to pitch up, putting the Oka into a stall that Kariya was unable to recover from, the plane crashing into the ground. Kariya was pulled from the wreckage but within a few hours had died from his injuries.

By December 1944, Kugisho had produced 151 Okas and the Dai-Ichi Kaigun Kokusho production was also well under way. Attempts were made to deploy the Oka to units outside of the Japanese home islands. Fifty were dispatched to the Philippines aboard the carrier Shinano, but on 29 Novem­ber 1944 the ship was sunk en route. Only a handful would reach other bases, notably in Okinawa and Singapore, and none would see combat. Even though the 721st had yet to see combat, there were some who realised that the G4M2e bomber would be easy targets for enemy Fighters and the odds of actually reaching the target were small. Conse­quently, morale dropped as the Oka was seen as a waste of a pilot who could be used to bet­ter effect elsewhere. The vulnerability of the G4M2e was vividly displayed when the 721 st went into battle for the first time on 21 March

1945. Attacking US Task Group 58.1, all 18 bombers (of which 15 were Oka carriers) were shot out of the sky by US Navy fighters along with their fighter escort before they could get within attack range. Again, the story of this and subsequent Oka missions are beyond the scope of this book but the inter­ested reader can find many excellent sources of information on the topic.

Following the Oka’s disastrous debut, reviews of gun camera footage from the US Navy fighters and from pilot debriefings revealed the existence of the new weapon for the first time to the Allies. At first it was thought that the Oka was simply a large, anti­ship bomb. This would change when four to six examples were captured near Kadena Air­field after the Allied victory at Okinawa. Only then was the aircraft’s true nature made known to Allied intelligence. The Oka was subsequently given the codename Baka by the Allies, the word baka meaning ‘fool’ in Japanese.

Production of the Oka Model 11 ceased in March 1945 with the Dai-Ichi Kaigun Koku Gijutsu-sho having built 155 and the Dai-Ichi Kaigun Kokusho constructed a total of 600. One Oka Model 11 was fitted with sheet steel wings made by Nakajima but no other exam­ples were produced with this feature. To help improve the training regimen, once the Dai – Ichi Kaigun Koku Gijutsu-sho had completed their run Oka production was switched to the Oka MXY7 K-l trainer. In all, 45 of the K-l would be completed and placed into the pilot training program.

Clearly, the G4M2e carrier aircraft was too slow and easy prey for defending Allied fighter protection. In addition, the short range of the Oka Model 11 compounded the prob­lem. Consequently, KQgisho decided to utilise the superior Kugisho P1Y Ginga (Allied code – name Frances) as the carrier aircraft and also to give the Oka a longer range. This adapta­tion was called the Oka Model 22.

The primary change in the Oka Model 22 was the use of the Tsu-11 thermojet engine in

Oka Model 22 in the colours of the example found at the close of the war and now on display at the Smithsonian Air and Space Museum.

place of the rocket boosters. This consisted of a lOOhp Hitachi Hatsukaze [Ha-11-11] 11 4- cylinder, inverted inline engine driving a sin­gle-stage compressor. Fuel was injected into the compressed air that was then ignited, pro­ducing up to 200kg (440 lb) of thrust. To com­pensate for the weight of the engine and fuel, the warhead had to be reduced to 600kg (1,323 lb). Finally, as the P1Y was smaller than the G4M2e, it was necessary to reduce the wing span by lm (3.2ft), although the length of the Oka Model 22 was increased by.8m (2.6ft). These changes improved its range of up to 129km (80 miles), although 65km (40 miles) or less was considered achievable under combat conditions. A rocket booster could be fitted to the underside of the fuse­lage to increase speed during the terminal dive.

Once the design of the Oka Model 22 was finalised, Kugisho began a production run of 50 aircraft even before flight testing was underway. Aichi Kokuki K. K. was contracted to construct a further 200 Model 22 aircraft, but due to US B-29 bomber raids Aichi’s pro­duction lines would never enter operation. Once the first handful of Oka Model 22 aircraft had been made available their testing began. Thanks to its short wings, a high stall speed of 334km/h (207mph) and high landing speed made a soft landing impossible. Test pilots were instructed to abandon the Oka rather than make a landing. Lieutenant Kazutoshi Nagano took the Oka Model 22 up for the first time on 26 June 1945. The flight would also be his last. After being released from a modified Kugisho P1Y1 at 3,658m (12,000ft), the Oka went out of control (another source states that the wing rocket boosters fired acciden­tally, causing the Oka to crash into the Ginga, damaging the Oka’s controls). With no ability to regain level flight from the plummeting Oka, Nagano was able to extract himself from the stricken aircraft but his parachute only partially opened before he hit the ground and was killed. A second test model was ready in August 1945 but the war ended before it could fly. Although fifty Oka Model 22s were built, the carrier, the Kugisho PI Y3 Model 33, would never leave the drawing board. The com­pleted Oka Model 22 were retained in Japan for use against the expected Allied invasion force.

Kugisho continued to investigate ways to improve the performance of the Oka and a series of models were planned around the Ktigishb Ne 20 turbojet. The first was the Oka Model 33 which was simply the Oka Model 22 enlarged to accept the Ne20 (or as one source states using the Nel2B jet engines that had been built prior to the shift to the Ne20 development). For a carrier, Kugisho planned on using the Nakajima G8M1 Renzan (known as Rita to the Allies) but with the fail­ure of the Renzan to enter production, the Oka Model 33 was quickly shelved without any prototype being constructed. This was followed by the Oka Model 43A. Larger in dimensions in comparison to the Oka Model 22, the Oka Model 43A was designed to be launched from submarines such as the Sen Toku class. To facilitate storage on such boats the wings were foldable, but with the Allies in complete control of the seas the Oka Model 43A was soon put aside and work begun on the Oka Model 43B instead.

