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|Bomber B (aviation design competition)|
|The Junkers Ju 288 V2 (Second prototype Ju 288)|
|Proposals||Arado, Dornier, Focke-Wulf and Junkers, Henschel|
|Prototypes||Dornier Do 317
Focke-Wulf Fw 191
Henschel Hs 130
Junkers Ju 288.
Bomber B was a German military aircraft design competition dating to just before the start of World War II to develop a second-generation high-speed bomber. The new designs would be a direct successor to the Schnellbomber philosophy, using high speed as its primary defence. But the Bomber B would also be a much larger and more capable platform, allowing it to carry bomb loads of an equivalent mass to any existing medium and heavy bomber airframe designs then being used or even considered by the Luftwaffe, but not quite matching the combat radius figures of the four-engined "heavies" of the Allied air forces. The winning airframe design was intended to form the backbone of the Luftwaffe bomber force, replacing the wide collection of semi-specialized twin-engined medium bomber designs intended for replacement, as many of them had origins going back as early as the mid-1930s, shortly after the Luftwaffe's founding. The Reich Air Ministry was so hopeful about the outcome that more modest projects were generally cancelled outright, so when the project eventually failed to deliver a working design, especially in the preference for a pair of high-output engines - meant to be capable of at least 1,500 kW (2,000 hp) each, if not more - to power the design, the Luftwaffe was left with hopelessly outdated aircraft.
By the late 1930s, airframe construction methods had progressed to the point where airframes could be built to any required size, founded on the all-metal airframe design technologies pioneered by Hugo Junkers in 1915 and constantly improved upon for over two decades to follow - especially in Germany with aircraft like the Dornier Do X flying boat and the Junkers G 38 airliner, and the Soviet Union with the enormous Maksim Gorki, the largest aircraft built anywhere in the 1930s.
However, powering such designs was a major challenge. Mid-1930s aero engines were limited to about 600 hp and the first 1000 hp engines were just entering the prototype stage - notably the Rolls-Royce Merlin and Daimler-Benz DB 601. But even the latest engines were limited in the sort of designs they could power; a twin-engine aircraft would have about 1,500 kW (2,000 hp) in total, the same power as a mid-war single engined fighter aircraft like the Hawker Typhoon or Republic P-47 Thunderbolt. Although using a larger number of engines was possible, and achieved in some airframe examples for both the United Kingdom and the Third Reich, the mass production by the aviation engine industries of both nations to build the numbers of engines needed to equip a large, four-engined heavy bomber fleet as the Americans were already starting to do was a serious problem. The first 1,500 kW output aircraft powerplants to be conceived anywhere would start their emergence within the United States during 1937, with the American Wright Aeronautical and Pratt & Whitney firms testing twin-row, eighteen-cylinder air cooled radial engine designs of at least 46 litres in displacement each, that would emerge as major factors in American air power during the war years.
The United States, confident in its ability to produce aviation engines, opted for four-engine designs with heavy defensive firepower, as seen in the Boeing B-17 Flying Fortress. The United Kingdom and Germany largely focused on twin-engine bombers with shorter range intended for a European war. The advantages of larger designs were evident, and both countries experimented with four-engine designs in the pre-war era, but the strain on the production capacity of each nations' aviation industries for airframes, and the engines to power them, was always a concern.
One way to address this issue would be to build larger engines. If one had access to an engine in the 2,000 to 2,500 hp range, a twin-engine aircraft would have considerably more extra power, allowing for much greater payloads. Yet such an engine, in theory, would not take any longer to produce than a 1,000 hp design, it would simply be larger. By the late 1930s, engines of this sort of power class first started to be seriously considered, and both the British and Germans drew up bomber designs based on them.
In the UK, Avro and Handley Page both drew up proposals for a large bomber based on two Rolls-Royce Vulture engines. The Vulture was essentially a quartet of six-cylinder-long cylinder blocks connected together onto a common crankcase and a single crankshaft, to make a larger displacement X-block design. As the bomber matured, problems with the Vulture became evident. Contrary to hopes, simply bolting together two engines to produce a working larger design led to the appearance of all sorts of additional problems. Development of the Avro Manchester pressed ahead, but Handley Page was asked to adapt their design for four smaller engines instead. When the Manchester flew with all of the problems with the Vulture remaining, it too received a four-engine remake. The resulting Avro Lancaster and Handley Page Halifax designs formed the backbone of RAF Bomber Commands efforts for the rest of the war.
