Friday, May 15, 2020

Fake Japanese Super-heavies

In 1939, the Imperial Japanese Army suffered total strategic defeat at the hands of the Soviet Union surrounding the village of Nomonhan, in Manchoukuo. This had been Japan’s first significant deployment of armour against a modern army. Tank development in Imperial Japan was still in its initial stages by the time the border conflict at Nomonhan began.

Japan had constructed a series of fortifications in Manchuria and Central China under the supervision of the Continental Fortification Research Committee, a department under the Imperial Army. This committee oversaw all major defenses in the region against the Soviet Union and Chinese warlords. Shortly after the Nomonhan Incident, the committee submitted an idea to construct special purpose tanks that were capable of breaking through heavily defended Soviet lines and force the Soviet armies back out of taken Manchurian lands. 

Historical Background

Colonel Hideo Iwakuro, an officer in the Military Affairs Bureau Tank Research Team, took an interest in the concept of a super-heavy tank capable of pushing through Soviet defensive lines, unmatched in firepower and armour. Japan built and tested an array of multi turreted heavy tanks from 1925 to 1938 in an attempt to implement a breakthrough vehicle. However, these tanks all had the same underlying issues: their armour was too thin to protect against anti-tank guns, and their armaments were inadequate. Due to weak engines, these prototypes lacked sufficient speed and mobility. But due to the large size of these engines, they forced the chassis to be larger. 

In the early months of 1940, Colonel Iwakuro along with a team of twenty engineers from the 4th Technical Research Group designed the superheavy tank at their headquarters in the Tokyo area. The superheavy tank project proceeded under the temporary name Mi-To (later, O-I). Construction work would be performed by the Sagami Army Arsenal in secret. As was customary with Japanese heavy tanks, the Mi-To had a multi turreted layout with two auxiliary turrets in the front part of the chassis, one primary turret in the center of the hull, and a machine gun turret located at the rear of the chassis. The tank design was heavier and larger than any other tank of its time. Basic in its shape, the vehicle kept a box-like appearance for ease of manufacturing and assembly of its plating.

Technical drawing document of the O-I super-heavy tank. One of various drawings purchased by Finemolds in 2014. (Source: author's collection)

The tank was sent to the Sagami arsenal for military trials upon completion of the prototype. Here, the tank was renamed the O-I. Due to faulty performance of the engine and the unsustainable weight of the tank, trials were concluded as a failure. The vehicle was then kept in a state of disrepair for the duration of the war. Upon the surrender of Japan, the records and materials of the project were kept privately near Sagami. Thought to have been destroyed after the war, for decades the vehicle was shrouded in mystery. Official documented records of the vehicle and plan were minimal in the remaining archives of Japan. Only through the writing of Tomio Hara, the head tank developer for Japan during the war, had the tank been mentioned in detail. However, in 2014, President Kunihiro Suzuki of FineMolds corporation purchased the remainder of the O-I’s documented reports from their holding place at Wakashishi Shrine in Fujinomiya. These were under ownership of Wakashishi Shrine museum staff. Along with the tank's reports, the last remaining physical piece of the vehicle, a track link of the prototype, also remained at Wakashishi Shrine on display.

Kunihiro Suzuki officially revealed and published these findings in the summer of 2015. Prior to this, information on the vehicle remained subject to great skepticism. Authors of various books and other media interpreted the tank in their own artistic licensing. Because of this, many false versions of the tank exist. Some of these interpretations have been regarded as separate projects, or the real accounts of the tank and its history.

Fake Iwakuro 100 ton  
(Commonly called O-Ni, or Type100 O-I, or O-I 100 ton)

Popularized by the Japanese book Tank and Tank Battles (2012), the Iwakuro 100 ton tank was described as the first super-heavy tank built by Japan. This publication contained extensive information detailing the development of Japan's tank projects. Mitsubishi engineers that took part in development were gathered and interviewed for information. One of these workers, Shigeo Otaka, described the O-I prototype and the information gathered from mobility tests conducted in late 1942. This corresponded with his accurate sketches of the tank.

Otaka recalls the vehicle as weighing 100 tons, significantly heavier than any other vehicle the Japanese army had employed. Development of the project had been under the supervision of Colonel Hideo Iwakuro in 1939. Iwakuro sought to develop a new breakthrough tank heavier than any other vehicles. This super-heavy tank was written to be as long as 10 meters, significantly longer than tanks of other nations. The tank’s height and width were listed as 4 meters.

Artistic interpretation of the O-I called the 100 ton tank. (Source;戦車と戦車戦 2012年)

In the book it is mentioned that the vehicle’s armament consisted of two 47 millimeter guns, a single 105 millimeter cannon, and two to three machine guns. The publication’s artistic reference pictured the Iwakuro 100 ton as a long yet thin super-heavy tank with conically designed turrets. The hull of the drawn tank depicted a single-step body with a recess in the front to accommodate two auxiliary turrets housing the 47 millimeter guns. Based on the artistic drawings, some depictions of this vehicle illustrate machine guns placed on the front central hull, but others do not have these present.

