Did DB 605 Power a Japanese Fighter?

I am in the process of writing a broad article about the Japanese Army’s relationship with the German engine ‘DB 605’ and later similar developments. This will cover procurement, replication attempts, the Army’s outlook, and ultimate decisions on the future of high-power V engines. At the moment, this article is difficult to complete.

In the meantime, I am splitting off this relevant story to pare down the size of the DB 605 article, which will be more oriented towards a larger picture. This article is concerning the strange theory that a Japanese plane was actually fitted with the ‘DB 605’ for flight.

The Type 3 Fighter (Ki-61) is well known as the only liquid-cooled fighter plane deployed by Japan during World War II. The initial mass production model Ki-61I was fitted with the ‘Ha-40’, a Japanese domestic version of the German inverted-V engine ‘DB 601 Aa’. Development on the performance-enhancing variant of this plane started in April 1942 as the Ki-61II. The main improvement was a new engine: the ‘Ha-140’, which advanced from the Ha-40’s 1,175 takeoff horsepower up to 1,500 takeoff horsepower. The Ki-61II design also featured an enlarged main wing (from 20 m² to 22 m²), which allowed 20 mm machine cannons to be installed.

Eight prototypes of the Ki-61II were completed at Kawasaki Gifu from the period of July 1943 to January 1944. It can be understood that the benefits of increasing the wing size did not out-weigh the performance detriments, as Kawasaki quickly decided to revert to the original Ki-61I wing before the flight exam was completed. The variant with the reverted wing size was designated Ki-61II Kai, and the design process started in February 1944. Nine prototypes of this plane were completed from April to July 1944, leaving the total count of ‘Model 2’ prototypes at 17.

Unusual Performance From Ki-61-II?

The first in-house flight of Ki-61II seems to have taken place in August 1943, and the Army Examination Department’s official review began near the end of that year. Around May 1944, test flights with Army between multiple examples of the Ki-61II and Ki-61II Kai happened at Kawasaki Akashi – the location where they were equipped with the sparsely available Ha-140 engines. The opinions on the prototype Ki-61II and II Kai are not well recorded, but generally it seems that these prototypes were reasonably reliable and featured most notably improved climbing performance.

In the book ‘Unknown Sword’ (未知の剣), a collection of stories from the members of the Army Air Exam Dept, there is an interesting remark about these planes in particular:

On a different day, Flight Lieutenant Kumagai piloted the -II Kai, and a pilot from Kawasaki piloted the -II. A race of horizontal top speed at the altitude of 2,000 meters turned an unexpected result. The -II, which has larger wings and should have been slower, overtook the -II Kai.

未知の剣 (translation)

Why would the Ki-61II, with the same engine but greater aerodynamic drag, exceed the speed of Ki-61II Kai? It could be something as basic as the engine in this -II Kai being a less refined unit that did not deliver the specified power (or simply malfunctioned). The II itself is a quite mysterious plane, and you may be surprised to learn that there is not a single known photo of it. The book does not offer any more detail or speculation about the event, but this anecdote of the unusual performance reminded me of something more interesting: The theory that the Ki-61II prototypes were fitted with DB 605 A engines imported from Germany.

This theory is not one that I can give much credence. However, its existence relates to the mystery of a ‘Japanese DB 605’, and moreover, it is most certainly unknown to English readers. In this article, the ‘Ki-61II fitted with DB 605’ theory will be explained, and analyzed.

‘Ki-61II Fitted with DB 605’ Theory

The theory in question originates from the power plant volume of the magazine pair “Hien Restoration Record” (飛燕修復の記録) published by Model Art in 2018. Firstly, it should be said that the Hien Restoration Record is a fantastic publication brimming with new information and unseen photos relating to Ki-61 and the restoration of the surviving II Kai prototype by Kawasaki. Nonetheless, this particular section is doubtful.

