9/5/21
This article will encompass the whole history of the Chi-To medium tank to the best of my ability, and also describe the general policy shift of Japanese armored vehicle development which directly affected its course. Due to the amount of information, this has become much longer than I originally anticipated (over 10,000 words...). Hardly a digestible article at this point, this page at least serves as a collection of information.
Please note that the information available for this vehicle is incomplete, and there are various theories that are still under research by many people, things may be subject to change. In this article, I am mainly operating on the theory that only one vehicle was ultimately completed. Based on the verifiable data I have encountered, this seems the most likely. However, there is also considerable (but, mostly anecdotal) evidence that multiple examples may have been in operation, which will be explained.
If corrections are found to be necessary, they will be noted below.
Background
Shift to High-Velocity Tank Guns
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| Soviet BT tanks at Khalkhin Gol. |
All Japanese tanks in service at this time, chiefly the I-Gō, Ha-Gō, and Chi-Ha, had been developed with pure infantry support in mind. As a result, they were armed with medium velocity 37 or low velocity 57-millimeter guns that were intended to knock out enemy gun emplacements to support the attack of infantry. During the battle at Nomonhan, they were pitted against Soviet tanks and anti-tank guns of 45 millimeters with a high velocity of 760 m/s.
The difference in performance was undeniably felt in battle, but it did not cause a total policy shift in the role of tanks at this time, because it was believed that upcoming quick-firing guns would be sufficient for anti-tank purposes when equipped. There was truth to this at the time, as even the current towed Type 94 37 mm Gun proved adequate against Soviet armor at ranges exceeding 1 kilometer, knocking out hundreds of vehicles. There was already an experimental 47-millimeter quick-firing gun currently under development, comparable to the Soviet gun, and this would become the basis of the next Japanese anti-tank gun - the Type 1 Motorized 47 mm Gun.
Successor to the Chi-Ha
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| Type 1 Medium Tank 'Chi-He'. |
The development of the next vehicle to succeed the Chi-Ha as Japan's primary medium tank began in 1940: that is, the 'Chi-He' (6th medium). In 1941, it was already designated the 'Type 1 Medium Tank' before even a prototype was made, and the full-scale wooden mockup was completed in August of that year.
As a direct evolution of the Chi-Ha, the chassis of the vehicle remained highly similar. The main improvements were as follows.
- Armor: Riveted construction of the Chi-Ha was changed to full welding, and the shape of the hull was simplified to facilitate mass production. Frontal armor thickness increased from Chi-Ha's 25 millimeters to 35 millimeters.
- Turret: Changed to a larger and up-armored turret design based on that of the prior experimental (canceled) medium tank 'Chi-Ho'.
- Armament: Equipped with the Type 1 47 mm Tank Gun, which was developed parallel with the new Type 1 Motorized 47 mm Gun. The tank gun has a velocity of 810 m/s, and is the first formal Japanese tank armament with a focus on anti-armor performance.
- Engine: Changed from Chi-Ha's Mitsubishi SA12200VD (170hp) to the Mitsubishi AC (240hp), significantly improving mobility.
The design of the Chi-He was improved throughout 1942 and 1943 as prototypes were made; most notably the armor was increased to 50 millimeters when this thickness became viable. The overly long development process, most likely delayed by the start of the Pacific War, was finally completed in June 1943. Even then, mass production did not start until the year 1944 because of the low priority of tank production compared to aircraft and guns at this time.
At this point, the successor of the Chi-Ha was already obsolete, and the M4 Sherman was deployed in large numbers to the Pacific Theatre. However, work on the next medium tank after the Chi-He had begun three years prior. This is what became the 'Chi-To'.
New Medium Tank (Kō), 'Chi-To' Early Plans
The concept of the next medium tank after the Chi-He first appeared in the weapons research plan of the fiscal year 1941 as the 'New Medium Tank (Kō)'. Research and development work on this tank started in April of 1942 under 2nd Section lead Colonel Kozo Murata at the 4th Army Technical Research Institute (#12, *6).
At the beginning of 1942, only the 47-millimeter gun was considered for applications in medium tanks according to the research policy, and this was initially to be the sole armament of the New Medium Tank (Kō). But by the 26th of August 1942, a report referring generically to the development of a new medium tank gave the following specifications (*3).
Weight: 20 tons
At the outset, the specs of 'Chi-He'
and 'Chi-To' were almost identical (*3).- Speed: 40 km/h
- Armor: 25-50 mm
- Armament: 47K (47 mm gun), MG
- Completion Date (Scheduled): August 1943
- Notes: Some medium tanks may be equipped with 57K [57 mm gun]
As you can see, from this point onward a new 57-millimeter gun has entered consideration for medium tanks. This is a new long, high-velocity gun, considerably different from the short 57-millimeter seen on previous tanks such as the Chi-Ha, long obsolete for the latest tanks at this point.
In September, the new medium tank was now defined specifically as the 'Chi-To' (7th medium), with the placeholder designation 'Type 〇〇 Medium Tank'. The specifications were largely identical to the ones listed in August, with the differences below (*3).
- The main force of tank divisions from FY1944, along with Chi-He
- Armament: A47 (A57) M7.7 [47 mm gun, 57 mm gun considered, and 7.7 mm MG]
As of 1942, the Chi-To plan was a somewhat unambitious development as the direct successor to the Chi-He. The armoring of 50 millimeters at the front is thicker than the 35 millimeters of the early Chi-He plan, but the same as the definitive Chi-He specifications from 1942. At this time, armor plates above 50 millimeters thick were only developed by naval arsenals for use on ships, and 75-millimeter tank armor was not available nor thought to be necessary yet.
The notable direct improvements over the Chi-He were to consist of an electric turret drive, synchronous mesh transmission (probably studied from captured American M3 Stuart light tanks), and hydraulic steering, the latter of which was originally planned for the Chi-He but could not be developed on time. The overall weight of the vehicle was projected at about 20 tons due to the larger chassis and improved systems compared to the 17 ton Chi-He. The engine equipped to this vehicle in the earliest plan is not clear, but it was supposed to maintain a similar level of mobility and top speed as the Chi-He.