Artistic interpretation of the O-I
 called the 100 ton tank. (Source;戦車と戦車戦 2012年)
In the summer of 2015, the video game company Wargaming Limited introduced the Japanese 100 ton O-I under the name “O-Ni”. This vehicle matched visual appearance and statistics to that of the 100 ton’s book counterpart. The name of the vehicle should be noted, as it does not match the Japanese Iroha naming classification system. The Japanese military organized vehicles by two categories; class and the number of the class to be introduced. The O-I had been the first super-heavy tank Japan built. Because of this, the tank received the designation O (meaning super-heavy), and the letter I (representative of 1). The O-Ni under this system would translate to Super-heavy two. While this would be accurate naming of the tank, because it is a misinterpretation of the historical vehicle, no such tank would have existed.

Historical Context

The O-I consisted of a 4 turret format on a two-step hull.  Two auxiliary turrets were coupled in the front of the hull. These turrets were constructed to carry Type1 47 millimeter tank guns. The Type1 entered service in 1941 as the standard anti-tank cannon for the army. The purpose of these anti tank turrets was to engage enemy vehicles, such as armoured tanks. One central turret stationed in the upper step of the tank was present. Initial drafts of the vehicle requested for this turret to have been armed with the army’s Type96 150 millimeter howitzer. This was to be prioritized for use against armoured fortifications. The fourth turret had been placed on the aft end of the tank. This turret had been armed with two 7.7 tank machine guns. These were intended to provide vehicle defense from enemy engagements to the vehicle’s flank.  

Drawings of Engineer Shigeo Otaka of his
memory of the O-I.  (Source;戦車と戦車戦 2012年)
The proportions of the O-I tank had been a matter of concern for the Mitsubishi engineers that took part in the vehicle’s development. The proportions of the vehicle were documented at a width of 4.84 meters , height of 3.63 meters, and length of 10.120 meters. Due to the size of the tank, significant amounts of steel were required for construction. With the vehicle being privately funded through an embezzlement scheme, delays due to lack of funding were frequent and costly to the tank’s assembly. Throughout development, the decision to cancel the project was considered on multiple occasions. 

The weight of the O-I at the date of its mobility testing in 1943 was 96 tons. This weight accounted for only the unarmoured chassis, compared to the designed gross weight of 150 tons total. Construction complications resulted in a point of delay, halting further assembly of the prototype. Mitsubishi documents the weight of the prototype vehicle in a structured list. Unaccounted in this listing had been that of the central turret. Due to the resources required to construct the armoured turret, a temporary wooden scaled replacement had been made on June 4th 1942. Fitted to the prototype on July 10th, the 96 ton prototype was then ready for mobility tests and inspection by ranking army officials.

Description of the tank's armour plating by Finemolds.
 日本の重戦車 (2016年) 
According to Shigeo Otaka in the interview, the O-I had been assembled with mild tempered steel plating. Upon complete assembly of the body, armour plating of 75 millimeters replaced various sections of the tank’s front and aft panels. Additional armoured plating of 75 millimeters were to be added onto the tank’s body by bolting them to the underlying armour plating. The vehicle sides were replaced with 35 millimeter armoured steel plating with an additional 35 millimeter skirting layer bolted on top of the hull. Flooring of the prototype was constructed using a three staged plate assembly to reinforce the vehicle.

Transportation of the tank had been a key requirement in the development of the O-I. Due to the excessive weight of the vehicle, transportation via army railways became impossible. Separate transportation of various components at a time was required. Modular armour plating had been requested early in the vehicle’s development to make the practicality of the vehicle effective on the frontline. On May 26th, 1943, disassembly of the O-I had begun for transportation to the Sagami Arsenal outside Tokyo. The tank was disassembled and transported by June 9th. Assembly took place from July 1st to the 20th, after which it was to be subjected to official army trials.

3D model of 1/72 scale of the O-I tank. Produced by Finemolds using documentation obtained by its president in 2014.
 ( Finemolds)

Fake 120 ton O-I   
(Commonly called O-Ho, or Type120 O-I)

The first representation of this interpretation comes from the book series Japanese Fighting Vehicles of WW2, written by Yusaku Shimada. The book describes the 120 ton O-I as a tank project starting development in 1944, after the failed trials of the 100 ton tank. It was said to be a secret project, sent to Manchoukuo upon completion and used against the Soviet army in 1945. The vehicle is written to have been designed with aid of Nazi Germany by an exchange of super-heavy plans to Japan. The tank is often described as well protected by armour plating ranging from 200 millimeters to 300 millimeters.