What follows is an overall paraphrased summary of that hypothesis:

• According to historical records and Takeo Doi’s recollection (Ki-61 chief designer), the Ha-140 was not completed in time for the eight Ki-61II prototypes. Rather, it was only built by June 1944, but Ki-61II flew in 1943.
• As Ha-140 couldn’t have been available at the time, the Ki-61IIs were probably fitted with eight of a total ten imported DB 605 A engines instead.
• The reason these planes were completed as such, despite the intention of ultimately using Ha-140, was for research purposes.
• The Army didn’t extensively test and reach the results of Ki-61II/II Kai’s examinations until mid-1944, when Ha-140 was ready.
• The DB 605 engines were probably named ‘Ha-140’ in records to hide the fact that they were implemented in aircraft without a license, and the secret remained even after the war to avoid issues between the companies.

Before analyzing these points, let’s also compare the Japanese Ha-140 and German DB 605 A engines. The Ha-140, like the DB 605, is a 12-cylinder liquid-cooled inverted-V engine, and a descendant of the DB 601. Ha-140 was designed to output a maximum power of 1,500 metric horsepower, which is roughly equivalent to the 1475 metric horsepower of the initial DB 605 A. The nominal/military power output of these engines is also nearly identical, with the same 1,250 metric horsepower at full pressure altitudes of 5,700 or 5,800 meters. However, the Ha-140 cannot be simply considered a ‘Japanese DB 605’.

While the DB 605 featured larger cylinder bores than the DB 601, the Ha-140 shared the same bores as the Ha-40, in other words, the same as DB 601. The power improvement of Ha-140 was rather attained only by enlarging the supercharger, raising the manifold pressure substantially as a result (aided by water injection), increasing the compression ratio, and the RPM. More accurately, it is an “up-rated DB 601”, while the DB 605, also exhibiting a higher RPM and larger supercharger than DB 601, is a further departure with a higher improvement cap due to its greater displacement.

The initial DB 605 A considered here did not have water-methanol injection, meaning it was relegated to about 150 mmHg less boost pressure than the Ha-140 at each power setting. Nonetheless, the power loss was mostly offset by the displacement in comparison. Future developments of the DB 605 with water-methanol injection and a further improved supercharger were eventually able to exhibit about 1,800 horsepower, or even 2,000 horsepower when aided by high-octane fuel.

It’s important to emphasize that these figures represent “performance on paper”, as all of these engines faced a range of problems in real world use.

In summary, the Ha-140 and DB 605 A are closely performing engines on paper, although the means of obtaining this performance are different. The Ha-140 has a superior supercharger which was leveraged with water-methanol injection, while the DB 605 A has greater displacement. The Ha-140 could be called an “end-stage DB 601”, while the DB 605 A was the first in a series of iteratively improved models. Dimensions and mass are naturally also relatively similar between these two, though the DB 605 A is slightly larger, and it’s easy to see where the idea of interchanging them could arise.

Analysis

With the main premise of the theory now outlined, let’s investigate each point and its justification.

1. Did DB 605 Arrive in Japan?

Firstly, in order for this theory to hold any weight, it is obviously necessary for DB 605 A engines to have been imported to Japan at all. It is recorded in Japanese record that the Japanese Army purchased at least five DB 605 engines in 1942. American intel gained from Germany after the war also supports this event.

More relevantly, Daimler-Benz itself recorded the supply of ten total DB 605 engines to Japan during the period of 1942-1944, in this incidence: five in 1942, three in 1943, and two in 1944. However, records of ‘purchases’ and ‘supply’ are not really overt proof that these engines actually made it into Japan successfully.

Is there any explicit evidence of a DB 605 in Japan, such as photographs or inventory records? As much as I have been able to observe, the answer seems to be a cautious “uncertain”. In the ‘Hien Restoration Record’, a photo of a DB 605 A (Wrk.Nr. 78006) from the materials of the Kawasaki company is used as the evidence that DB 605 arrived. However, with an abundance of skepticism, the possibility that these were just photos imported from Germany prior to the actual engine shipment could also be considered.

