Enter the Japanese – Testing the Japanese engines
BMW were a little sceptical when the Air Arsenal-led multi-manufacturer team arrived in late 1940, bringing with them a number of representative and prototype engines. While Daimler-Benz quickly peeled off with the inline team, BMW worked with the radial team. They were startled at the lack of finish on the Japanese engines. What they observed was though there were far more fins/surface area than a comparable American, French, British or German engine, the fins were far cruder. Many ran into each other and showed the fact that they were from a casting process with very little cleanup beyond manual filing.
What really surprised BMW's engineers and engine design staff was their simplicity. They compared a French Ghome-Rhone 14M engine with the Nakajima Sakae engine, a developed version based on that engine. The difference is that Nakajima took that engine, retooled it and made it so that instead of running at 700 hp it ran at 940 hp and required half the time to build. They then expanded the engine into the Sakae 21 series that had 1040 hp. Meanwhile Mitsubishi showed BMW an engine, Kinsei 43, which although not based on the GR-14, was similar in size and produced 1740 hp using 10 kg less weight and took less man-hours to manufacture than the Nakajima.
At this point, many technical lightbulbs began to light up in the heads of BMW's design staff.
As for the Japanese, they looked at BMW-801 and asked many questions, most of which started with 'Why on Earth?' The engine, though producing about 1500 hp, was a gas hog (even though it was already used in designs like the Fw-200) by Japanese standards as among other things BMW used a very rich mix to assist in cylinder cooling. While a standard matter for aero engines, the Japanese demand for range had led them away from this particular path. Only the planned C and D versions of the engine promised to be somewhat fuel efficient, but only at low altitude.
The first point the Japanese made was that with BMW did not appear to have embedded their operational philosophy into their design philosophy. This brought about embarrassed silence and meek questions about that that comment meant. The Japanese responded that they had adopted what the operators said they wanted, codified it, and applied it to their design philosophy. What the operators wanted was simple, powerful, easy to maintain, low fuel consumption, rugged engines which worked in the field. The companies took this and applied it, also applying cost reduction as part of the design process. Once they understood that BMW did not do this, they noted that this explained the expensive and unnecessary over-engineering observed on the BMW-801, as well as the poor selection and layout of its auxiliaries.
The Japanese engines were then put onto the test stands in Germany and started up. The first one was a standard Mitsubishi MK4R-A 'Kasei 23E15' bomber engine which the Japanese team assured BMW was supposed to produce 1530 HP, although they also had a prototype R engine rated at 1750HP. After some issues with running incorrectly it was producing almost 1900 hp. The initial problems with running rough the Germans had seen earlier when they had to come up with tuning settings for their engines when going from 87 Octane to 100 octane. After discussions with the Mitsubishi representatives the mutually appalled Japanese and Germans realised that they were not using remotely comparable fuels. The Germans were astounded to be told the engine was tuned to standard Japanese bomber fuel – 72 octane. The Japanese were astounded to find out that the Germans used 100 octane. The Germans then realised the implications: what they had on the stand was an engine which produced 1530 HP on 72 octane fuel when their best radials produced 1539 HP on 100 octane.
Next they threw on the Nakajima NK9H 'Homare 21' engine which was supposed to produce 1990 HP. Instead it produced power over 2200 hp. The German engineers after seeing that would not even let them try to run the Mitsubishi MK9A 'Ha-211 Ru' engine which is supposed to run at 2200 hp. The German test stands were only rated to 2400 hp and they needed to be strengthened, which took a week. RLM was told immediately and a large RLM technical team quickly arrived.
By this time everything was on the table. The Japanese knew that the Germans could assist them in improving the octane ratings of their fuel, while both BMW and RLM knew that the long-established linkages between German and Japanese aviation industries had borne extremely valuable fruit. Both knew that they had things to exchange at the technical level. Quietly and behind the scenes, the BMW technical staffs began to work with their Japanese counterparts arranging technology swaps.
RLM now knew that the Japanese were using octane fuel in the 72-86 octane band and no better, and that they had mature, technologically advanced radial engines that could make the Fw-190 even faster and revolutionise the German fighter force. What to do? They discussed the options, which were to study the engines and adapt their ideas to new construction, license the engines and build them in Germany, using the secret of lead additive as a bargaining chip to buy the licenses? As technicians, the only thing they all rejected was ignoring it and continuing on as before.
As this point, Ambassador Onishi arrived with his close friend Heinrich Himmler – which made the policy issue political and at Hitler's level. Frantic RLM staff immediately advised Göring, who arrived post-haste. He had quickly sent a brief to Hitler himself so as to head off or at least balance Himmler.