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| Type 1 47 mm Tank Gun. |
The primary planned armament during 1942 was the 'Experimental Coaxial 47 mm Tank Gun'. This consisted of a Type 1 47 mm Tank Gun paired in its mount with a Type 97 in-Vehicle Heavy Machine Gun. In all Japanese medium tanks serviced prior, the turret HMG was located at the side or rear due to the issues with installing a coaxial machine gun in the characteristic 'shoulder mount' of Japanese tank guns. This created a situation where the gunner would have to swivel the turret completely around to engage infantry, often to find the target vacant by the time it was.
Solutions to this problem were initially developed for the 37 millimeter tank guns equipped to light tanks (first appearing as the Type 100 37 mm Gun on the Type 98 Light Tank 'Ke-Ni'), and later the 47 millimeter gun for medium tanks. The prototype for this coaxial 47 mm gun configuration was built in November 1942 at the Osaka Army Arsenal and saw trials in early 1943, development was expected to be completed in June 1943.
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| Experimental Motorized 57 mm Gun. A towed anti-tank gun developed from 1941 to combat new heavy tanks. |
The alternative gun under consideration was the 'Experimental 57 mm Tank Gun'. This gun was developed from March 1941 in parallel with the towed Experimental Motorized 57 mm Gun and shared the same ammunition. Both were long-barreled, quick-firing guns intended to deal with the heavier armored vehicles that were expected to appear at the time. The towed version was expected to penetrate 60 millimeters of armor at 1,000 meters with a muzzle velocity of 850 meters per second.
The tank gun was developed in two models, Kō and Otsu, with respective velocities of about 800 meters per second and 900 meters per second. The model equipped to Chi-To at this stage of the plan would be the smaller Kō, which was originally intended for installation in turreted gun tanks, while the larger Otsu was for turretless SPGs and tank destroyers.
The design of the 20-ton Chi-To itself was completed in January 1943. However, by this time it seems that the design was already considered obsolescent due to the flow of development in foreign countries. The requirements were changed: it was now necessary to bolster the front armor to 75 millimeters, and the 57 millimeter tank gun was definitively selected as the armament. The projected weight of the vehicle increased to about 25 tons, and this improved design was started in February.
The Fiscal Year 1943 Research Plan of the Army Ordnance Bureau was established in March, and the Chi-To was now described as having "improved armor and armament compared to the Chi-He while emphasizing mass production to the utmost". At this time, the Chi-To was still the most advanced Japanese medium tank under development. However, when the improved 25-ton design was completed in June 1943 (depending on the material it may be August), there was a total policy shift in the development of future Japanese tanks.
1943 Policy Shift to Anti-Tank Combat
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| The armor of the M3 Stuart was defeated by the Type 1 47 mm Tank Gun at all combat ranges. |
Up until the middle of the Pacific War, the most powerful allied tank encountered by Japanese forces on a regular basis was the American M3 Stuart light tank. The 38-millimeter frontal armor of this tank proved to be a struggle for the Japanese 37-millimeter guns widely distributed at the outset in some conditions but was easily perforated by the tank and ground-based versions of the 47-millimeter gun at all incidences and combat ranges, which were deployed in the first half of 1942.
The sufficient results of the existing Japanese tanks early on in the Pacific War, along with the massive shift of production priority to aircraft and guns, led to a period of relative stagnation in tank development and deployment of new models. The threat of the Soviet Union, which until this point was the main drive behind Japanese armored innovation, had all but disappeared for the time being due to the decision to prioritize the Southern Expansion Plan and the fact that the Soviet Union was now heavily occupied in war with the German Reich.
Nonetheless, the Japanese Army continued to keep the tabs they could on the development of foreign armor and doctrine around the world, using what could be gleaned from intelligence data and the study of ongoing warfare. By 1943, studies of new tank models deployed by Germany and Russia on the Eastern Front along with data obtained regarding new allied armor developments revealed the inadequacy of Japanese quick-firing guns (both those currently deployed and under development) against the latest foreign heavy tanks. Furthermore, the importance of emphasizing anti-tank capabilities in modern armored warfare was realized, as tank armor and firepower rapidly increased.
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| Intelligence page on the Soviet KV-1. A heavy tank with (initially) 75 mm of armor, famous for its resilience early on. |
These studies were implemented in the research policy revision of June 1943, which radically shifted the priority of Japanese tank designs from a focus on 'infantry support' to a focus on 'anti-tank combat'.
First off, the Experimental Motorized 57 mm Gun which had been under slow development since 1941 was now completely inadequate for dealing with the latest tanks of foreign powers. It was suspended, and research began on an exponentially more powerful self-propelled 10-centimeter anti-tank gun. A penetration of 200 millimeters of armor at 1,000 meters was thought necessary to deal with new heavy tanks.
Sweeping changes were made to the medium tank class to prioritize anti-tank combat ability. Two types were now established, a medium tank with a weight of up to 35 tons, and a medium tank with a weight of up to 25 tons. The large 35-ton medium tank was to be equipped with a 75-millimeter tank gun and protected by 75 millimeters of armor. The 25-ton type is the latest Chi-To plan described in the last section, with a 57 mm tank gun and 75 mm of armor. The armor for both types was limited to 75 millimeters at the maximum due to weight limitations and the necessity to maintain good mobility (at least 40 km/h) with the engine power available.
A 75-millimeter long-gun was the largest caliber possible to mount in a rotating turret at this time, but it was thought to be insufficient for dealing with new heavy tanks. To get the necessary 10-centimeter long-gun onto a tank platform, a new gun tank (tank destroyer) with a fixed superstructure was devised. This gun tank would have more protection than the medium tanks for defense against heavy AT guns at long ranges, about 120 millimeters of armor, and be equipped with a tank version of the 10 cm anti-tank gun.
The research policy explains that the 35-ton medium tank and the 10-centimeter gun tank were to become the core of Japan's future armored forces. While the research on the 25 ton Chi-To was continuing as it was 'already being prototyped', its performance was not expected to impress. So, while the Chi-To was originally planned to become the main force of Japan's tank divisions as the successor of the 'Chi-He', it was now considered as merely an auxiliary medium tank to support the larger class.
The research policy of June 1943 was implemented as the FY1943 Research Plan Revision the following month. In this research plan, the 35-ton medium tank concept became the 'New Medium Tank (Otsu), Chi-Ri' armed with the Experimental 7.5 cm Tank Gun (Long), and the gun tank became the 'New Gun Tank (Kō), Ho-Ri' armed with the Experimental 10 cm Tank Gun (Long).