Artistic interpretation of the 120 ton O-I. (Source; 帝国陸軍陸戦兵器ガイド 1997年)

Illustrated in Japanese Fighting Vehicles of WW2, the 120 ton is designed with a two-step hull, mounting two auxiliary turrets and a primary central turret. Both auxiliary turrets were mounted on the upper step of the tank’s body, each mounting 47 millimeter guns. One was positioned to the front, with the second to the aft of the tank. The primary turret housed a 105 millimeter gun coupled with machine guns. The forward lower step of the tank is drawn in similar fashion to standard Japanese vehicles such as the Type97 Chi-Ha and Type3 Chi-Nu medium tanks. This bottom step was designed to house the front transmission of the vehicle. Machine guns are spread out around the tank, such as in the rear central turret and front hull of the tank.
Artistic interpretation of the 120 ton O-I.
(Source; 帝国陸軍陸戦兵器ガイド 1997年)

A second drawing of the tank had three turrets again but in a different format. There were two auxiliary turrets; one mounting a 47 millimeter gun, and the other a machine gun. In the case of this interpretation, bulges were made on the upper step of the hull to accommodate an auxiliary turret crew. While the cannon is mentioned as the 105 millimeter gun, the drawing illustrates the Japanese Type5 75mm tank gun serviced with tanks such as the Type4 Chi-To and Type5 Chi-Ri. Other depictions instead show the 120 ton tank with the experimental high velocity 105 millimeter tank gun. The tank is written to have been built by late 1944, weighing 140 tons total.

Rumours claimed that only the turret of this vehicle was transported to Manchuria. This rumour erupted due to photography of Soviet soldiers in Manchoukuo standing in front of a bunker with a steel turret.

In the summer of 2015, the video game company Wargaming Limited introduced the Japanese 120 ton O-I under the name “O-Ho”. This vehicle matched visual appearance and statistics to that of the 120 ton’s book counterpart. The name of the vehicle should be noted, as it does not match the Japanese Iroha naming classification system. The Japanese military organized vehicles by two categories; class and the number of the class to be introduced. The O-I had been the first super-heavy tank Japan built. Because of this, the tank received the designation O (Meaning super-heavy), and the letter I (representative of 1). The O-Ho under this system would translate to Super-heavy five. The reason for this mistake is not clarified. However because the tank is a misinterpretation of the historical vehicle, no such tank would have existed.

Both O-Ho and O-I published by the video game company Wargaming Limited. It can be seen the visual difference between
the 120 ton tank artistic interpretation and the correct model of the O-I tank that had been designed and built. (

Historical Context

The claims of the 120 ton O-I generally originates from the words of Tomio Hara in his book, Japanese Tanks (1978).  In this book, Tomio Hara briefly writes on the history of the O-I tank along with the specifications of the prototype and project. Upon failed prototype trials conducted in 1943, the Japanese army attempted to fix the damages and underlying issues. By 1944, a proposal had been made to make alterations of the tank and rebuild the prototype. These changes, however, did not happen. The Army ended up disbanding the program due to the need for such a tank no longer existing.

The construction of the suspension, hull and secondary turrets of the Mi-To finished in early 1943. The chassis weighed 47 tons empty without additional armour. The tank weighed 97 tons when configured for mobility and defensive tests, but this did not include the weight of the turrets. The Mi-To weighed 120 tonnes when the three auxiliary turrets and other pieces of armour plating were installed. The primary turret and additional 75mm plating would have given the tank a weight of 150 tonnes, as the original design intended. 

Interior module placement of the O-I tank. It can be seen here the 4 turret placement in its correct figuration. One of various drawings purchased by Finemolds in 2014. (Source: author's collection)

Removed parts were transported by truck to the Sagami Arsenal. All parts were covered by a wooden box at the rear of the trucks to hide components, and prevent citizens or potential spies from discovering the super heavy project. The main hull was placed on a 30 ton trailer, and towed by a 13 ton tracked towing tractor. The turrets were towed by Type 95 Ha-Go light tanks and a Type 97 Chi-Ha. Moving all the parts from Mitsubishi’s construction center to the arsenal took 19 hours. The tank was not completely disassembled until June 9th. Once at Sagami, re-assembly began on July 1st. Once completed on the 20th, the trials were scheduled to begin on the 1st of August.

Weight distribution of the prototype.
(Source: author's collection)
On the day of the trial, high ranking officials in the Army were present to witness the first Japanese superheavy tank in motion. The Mi-To received a name change to O-I to follow Japanese naming convention that was standard. Tomio Hara, head of the Sagami Army Arsenal, was also present for the trials. 

