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Kugisho High-Speed Projects
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
90
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.
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Kugisho High-Speed Aircraft Project – data
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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
|
Powerplant Unpowered except for one modified with a Nihon Hainenki Semi 11 ([Ha-90] 11) 4-cylinder, air-cooled engine developing 22hp and driving a two-bladed, fixed stroke wooden propeller
Deployment None. The MXY6 was purely a proof of concept glider. |
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Oka Model 11 – data
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‘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
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Type Crew |
Special Attacker One |
|
|
Powerplant |
Three Type 4 Mark 1 Model 20 solid fuel rockets, |
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each developing 267kg (5881b) of thrust, for a |
total of 801kg (1,7641b) |
|
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Dimensions |
||
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Span |
4.11m |
13.5ft |
|
Length |
6.88m |
22.6ft |
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Height |
1.12m |
3.7ft |
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Wing area |
3.99m! |
43ft2 |
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Wing loading |
399.78kg/m! |
81.9 lb/ft2 |
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Power loading |
1.99kg/hp |
4.4 lb/hp |
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Weights |
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Empty |
535kg |
1,1791b |
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Loaded |
1,600kg |
3,5271b |
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Useful load |
915kg |
2,0171b |
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Performance |
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Max speed |
642km/h |
399mph |
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at 4,000m |
at 13,125ft |
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Cruise speed |
443km/h |
275mph |
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at 4,000m |
at 13,125ft |
|
|
Max range |
lllkm |
69 miles |
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Ceiling |
8,500kg |
27,887ft |
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Armament 600kg (1,3221b) explosive warhead |
Oka Model 22 – data
Type Special Attacker
Crew One
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Powerplant One Tsu-11 thermojet developing 200kg (440 lb) ol thrust Dimensions
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Type Special Attacker
Crew One
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Powerplant One Ne 20 axial-flow turbojet developing 475kg (1,047 lb) of thrust or one Ne 12B jet engine developing 320kg (7051b) of thrust
Armament 800kg (1,7631b) explosive warhead |
Oka Model 43A – data (estimated)
Type Special Attacker
Crew One
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Powerplant One Ne 20 axial-flow turbojet developing 475kg (1,047 lb) of thrust
Armament 800kg (1,7631b) explosive warhead |
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.
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Oka Model 43B – data (estimated)
Armament 800kg (1,7631b) explosive warhead |
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Type Crew |
Special Attacker One (or none) |
|
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Powerplant |
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One Ne 20 axial-flow turbojet developing 475kg (1,047 lb) of thrust; one |
||
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Type 4 Mark 1 solid fuel rocket, developing 267kg (5881b) of thrust |
||
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Dimensions |
||
|
Span |
6.43m |
21.1ft |
|
Length |
7.77m |
25.5ft |
|
Height |
1.43m |
4.7ft |
|
Wing area |
8.99m2 |
96.8ft2 |
|
Wing loading |
N/A |
|
|
Power loading |
N/A |
|
|
Weights |
||
|
Empty |
N/A |
|
|
Loaded |
N/A |
|
|
Useful load |
N/A |
|
|
Performance |
||
|
Max speed |
N/A |
|
|
Cruise speed |
N/A |
|
|
Max range |
277km |
172 miles |
|
Ceiling |
N/A |
|
|
Fuel capacity |
400 litres |
105.6 gallons |
|
Oil capacity |
16 litres |
4.2 gallons |
|
Armament |
||
|
600kg (1,322 lb) explosive warhead |
|
Oka K-l – |
data |
||
|
Type Crew |
Trainer One |
||
|
Powerplant |
None |
||
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Dimensions |
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Span |
5.12m |
16.8ft |
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Length |
6.06m |
19.9ft |
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Height |
1.12m |
3.7ft |
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Wing area |
6.00m2 |
64.6ft2 |
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Weights |
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Empty |
730kg |
1,6091b |
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Loaded |
2,120kg |
4,6731b |
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Useful load |
150kg |
3301b |
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Performance Max speed Cruise speed |
N/A 147km/h |
91mph |
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Landing speed |
200km/h |
124mph |
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Armament |
None |
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.
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Oka 43 K-l Kai – data
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