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| 'Chi-To' (37 戦) and 'Chi-Ri' (14 戦) in the 7/1943 research plan revision. |
As of July 1943, the first prototype of the Chi-To was scheduled to be complete in June 1944, and the whole project was to be finished in March 1945. In the next revision of the FY1943 Research Plan which appeared in August 1943, this schedule was slightly adjusted - the prototype was to be completed in March 1944, 3 months earlier. Work on constructing the prototype began at the Mitsubishi Heavy Industries Tokyo Machinery Works (also called the Maruko Plant), using mild steel in consideration of easy procurement and modification.
The Ambiguity of Chi-To's Armament
As previously mentioned, the towed Experimental Motorized 57 mm Gun had been canceled in June 1943, and with it, the tank gun models, which were originally intended for tank destroyers, were also suspended. Instead, the Chi-To was now to be armed with the new 'Experimental 5.7 cm Tank Gun Maru-Shin' (Maru-Shin being the character 新 / shin ['new'] inside the character 〇 / maru ['circle']). This gun was largely similar in performance to the prior 'Kō' model of the 57 mm Tank Gun, but specifically designed for the Chi-To. This gun was expected to complete development in March 1945.
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| Experimental 7.5 cm Tank Gun (Long). |
However, the 57 mm caliber was now widely considered obsolete for anti-armor performance. Even in regards to the Chi-To, which was no longer intended to become the primary Japanese medium tank, there seems to have been dissatisfaction with this gun early on. In the 'Chi-To Corrective Research Implementation Plan' that was made at the beginning of August 1943 (#8), there was already a notation that the gun should be changed to the '7.5 cm (Long)', which is an abbreviation of the Experimental 7.5 cm Tank Gun (Long) developed for the Chi-Ri. Also, it was instructed to conduct research on thick steel plates and cast steel for the gun turret, which seems to be the first mention of the large cast turret that will be introduced later.
Briefly, the Experimental 7.5 cm Tank Gun (Long) was a 75 millimeter, 56 caliber length tank gun that started research in July 1943 after the policy revision. It was developed with a planned muzzle velocity of 850 meters per second and was based on the Type 4 7.5 cm Anti-Air Gun, which itself was based on the Swedish Bofors 75 mm Lvkan m/29. The expected penetration at 1 kilometer was 80 millimeters. For installation in the Chi-Ri, this gun was also equipped with an autoloading device.
When the Army 1st Technical Arsenal put in an order for eleven 7.5 cm tank guns from the Osaka Army Arsenal on April 25th, 1944, six of these guns were to be allocated to the Chi-To, once again affirming the strong push to use this gun. Then in May, days before the completion of the Chi-To prototype, there was an instruction that the 7.5 cm long-gun should be equipped for the next 5 prototypes (described as 'prototype production vehicles'), along with a remodeling of the current vehicle being constructed.
The 4th Army Technical Research Institute ordered six large cast turrets for housing the 7.5 cm long-gun, and six sets of armor plates 50 millimeters thick or more from the Osaka Army Arsenal. Plates 35 millimeters or less for the 6 armor sets were consigned to the Japan Steel Yawata Works.
In view of the above, it can be considered that the change in plan to equip a 7.5-centimeter long-gun to the Chi-To was effectively made in April 1944. However, the 7.5-centimeter long-gun was not yet built at this time, so the Chi-To prototype continued to be equipped with the 5.7 cm Tank Gun Maru-Shin for the time being, even though its practicality was effectively out of the question.
Chi-To Prototype No. 1
The prototype of the Experimental 5.7 cm Tank Gun Maru-Shin was completed in March 1944 and was followed shortly by the completion of the Chi-To prototype on May 8th, 2 months behind schedule. The vehicle that came out of the factory at this period had a significantly larger body than the Chi-He, with a length of 6.34 meters, a width of 2.87 meters, and a height of 2.67 meters. The welded turret, vacant of a cannon at this point, was based on the design of the Type 2 Gun Tank 'Ho-I', with increased frontal armor thickness consistent with the hull, 75 millimeters. The chassis was powered by the 412 horsepower Mitsubishi AL air-cooled diesel engine. |
| Representation of Chi-To, 1st Prototype, from 'Japanese Tanks'. |
Basic functional tests were carried out from May 16th to 20th at the 4th Army Technical Research Institute and the Sagami Proving Grounds. The appraisal of the vehicle chassis itself was, overall, excellent. In mobility trials, the tank reached a maximum on-road top speed of 46 kilometers per hour. The new Mitsubishi engine functioned well with sufficient cooling, and in particular, the ease of operating the hydraulic steering mechanism and the function of the synchro-mesh transmission was highly praised in comparison to prior tanks. On the 25th, a display test was conducted for the general manager, and once again good results were obtained.
The Experimental 5.7 cm Tank Gun Maru-Shin was installed in the turret on May 29th, and 89 rounds were fired in the test. During the tests, 5 rivets in the turret were loosened due to firing, which caused problems with rotating on inclined surfaces. Also, the 5.7 cm gun only achieved a muzzle velocity of up to 798 meters per second, which was lower than the planned value of 810 meters per second (#13). Generally speaking, the power of the tank gun was not sufficient even if it could perform as planned, and it was inevitably canceled as only a prototype. The Chi-To in this configuration was no longer adequate for the times.
After final disassembly and reassembly, this tank was returned to Mitsubishi from the 4th Army Technical Research Institute on July 20th, 1944. From this point, a Type 90 Field Gun (likely Type 3 7.5 cm Tank Gun Model II, if true) was allegedly mounted onto the vehicle at Mitsubishi during August and conducted a firing trial (#3). While there is evidence even in primary sources that the Chi-To had considered mounting a Type 90 Field Gun at some point, whether this was actually carried out is a little dubious. The Type 3 7.5 cm Tank Gun Model II prototype was only available for a very short period of time before it was installed on the prototype of the 'Chi-Nu' in September.
Experimental 7.5 cm Tank Gun (Long) Model II, Chi-To's Gun
The first two prototype units of the Experimental 7.5 cm Tank Gun (Long) developed for the Chi-Ri were completed in July 1944 by the Osaka Army Arsenal. The completion test was conducted on July 14th, followed by a specification measurement and functional test from the 14th to 15th, and firing tests at the Otsugawa Range from the 16th to the 18th (#5).
As a result of the firing tests, numerous issues were identified that needed to be remedied. On August 26th, a corrective functional test was conducted to determine the effectiveness of the countermeasures. The results were that the functions and endurance of the gun were now generally good, but the autoloader was still defective. So on the 30th, an agreement was made between the 1st Army Technical Research Institute and the Osaka Army Arsenal on the next corrections to be done, during which the modification to the gun for installation in the Chi-To was also outlined.