Conditions for the trial were poor. Weather during the previous days caused the soil to soften, and the terrain became uneasy. The road at the proving grounds was narrow, and when the O-I began moving down the road, the tank started to drift. This damaged the roadwork, resulting in the tank sliding off the road and into the soil. A change in the trial’s planning focused on the O-I traversing around in the dirt next to the facility it was kept in. Due to the surrounding buildings, its top speed could not be showcased, and traversing was difficult. The weight of the vehicle caused the land to start sinking in during pivots. The O-I reached the first three gears in its mobility test. At the end of the first day of the trials, the O-I had been put back into its garage, and the engineers noticed that the seventh lower track roller’s bearing was damaged, and it was caught between the drive sprocket and the outer plate skirt. The outer plate had broken through, and the gear of the drive sprocket broke apart into two pieces.  The wheels were of cast construction and lacked rubber lining. During movement, the O-I chassis experienced severe rocking motions. Heavy duty shock absorber assemblies were placed inside the vehicle. The top and bottom of each shock absorber’s spring were connected to the suspension by two arms extending from the front and rear.

Engineer's notebook pages documenting the events of the O-I's mobility testing. The engineer's notebook was part of the collective notebooks documenting the O-I's development history. These were purchased in 2014 by Finemolds.
(Source;日本の重戦車 2016年)  

From August 3 to August 8, the engineers began repairing the damage that had been done to the tank. 32 of the total 64 bearings were broken during the tests due to the bearings having insufficient strength. The excessive weight of the O-I caused the entire right side of the tank to collapse when at an extreme angle. Engineer Shigeo Otaka recalls the O-I undergoing immediate repairs to continue its trials. However, due to the steep cost of the repairs, they were put on hold for over a year, and the tank stayed at Sagami Arsenal until March of 1945. The war was nearing its end, and the usefulness of the O-I in Manchuria began to fade. The Soviet Union was no longer Japan’s primary enemy, and the development of more powerful anti-tank guns by the Soviet Union made the armour protection of the superheavy tank obsolete. The project to build a single prototype unit came at a steeper expense than originally thought possible, and mass production was not realistically possible for Japan during wartime. The project was thus deemed a failure.

Fake Type4 / Type5 Superheavies

In the summer of 2015, the video game company Wargaming Limited introduced the Japanese super heavy tanks to their game. Of these vehicles, two tanks that had been added were called the Type4 and Type5. These super heavy tanks represented Japan’s final experimental super heavy tanks of the war. According to Wargaming Limited, the tank’s histories were described as follows:

"This is it. The top of the Japanese super heavy tank line. The Japanese Maus. The Type 5 Heavy was designed to defend coastal positions and to break through the enemy’s defences, which is reflected in the characteristics of this behemoth. " -

"The Type 5, also known as the Type 2605, was one of the variants of the O-I superheavy tank, developed during WWII. The vehicle was planned to be used for breaking through fortified enemy lines and for coastal defense." -

Both Type4 and Type5 published by the video game company Wargaming Limited. (

Supposedly, the Japanese decided to procure more super heavy tanks after the failure of the O-I’s development. The intentions of these new tanks were described as attempts at coastal defenses against Soviet invasions. By 1945, the Japanese army resorted solely to the defense of the home islands and remaining territory. However according to Wargaming Limited, these tanks were aimed for offensive operations.

The company released documented Soviet reports on captured Japanese super-heavy project plans in late 2013. These materials claimed to have been procured by a Soviet engineer by the name of Grigoriev belonging to an engineering division. A report on the findings were documented in a Soviet written report titled “Cooling of Japanese heavy tank projects”. In the file, it is claimed that two new super-heavy tanks were designed with intentions of mounting German BMW 12 cylinder engines, as opposed to the O-I’s Kawasaki 550 horsepower (rated to 600) engines. Further details were reported of advanced ventilation systems used on the tank design. A second variant of the tank existed with a 35% size reduced engine bay. The vehicles were claimed to have weighed over 200 tons. The engineer claims to have seen a wooden mockup of the vehicles in Manchoukuo with experimental engines. During the Soviet invasion, findings of the built turret of these tanks existed, with naval guns mounted for coastal defense.

Supposed technical drawing by Soviet engineers in 1945 detailing the cooling system of the Type5 heavy tank. These were
presented by Wargaming in 2013, showing no source material suggesting the authenticity of the claims.

Historical Context

The authenticity of these Soviet documents comes into question due to various errors in their claims. After the failed testing of the O-I prototype in 1943, the Army expressed concerns regarding the usefulness of the vehicle, considering the war’s deteriorating condition. A brief proposal at revising the tank’s mechanical issues had been proposed in 1944, however, this was quickly denied. After this, the Japanese army focused solely on tank projects aimed at the defense of the home islands against allied invasions. Super-heavy tank concepts were not pursued and forgotten with the O-I’s cancellation.

Questions as to how the Soviet military obtained information on private tank projects conducted in Japan leaves skepticism. Research and development of tanks existed solely in arsenals in Honshu, not outside the home islands inside Manchoukuo. Documents regarding the tanks cooling system do not exist inside Japan’s national archives. It is unlikely that they would be conveniently placed in the area of the Soviet invasion of Manchoukuo to obtain. 