This version was designed to operate from caves and launched by a catapult. It retained the folding wings to allow the production lines to be set up in cramped, underground sites or caves as well. Unlike the previous models, the Oka Model 43B was all metal, used a central skid and in order to better facil­itate target penetration the pilot could jettison the wing tips. Like the Oka Model 22, a rocket booster could be carried under the fuselage. A full scale wooden mock-up was completed in June 1945 and was promptly approved for production. Aichi were tasked with construc­tion of the Oka Model 43B at their Gifu and Oyaki factories but the war ended before the first prototype was completed. However, a catapult ramp was built at Takeyama, near Yokohama, and pilots destined for the Oka Model 43B were being given instructions on catapult launching as they waited for their air­craft to be delivered.

A hybrid Oka was considered which was called the Oka Model 21. The Tsu-11 engine was to be removed from the Oka Model 22 to be replaced by the standard rocket booster system as used on the Oka Model 11. This may have been contemplated as production of the Tsu-11 engine was slow and was not keeping pace with the Oka Model 22. The proposal, however, never proceeded past a single prototype.

Whereas all previous Okas, with the excep­tion of the Models 43A and 43B, required modified bombers to carry them aloft and launch, the Ne20 turbojet equipped Oka Model 53 was designed to be towed into the air. As such any aircraft, with the addition of a tow line and having enough power, could be used to tow the Oka Model 53 into the air. Nothing came of this design due to the end of the war. However, it is worth noting that some contemporary illustrations show the Oka Model 53 without a cockpit, which would turn the type into a glider bomb. For guidance, it is speculated that upon release from the tow aircraft, it was either radio controlled from a parent plane or used infrared or acoustic homing to guide itself to the target. This con-

cept has not been verified in wartime Japan­ese sources and could be post-war conjec­ture.

A derivative of the MXY7 K-l was planned and this was known as the Oka Model 43 K-l Kai Wakazakura (meaning ‘Young Cherry’ in Japanese). This was to be the definitive trainer for pilots destined for operational Oka models. A second cockpit was installed in the nose in place of the warhead, flaps were fit­ted to the wings to help with landing and, like the K-l, the Model 43 had a central landing skid with wing bumpers. It also included a single Type 4 Mark 1 Model 20 rocket in the tail to allow the student to get a taste of powered flight. By the close of the war only two of the Wakazakura trainers had been completed.

Perhaps one of the more unusual uses for the Oka occurred in Singapore. The handful of Oka Model 11 aircraft that were received by units in Singapore were, for the most part, grounded because they did not not have their

G4M2e parent aircraft. In order to get some use from the Okas, mechanics planned to fit them with floats cannibalised from unser­viceable or available floatplanes such as the Aichi El ЗА (known as Jake to the Allies). It is not known exactly how the floats were to be installed but crude fittings could have been fabricated to attach a float under each wing. It is believed that the float equipped Okas were to be positioned along the Straits of Johor that separate Johor from Singapore and be used in conjunction with Shinyo special attack boats. Another unknown is how they would have performed given the short burn time of the rocket boosters let alone handling qualities across water. It can be surmised that performance would have been very poor. By comparison, the German Tornado attack boat used two floats from a Junkers Ju 52/3mg5e and was powered by an Argus 109-014 pulse-jet. Trials would prove a failure as the boat could not operate on anything but calm seas without capsising.

Developed and produced from 1941 to 1942, the To-З was a cluster munition and initially designed to combat aerial targets. However, the To-З turned out to be more effective against ground targets. Thirty to forty To-3 bomblets could be carried in a dispensing container or modified underwing bomb racks could carry several of the To-З bomblets. Once released, either from the container or the rack, the To-3’s vane would spin, arming the munition. Use of the To-З was performed on a limited basis and production ended in

1942.

Та (IJA)

Evolving from the To-З was the Та. Each Та bomb was a 40mm hollow charge weighing.33kg (.741b). It used fixed tail fins. When attacking airborne targets, canisters contain­ing between 30 and 76 of the Та bombs would be released. A burst fuse would open the can­isters, usually just after release, to scatter the Та bombs. The loaded canisters weighed 30kg (66 lb) and 50kg (110 lb) respectively and the Та was first used operationally in 1943 with Japanese pilots claiming a number of kills using the weapon. Typically, the Та was far more effective when used against more densely packed bomber formations.

Та-105 (IJA)

The Та-105 came from the need to attack Allied shipping during the expected invasion of Japan. The Ta-105 was the Та bomblet enlarged to carry a more substantial payload. The bomblet had a 100mm hollow charge that was capable of penetrating up to 140mm of armour plate. Unlike the Та, the Ta-105 used folding tail fins that deployed upon release. Each canister contained 21 Ta-105 bomblets.

Though the Japanese had no interest in the ‘Tante Ju’, the Allies thought the Japanese would be using the transport in action. This idea may have stemmed from a May 1939 flight made by a Ju 52/3m to Japan to bolster trade relations. As such, the plane was code­named Trixie.

Junkers Ju 87A ‘Stuka’ dive bomber

In 1940, a single Ju87A was sent to Japan for evaluation. By 1939, all A models had been withdrawn from frontline German service, and after flight testing and study the plane was put into the collection of the Tokorozawa museum. However, it was lost when the museum was bombed. The Allies, believing the Japanese would be using the Ju87, assigned it the codename Irene.

Junkers Ju 88A-4 bomber

The Japanese acquired a single example of the Ju88A-4 in 1940 for the purposes of test­ing and evaluating the aircraft as well as for the study of the design. The Japanese had no intentions of producing the bomber but nev­ertheless the Allies thought it likely they would and thus gave the Ju 88 the codename Janice. As an aside, one intelligence report states that the Mitsubishi office in Berlin had a number of ‘Ju 88K-5’ (the export version of the Ju88A-4) aircraft and parts shipped to Japan, perhaps in 1943, though this has never
been verified and only the single Ju 88A-4 is known to have been delivered.