1. Experimental 7.5 cm Tank Gun Model I (for Chi-Ri)
The one currently being prototyped (with an autoloader)
2. Experimental 7.5 cm Tank Gun Model II (for Chi-To)
The one where the autoloader is removed from the current prototypes and a new
III. From now on, the 2 guns being prototyped will be treated as follows.
1. One gun will complete research as the Model I.
2. One gun will have the autoloader removed and a counterweight attached as Model II.
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| Experimental 7.5 cm Anti-Tank Gun Model II photographed at the end of the war. There are no photos of the 7.5 cm Tank Gun (Long) Model II by itself, but the AT Gun Model II is extremely similar, also featuring a rear counterweight. |
The second unit of the Experimental 7.5 cm Tank Gun (Long) Model I was selected for remodeling and was completed as the Experimental 7.5 cm Tank Gun (Long) Model II in October 1944 by removing the autoloader and attaching a counterweight. Functional and firing tests were conducted with both guns from the 26th to the 30th at Irago Range, and the elevation mechanism of the Model II was tested. As a result, although the autoloader of the Model I gun was still defective, it was concluded that the Model II gun for the Chi-To was ready for practical use.
Coaxial machine gun mounts were not developed for large Japanese tank guns because of the elimination of the shoulder mount and trigger firing method, which allowed precise aiming of smaller tank guns. The gunner remotely operating a Type 97 in-Vehicle HMG with traverse handles, which only loads from 20 round magazines (and thus, requires very frequent replenishment), would be impractical.
Penetration Ability
The armor penetration of the 7.5 cm Tank Gun (Long) and its anti-tank counterpart is not something that has been definitely established due to the lack of specific data. Still, using the available sources we can get a general idea of its capabilities in certain conditions.
At the start of development, the performance target for this gun was a muzzle velocity of 850 meters per second, with a penetration of 75 millimeters at 1 kilometer. It was estimated by the research council that the actual penetration with this velocity would be about 80 millimeters at that distance. However, the muzzle velocity of the real gun was about 820 meters per second with AP shells. Calculations aside, let's look at the actual data.
The penetration of the 7.5 cm anti-tank gun, which has the same barrel length as the tank gun version, was recorded to be 100 millimeters at 1,000 meters (#7). The type of AP shell used to achieve that number was not recorded, but given the performance, it is likely Type 1 AP. Regardless, it can be seen that the estimation made at the start of development was considerably below the actual ability of the gun, even though the velocity turned out to be lower than planned.
The use of cast armor is in consideration of the main enemy, American tanks, which employed softer cast armor on a large scale. Cast armor is of course inferior to rolled armor steel in general, but it seems that the Japanese standard of cast steel used in this chart is also inferior to the actual strength of American cast armor. According to a Japanese gun training reference created in 1945, cast steel was 20-25% inferior in bullet resistance to rolled alloy steel, but the type used by American tanks was noted to be less inferior than that, although the values were unknown (*19).
If we remove 25% from the values on the chart, we get expected penetration numbers of 150 millimeters at 100 meters, 135 millimeters at 400 meters, and 105 millimeters at 1,000 meters against rolled armor steel, which is roughly in line with other values, if not slightly higher. A slight increase in expected penetration may be due to an even higher inferiority ratio of the cast plate, or potentially the use of stronger shells. To increase the strength of Type 1 AP shells and reduce shattering on impact, versions made out of stronger tungsten-chromium or nickel-chromium alloy were developed, which are usually identified as 'Toku Kō' and 'Toku Otsu' respectively. It’s possible that these shells were formally designated as the 'Type 1 AP Improved' (#12). For reference, Type 1 AP fired from the older Type 90 Field Gun achieved 70 millimeters of armor penetration at 1,000 meters, while the ‘Toku Kō’ shell fired from that gun achieved 85 millimeters at the same distance, an increase of about 20% (#11).
Anyway, from the report featuring the chart in question, it was concluded that the gun could defeat the inclined front armor of the M4 Sherman at ranges of up to 300 - 500 meters, and any side, rear, or flat frontal areas at 2,000 meters and more. 300 - 500 meters may seem a bit low for the values this gun achieved, and there is a reason for that. In this document, the armor of the Sherman is somewhat overestimated at 65 millimeters angled 35 degrees from the horizontal. The shells of Japanese tank guns were not capped, so a steep angle such as this would decrease penetration significantly. However, the actual frontal angle for nearly every Sherman with 65 millimeters (2.5 inches) of armor was only about 45 degrees from the horizontal. The sole exception being the M4A1(76)W, which can reach 35 degrees from the horizontal in specific areas due to the rounded hull.
There was also a shell called 'Type 4 AP' produced at the end of the war, which is a solid shot round. Because there is no internal cavity for explosive filler, the weight and density are higher than the Type 1 AP shells, and the penetration will logically be higher. This shell was used in some of the firing trials, but there is no known penetration data as of now.
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| 'M4 Sherman' vs 75 mm Tank Gun (Long), Type 1 AP |
Situation up to the Next Vehicle
Before explaining the second Chi-To prototype, first, we must return to the six cast turret and armor sets ordered by the 4th Army Technical Research Institute around May 1944.
As briefly noted before, there were five more prototypes of the Chi-To scheduled to be completed after the first mild steel unit. This can be confirmed by Sagami Arsenal's production situation, where as of December 1944, five more Chi-To tanks are planned to be completed in the last 3 months of the fiscal year 1944, which is January to March 1945 (*8). Also, it can be confirmed that Yawata Works completed their share of the armor in the first half of the fiscal year 1944, which is April to September 1944 (*12). But why then, were six sets of tank armor produced in 1944 when only five more prototypes were scheduled?
Well, as stated prior, the concept of equipping the Chi-To with the 7.5 cm gun in a large cast turret seems to have been floated, surprisingly, since August 1943. This is well before the completion of the prototype. Also, when the decision was made to equip the 7.5 cm gun in early May 1944, it was explained as "In order to equip the 5 prototype production vehicles and the newly remodeled vehicle with a large caliber gun in response to the state of the war...", which seems to express that the first prototype would also be remodeled to equip the 7.5 cm long-gun.