Photograph of two Soviet soldiers infront of a captured Japanese fortification in 1945. Location of the photograph is Koto fortress on the coast of Manchoukuo. The turret has been claimed to have been that of the O-I/Type4 and 5 heavies. However this has not been proven. (

Commonly discussed is a photograph from the Soviet invasion of Manchoukuo in 1945, showing Soviet soldiers posting for a photograph in front of a captured Japanese bunker defense. This bunker mounted a large steel turret equipped with a 140 millimeter coastal naval gun. The photograph is taken at the Japanese fortress of Koto on the Manchurian coastline. Soviet forces captured the fortress containing one of the last remaining Japanese holdouts in the region. Various coastal defense turrets were positioned at Koto fortress for the invasion. These turrets consisted of thinly armoured, housed naval batteries alongside the Manchurian coastline. While the turret photographed is likely to have been real, it is also likely that it belonged to no such tank project. The Imperial Japanese army would not have used Japanese naval guns in their vehicles. Due to standards and regulations, only army equipment was permitted to be used within the army. 


- 第8編  技術関係事項Ref. C15120358000
- 日本の戦車 (1978年) 
- 日本の重戦車 (2016年) 
- 日本陸軍の火砲 (2012年)
- 戦車と戦車戦 (2012年)
- 帝国陸軍陸戦兵器ガイド (1997年)
- 日本陸軍超重戦車 株式会社学研プラス (2016年)

Thursday, March 5, 2020

Japan's First Jet Plane [1/3]: The Unofficial Development of 'Kikka'

From June 19-20, 1944, a large scale air attack against the US Navy task force that was covering the invasion of Saipan Island in the Marianas ended in a devastating defeat for the Japanese Navy.

The 1st Mobile Fleet, the main carrier aviation force of the Japanese Navy, failed to effectively cooperate with the exhausted land-based 1st Air Fleet which was supposed to offset the US numerical superiority. As a result, the 1st Mobile Fleet launched the attack alone, and was annihilated by the US Task Force 58 which outnumbered it nearly two to one in an event appropriately dubbed by US aviators as the "Great Marianas Turkey Shoot".

This 'decisive battle' Operation Mk. A (ア号作戦) ended quite decisively in favor of the American forces,  with the loss of the Japanese fleet carriers "Taihou", "Shoukaku", and "Hiyou" along with the grand majority of Japan's carrier aircraft groups, forcing the Japanese Navy's carrier aviation force into a situation where its reconstruction was impossible. The subsequent fall of the Marianas triggered the collapse of the Tojo Cabinet and placed the mainland of Japan within the range of B-29 strategic bombings for the first time.

However, despite the heavy defeat, the land-based units of the Navy's Air Service still remained, and the Army's Air Service had not been subject to the annihilation at the Mariana Islands. Moreover, Japan's 'life-line', the southern resource transfer route, was still intact. Though ultimately set for a total defeat, Japan still possessed the ability to continue the war for the time being, and to this end, the characteristically Japanese plan for a 'decisive battle' against the US Navy was borne once again.

It was plainly clear to the Japanese military that the location of this air battle would be the Philippines, where the US forces would undoubtedly move in to sever the aforementioned southern resource transfer route, by estimation, about 6 months after the fall of the Marianas Islands. The Army and Navy launched joint research into the preparation of military forces for this event in August of 1944. This newly planned decisive air battle was recognized as the last time the Japanese military could be capable of dealing a critical blow to the US Navy.

Special Weapons Development for the Decisive Battle

To increase the Japanese air power in the Philippines for the next decisive battle, the Army adopted extreme prioritization measures. Part of this prioritization was the first formal application of deliberate suicidal aerial ramming attacks, to which the Navy followed a similar course. The reason for officially adopting the use of suicidal ramming attacks was a desperate move to make victory achievable, under a situation where few skilled pilots remained, and the ability of production was declining. The first official 'ramming' weapons of each service, the 'To-Gou' plane (modified Ki-67) and the flying bomb 'Maru Dai' (later Ouka) respectively, were ordered into development at the same time in the middle of August 1944.

In conjunction with these independent developments, a strategy to conduct joint research between the Army and Navy on new ramming aircraft was being conceived at this time. The first known concept of these new ramming aircraft appeared in a summary of meetings held by the Navy Aviation HQ on August 18th. The following transcription was recorded:
  Model 2: For use against ships near to the coast.
  Model 3: For loading onto a carrier.
  Model 4: Single-seat fighter rocket airplane.

Due to the deterioration of the war situation
 and the resignation of the strongest
supporter, Hideki Toujou, the design team
 of Nakajima's 'Fugaku' super-heavy
 bomber was dismantled by August 1944.
Of most interest in this instance is the Maru-Dai Model 2, which may have been the earliest name and conceptual appearance of Japan's first jet aircraft, based on its mission statement. However, the designation 'Maru-Dai' was short-lived, and the concept of such aircraft emerged again as 'Prosperous Country' (興国)weapons.