Junkers Ju 290 long range heavy bomber

The Ju 290 was initially a heavy, four engine transport aircraft that was reworked into a long range maritime reconnaissance and bomber aircraft. It was felt that by October 1943, the Japanese were in possession of the complete details of the Ju 290. Even if this was the case, it would appear the Japanese did not act on the information and they may have been more interested in the Ju 390. There is no evidence to suggest that the Ju 290A-7 was ever adapted as a ‘nuclear’ bomber for the Japanese, especially in light of the fact few A-7 models were ever completed. Some sources suggest that Ju 290 flights were made into Manchuria carrying documents and other intelligence, possibly in exchange for raw materials from Japan, but information has never surfaced confirming these flights and popular opinion in the face of current evi­dence is that they did not occur.

Junkers Ju390A-l reconnaissance bomber

In 1944, the IJA was very interested in the potential of the Ju390 as a strategic bomber and sought to obtain the rights to the aircraft. In the fall of 1944, the Japanese acquired the manufacturing licence and design plans for the Ju390A-l long-range bomber reconnais­sance aircraft. By 28 February 1945, Major – General Otani of the IJA was to have collected the plans and licence from the Germans but it is unknown if this ever occurred. In any case, the Ju 390 V3, which was to be the pro­totype for the bomber reconnaissance design, was never built.

Contemporaries Daimler-Benz Projekt E and Projekt F (Germany), Messerschmitt Me328C (Germany)

Deployment

Varied. The Oka Model 11 and MXY7 K-l saw operational service. The Oka Model 22 was deployed but did not see action. The Oka Model 21 remained a single prototype. The Oka Model 43B prototype was incom­plete at war’s end. The Oka Model 33,43A and 53 remained designs only. The Oka Model 43 K-l Kai was too late to be issued to training units.

Survivors

Kugisho MXY7 Oka (FE-N50)

The ‘N’ is assumed to signify ‘Navy’ as in the US Navy, thereby denoting the US Navy was evaluating this particular aircraft. This MXY7 was listed on a 10 March 101946 report allowing it to be released to aviation indus­try. On 1 August 1 1946, an inventory reported it at MAMA and by 18 Sep­tember 1946 was slated for the museum and storage at Park Ridge. No further trace of the MXY7 is known.

Kugisho Oka Model 11 (no tail number assigned)

One of a number of Oka aircraft taken to the US, this one, serial number 1049, was obtained by Edward Mahoney. It was restored and remains on display at his Planes of Fame Museum in Valle, Arizona.

KQgisho Oka Model 11

Bearing the serial 1018, this Oka is currently on display at the Marine Corps Base Quantico, near Triangle, Virginia.

Kugisho Oka Model 11

Originally this Oka was in the collection of the Victory Air Museum located in Mundelein, Illinois. It closed its doors in 1984 and the aircraft was sold off. It was last obtained by the Yanks Air Museum in Chino, California.

KQgisho Oka Model 11

Shipped to India in September 1947 by the No.4 Squadron of the Indian Air Force following their duties in Japan as part of the British Common­wealth Occupation Forces, the Oka is currently on display in the Indian Air Force Museum at Palam Air Force Station, New Delhi, India.

Kugisho Oka Model 11

Another Oka in England, this time displayed at the Fleet Air Arm Museum in Yeovilton, Somerset, UK.

Kugisho Oka Model 11

This example of the Oka is in the collection of the Museum of Science and Industry in Manchester, UK, with the registration number of LI 996.53.10. Since 1961, this Oka has passed through a number of muse­ums before it reached its current location.

Kugisho Oka Model 11

This Oka is housed in the collection of the Defence Explosive Ordnance Disposal School. The school is currently located in Chattenden, Kent, but is to be relocated to St. George’s Barracks, Bicester in Oxfordshire, UK.

Kugisho Oka Model 11

The fourth Oka in the British Isles is on display at the Royal Air Force Museum at Cosford in Shropshire. Prior to this, it was housed at the Rocket Propulsion Establishment in Westcott, Buckinghamshire.

KQgisho Oka Model 11

This, the only known genuine Oka in Japan (a replica is used at the Yasukuni Shrine in Chiyoda, Tokyo as well as the Oka Park in Kashima City, Ibaraki Prefecture), is currently housed at Iruma Air Base, a Japan­ese Air Self-Defence Force facility in Iruma, Saitama Prefecture. During the war it was at the IJA base Irumagawa Airfield.

KQgisho Oka Model 22

Restored and on display at the Smithsonian Air and Space Museum. KQgisho MXY7 K-l

With a serial number of 5100, the MXY7 K-l is housed in the Navy Museum at the Washington Navy Yard in Washington, D. C.

KQgisho MXY7 K-l

This trainer is currently on display at the National Museum of the United States Air Force, located on the property of the Wright-Patterson Air Force Base in Riverside, Ohio, which is just outside of Dayton, Ohio. At one time it was painted as an operational Oka but has since been returned to the orange colour scheme of a trainer.

KQgisho Oka Model 43 K-l Kai

At present, this trainer bearing the serial number 61 is in storage and unrestored at the Paul E. Garber facility.

(Note: The KQgisho Oka Model 11 displayed at the Wings of Eagles museum in Horseheads, NY, is a replica.)

The design and development of the Mitsubishi J8M1 SyCisui presented a challenge. Despite the information available to the Japanese on the Messerschmitt Me 163B, upon which the J8M1 was based, the concept of a tailless fighter, let alone a rocket powered one, was new and untested. What was required was a means to verily the design of the J8M1 and in doing so provide a way to train pilots who would be fly­ing a plane that was unlike any they had ever down before.