In Germany, the original Bomber A design program in the summer of 1936 had led to the Heinkel He 177, powered by two Daimler-Benz DB 606 engines. The 606 was an attempt to use two separate Daimler-Benz DB 601 powerplants mated to a common gear reduction case to arrive to a 24-cylinder powerplant as with the Vulture, although arranged in an inverted W-block layout instead of the X from the use of two separate powerplants, even with the Daimler-Benz firm working in parallel with an X-engine design of their own from 1939 through September 1942. Like the Vulture, DB engineers found the DB 606 "power system", weighing in at a massive 1.5 tonnes apiece simply did not work well, particularly when the airframe mounting them possessed a deficient powerplant accommodation design that prevented adequate maintenance access and ventilation (as the He 177A series did), and was subject to considerable development issues from such deficiencies with the Greif. Production of the He 177 was pressed on anyway, and in service it was plagued by engine failures, overheating and in-flight fires, earning it the nickname "Flaming Coffin" by its own crews. Unlike the British, and Ernst Heinkel's own complaints in November 1938 over what Reichsmarschall Hermann Göring would himself later consider to be "welded-together engines" by August 1942, the German government's military aviation agencies responsible for making the He 177A combat-ready never placed any serious efforts into a "separate four-engine version" of the production He 177A until the 1943-44 timeframe, and inefficiently spreading the Heinkel firm's own efforts to develop such an aircraft along three simultaneous lines between the He 177B (the first of which flew in late December of 1943), an Amerika Bomber contract contender, and a high-altitude heavy bomber series of development programs.
High-output aviation engines for the Luftwaffe
Nazi Germany's own attempts during the late 1930s to create aviation powerplants in the over-1,500 kW (2,000 hp+) power output class initially started with the foreknowledge that unlike the United States, the Third Reich did not have the large production capacity for aviation powerplants as the Americans did, and twin-engined aircraft powered with two 1,500 kW class powerplants were more realistic for heavy bomber-sized airframes, given their own restricted production capacity in comparison with the American aviation industry. The first attempts by the Third Reich to get to over-1,500 kW power output levels began, roughly simultaneous with both the RLM's original "Bomber A" heavy bomber specification of June 3, 1936, and in August of the same year with the private venture Heinkel He 119 high-speed reconnaissance aircraft projects' need for such powerplants. The need for such high-output powerplants was initially intended to be met by the Daimler-Benz firm in 1936-37, through mounting a pair of Daimler-Benz inverted V12 engines, in a "side-by-side" manner within a common mount, and mechanically coupling the two powerplants together at their forward ends into a single "power system", using a common gearbox at the front of the arrangement with a single propeller shaft. The first example of such a "coupled" powerplant was created by combining two Daimler-Benz DB 601 engines in the aforementioned manner to create the "DB 606" before World War II in February 1937, in time for both airframe designs to have their powerplants ready and in the initial stages of production. This line of thinking led to the continuation of the format in creating ever more powerful "power systems"; using the Daimler-Benz DB 605 design in a similarly-twinned format resulted in the "DB 610" in June 1940, and the Daimler-Benz DB 603 twinned-up in such a form to create the most powerful of the cumbersome "coupled" power systems, the "DB 613" in March 1940. None of these "power systems" really ever had all their design flaws worked out during the war years, some of which were also related to airframe design issues in how the "power systems" were mounted onto the airframe, and housed in their nacelles. Such issues in airframe powerplant installation design did not exist in the prototype airframes for both the Heinkel He 119 reconnaissance design, nor in the Messerschmitt Me 261 long-range design due to those designs having well-laid out DB 606 engine installations — however, such issues with poor engine accommodation design for such a heavy and complex "power system" as the DB 606 partially contributed to the endlessly troubled career of the Luftwaffe's only heavy bomber design to see production, the Heinkel He 177A. The operational front-line versions of the Greif only used the Daimler-Benz produced coupled engine systems for propulsion, resulting in Reichsmarschall Hermann Göring's complaints in August 1942 that derisively labeled the Daimler-Benz firm's "coupled" engine concept, as used in the He 177, as "welded-together engines", with each "power system" weighing roughly 1.5 tonnes apiece.