On August 25th, a team of surplus Nakajima Airplanes engineers from the suspended 'Fugaku' bomber project, along with engineers from Kawanishi, were convened at the Kuugishou (1st Aviation Technical Arsenal, at Yokosuka) for a research task. The Nakajima team consisted of Engineer Kenichi Matsumura and 5 other individuals, and a second Nakajima team of four people was convened on September 1st. At these meetings, multiple types of special weapons were explained, but the main task given to Nakajima and Kawanishi's engineers was the research of a simplified attack plane for the upcoming Philippines decisive battle, called the 'Prosperous Country Mk. 1 Weapon'. However, the basic concept of the 'Prosperous Country Mk. 2 Weapon', a new grade of special attack plane utilizing the novel technology of jet propulsion, was also explained during this time at the Kuugishou.

Jet-propelled Special Weapon 'Prosperous Country Mk. 2'

It may be questioned why an advanced new technology such as the jet engine would be put into the development of special attack weapons. The explanation is in the reality of the situation of Japanese jet engine development at the time of August 1944. The only engines becoming available at this time were of poor thrust output, so creating a fighter-jet like the 'Me 262' was impossible. Thus, the only practical application of these engines, more so when considering the ease of manufacturing and fueling jet engines compared to their piston counterparts, was into small special attackers.

Engineer Shigeru Arai, chief of the Nakajima Koizumi prototype factory, traveled to Kuugishou on September 12 to prepare production plans for the Nakajima model of 'Prosperous Country Mk. 1'. On the 14th of the same month, Kenichi Matsumura returned to the Nakajima Koizumi factory and made an interim report. Matsumura's interim report describes three types of special weapons planned, the simple attack plane 'Prosperous Country Mk. 1', current planes remodeled to carry 800kg bombs, and 'Prosperous Country Mk. 2'.
"Research on TR30, TR10 Model is Mk. 2 Weapon Nakajima Plan"
TR10 Centrifugal Turbojet
Engineer Matsumura's words may suggest that the Mk. 2 Weapon was planned by other companies than just Nakajima, and moreover, that outside of Nakajima's plan, this plane may have not necessarily been a jet at all. However, no other plans than Nakajima's are known to be recorded. Most importantly, this interim report explains that the first jet engines planned for the Mk. 2 Weapon were the TR10, and more unexpectedly, the TR30.

The TR10 was the improved model of the Japanese Navy's first jet engine, the centrifugal turbojet TR, which had been tested by 3 examples in the spring of 1943. Using what had been learned from the failures of TR, a mass production of 70 TR10 engines for rapid trial and error testing had been planned from July to August 1944, but failed to materialize. The first TR10 engine was finally trial-run in September of that year. A variant known as the TR10 Kai also finished design in this month, by implementing 4 axial stages in front of the centrifugal compressor, and a further strengthened model of this, TR12, was being planned. The intended thrust output of TR10 was 300kgf, with a maximum RPM of 16,000 (TR10 Kai: 15,000), and the overall weight was 250kg.  TR10 and its derivatives were deeply flawed engines that never escaped issues of performance and breakage. It is consequently easy to understand the fundamental issue with the TR30, which was in an almost exact sense, the TR10 Kai engine upscaled by a factor of 3. The estimated thrust output was a formidable 850kgf, but it can be said that this engine was already destined to fail before it had even reached the prototype stage.

Tsu-11 Axial Motorjet
Engineer Matsumura's interim report also explains that the Navy was aware that the necessary jet engine would not be available in time for the prototyping of Mk. 2 Weapon, and a tentative plan was prepared to provisionally equip prototypes with 'Hatsukaze' engines, and later swap over to the intended turbojet engines when available. The meaning of this 'Hatsukaze' provisional plan was to initially outfit the aircraft with Tsu-11 engines. Tsu-11 is a low-power motorjet engine often informally referred to as the 'Hatsukaze Rocket'. The simple configuration of this engine is a 160hp Hatsukaze piston motor driving a single-stage compressor, followed by a combustion chamber to the nozzle. The static thrust output is 230kgf. There were plans for both two and four of these engines to be equipped.

On September 16, the head of the Kuugishou, Vice Admiral Misao Wada, requested for the head of the Koizumi prototype factory, Shigeru Arai, to rapidly commence development on the Mk. 2 Weapon. Vice Admiral Wada instructed Arai to research the possible preparation of production, with 3 planes in December, 100 the next January, 200 in February, and 300 in March. The unrealistic demand for production to begin in just three months clearly introduces the ambitious encouragement of Vice Admiral Wada, who became the leading character in the advancement of development for the Mk. 2 Weapon.