Of course, it is known that the prototype was completed initially and tested with a 5.7 cm gun, but the desire to remodel it even before it was completed explains the sixth cast turret order. As for the sixth set of hull armor, since the first prototype of the Chi-To was completed out of mild steel, it may be that there were also plans to renovate the entire vehicle with bulletproof tank armor. In the production plan for January-April 1945 (*9), it can be seen that the plan changed to complete 6 Chi-To vehicles from January to March, so it seems that the remodel of the first prototype was added to the plan.
It is important to consider that the first vehicle was most likely remodeled because it calls into question whether the 'second prototype' equipped with the 7.5-centimeter tank gun was an entirely new chassis or simply a remodel of the first vehicle. In my opinion, there is not enough information to completely confirm either way, but the next tank will be referred to loosely as 'prototype no. 2' just for differentiation, which is the common theory.
Chi-To Prototype 'No. 2'
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| Chi-To prototype armed with the Experimental 7.5 cm Tank Gun (Long). The only photo in 'operating' status, its origins are unknown - it may be only a model. |
The hull of this vehicle was the same layout as the first prototype, but the turret is a much larger structure formed by bolting/welding together 3 cast pieces (the rear, and each side), the roof plate, and a 75 millimeter plate at the front. This was the first application of cast steel in Japanese tank armor, employed with the intent of easing mass production.
However, due to the infancy of Japanese steel casting methods compared to other countries, a single-piece casting of the turret was not possible. Even when casting the turret in three pieces, there was great difficulty and each piece often ended up centimeters off of the construction drawings. For these reasons, the actual production benefit of casting turned into a detriment, which is probably the reason that a unit could not be assembled until February 1945 (the original schedule was September 1944).
The testing of this vehicle was scheduled to take place from February 26th to March 30th, including basic tests, mobility testing from Hakone to Gotemba, a firing test at the nearby Fuji-Susono Training Grounds, and ballistics tests at the Irago Range. At the firing trial in Fuji-Susono on March 9th, 24 high explosive shells and 2 anti-aircraft sharp nosed projectiles were fired, and as a result, it was judged that the automatic opening/closing mechanism of the breech block was defective. Also, from this point onward, the name of the Experimental 7.5 cm Tank Gun (Long) Model II was changed to 'Experimental Type 5 7.5 cm Tank Gun (Long) Model II'.
After completing the trial at Fuji-Susono, the Chi-To was loaded onto a train car on March 16th and unloaded at Tahara Station. From there, the tank was driven to the Irago Range about 25 kilometers away. The firing tests started the next day, and 72 Type 1 AP shells and 68 Type 4 HE shells were fired during the period. After completing the testing of the gun mounted to the Chi-To on the 19th, the turret was removed and installed onto a Type 3 Medium Tank 'Chi-Nu' chassis the same day to ascertain the practicality of using the gun on this vehicle; this modification has been called the ‘Chi-Nu Kai’. Surveying the results on March 20th, the Type 5 gun testing on both the Chi-To and Chi-Nu Kai was excellent with no issues. However, as there were plans to mount the gun onto the standard ‘Chi-Nu’ turret, the Type 5 gun’s design would need to be modified for production.
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| 'Chi-Nu Kai' is a name for the Chi-Nu vehicle which mounted the Chi-To turret. There is no official record of this designation, and it was most likely made in posterity for simplicity. |
With trials completed, the Chi-To was loaded onto a train car at Tahara station on the 21st and returned to Mitsubishi Heavy Industries Tokyo Machinery Works. The subsequent activities of the tank are not recorded for several months.
In July 1945, a Chi-To was reported to be under the ownership of the Chiba Tank School in Chiba city, and a practical test (equivalent to service test) was conducted this month at Sankakuhara. In the mobility test, even 5 tons heavier due to the new cast turret and 7.5 cm gun, the excellent mobility of the Chi-To in comparison to its weight was again affirmed - the hydraulically-assisted steering was said to be light and easy enough to operate with a single finger. A firing test was then planned to take place at Katakai Beach on August 15th, but the war ended that day and it was not carried out (#6).
Variants
The research of variants based on the chassis of the Chi-To began very early into its development. This is because the Chi-To was initially conceived as the future main-line Japanese medium tank - just as the Chi-Ha had formed the basis of most auxiliary tanks during its time, the Chi-To chassis would facilitate the next generation of these vehicles. By the time of September 1942, when the Chi-To was still a 20-ton design with a 47 mm gun, the basic outlines of several variants had been laid out for future research (*3).
However, by the FY1943 research plan of March 1943, only the Ho-Chi gun tank seems to have progressed to concrete development. By this time, the project name was 'New Gun Tank, Ho-Chi', and the specifications changed slightly. In addition to the 7.5 cm gun, it now included the possibility of mounting a 105 mm howitzer, described in the plan as only the '10 cm Tank Gun' with a low velocity of about 330 meters per second. The project completion date of the Ho-Chi was scheduled to be September 1945 at this time.
In the revision of the research plan that appeared in July 1943, the tank appears again as 'New Gun Tank (Otsu), Ho-Chi'. 'Otsu' was probably added to distinguish it from the other New Gun Tank that appeared at this time, the Ho-Ri, which was designated 'Kō'. However, in the following August plan revision, Ho-Chi disappeared. As such, the Chi-To's first prominent variant was suspended far before a prototype could be constructed.
According to secondary sources, the name ‘Ho-Chi’ was later re-used to designate the Type 5 15 cm Self-Propelled Gun, which is an entirely different vehicle based on the Chi-Ha chassis, but this has not been confirmed.
Experimental 10 cm AT-SPG, Ka-To
The requirements for the 10 cm anti-tank gun were initially to defeat 200 millimeters of armor at a distance of 1,000 meters. However, in consideration of development time, the requirements were eventually pared down to a muzzle velocity of about 900 meters per second. This was expected to penetrate 150 millimeters of armor at 1,000 meters.
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Ka-To side view schematic. |
The armor plates for the Ka-To were built at the Yawata Works in mid-1944, and the prototype vehicle itself was initially supposed to be completed in November 1944. But neither the vehicle nor the gun was completed by 1945, and the chassis was delayed to June 1945. Two of the 10-centimeter anti-tank guns were finally built in May 1945, but the vehicle was never completed. At the end of the war, the parts for the Ka-To were at Mitsubishi's Tokyo Machinery Works, and the chassis was considered to be '50% complete' (*20). According to the 4th Army Technical Research Institute, the exact situation is that the parts were only prepared, and the engine was completed (*23).