Therefore, Kugisho was given the task of cre­ating a glider that was to be a copy of the J8M1. The main purpose was to assess the flight char­acteristics of the tailless fighter given that the Japanese did not have extensive experience with such aircraft. Data collected from flying the glider would in turn be reviewed and applied to the J8M1 fighter prior to series pro­duction. In addition to serving as a proof of con­cept vehicle, the glider would provide the means to train new pilots in flying the aircraft since it was like no other fighter then in service in the 1JN and 1JA. By using the glider as a trainer, pilots could better transition to the J8M1 and therefore minimise operational mistakes.

Kugisho assigned the glider construction, called the MXY8 Akigusa (meaning ‘Autumn Grass’), to engineer Hidemasa Kimura. Kimura utilised wood with some cloth covered sur­faces in the design of the MXY8 and ensured that the glider was a near exact replica of the J8M1 in order to provide the most accurate flight data once testing got under way. By the close of 1944, the first MXY8 was finished and another two were nearing completion.

In December, the first MXY8 was taken to the airfield located in Hyakurigahara, which is about 79km (49 miles) northeast of Tokyo. It was here that the UN’s Hyakurigahara Air Group was stationed, operating in the defence of Tokyo. Also at the airfield was the 312th Kokdtai, a newly formed unit that was to be equipped with the J8M1 once it entered pro­duction. As such the 312th Kokutai was the per­fect group to begin testing the MXY8. The first flight, scheduled for 8 December 1944, was given to Commander One. Unfortunately, One was taken ill and was unable to fly so the mis­sion was assigned to Lieutenant-Commander Toyohiko Inuzuka. On the day of the flight, Inuzuka climbed into the cockpit of the MXY8 and once secure, a Kyushu K10W1 of the 312th Kokritai took the glider into the sky. At altitude Inuzuka was released from the tow plane and began his descent. After successfully bringing

MXY8 shown in the orange colouration as used on trainers and experimental aircraft.

the glider down, Inuzuka gave the MXY8 high marks, having found the handling and flight characteristics to be very acceptable.

The IJA, who were also slated to fly the J8M1 as the Ki-200, was provided with the second MXY8. Delivered to the Rikugun Kokuyijutsu Kenkyujo, the pilot selected to test the MXY8 for the IJA was Colonel Aramaki, and like Inuzuka, he felt that the MXY8 performed well. The only notable deficiency to be found by both Inuzuka and Aramaki was the tendency for the MXY8 to nose over into a dive. The third MXY8 to be built was delivered to the Naval Air Force.

The first MXY8 did not match the combat weight of the J8M1. To this end the UN wanted to modify the MXY8 so that it incorporated bal­last tanks which could hold enough water to fully simulate the combat weight of the J8M1 and be the definitive production model for use in training pilots. With the completion of the ini­tial three MXY8 gliders by Kugisho, a number of manufacturers were organised to begin the production of the revised ‘heavy’ MXY8 glider to meet the training needs of the 312th Kokutai and other units that would be flying the J8M1 and Ki-200. Maeda Kdkii Kenkyujo was tasked with producing the MXY8 for the UN and the MXY8 was to be built for the IJA by Yokoi KdkCi K. K. as the Ku-13.

Further flight testing by the 312th Kokutai found that the MXY8 experienced aileron flutter at speeds above 295km/h (183mph) (as a side note, the same problem was encountered in the Messerschmitt МеІбЗА VI first prototype during testing). This and other minor problems were noted, analysed and corrected, and the flutter issue was resolved by closing the gap between the wing and the aileron (the Me 163A VI was rebalanced). In the meantime, the MXY8 was being flown by Naval Air Force pilots at the Kashiwa airfield in Chiba Prefecture. However, the pilots were less enthusiastic on the design, especially after a crash involving one of the gliders that severely injured the pilot. Regardless, the Kaigun Koku Hombu assessed all of the data from the test flights and formally approved the MXY8 on March 1945 and work proceeded with full production of the MXY8 and Ku-13 that continued until the end of the war.

Contemporaries Messerschmitt Me 163S, Heinkel He 162S (Germany)

Data is for the MXY8. The specifications also apply to the Yokoi Ku-13. No specific data is available on the MXY9 Shuka.

Another version of the glider was investi­gated by the UN. Whereas the MXY8 was unpowered, the new version would have some means of propulsion. Designated the MXY9 Shuka (meaning ‘Autumn Fire’), the new design was to be an advanced trainer which, because it had the means to propel itself when it was airborne, would provide training with a modicum of power and offer longer flight times. It was envisioned that once training in the MXY8 was completed, pilots would transi­tion to the MXY9 for advanced training before moving to the J8M1 or Ki-200. The propulsion method proposed was the Tsu-11 thermojet. This was the same engine as used in the Kflgisho Oka Model 22 (see Page 70). However, the MXY9 was never

Given the expanse of the Japanese empire by 1942, the UN found that they had a need for a long range reconnaissance aircraft that could operate from land bases and fly at high speed to render it immune to interception. In the same year, the UN issued a 17-shi specifica­tion for just such a plane and Kugisho looked to provide the response.

The 1942 17-shi specification called for the aircraft to have a maximum speed of 667km/h (414mph) at 6,000m (19,685ft) along with a mission profile of long range recon­naissance. The speed requirement stemmed from the need to be able to avoid intercep­tion; the intelligence it gathered would be useless if the aircraft was shot down before it could return to base. The initial design, the R1Y1 Gyoun (meaning ‘Dawn Cloud’; other sources use Seiun, meaning ‘Blue Cloud’), bore the designation Y-30 and was to be developed around a new Mitsubishi, 24-cylin – der, liquid-cooled engine that was projected to produce 2,500hp. However, delivery of the engine was not expected to be rapid and in
order to proceed with the R1Y1, Kugisho decided to utilise two Mitsubishi MK10A radial engines. In so doing, the R1Y1 took on the appearance of the Kugisho P1Y1 Ginga and with the use of two radials and the result­ing drag imposed by them the RlYl’s calcu­lated performance was projected to fall below the 17-shi specification. Consequently, all work on the R1Y1 ended and the project was abandoned.