Onishi, well versed in Byzantine Nazi internal machinations, made a modest proposal to both. He suggested that, as BMW was receiving much equipment removed from wrecked French factories, that they and Japanese industry, through the Japanese Government Military Arsenal system set up a joint venture company to build a 'new BMW engine' to fill the workaday 900-1200hp range with a cheap, simple, easily maintained engine designed to use the relatively abundant lower quality fuels. This was immediately agreed to as it was an obvious commercial goldmine. He then suggested that, as both Japan and Germany had mutually supporting interests under the Tripartite Axis, that they agree broadly to a mutual, no-cost exchange under the GJTCA of synthetic fuel assistance for radial engine assistance. His trump card was asking the senior BMW engineers what their 801 would produce if BMW incorporated useful technological tweaks from the Japanese design philosophy. When he replied that it would produce about 2000 to 2200HP using less fuel, be easier to maintain and production costs would drop by, he guessed, a tenth, it was all over bar the detailed negotiations.
This solved a serious problem developing within the German aircraft engine industry. They understood that what they had would peak in capability by early 1942, and that the best which could be expected from a German radial engine that could fit an aircraft like the Fw-190 was about 1730 hp from the BMW 801D-2. Beyond that point the German industry just could not produce a meaningful engine in a radial. Only Liquid cooled in-line engines had shown promise going to higher hp ratings in that engine size range.
Among those who understood this was Kurt Tank, who was already thinking about how to put an in-line engine on the Fw-190 and create the D longnose. What these tests showed in stark terms was that in Japan, the complete opposite had happened. They already had radials that were in the 2200 hp range and were working on prototypes in hp ranges far higher than this. As a result of the Military Missions, BMW, DB and Jumo obtained access to engines and the Japanese who designed them to study and the results shocked them. Tank realised that this would turn the Luftwaffe on its head.
This changed everything for the Germans, and it changed nothing. They certainly realised that they had missed an entire development path but there just was not a lot most companies could do about it – except BMW. Not only had BMW been down this path with the Japanese before (their close relationship with Kawasaki from 1928 had been the direct cause of improvements to the BMV VI V-12 (450hp-550hp) which had resulted in the 800hp BMW VII V-12), they were the only ones with a modern, high quality radial engine core. As the Japanese said, when you stripped the BMW-801 back to its core it was a very good engine indeed, and easily able to accept what the Japanese had developed in terms of cylinder super-cooling, layout and fuel management. In return, BMW was able to fix developmental problems with advanced Japanese engines like MK9 by designing a better fuel injection system for it (which they could do in their sleep), and correct the faults with the licence-built Japanese DB-601, the Ha-40.
The testing of Japanese engines in 1941 under the GJTCA (and after the revelations of the Balbo Mission) led to a ferment at engineer and technical levels within BMW. As was normal within the German aviation industry, BMW already had many years of very close association with Japanese companies.
BMW-800.
This was developed during 1941 under Air Arsenal auspices by Kawasaki and Nakajima as a commercial venture with BMW. It married French machinery stripped from their damaged radial engine producing facilities and the basic GR-14 design to the technical advances of the Japanese Sakae (itself based on the GR-14 root). This engine had the advantages of being cheap to produce, reliable and as close to no risk as it was possible for an aero engine to be. It used the cheap and simple 'cylinder super-cooling' technology the Japanese had developed in casting fins into their cylinders. It was kept low-cost and minimised demands on skilled manpower. BMW adopted the cheap Japanese approach of casting innumerable fins in, and finishing by hand-filing using unskilled labour. This approach was justified as being acceptable for a 'war emergency' utility engine.
This was its own engine and was not a licence built Nakajima Sakae. It was firmly based on the GR-14 design as adapted to BMW building practises and attracted significant attention and even some input from Hungarian and Italian engine designers. The basic comparison unit was the NK1F Sakae21, this itself being developed from the GR-14 root. The BMW-800 produced 1110hp on take-off using the low octane fuels it was designed for. However, the engine could also be retuned to use better fuels, obtaining a useful 1270hp when using 100 octane fuel.
It was aimed at the Luftwaffe's utility market and was used on trainers, transport aircraft, and specialist machines like the Hs-129, to which it gave a sparkling performance. It turned the Messerschmitt Gigant motorised glider into a transport aircraft.
The BMW-800 was produced in large numbers from early 1942. The first proof of concept versions were assembled from captured French components and hand-made parts from the BMW prototyping shops, but design was rapid and without the usual long debugging program as the concept was already mature. While this approach meant that performance was slightly below that of Sakae 21, BMW was not concerned with this. The engine's development costs were very low and it was what the Luftwaffe needed in this engine range. The engine was unique in being developed to a timeline set by construction of new BMW annexes and bringing into service of captured French equipment.
BMW-801D
The 801C was replaced in production with the BMW 801 D-2 series engines in early 1942, which ran on C2/C3 100 octane fuel instead of the A/B/C's B4 87 octane. BMW was tooling up for this production when the Japanese bombshell burst. They continued with 801D in order to buy time to properly test and absorb the new technical tweaks being learned, but immediately began development of the 801E, using the 100 octane experience of the 801D and the excellent basic 801 engine core.