A document concerning 'Army and Navy Emergency War Preparation' was dated on the 25th, relating to the 'Prosperous Country' weapons development.
"Considering the current national power and the tension of the current war situation, the Army and Navy will unite and strive for the urgent development of certain victory decisive battle military forces."
Attached was a priority ranking of 'Prosperous Country' weapons. The first rank, "Emergency Procedure 1" was assigned to the 'Mk. 1 Weapon' and 'Current Planes Remodeled' (Shiden, Raiden, Reisen with large bombs and RATO). The second rank, "Emergency Procedure 2" was attributed to the 'Mk. 2 Weapon'. It can be seen that the importance of the Mk. 2 Weapon was lower than the rest at this time. The engine is listed as both Hatsukaze Rocket and TR10.

Air Technical Arsenal Chief
Vice Admiral Misao Wada
On the same day, a construction meeting of the Mk. 2 Weapon held at the Kuugishou between the Navy and Nakajima confirmed that the previous plan instructed by Vice Admiral Wada could not be achieved. The Navy's production plan was greatly scaled back to expect the beginning the production of 'Hatsukaze Rocket'-equipped aircraft in January 1945, and the first 3 'TR12'-equipped aircraft to be delivered in February, 7 in March, 30 in April, and 100 in May. The respective TR12 engines were to be produced with 10 in January, 20 in February, 100 in March, 300 in April, and 750 in May.

Why is it, now, that the engine of the Mk. 2 Weapon was said to be 'TR12' rather than 'TR10'? There is no exact date of this change, because as the TR12 itself is only a slight modification of the 'TR10 Kai', it can be considered a natural gradient. When the improved 'TR12' exists, there would no longer be a reason for the 'TR10' to be equipped. Anyway, the main engine planned for the Mk. 2 Weapon was inaugurated the TR12, as recorded in the October 5th revised version of the previously mentioned document of 'Army and Navy Emergency War Preparation'.

On this day it was also reported that the primary drawings of the TR12-equipped aircraft were to be completed within December, but Vice Admiral Wada was holding onto a highly optimistic plan, and on October 8th gave a notice to complete the airframe outline drawings along with a wooden mockup during October, test fly the TR12 mounted to a Type1 Medium Attacker (G4M) the next month, and complete 30 planes in December. Relentlessly, the Vice Admiral ordered research to be advanced on this possibility, keeping the design department at Nakajima extremely busy.

Frontal projection of the Bell P-59 Airacomet.
Very little is certain about the design situation of the preliminary development stage for this airplane. However, according to the recollection of engineers involved in this project, working under Chief Engineer Kenichi Matsumura, there was initially a concept studied in which two TR10 engines were mounted side-by-side within fuselage, perhaps similar in format to the Bell P-59 Airacomet. Of course, there was also the alternative plan to mount each engine in an under-wing nacelle referential to the 'Me 262', which was ultimately adopted. Although the former plan was recognized as superior in terms of aerodynamics, the latter was more practical from the standpoint of technological simplicity and productivity. Around this early period of time, the top speed of the Mk. 2 Weapon while tentatively equipped with Hatsukaze rockets was projected to be about 556km/h at sea level with an 800kg bomb, and about 593km/h with a 500kg bomb. The airframe's empty weight was to be 1,480kg, overall weight 3,630kg, strength 7G, and top speed with TR12 engines, about 639km/h at sea level. It was also planned to equip takeoff assistance rockets (RATO).

A similar layout should not give the perception that this plane was a copy of any other aircraft. If it was not expressed clearly enough thus far, this 'Mk. 2 Weapon' was a purely Japanese design, stemming from a unique Japanese requirement. There is a common theory in western sources that upon learning of the German jet fighter 'Me 262', the Navy ordered a copy into development as this aircraft. When observing the actual situation, this theory is baseless, though a similar course was followed by the Army with the Ki-201. The true attribution of the Me 262 to the Japanese Navy was the inspiration to further advance the development of jet engines, due to the success of practical usage in Germany.

Nakajima Airplane Company prepared the arrangement of staff reassignment for the production of Mk. 2 Weapon on October 20, and Vice Admiral Wada visited the Koizumi factory the following 22nd, where he was informed of current circumstances with Mk. 2 Weapon design, including the shape of the wing planform, issues with the undercarriage, and issues with mounting the 'Kuurai Model 7' gliding torpedo. So, by late October, the shape of the main wing was still undecided. The equipment of a gliding torpedo to this plane is a strange matter that was never mentioned again, so it seems to have been dropped. On the 23rd, Vice Admiral Wada was informed of the state of progress toward the prototype. Returning to the Kuugishou, he dispatched a staff member to inform the factory that a plan research meeting would be held on the November 9.

The Plan of 'Mk. 2 Weapon' Becomes Formal

The agenda of this meeting attended by representatives of the Navy General Staff, Navy Aviation HQ, Kuugishou, and Nakajima was "the value of this weapon, issues with TR, and issues of bomb quantity". From this content, it can be seen that even while Vice Admiral Wada was enthusiastically promoting Nakajima's research, the Navy itself still needed to receive explanations on the importance of the 'Prosperous Country Mk. 2 Weapon' in general. In fact, despite currently being past the date by which, if following the earliest plans, actual examples of this plane would have been completed, the Navy had not even issued a formal instruction of prototyping, nor a plan request, to Nakajima. 'Prosperous Country Mk. 2' was still entirely unofficial. The fact that it had even progressed this far can be attributed almost entirely to the orders of Vice General Wada alone.