Even as Kugisho was working on the R1Y1, they were developing another design, the Y – 40, which was the result of an evaluation of the Heinkel He 119, two examples being pur­chased from Germany in 1940.

The He 119 was an attempt to create a fast, unarmed reconnaissance aircraft whose high speed would enable it to avoid interception and elude pursuit. To accomplish this, the He 119 used radical concepts to minimise drag and thus enhance speed performance. A pair of Daimler-Benz DB 601 engines coupled together drove a single propeller shaft. The engines were placed in the rear of the He 119
fuselage with the shaft running forward, through the middle of the cockpit, spinning a four-bladed propeller in the nose. To cool the paired powerplants, the He 119 used a wing surface evaporation system in which steam from the engines was circulated through the wings where it cooled and condensed back to liquid where it was pumped back to the engines. To cool the engines when on the ground or during take-off and landing (due to the lack of sufficient airflow across the wings at such times), a supplementary radiator was installed under the forward fuselage. The He 119 VI first prototype attained a top speed of 565km/h (351mph) at 4,500m (14,765ft). Unfortunately, Heinkel was forced into adding armament to the He 119 but this was done in a very minimal fashion. The V2, with a full functional bomb bay, was able to reach speeds of up to 585km/h (363mph) at 4,500m (14,765ft). The V4 was used as a record breaker, briefly holding the record for speed with a 1,000kg (2,2051b) payload on a closed 1,000km (621 mile) circuit with the average

speed of 504.97km/h (313.78mph). Later the V4 was wrecked in a crash during an attempt to better that time. The record was set on 22 November 1937 and the successful aeroplane was listed as the ‘He 606’. The V3 was built as a float-plane, intended to best the 1,000km (621 mile) seaplane speed record. Ultimately, the V5 through to the V8 would be the last He 119 aircraft built because the Luftwaffe showed no further interest in the aircraft.

Following testing in the summer of 1938, a delegation from the UN was able to inspect the He 119 at Marienehe in Germany. The Japanese were most impressed by the range offered by the He 119 as well as its speed. Of interest were the coupled DB601 engines. After reporting their positive findings, nine technicians from the Dai-Ichi Kaigun Кбкй Gijutsu-sho flew from Japan to Germany to study the He 119 further. Commander Hideo Tsukada arranged to obtain the manufactur­ing licence for the He 119 and also the pur­chase of the He 119 V7 and V8. Both aircraft were crated for shipment and sent to Japan arriving in May 1940. Reassembled at Kasum – igaura, KQgisho began flight trials under the leadership of Major Shoichi Suzuki. During the brief trials, one He 119 was lost to landing gear failure (the He 119 used a special, retracting telescopic oleo leg in order for the long landing gear to fit into the wings). In the end plans to manufacture the He 119 in Japan did not come to fruition.

Although the He 119 was rejected, the study of this aircraft resulted in the development of the Y-40. Like the He 119, the Y-40 was to be a fast, unarmed two-seat reconnaissance air­craft using coupled engines placed within the fuselage behind the cockpit and driving a pro­peller via an extension shaft. The UN’s 18-shi specifications for a long range reconnais­sance aircraft were based on the Y-40.

The Y-40 project, by now called the R2Y1 Keiun (meaning ‘Beautiful Cloud’), was led by Commander Shiro Otsuki and his design team made good progress. The Keiun was to be equipped with two Aichi Atsuta 30 engines coupled together in a combination known as the Aichi [Ha-70] 10. The 24-cylinder, liquid – cooled [Ha-70] 10 was rated at a maximum of 3,400hp and drove, via the extension shaft, a 6-bladed propeller. The Keiun did not use the same method of cooling as the He 119. Instead, it relied on air intakes and a radiator bath underneath the fuselage, and it also dif­fered from the He 119 in that the Keiun used a tricycle landing gear system and was not a tail sitter.

By the fall of 1944, the war situation for Japan was deteriorating. With the loss of terri­tory to the advancing Allies, the UN no longer saw a need for a long range reconnaissance aircraft. Following the defeat of the Japanese in the Marianas Islands (following Operation Forager), the fate of the Keiun was all but sealed. The UN had no need for such a plane as existing designs would be adequate for the dwindling Japanese holdings. In addition, the need for fighters and bombers was rather more urgent than reconnaissance aircraft.

But Otsuki and Kugisho did not let the Keiun fall by the wayside. In late 1944, KQgishd approached the UN and informed them that the R2Yl’s airframe was readily adaptable to other roles, including that of a fast attack bomber. To heighten the interest, it was proposed that the [ Ha-70 ] 10 engine be replaced with two Mitsubishi Ne330 axial – flow turbojets, each of the engines being slung under the wings in nacelles. The fuse­lage space vacated by the Aichi engine would be replaced with fuel tanks. For weapons, the aircraft would carry one 1,800kg (7641b bomb) and have a cannon armament in the nose. With the introduction of the Ne330 engines, the maximum speed was expected to be 495mph, superior to the projected 720km/h (447mph) top speed of the Aichi engine model. With these advantages in mind, the UN approved that work should begin on designing the R2Y2, the turbojet powered Keiun which was sometimes referred to as the Keiun-Kai, as well as per­mitting the R2Y1 to be completed as an air­frame demonstrator to test the handling characteristics.

In April 1945, the first prototype of the R2Y1 was completed and moved to Kisarazu in Chiba Prefecture to begin flight testing. Initial taxi trials, conducted by Kugisho test pilot Tereoka, showed that the nose wheel had a bad shimmy when in motion and the Aichi engine was prone to overheating. The latter was either due to a lack of airflow through the radiators and inlets during taxi tests or through a poorly designed cooling system. Nevertheless, despite the problems testing of the Keiun continued.