Simultaneously with the early development of the "coupled" engines, was Daimer-Benz's attempt at creating an over-1,500 kW output aviation powerplant using only one common crankcase, the twenty-four cylinder Daimler-Benz DB 604, with four banks of six cylinders each, arranged in an upper and lower set with a 60º angle between each bank per set. Possessing essentially the same displacement of 46.5 litres (2830 in2) as the initial version of the Junkers Jumo 222, the DB 604 was intended to have A and B versions of opposite rotation, like the coupled engines did, but as its protracted development was diverting valuable German aviation powerplant research resources, and with more development of the DB 610 coupled engine giving improved results at the time, the Reich Air Ministry stopped all work on the DB 604 in September 1942.
The BMW company's work with radial engines resulted in the BMW 802 eighteen-cylinder air-cooled radial engine design from the mid-war years, which was close to being a high-output aviation engine, being in the same general class in the number of cylinders and output power as the American Double Wasp powerplant (which had itself achieved a 2,000 horsepower output rating as early as 1939) while having a 53.67 litre (3,275 cu. in.) displacement, close to that of an even more powerful 18-cylinder twin-row American radial, the Wright Duplex-Cyclone. The largest displacement radial piston engine designed and built in Germany for aviation purposes during the war years, the incredibly complex Wasp Major-class BMW 803 28-cylinder liquid-cooled radial was proving to be a total failure in testing of its prototypes.
The Junkers company's own 24-cylinder Junkers Jumo 222, liquid cooled six-bank inline engine, with four cylinders in each bank, came the closest to being Nazi Germany's only production, single-crankcase design high-output powerplant candidate during the war years, intended to power not only the Junkers Ju 288, but also many other German multi-engined advanced combat aircraft projects. The 222 was a remarkably compact and efficient engine design, being almost identical in cylinder number, displacement and weight to the British Napier Sabre H-type four-bank inline engine, and the best attempt at creating a German aviation engine that could routinely exceed 1,500 kW output at altitude, but as with the BMW designs and even the later Heinkel HeS 011 advanced turbojet engine, never came close to being a production-ready aircraft powerplant, with just under 300 examples of the Jumo 222 produced in total between several different versions.
The original "Bomber A" pre-war medium bomber competition was intended to create Germany's first truly effective bomber. Previously they had developed a collection of much smaller designs, some of them developed from pre-war passenger and courier aircraft. Their first purpose-designed type, the Junkers Ju 88, had limited range and payload, forcing the Luftwaffe to maintain the Heinkel He 111 for other missions. Also facing limited availability of both designs, the early-war Luftwaffe was forced to use a collection of different aircraft, a problem no one in the Luftwaffe was at all happy with.
The Ju 88 was just entering service when Germany's own aforementioned 1,500 kW+ output class power plants started bench testing. Compared to the Jumo 211s in the Ju 88, a pair of such engines in a bomber's airframe would more than double the power when compared to a pair of the earlier inverted V12 powerplants, upwards to 5,000 hp (3680 kW). With this sort of power, a significantly more capable design could be built, one with considerably larger internal space for a much large bombload, more fuel for longer range, and even better speed.
Junkers had been studying dramatically more capable versions of the Ju 88 powered by their relatively compact Jumo 222, or the four-crankshaft Jumo 223 diesel engine design from late 1937. No serious work was undertaken, but after Heinrich Hertel left Heinkel and joined Junkers in 1939, the EF 74 design was submitted to the RLM in May 1939. Accordingly the RLM sent out the specifications for Bomber B in July 1939. The specification called for a new medium bomber with a maximum speed of 600 km/h (375 mph), able to carry a bomb load of 4000 kg (8,820 lb) to any part of Britain from bases in France or Norway. To improve crew performance and defensive firepower, the designs were to have a pressurized cabin with remotely aimed armament. With the extended range, larger payload and better performance, Bomber B would replace all existing bombers in service.
Arado, Dornier, Focke-Wulf and Junkers all responded with designs, and Henschel later added its own entry (the Hs 130). However, it was clear even at this point that the call for designs was to some extent a formality, as the Junkers design had already been selected for production. The Ar 340 was dropped in the design stage and Do 317 was relegated to low-priority development, while prototype orders were placed for the Fw 191 and the Ju 288.