Mass Production Plans (How Many Chi-To Were Built?)
Chi-To Production Situation
As explained prior, just before the first unit of the Chi-To was built in May 1944, the plan was to complete an additional five prototypes (described as 'prototype production vehicles') and also to renovate the first prototype to the new standard cast turret and 7.5 cm gun already decided.
However, Sagami's production report from December 22nd, 1944 states that while one vehicle was planned from April-November 1944, it was not completed. Mitsubishi similarly did not record the construction of the first Chi-To in their tables prepared at the end of the war. Apart from simply an error in the records, it may be interpreted that due to the specifications changing before the first vehicle was completed, it was not considered 'finished' in its 5.7 cm gun-equipped form. Regardless, a sixth prototype (which seems to represent the renovation of the first vehicle) was subsequently added into the production plan for January-April 1945 made on December 26th.
Then, the 'second prototype' was completed in February 1945, which is officially recorded by Mitsubishi, but this may actually be the remodeling of the first prototype as explained in the section 'Situation up to the Next Vehicle'.
The full-scale mass production plan of the Chi-To first appeared in late February 1945 as a plan for Sagami Arsenal to complete 200 vehicles in the fiscal year 1945. At this time, it was planned for the first 10 vehicles to be completed during September 1945, with 70 more from October to December, and the last 120 during January to March of 1946. |
| By early 1945, the production of the advanced 'Chi-Ri' was suspended. |
By the time of the next production report in late March, it seems that the priority of Chi-To's production had increased, and a more detailed breakdown including the manufacturer allocation was given.
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| Sagami Arsenal Chi-To Production Plan as of March 31, 1945 |
All production of the Chi-To was to be handled by civilian plants under the jurisdiction of the Sagami Arsenal, with the parent company Mitsubishi producing 170 of the tanks, and Kobe Steel supplementing production with an additional 30 vehicles. The first 5 vehicles were scheduled to be completed by Mitsubishi in August, ramping up to 30 vehicles per month by October (35 including Kobe Steel), before tapering off in February 1946 (*13).
Also, the first 5 production vehicles are indicated by brackets to be carried over from FY1944, confirming that only one of the six prototypes that were planned from January-April 1945 was completed in that period, while the remaining five were delayed. This can be interpreted in two ways: either the first prototype was refurbished during February, or a new vehicle was completed then, and the original prototype remained as it was during early 1945. Thus, either one or two individual prototypes were completed.
The next important record of the Chi-To's production plan after the one established in late March is from an ordnance procurement order given in June 1945. In this brief document, a monthly schedule is not given, but, interestingly, the number of Chi-To to be produced has reduced from 200 to 195. In my opinion, this is probably because of the emergence of the Chi-To mass production model drawings (explained in the next section) around this time. This would render many of the armor plates manufactured in 1944 for the 5 planned prototypes, which were carried over as production models, obsolete.
As for the result of the mass production plan, according to every known production chart compiled by Mitsubishi and Sagami at the end of the war, zero production vehicles were completed. Assuming the 5 vehicles removed from the plan by June were the five carry-overs planned for August, it is consistent with the schedule that 0 vehicles would be completed by August 15th when the war ended. Nonetheless, it is almost certain that production was not proceeding as planned, and it is highly unlikely that a vehicle would have been completed even in the following September.
Mitsubishi reported no incomplete Chi-To vehicles in their inventory at the end of the war, which is actually quite unusual. If Mitsubishi was in possession of no Chi-To parts as of August 15th, and there is no mistake in the record, it would suggest not only a major delay in production but that all parts for the five never completed prototypes were also discarded. Osaka Army Arsenal, where the thick armor plates of Chi-To would most likely be mass produced, was unaffected by bombing raids until the end of the war, and the Yawata Works, which is known to have scheduled the mass production of the thinner plates for the Chi-To, was also relatively unaffected, but suffering from fuel shortages. It may be that the emergence of the Chi-To mass production redesign, which has many different parts than the prototypes, caused an upset. |
| Chi-To turrets were in the Japan Steel Works Muroran Plant at the war's end. |
The summary given by the 4th Army Technical Research Institute as a response to the USSBS investigation at the end of the war is as follows (*23).
1 prototype was finished, reached the stage of production and started work on maintenance.
Chi-To Production Model
But what kind of Chi-To was planned to be produced during the fiscal year 1945? A collection of diagrams from Mitsubishi called the "Chi-To Explanatory Drawings" gives all of the insight into this. Due to applying the utmost optimizations for mass production, what is presumed to be the mass production model of the Chi-To turned out to be significantly different than any of the prototypes completed.
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| Chi-To Mass Production Model (Mitsubishi Drawing). |
The most apparent change is that the turret was changed from the problematic 3-piece cast design of the 1945 prototype. This change is an obvious one, as the cast design turned out to be difficult to manufacture. The new turret is a version of the Chi-Nu turret with increased armor corresponding to the specifications of the Chi-To, which is of course, fully welded. The possibility of mounting the 7.5 cm Tank Gun (Long) Model II in the turret of the Chi-Nu had been examined since at least February 1945. The removal of the lower-left cylinder below the gun was probably one of the modifications made to the production drawings of the gun to fit in this turret.
A less noticeable change regarding the turret layout is that the electric drive seems to have been removed, and the manual traverse wheel was enlarged in response. While this would undoubtedly lower the traversal speed of the Chi-To's turret, it also simplifies the production process, and probably was not a major impediment considering that this tank would be deployed in a largely defensive role.
As for the hull of the vehicle, the slope angle of the upper front plate was increased to improve protection. The side armor above the tracks was also changed from a notched design to a more efficient flat single-plate. Various other production optimizations were done, such as removing the machine gunner's hatch (the driver never had a hatch in the first place), simplifying the structure of smaller plates, and specific changes to the engine compartment and mufflers. The fuel capacity was reduced from 400 liters to 250 liters.
The only performance improvement of this vehicle over the prototype seems to be the defense of the frontal hull plate, and perhaps marginally improved turret armor strength due to the elimination of casting, while the survivability, range, and turret slewing rate have decreased. It can be seen that the changes to this model were made emphasizing production ability to the utmost over all other aspects, which is natural considering the state of the war and the waning of the Japanese economy.