On 8 May 1945, Lieutenant-Commander Kitajima, another Kugisho test pilot, took the Keiun on its first flight. Kitajima noticed that the oil temperature was rising rapidly and he cut short the flight, landing the Keiun before the engine suffered damage. Mechanics and engineers continued to try and solve the cool­ing problems, but a few days later the engine caught fire during ground testing, completely destroying the power unit. Then before the Keiun could be returned to KQgishd to receive a new engine, the aircraft was destroyed by a US bombing raid.

Even before the destruction of the first R2Y1, a second was being constructed and design work for the R2Y2 was underway.

Contemporary sources show no less than four versions for how the R2Y2 may ultimately have appeared. The first had the Ne330 engines in underwing nacelles. The second version showed the two engines buried within the fuselage with wing root air intakes and narrow jet nozzles. The third removed the wing intakes and replaced them with a nose intake, but it retained the narrow noz­zles. Finally, the fourth was similar to the third save that the engine nozzles were larger. The first design is considered by most to be the ini­tial R2Y2 concept while the other three are subject to debate. In part, this is due to the fact that the Japanese had very little time to explore various installations of turbojets in airframes. The easiest means to place turbo­jets on aircraft was by using nacelles and this was seen in the Nakajima Kitsuka, Nakajima Ki-201 Karyu and proposed KQgisho Tenga and Kawanishi K-200.

Even the Germans with their turbojet expe­rience did not fully understand the effects of a long nose intake feeding a high perfor­mance jet buried in a combat fighter’s fuse­lage. Messerschmitt, when they began to study how to start the P. l 101 second genera­tion jet fighter, catalogued the obstacles that needed to be overcome. They included the effects of engine operation on the fuselage integrity, ensuring the nose intake was prop­erly positioned and shaped for maximum air­flow, making sure the intake tube was made as smooth as possible to minimise air restric­tions, how to protect the rear of the aircraft from the heat generated by the exhaust thrust, the effects of reduced airflow on thrust due to flight angles and more. The Germans were at least able to devote some time to investigating these problems and providing solutions to them. This was time however, that the Japanese simply did not have. Up until the construction of the P. l 101 VI and the planned Focke-Wulf Та 183, all of the wartime jet designs flown by the Luftwaffe had nacelle mounted turbojets. The Japan­ese may not have been made fully privy to the latest German jet engine technology as it per­tained to long intakes before the war ended. It is within reason to suggest that the R2Y2 with the wing root intakes could have been under consideration since it would be a logi­cal development, especially since such intake arrangements were not entirely new. The third and fourth designs may or may not have been post-war conjecture.

Unfortunately for KQgisho and the UN, the R2Y2 would never be brought to full produc­tion. With the end of the war, the second R2Y1 prototype remained incomplete and the R2Y2 would forever remain a design board aircraft.

Kugisho R2Y Keiun – data

Contemporaries

Messerschmitt Me 509 (Germany), Tupolev Tu-91 (NATO codename Boot) (Russia), Messerschmitt P. l 100 (Germany)

The specifications in parenthesis are for the R2Y2 with the underwing turbojets.

Type Long range reconnaissance aircraft (attack aircraft)

Crew Two

Powerplant One Aichi [Ha-70] 10,24-cylinder, liquid-cooled engine developing 3,400hp at take-off and 3,100hp at 3,000m/9.845ft, driving a 6-bladed metal propeller (two Ne330 axial-flow turbojets developing l,320kg/2,9101b of thrust each)

Armament

None (one 800kg/l,764 lb bomb and a battery of forward firing cannon)

Note

Concerning the three other R2Y2 jet variants with internal engines, little is documented and much is open to conjecture. In some instances, the wing span, length, height and wing area are listed as being the same for the R2Y1 but the speed is given as being a maximum of 800km/h (497mph).

Deployment

None. One prototype of the R2Y1 was built and flown while the second R2Y1 prototype was unfinished by the end of the war. The R2Y2 stayed on the design board.

Of the many Japanese experimental aircraft of World War 2, perhaps none is more of a mystery than the Kugisho Tenga. The Tenga (which can mean the Milky Way as one trans­lation of the kanji) was to be a first for Japan: a turbojet powered bomber. To realise this ambition as quickly as possible, the Japanese intended to use one of their latest and best bomber designs – the Kugisho P1Y Ginga (‘ginga’ also means Milky Way) – as the basis for the Tenga.

If one examines the aircraft available to the Japanese during the war, the distinct lack of a medium bomber quickly becomes evident. Whereas most of the warring powers oper­ated medium bombers (for example, the Martin B-26 Marauder, the Junkers Ju88 or the Vickers Wellington), the Japanese were very late in bringing such aircraft to the front. The IJA brought the Ki-67 Hiryti medium bomber into service in 1944 and so the UN looked to the Kugisho P1Y Ginga as their answer to the need for a medium bomber.

Development of the Ginga began in 1940 as the Dai-Ichi Kaigun Kokti Gijutsu-sho’s attempt to meet a 15-Shi specification for a medium bomber capable of high speeds, the ability to conduct low-level bombing and tor­pedo missions, and the capability to perform dive bombing. With Tadanao Mitsuzi and Masao Yamana at the helm the Y-20, as the Ginga was called at this stage, emerged as an aerodynamically clean, mid-wing, twin – engine design. Despite its relatively small size, the Ginga had fourteen fuel tanks (of which only eight had some protection from battle damage), a modicum of armour for the pilot (which consisted of a 20mm thick plate behind his head), a light defensive armament of a 7.7mm machine gun in the nose and in the rear of the cockpit, and the ability to carry a single 800kg (1,764 lb) torpedo or two 500kg (1,1021b) bombs. With the two Nakajima Homare 11 18-cylinder, air-cooled radial engines developing 1,280hp each, speed was estimated at 556km/h (345mph).