Overall, though, I find it to be very possible (even probable) that some or all of these ten DB 605 A engines actually made it to Japan. The reasoning is simply the fact that blockade runners were typically successful in their technology exchange missions until the year 1944, and we know that Daimler-Benz released DB 605 engines to Japan prior.

Allowing the possibility that DB 605 A arrived, there is also another question: would all ten of these engines have really been allocated to the Japanese Army with Kawasaki? The Navy, after all, had their own domestic developments of the DB 600-series with Aichi. It also seems that when four DB 603 engines were later purchased in 1944, two of these were to be allocated to the Army, and two to the Navy. I can only verify in documents that the Army intended to purchase five DB 605s, so the possibility certainly remains that some were intended for the Navy. In that case, of course, it would be impossible to fit all eight prototypes with DB 605 engines.

2. Was Ha-140 Not Completed Until 1944?

This theory requires the assumption that the Ha-140 engine was not completed in time for the flight of Ki-61II in August 1943. Or, at the very least, that there were not enough prototype airworthy engines to power these planes. To support this, first the writing of Takeo Doi in his book “Recollecting 50 Years of Airplane Design” was quoted as follows:

The first unit of Ki-61II flew in August 1943, but due to delay in the completion of Ha-140, only eight planes were completed by January 1944.

飛行機設計５０年の回想 (translation)

In ‘Hien Restoration Record’, this delay of ‘completion’ seems to be interpreted as “completion of the first unit”. Regarding production records used to support a very late completion date for Ha-140, the following sources and their production charts were used:

1. Bunrindo “Famous Airplanes of the World” Issue 98: Type 3 Fighter
   • 179 total engines built, first six in 6/1944, after which it was mass-produced every month.
2. Akashi Factory 50 Year History
   • 199 total engines built, in the overall period of 1942-1945.
3. NIKKI 50 Year History
   • 199 total engines built, four in 1942, sixteen in 1943, the rest 1944-1945.

As explained, this theory supposes that Ha-140 was not completed until 1944, and resultingly considers it unthinkable that there were engines built in 1942-1943, especially not airworthy models ready for the flight of Ki-61II. The records that do list Ha-140s built as early as 1942 are explained as imported DB 605s named ‘Ha-140’ for confidentiality. With the basics laid out, let’s now consider each point.

To start out, it is obviously a very unrealistic idea that the Ha-140 was immediately passed from a paper design to mass production in 6/1944 without any prototype development period. In my opinion, Takeo Doi’s words about “delay in the completion of Ha-140” during 1943 are definitely just referring to the completion of the entire project, i.e., passing service trials and entering mass production. It is very necessary for prototype Ha-140s to have been manufactured months or even years in advance of the beginning of production. However, were enough engines built to allow the capacity of fitting all eight Ki-61II prototypes?

To introduce some context about Japanese developments to enhance the power of DB 601-type engines, an imported DB 601 Aa was up-rated from 1,175 to 1,550 horsepower by late 1942. The basic method was largely the same as that subsequently employed in the development of Ha-140: raising the RPM, enlarging the supercharger (to allow increased manifold pressure), and introducing methanol injection. The purpose of this engine was to power the high-speed research plane ‘Ki-78’ developed in cooperation by the Kouken and Kawasaki. Although this was just a test engine without the necessary endurance or practicality for service, it could be called a ‘pre-prototype’ of the Ha-140.

Now let’s look at a couple of separate historical records concerning Kawasaki Akashi’s engine production during the war, to try and establish how many Ha-140s were built before mass production started in mid-1944. Both of these reports were prepared by the Japanese immediately after the war for the US Strategic Bombing Survey (USSBS).

Report #1 presented in the table to the right is from a record of production for all Japanese Army aero-engines. In this report, it was recorded that the first prototype of Ha-140 was built in November 1942, followed by 31 other prototypes until September 1944, when mass production was actually attained.