Considering that the production drawings of the Type 5 7.5 cm Tank Gun for installation in the Chi-Nu turret were not completed by March 20th, 1945, it can be interpreted that the Mitsubishi Chi-To production drawings were probably finalized sometime from April to August 1945.
Type 5 Tank Gun Production Situation
If the mass production of the Chi-To was to proceed smoothly, the production of the main cannon 'Type 5 7.5 cm Tank Gun (Long) Model II' was paramount. Looking at the plans and actual results of the time, it can be seen that there were also major issues here.
As mentioned, the first two prototypes of the parent of this gun, the autoloading 'Model I' for Chi-Ri, were completed in July 1944. In October the second gun had its autoloader removed and was remodeled to 'Model II' for Chi-To. Apart from that, 11 other guns had also been ordered in April 1944, six for Chi-To, and five for Chi-Ri. According to Osaka Arsenal's manufacturing documents in December 1944 (*8), these 11 guns were supposed to be completed during the period of January-March 1945. A few days later (*9), this schedule was slightly staggered back to only completing 10 guns from January to April. However, this did not proceed as planned.
The full-scale mass production plan of the Type 5 7.5 cm Tank Gun was made near the end of the fiscal year 1944 at the same time as the Chi-To's mass production plan. It was scheduled for Osaka Army Arsenal to produce 200 guns, equivalent to the amount of Chi-To, with the first guns being completed in June 1945.
An agreement was made between the 1st Institute and Osaka Arsenal on March 7th, 1945, where it was decided that the design of the Model II tank gun would be modified for installation in the turret of the Chi-Nu. This was not only for the Chi-To production model, which used the Chi-Nu turret design, but also for installation onto the Chi-Nu itself. The first 11 guns produced were now all to be Model II, and the 200 guns scheduled to be built in the next fiscal year would also be entirely Model II. At this point, it's evident that the Chi-Ri was suspended.
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| (This is not a real diagram, it is made by the author.) Rough representation of Chi-To with a Type 5 Tank Gun base using the Type 90 Field Gun barrel. |
Also, on May 14th, 1945, a plan was examined to further increase the number of gun production to 300 by diverting the barrels of Type 90 Field Guns onto the base of the Type 5 Tank Gun (*11). While it is unclear if this plan was officially adopted, considering the terrible state of production at the end of the war, it is easy to imagine that many Chi-To tanks may have appeared in this form if production had been attained. Of course, the performance of the gun would have deteriorated in this configuration.
Based on the production results provided by the arsenals at the end of the war, only two production examples of the Type 5 7.5 cm Tank Gun (Long) Model II were actually completed, one in April and one in May 1945. In the inventory of Osaka Arsenal, there was only one more unfinished Type 5 at approximately 70% completion. Including the converted prototype which was mounted to the Chi-To prototype in February 1945, this means that only three guns compatible with the tank were ultimately completed, and it can be considered that mass production was practically not attained at all.
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| Until the end of the war, only two production examples of the Type 5 Tank Gun were completed. |
Post-War Activity
Lake Inohana Mystery
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| The sinking location is in strait which connects Hamana and Inohana, on the Inohana side (north). |
Since the end of the war, there have long been rumors that three tanks were sunk at the area around Seto Bridge in Lake Inohana, as many residents at the time witnessed the event. According to the stories, one small vehicle was sunk in a shallow area and later withdrawn and scrapped. The other two vehicles sank into a deeper area and could not be seen.
Mr. Ohira first publicly testified about being involved in the sinking of these tanks in a local television program aired in 1989. However, this did not make much of an impression at the time because the vehicles in question were not named. When he was later interviewed for the Chunichi Shimbun in 1998, more details emerged and the story became famous.
With the help of the reporter, Mr. Ohira identified the three vehicles from images of various Japanese and foreign vehicles as a captured British Windsor Carrier, a Type 97 Medium Tank (Chi-Ha), and a Type 4 Medium Tank (Chi-To). In regards to the Chi-To, Mr. Ohira said that he remembered specific details such as the shape of the turret, the 30-ton weight, and 7 roadwheels. He also remembered that it was rumored within the unit as "A secret tank with only two examples in Japan". The tanks were said to have been sunk around August 23rd, to keep the military secret from the US. However, Mr. Ohira did not recognize this tank as the 'Chi-To', but as the 'SK'.
Mr. Ohira was 81 years old at the time of this interview, and 53 years had passed since the end of the war. Whether his identification of the tank as a Chi-To was an error in memory cannot be known unless the vehicles are excavated. However, the details in his recollection are interesting, and the alignment is hard to ignore.
Efforts to locate the tanks remaining in the lake were carried out as a regional revitalization project by the group 'SM@Pe' from 2012 to 2015. However, none of the objects identified by magnetism or sonar proved to be related to the Chi-To, or any tank for that matter, and the search was eventually halted due to exhaustion of funds with no meaningful result. Either the tanks are now completely buried by over 80 years of sediment, or there are no longer tanks in the lake.
If it is correct that a Chi-To was sunk into Lake Inohana, then it would be necessary for two separate prototypes to have been completed, one being serviced with the 24th Tank Regiment operating in the area, but it is impossible to confirm.
Chi-To at Chiba
At the end of the war, the known Chi-To prototype was in service with the 28th Tank Regiment located at Chiba City, where it had been transferred from the tank school and was positioned for the envisioned decisive battle for the Japanese mainland. The 28th Tank Regiment was a unit formed by the mobilization of the former Chiba Tank School Teaching Unit. Several Japanese photographs were taken immediately after the war when photo restrictions were relaxed, and after that, the vehicle was requisitioned by the US military.
US military personnel reportedly loaded the Chi-To onto an M26 'Dragon Wagon' and brought it to a holding area. The tank seems to have been subsequently transferred to multiple depots before eventually being rounded up with various other armored vehicles at the Tokyo 1st Arsenal to be shipped to the United States for technical evaluation.