The first prototype was completed in August 1943 and flight testing began shortly afterwards. Test pilots found that the Ginga possessed excellent speed and also dis­played good handling qualities. Ground crews on the other hand had anything but good things to say about the aircraft. The Homare 11 engines and the hydraulic system used in the Ginga were a constant mainte­nance hassle, requiring far more time and effort to maintain than was considered rea­sonable. So bad were the problems, the IJN postponed its acceptance of the aircraft.

Despite the problems production got mov­ing and design changes saw the machine guns replaced with Type 99 Model 1 20mm cannons and 13mm Type 2 machine guns. Other changes included revised engine cowl­ings, replacing the retractable tail wheel with a fixed wheel, moving from flush riveting to flat-head riveting, incorporating a bullet-proof panel in the windshield and also replacing the Homare 11 engines with the Homare 12 which could produce l,825hp.

After these modifications, the IJN finally accepted the P1Y1 Ginga bomber into ser­vice. But the type was still nagged by prob­lems, notably the Homare 12 engines which rarely produced the horsepower they were rated for. Such issues delayed the Ginga entering combat until the spring of 1945. Even though the Ginga would see battle for a mere six months the design nevertheless proved to be a capable medium bomber and one which the Allies respected when they encountered it. When the Allies first heard of the plane they thought it was a heavy fighter and assigned the codename Francis to it (after Francis ‘Fran’ Williams of the Material Section of the Directorate of Intelligence, Allied Air Forces, Southwest Pacific Area). However, when the Ginga was finally spotted after 1943, it was realised that it was a bomber and the name was changed to the feminine version of Francis – Frances.

The Ginga was developed into several vari­ants and there were plans to use the bomber as a carrier for the Oka Model 21 and Oka Model 22 suicide aircraft. Kawanishi built a night-fighter/intmder version as the P1Y2-S, which entered service with the IJN as the Kyokko (meaning ‘Aurora’) in the summer of 1944. The Kyokko was fitted with Mitsubishi Kasei 25a l,850hp, 14-cylinder radials

because the Homare 12 could not be assem­bled fast enough to meet demand. Weapons included two forward oblique mounted 20mm Type 99 Model 2 cannons firing upwards and the nose cannon was removed. First flown in July 1944, it was found that the Kyokko did not perform well at the high alti­tudes where the Boeing B-29s roamed. This revelation was so disappointing that the upward firing cannons were removed and the Kyokko returned to its bomber role as the Ginga Model 16 (P1Y2). Nakajima also built a similar night-fighter version as the P1Y1-S Byakko (which meant ‘White Light’). The Byakko fared little better than the Kyokko and did not see service. Other modifications and plans included upgrading the engines to the Homare 23, Kasei 25c or the Mitsubishi MK9A,

the idea of an attack model with ten to sixteen forward firing 20mm cannons and using steel and wood in the aircraft’s construction. The most interesting was the P1Y3 Model 33. This version was to be built from the ground up to carry the Oka and would have had a special bomb bay to accept the Oka Model 21 or 22 with increased wing span and an enlarged fuselage. The P1Y3 never left the drawing board.

With the Ginga’s success in terms of per­formance, it’s easy to see why there was interest in converting it to turbojet power. The concept of the Tenga was certainly real. But outside of the name and the basic intent to replace the radial engines with turbojets, nothing else is known. Therefore, one has to review other designs to make assumptions on what kind of task the Japanese might have faced in making the Tenga a flying reality.

The first point to consider would be that the Kugisho Ne 20 turbojets, then in production for use in the Nakajima Kitsuka, would not have been sufficient to provide the Tenga with any meaningful speed if mounted one per wing. One Ne20 produced 487kg (1,074 lb) of static thrust and two could propel the Kitsuka to 623km/h (387mph), which was not particularly significant over conventional high-performance propeller driven aircraft. The Kitsuka was a much lighter aircraft and a twin turbojet Tenga using Ne20s would not have been feasible.

It would have needed some of the pro­jected advances in the Ne 20’s development to come closer to reality to provide the Tenga with a meaningful system of propulsion. The Ne 30 turbojet was expected to generate up to 850kg (1,873 lb) of thrust (better than the Ger­man BMW 003 turbojet rated at 800kg/l,7631b) while the Ishikawajima Nel30 was projected to produce 900kg (1,9841b) of static thmst, comparable to the Junkers Jumo 004 engine. The Nakajima Ne 230 and the Mitsubishi Ne 330 were esti­mated to be able to produce 885kg (1,951 lb) and 1,300kg (2,8661b) of thrust respectively with the Ne 230 sacrificing thrust for a lighter weight.

It is said that the Ne 30 would have been the initial choice to power the Tenga had it been available. In comparison, the German Arado Ar234B jet bomber used two Jumo 004 engines. It was similar in size to the P1Y1, the notable differences being a smaller wing span, the loaded weight was nearly 680kg (1,5001b) lighter and the Ar234B had far less wing area. Together, the two Jumo 004 engines could move the Аг 234B at speeds up to 742km/h (461mph). Certainly, two Ne30 engines would not have provided such a speed when mounted to the P1Y1 but it would have been the logical starting point. Quite possibly, RATO units may have been needed to boost the Tenga off the ground. Clearly the Ne 130 would have been a better selection and with the Ne330, the Tenga would have enjoyed a noticeable speed improvement. Problems with the develop­ment of the Ne 30 engine are cited as a reason for the Tenga project being cancelled. Indeed, the Ne 30, an off-shoot from the Ne 12 program, never advanced, being surpassed by the Ne 20, Ne 130, Ne 230 and Ne 330 devel­opments.

But could the basic airframe of the Ginga be used with radial engines replacing with turbojets? It may have been attempted had the Tenga advanced in design. Even changing the radials for turbojets would have necessi­tated fairly significant adjustments in the wings to accommodate them but at least redesigning a wing to accept turbojets is a simpler task than redesigning the entire aircraft.