Report #2 is a record submitted by the Kawasaki Akashi factory about its own engine production during the war. This record instead states that only three prototypes were made during January-April 1943, followed by roughly the same pattern of mass production starting in September 1944.

Such huge discrepancies in record between Japanese documents are unfortunately common, as the contemporary records were often destroyed and reproduced from estimation.

Which report is correct? Considering that the prior ‘Ha-40’ also had 30+ units built before mass production, it seems almost impossible to me that the more troublesome Ha-140 would only require three. Presuming that about 30 prototypes of Ha-140 were actually produced from 1942 to mid-1944, it is of course easily possible that a handful of these engines were airworthy in time for the construction of Ki-61II prototypes.

Furthermore, this also reasonably explains why the last two units of Ki-61II were recorded as sitting engineless at the Kawasaki Akashi engine factory in May 1944, despite being completed in January. By January, only the first 13–16 prototypes of Ha-140 had been manufactured, meaning that the pool of currently airworthy engines was very likely not enough to fit all eight prototypes. The last examples of Ki-61II sat headless at Akashi for a few months after completion as a result.

Why Was the Examination of Ki-61-II so Late?

The last point is that the examination of Ki-61II was not carried out in earnest and completed until 1944, although prototypes were flying by the third quarter of 1943. The theory proposes that this is because the Ki-61IIs were never considered for service, as they were research planes fitted with DB 605, so service evaluations were not necessary until the Ki-6II Kai with Ha-140 engines appeared.

The true reason for the delayed evaluation can be easily understood with the fact that the Ki-61II was very rapidly replaced with the Ki-61II Kai plan in February 1944. The Ki-61II Kai was better for evaluation as it featured the same wing size as the prior Ki-61I series, reducing the drag disparity and making the performance enhancement of the Ha-140 over the Ha-40 much clearer to observe. It would have been meaningless to complete the evaluation of Ki-61II itself, which was already ‘abandoned’.

As previously mentioned, a sizeable pool of the original Ki-61IIs for flight testing also did not exist until 1944. Though all the Ki-61IIs (non-Kai) were completed by January 1944, they were not all fitted with engines immediately due to a lack of airworthy Ha-140s for each airframe. The fact that a few of these planes sat ‘headless’ for months after completion further evidences that DB 605s were not mounted as a stand-in.

Conclusion

The theory that the Ki-61II prototypes were fitted with imported DB 605 A engines essentially does not exist in any capacity outside the book it is introduced in. Nonetheless, as I am writing an article directly concerning the DB 605 A in Japan, I thought that it would be interesting to introduce and ‘debunk’.

Although the theory initially seems almost reasonable (assuming all the factors introduced with it are correct), when considering the existence of contradictory production records and the logical conclusion that a much larger pool of Ha-140 engines was very likely produced, it quickly loses its basis. As interesting as the idea of a Ki-61 equipped with the German import DB 605 is, there is no considerable evidence or reasoning to propose that such a thing was necessary, much less actually carried out, at all.

Sources

• Rep. Telegrams – No.192-200 (Translation of Telegrams on Purchasing Weapons from Foreign Countries). 1942.
• Rep. German Technical Aid to Japan. 1945.
• Rep. Report on Visit to Daimler-Benz A.G. at Stuttgart-Unterturkheim. 1945.
• Rep. PRODUCTION, PLANNED AND ACTUAL. REPORT NO. 19C(6). 1945.
• Rep. PRODUCTION FIGURES ON ENGINES, PROPELLERS, AND AIRCRAFT. REPORT NO. 15A(3). 1945.
• Doi, Takeo. 飛行機設計５０年の回想. Toukyou, Kantousha. 1989.
• Watanabe, Youji. 未知の剣. Toukyou: Bungeishunjuu. 2002.
• 飛燕修復の記録 – 動力編. Toukyou: Model Art. 2018.