On the 15th of November, a meeting was held regarding 5 types of engines that could be used for the Mk. 2 Weapon, and finally on December 9th, a second plan research meeting was held which confirmed the current plan of the aircraft would be proceeded. The production schedule was moved to the end of February 1945 for unit 0 (strength tester), and the end of March 1945 for units 1-3. Nakajima also submitted the required personnel (approximately 950 people) for prototyping and production, floor space (800m² for parts, 1,150m² for assembly), estimated materials for machinery, etc. At last, on December 25th, an official plan request was drafted by the Navy at the Kuugishou, and the contents of this draft are written below.

Experimental "Kikka" Plan Request Draft
December 25 1944
1. Purpose
To obtain a land-based attack plane suitable for attacking enemy warships
coming in close range, and suitable for large quantities of production.
2. Form-Type
Turbine Rocket Twin-Engine Monoplane Model
3. Main
As small as possible, dimensions at the time of [wing] folding
5.3 meters width, 9.5 meters length, and 3.1 meters height, or less.
4. Equipped
TR12 Model x2
5. Crew
1 person
6. Performance
Flight Performance 

(with a No. 50 bomb
unless specified).
1. Top Speed
275 knots (579km/h) at sea level
2. Cruising
No. 50 bomb condition,
maximum 110 nautical miles (204km) sea level 
No. 25 bomb condition,
maximum 150 nautical miles (278km) sea level
3. Climbing
No particular numbers shown, but not too little at the
time of retracting undercarriage immediately after takeoff.
4. Takeoff
   Run Distance
Within 350m in no wind,
when using takeoff acceleration devices.
5. Landing
Within 80 knots (148km/h) at light load.
Stability and
To be able to turn easily, and have basic aerobatic potential.
7. Strength
Category III
8. Armament
Bomb Armament
Type2 Large Bomb Suspension Rack
Type97 Twin-Drop Controller
Tentative Name Type4 No. 50 Mk. 8 Bomb or Type3 No. 25 Mk. 8 Bomb
Radio Armament
Type3 Air Mk. 1 Radiotelephone (Reciever)
9. Defense
1. Pilot
Equipped with 70mm thick bulletproof glass at the front of the cockpit,
and 12mm thick bulletproof steel plates below and behind the cockpit.
2. Fuel Tank
To be 22mm thick inner-bag type bulletproof tank.
10. Instruments
Tachometer, Fuel temperature gauge, Fuel pressure gauge,
Oil pressure gauge, Oil temperature gauge.
Speedometer, Pitot tube electric heater, Altimeter,
Artificial horizon, Type0 High Altitude Compass Model 1.
Others to be determined at mockup inspection.
11. General
Type0 Parachute, Automatic fire extinguisher, Mk. 3 Dry Storage Battery,
Lifesaving Raft Model 1, Reserved weight 30kg.
12.  If the characteristic performance is satisfied, aim to extend the cruising power as much as possible.

This plane once known as the 'Prosperous Country Mk. 2 Weapon' was now officially designated by the Navy as 'Kikka'(橘花). Kikka is a name literally translating to 'Tachibana Blossom'. Tachibana is a type of orange with deep cultural significance in Japan, being associated with immortality in ancient mythology, and famously having a symbolic place on the western flank of the Shishinden at the Kyoto Imperial Palace.

In terms of designation nomenclature, the name 'Kikka' denotes the plane as a special attacker of the same purpose as the "Ouka", "Touka", and "Baika" special ramming planes. However, it is noteworthy that the purpose statement of this plane does not deliberately mention ramming attacks, and in addition, the  'Type3 No. 25 Mk. 8' and 'Tentative Name Type4 No. 50 Mk. 8' bombs are bouncing bombs which skip over the surface of the sea, and also do not necessitate a ramming attack.

On the same day of creation, this draft was handed over to Chief Engineer Kenichi Matsumura at the Kuugishou, and he brought it back to Koizumi factory. With the delivery of this draft, after almost 4 months of unofficial development encouraged by the enthusiasm of Vice Admiral Wada alone, the plan for ‘Kikka’ was finally officially promoted by the Navy. The year changed, and on January 4th, when the last embers of the Philippines air battles that the Kikka had been concieved for were extinguished, a 'Kikka' Plan Request Council was held at the Kuugishou, and the contents of this request draft were finalized.

While the 'Prosperous Country Mk. 2 Weapon' was too late for the Philippines decisive air battle, it was now to be further promoted as the 'Kikka' for the next decisive battle of the Japanese home islands.

Part 2 will cover the development of 'Kikka' as an official project, from January 1945 to the end of the war.