However, if one examines the history of combat aircraft, you would be hard pressed to find a conventional combustion engined bomber switching to jet power merely by changing the engines and adjusting the wings. For instance, not even among the dozens of jet bomber projects undertaken by the Germans did a piston-engined bomber switch its engines for turbojets without heavy modifications, if at all. One such example was the Messerschmitt Me 264 which used four Junkers Jumo 9-211 radial engines when the first prototype was flight tested. However, the proposed four turbojet engined version bore little resemblance to the original design.

Perhaps the only notable propeller to tur­bojet design created by adaptating an existing airframe was the Russian Tupolev Tu-12 whose heritage was owed to the Tu-2, one of the premiere Soviet light bombers. Built from 1941 through 1948, the Tu-2 possessed fast speed, excellent agility and had a substantial weapon fit and bomb carrying capacity. When Tupolev answered the call to produce a jet bomber, he took the Tu-2 as the basis for his Tu-12. He used the fuselage, wings and tailplane of the Tu-2 and adapted them to suit the installation of two Rolls-Royce Nene-1 tur­bojets and the higher speeds that would result. Although one can certainly see the lin­eage of the Tu-2 in the Tu-12, the aircraft still required a general redesign to cope with the new engines and the associated handling characteristics and was not simply a case of swapping the radial engines for turbojets. The design of the Tu-12 began in 1946 and the first flight took place in June 1947.

It is not unreasonable to conclude that the initial Tenga designers may have tried to utilise as much of the Ginga as possible, offer­ing the benefit of an airframe already in pro­duction with proven airworthy characteristics. This would have reduced development time when the need for such a bomber was most urgent. It would have also served as a starting point for aerodynamic testing. Still, when one reviews the jet bomber proposals of other nations, the num­ber of piston engine to jet engine concepts can be counted on a single hand. For the majority, the jet bomber was designed from the ground up instead of being adapted from an exisiting aircraft. The designers of the Tenga may have come to the same conclu­sion had they had the opportunity to continue their work. If so, the final Tenga design and prototype may have borne little resemblance to the Ginga with which it shares its name.

This munition was the UN’s version of the Та. It was a heavier bomblet and weighed 1kg (2.2 lb). A canister would carry up to 40 of the Model 1 bomblets. Like the Та, a fuse would open the canister deploying the bomblets onto their target.

Type 2 No.6 21-Go Model 2 (IJN)

This was the same bomblet but instead of the hollow-charge in the Model 1, the Model 2 used a solid black powder charge. This change was made because the IJN felt that an armour piercing charge was not necessary against aircraft that were typically unar­moured. This did, however, raise the weight of the Model 2 and a canister could only hold 36 of the bomblets, the total weight of the loaded canister being 49kg (108 lb).

Type 3 No.6 Mk.3 Model 1 (IJN)

This was basically a simplified Type 99 No.3 Mk.3 bomb as it used a wooden nose and did away with the impact fuse. The bomb was 1 m (3.3ft) long and weighed 56kg (1241b). Its design commenced in 1943 and entered ser­vice in 1944. Colouration was the same as the Type 99.

Type 3 No.6 Mk.27 Model 1 (IJN)

Development of this anti-bomber rocket bomb began in January 1944. The cone – shaped nose contained 140 iron pellets embedded in 4kg (8.8 lb) of white phospho­rous. A 10kg (221b) propellant charge moved the bomb to a velocity of 270m/sec (885.8ft/sec). On detonation the pellets were scattered in a 60° radius and the bomb was provided with the fittings for rail launch­ing by fighters. The IJN accepted it for service in February 1945 and final testing was completed in April 1945. Testing was con­ducted by Dai-Ichi Kaigun Koku Gijutsu-sho, the Second Powder Factory Arsenal, and Kashima Bombing Experimental Field. The bomb was 1.4m (4.6ft) long and weighed 60kg (132 lb).

Type 3 No.6 Mk.28 Model 1 (IJN)

Using the Mk.19 as a basis, the Mk.28 was a small, anti-bomber rocket bomb with a.6kg (1.31b) explosive warhead A 2kg (4.41b) pro­pellant charge moved the bomb to a maxi­mum velocity of l,312ft/sec. Dai-Ichi Kaigun Koku Gijutsu-sho built the bomb and its clearence testing was undertaken by Dai-Ichi Kaigun Кбкй Gijutsu-sho, the Second Powder Factory Arsenal, and Kashima Bombing Experimental Field. Testing was completed late in 1944 but the rocket bomb was not accepted for service. Its length was .7m (2.3ft) and weight 7.3kg (161b).

Type 5 No.25 Mk.29 (IJN)

Designed for use by special attacker aircraft, the Mk.29 contained 1,100 pellets packed in 50kg (1101b) of white phosphorous. After fly­ing into a bomber formation, the pilot would use a pull cord to detonate the bomb while it was still attached to the aircraft. The bomb could also be released where the tail fuse would trigger the bomb explosion. Unlike other pellet dispersing munitions, the Mk.29 flung them at right angles to the bomb casing instead of in a downward cone. Initial testing was begun in 1944 but only one prototype was air dropped and the type did not enter service. The Mk.29 was 1.6m (5.1 ft) long and weighed 250kg (551 lb).

The Ju 488 was a design by Junkers to rapidly produce a long range bomber using compo­nents of aircraft then in production. Combin­ing parts from the Ju388K, Ju 188E, Ju88A-15 and Ju288C bombers, construction of two Ju488 prototypes was begun (V401 and V402), but prior to completion these were destroyed by French resistance forces in July

1942. In November 1944, the Ju488 program was cancelled. In January 1945, the Ju488 design was offered to Japan but neither the IJA nor IJN took any interest in it. This rejec­tion sealed the fate of the project.

Messerschmitt Bf 109E-7 fighter

In 1941, the Japanese obtained three Bf 109E-7 fighters. These were pitted against various Japanese fighter designs for compar­ison purposes. Despite the Japanese having no intention of licence building the fighter, the Allies anticipated that they would be encountered in combat and gave the plane the codename Mike.