With the Focke-Wulf and Dornier projects as first and second backups, the Technisches-Amt technical development office of the RLM started using these other designs as experimental testbeds. For instance, the Fw 191 was based around an all-electric platform that replaced hydraulics wherever possible. The Fw 191 thusly earned the nickname of Das Fliegende Kraftwerk (the flying power station). However this dramatically increased the complexity of wiring the planes, and the chance that one of the many motors would fail was considerable. But that was not terribly important—it was felt that the Junkers design would work anyway.
The end of the project
Prototype airframes of the Ju 288 and Fw 191 designs were ready mid-1940, but in a taste of things to come, neither the Jumo 222 nor the DB 604 were ready to be installed. Instead of waiting, both teams decided to power their prototypes with the BMW 801 radial engine, although with 900 hp less per engine and with the BMW 801 radials themselves barely out of initial development, the planes would be seriously underpowered. The first 222s did not arrive until October 1941, and some eleven months later the DB project was cancelled outright. By May 1942, things were getting desperate, and it was suggested that the Daimler-Benz DB 606 be used instead, even though it was considerably larger and heavier (at 1.5 tonnes apiece), and was well known to have serious problems when not installed and accommodated properly in well-designed nacelles. Prototypes of both designs with these engines were ordered, although the Fw 191 was just getting into the air with the BMW 801 radials at this point and the 288 was showing a continual tendency to break its main landing gear on touchdown.
Desperation set in at the RLM, who had no other designs "in the pipeline" to fill the gap left if Bomber B did not work, even though some minor designs like the Henschel Hs 130, usually powered with two DB 603 or 605 engines, and the Dornier Do 317, itself being tried with the same, trouble-prone DB 606 or 610 "welded-together engines" on some of its prototype airframes were also being considered. A slightly improved Ju 88 — based on the prototype-only Ju 88B design — was ordered as the Ju 188, and several prototypes of "stretched" versions of existing bomber designs with four engines were also ordered, as with Junkers' own Ju 488 in 1943-44.
In June 1943, the T-Amt finally gave up; by this point, even if the Jumo 222 started working reliably, as it had begun to do so in the summer of 1943, a shortage of the metals needed for the high-temperature alloys it used meant it would not be able to enter production anyway, with just under 300 development powerplants built. The three-year development period during wartime in Europe, with no combat-ready designs to show for the effort, meant that the Germans' Bomber B project was a time-consuming venture that delivered nothing, while also serving to ensure that no other designs were available in late 1943, when their existing twin-engined medium bombers — most of which were first developed in the mid to late 1930s — started to become hopelessly outdated.
With the failure of Bomber B, four engine versions of the He 177 — which had first begun to be officially considered as early as October 1941 with the "He 177H" paper-only derivative, the direct ancestor of the Heinkel He 274 high-altitude design project — were finally considered for the mainline variants of the He 177 itself through most of 1943. The trio of completed DB 603-powered He 177B prototypes would successfully start their flight tests by the end of 1943. However, production of the B-series He 177s by Arado Flugzeugwerke, the prime subcontactor for Heinkel's heavy bombers, was never undertaken as both the Arado firm had its own priority for a jet-powered bomber, and by early July 1944 the Luftwaffe's attention turned to fighter production.
References and notes
- Griehl, Manfred; Dressel, Joachim (1998). Heinkel He 177 - 277 - 274. Shrewsbury, UK: Airlife Publishing. p. 224. ISBN 1-85310-364-0.
- Griehl, Manfred; Dressel, Joachim (1998). Heinkel He 177 - 277 - 274. Shrewsbury, UK: Airlife Publishing. p. 52. ISBN 1-85310-364-0.
- von Gersdorff, Kyrill; Schubert, Helmut (2007). Die deutsche Luftfahrt: Flugmotoren und Strahltriebwerke. (in German). Bonn: Bernard & Graefe Verlag. ISBN 3-7637-6128-4.
- Gunston, Bill (1989). World Encyclopaedia of Aero Engines. Cambridge, UK: Patrick Stephens Limited. p. 27. ISBN 0-517-67964-7.
- Jane's Fighting Aircraft of World War II. Studio Editions Ltd. 1989. p. 296.
- "The Hugo Junkers Homepage - Engines: Jumo 222". The Hugo Junkers Homepage. October 29, 2012. Retrieved 4 April 2013.
- Griehl, Manfred; Dressel, Joachim (1998). Heinkel He 177 - 277 - 274. Shrewsbury, UK: Airlife Publishing. p. 177. ISBN 1-85310-364-0.