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| Left: Type 4 15 cm SPG Ho-Ro, Right: Type 4 Medium Tank Chi-To. The gun shield raises off the turret because it was originally made for Chi-Ri. Photo location: Chiba |
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| The huge power of Chi-To's main gun was recounted by one crew member in comparison to the Type 3 Medium Tank Chi-Nu. Photo location: Chiba |
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| Chi-To loaded onto a trailer by American military personnel. Photo location: Tokyo |
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| American posing with the Experimental Type 5 7.5 cm Tank Gun (Long). Photo location: Tokyo |
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| Chi-To next to an FB swamp vehicle at the Tokyo 1st Arsenal. 4 large bolts can be seen faintly under the rear turret overhang. Photo location: Tokyo |
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| The vehicle was mistakenly marked as 'Type 5 Heavy, 32 ton'. |
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| A less known detail is the marking 'loose turret' on the right side. |
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| US planview of Chi-To based on the prototype. (*24) |
As a result of postwar investigations, the US obtained 47 pages of technical and developmental documents regarding the Chi-To, which were processed through the Washington Document Center for assessment. The Chi-To prototype and other vehicles remained in Japan into the year 1946, during which they were finally shipped overseas.
The tank arrived at the Aberdeen Proving Grounds in Maryland by the beginning of 1947, and examinations were carried out. Unfortunately, I do not have the results of these tests, but given the state of US armored technology compared to Japan, it seems fairly certain that there was little to be gleaned from this vehicle.
By 1952, the Chi-To was no longer of any use for study. This tank, along with other late-war Japanese tanks such as the Chi-Ri, Chi-Nu, and Ke-To were scrapped at Aberdeen around October 1952.
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| Chi-To at Aberdeen, January 1947. |
Legacy
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| STA2, the second Type 61 prototype. |
As a remedy to this problem, the STA2 was modified with 'Chi-To Type' synchro-mesh transmission, and the Pontiac steering system was also replaced with the Chi-To's hydraulic steering system (#9). The functions were improved as a result, but by the time of the next round of prototypes in 1959/1960, the transmission of these new models (STA3 and STA4) was changed to a newly developed domestic synchro-mesh type with a 2-stage clutch, and the steering system was changed to a type made by Cletrac. Nonetheless, the application of wartime technology was interesting.
Also, although there is no direct relation here, the Type 61 tank successfully adopted a large turret formed by split casting, in consideration of mass production efficiency. The technology that was not practical in time for its first application, the prototype of the Chi-To, finally matured and was applied practically to Japan's first domestic postwar tank.
Type 61 Tank.
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| Type 61 Tank. |
References
Secondary
#1. Hara, Tomio. Japanese Tanks. Koujinsha, 1978.
#2. Shikenshi - History of the 4th Army Technical Research Institute. Shikenkai, 1982.
#3. Ground Power Issue 96/1.
#4. PANZER Extra Issue 00/6.
#5. Imperial Army Tanks & Gun Tanks - The Full Picture of Japanese Anti-Tank Combat Vehicles Comparable to the West. Gakken Plus, 2001.
#6. Sayama, Jirou. Introduction to Armor. Koujinsha, 2002.
#7. Shirai, Akio. Research of Japanese Army 'War Lessons'. Fuyōshobō-shuppan, 2003.
#8. Tank Armored Units, Senki Series 66. Shin-Jinbutsuoraisha, 2004.
#9. Hayashi, Iwao. Tank Development History of Postwar Japan. Mitsuhitosha, 2005.
#10. Sayama, Jirou. Introduction to Cannons. Mitsuhitosha, 2008.
#11. Sayama, Jirou. Artillery of the Japanese Army - Infantry Guns & Anti-Tank Guns. Koujinsha, 2011.
#12. Ground Power Issue 12/10.
#13. Maru Issue 19/12.
#14. PANZER Issue 21/4.
Primary
*1. 1941-1943 Binding of Materials Related to the Army Ordnance Bureau Part 1 (C14011059100)
*2. Army Technical Investigation 5th Section (C14010889500)
*3. 9.1942 to 9.1943 Binding Regarding Munitions Council (C12121561500)
*4. FY1943 Army Ordnance Bureau Research Plans (C14011078200)
*5. FY1943 Research Plans Document Binding (C14011074600)
*6. FY1943 (Revised) Research Plans, 4th Army Technical Research Institute (C14011080000)
*7. 9.1943-12.1943 Binding of Documents Related to the FY1944 Servicing Plan (1 of 3 Volumes) (C12121785400)
*8. 1944.12 Document Binding of Each Arsenal’s Submissions for the Manufacturing Department Managers Meeting (C14011009600)
*9. Military Secrets No. 1, January to April 1945 Maintenance Plan (C13120838900)
*10. 1944 Meeting Documents Regarding Manufacturing (C14010999100)
*11. Military Secrets No. 11, FY1945 Maintenance Plan Attached Tables (C13120840600)
*12. Status Report, Army Ordnance Bureau Yawata Branch (C14010919200)
*13. FY1945 Binding of Each Arsenal’s Construction Plans (C13120849200)
*14. FY1945 Parts Procurement, Maintenance Plan (C12121801400)
*15. FY1945 HQ, Arsenal Servicing Plan Charts, Artillery (C13120837700)
*16. FY1945 Procurement of Weapons and Ammo, Item and Number Tables (C14010917500)
*17. FY1945 Munitions Maintenance Situation Survey Charts (C13120835800)
*18. Binding of Tactics Research Reference Material (2) (C14060868200)
*19. Heavy Quick Firing Guns Training References (000000675992)
*20. Incomplete Weapons Catalog (C14010956500)
*21 Strategic Bombing Survey Group Submission Material, 1941-1945 Weapons Production Situation, HQ and Arsenal Submission Materials Comparative Tables, Department of Administration for Demobilized Soldiers and Civilian Employees (C15011298300)
*22 Sagami Army Ordnance Arsenal United States Strategic Bombing Survey Group Investigation Materials (Reserve) Ordnance Section (C15011257300)
*23. 1945.8.31 Army Technical Research Institutes Section (C15010408100)
*24. Ordnance Technical Intelligence Report No. 21 (Automotive Development of Combat Vehicles)
*25. Ordnance Technical Intelligence Report No. 144 (Type 5 Tank Gun Experimental)
Oh Boi, This Is Gonna Be A Good Read.
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteI separated the read in 2 different days, but it is a very good article, greetings!
ReplyDeleteHey what happened to Mai?
ReplyDeleteSad to see it scrapped just like that rather than sending it back to Japan. I would've loved to see more tanks akin to the Chi-To and Chi-Ri, particularly the rejected Plan 3 of the Chi-Ri which could've replaced the Chi-To, and a drawn-up diagram of the Chi-Se to replace the Chi-Ri.
ReplyDeleteVery good article, easy to read and understand. So I would like to bring your article to translate into Thai so that Thai people who are interested in Japanese tanks can read it. Will it be published on my Facebook?
ReplyDelete