BMW-801E
It used the excellent engine core of the 801 series engine to best advantage. This engine fully incorporated the technical lessons learned from examining Japanese Kasei series engines as well as the formidable eighteen cylinder Nakajima NK9K Homare 22 (2000hp on 76 Octane fuel) and Mitsubishi MK9A (2200hp on 76 octane fuel). These technical tweaks included cylinder super-cooling through use of cast-in finning (although the Germans could not resist an expensive final finishing stage which made it look much less crude although it actually did nothing to improve cylinder cooling), and above all else incorporation of the engine philosophy the Japanese had developed.
The result was an engine that was simpler to build, easy to maintain, cheaper to manufacture, and which used less fuel than the earlier 801 versions. The BMW-801E produced 2060hp, a figure which astounded RLM and simply thrilled the Luftwaffe. Better, it did this without the planned system known as MW50, which injected a 50-50 water-methanol mixture into the supercharger output to cool it and reduce backpressure. This was still developed but did not appear until later in the war. When it did, it boosted low and medium-altitude performance improved considerably, with takeoff power increasing by another 170hp. The BMW 801G and H models were E engines modified for use in bomber roles with lower gear ratios for driving larger propellers, clockwise and counter-clockwise respectively.
As a result of developing the 2000hp class BMW-801E, the Fw-190A-4 entered service in the second half of 1942. Comparison to the Fw-190A-2 is instructive. The A-2 had a maximum speed of 322mph at 3,280' and 389mph at 18,045'. The A-4 had the startling performance of 357mph at sea level, 415mph at 10,830', and 440mph at 21,650'. The A-2 was a truly formidable opponent to contemporary RAF Spitfires but the A-4 outclassed literally everything in European skies. Fortunately for the Allies, it was dedicated at least initially to the eastern Front. There, despite its initial small numbers, it caused a slaughter in the ranks of the Red Air Force during the autumn of 1942 unprecedented since the 'Fokker Scourge'. In one celebrated incident in the third month of the war there, six of these aircraft attacked a full regiment of 21 Yak fighters, and destroyed them all in four minutes.
One unexpected result of this was the creation of a Fw-190 fighter monoculture in Germany. It ensured the immediate obsolescence of the entire Messerschmitt single engine fighter line. Me-109 continued in production, but the Luftwaffe made it plain from early 1942 that the aircraft would leave production and that the company's factories would replace them with Fw-190 in the production halls. In the end, the Me-109 did not leave production until late 1943, simply because the powerful new BMW-801E series could not be produced in sufficient volume to justify this. The Me-109 series was also exported to Italy, Hungary and Rumania.
BMW-802
This was the license-built version of the Nakajima NK9 Homare 22, a mature 18 cylinder design which reliably produced 2000hp. BMW purchased licences for this engine as its own 18-cylinder program was in trouble, and this engine was mature. RLM insisted on this for insurance purpose, and also because they wanted this to offer a development path to 2500hp as a bomber engine. This also made sense as Homare came from the Sakae root, and this increased BMW's confidence and depth in this engine line.
BMW-803
This was the license-built version of the Mitsubishi MK9A, a 18 cylinder fighter engine design which produced 2200hp in its prototype version. It came from the Kinsei root, and this led to considerable interaction with the Italians, who were producing a variant of this engine. BMW purchased licences for this engine as its own 18-cylinder program was in trouble, and this engine offered more than the Homare 22. RLM insisted on this as they wanted this to offer a development path for fighters in the 2500hp-3000hp range as a fighter engine.
Supercharger development
With the engine now being used in higher-altitude fighter roles, a number of attempts were made to address the limited performance of the original supercharger. The BMW 801F was a modification of the E using supercharger gear ratios tuned to higher altitudes. Although takeoff power was unaffected, cruise power increased over 100 hp and "high power" modes for climb and combat were likewise improved by up to 150 hp. The F model was also used as the basis for the BMW 801R, which included a much more complex and powerful two-stage four-speed supercharger. Continued improvements to the basic high-altitude E model led to the BMW 801G, which dramatically improved performance across the board, with takeoff power increasing to 2,400 hp (1,790 kW). It was planned to use the F on all late-model Fw-190's, but the war ended before production started.
Turbocharger development
A number of attempts were made to use turbochargers on the BMW 801 series as well. The first used a modified BMW 801E to create the BMW 801J, delivering 2145hp, at takeoff and 1600 hp at 40,000 ft (12,200 m), an altitude where the E was struggling to produce 730 hp. The BMW 801F was likewise modified to create the BMW 801Q, delivering 1,875 hp at 40,000 ft (12,200 m), power ratings no existing allied engine could touch. However none of these engines ever entered production due to high costs, and the various high-altitude designs based on them were forced to turn to other engines entirely, typically the Junkers Jumo 213.