Bomber B

Bomber B project
The Junkers Ju 288 V2 (Second prototype Ju 288)
Project for	Second-generation high-speed bomber
Issued by	Reich Air Ministry
Service	Luftwaffe
Proposals	Arado, Dornier, Focke-Wulf and Junkers, Henschel
Prototypes	Dornier Do 317 Focke-Wulf Fw 191 Henschel Hs 130 Junkers Ju 288.
Predecessor programs	Schnellbomber

Bomber B was a German military aircraft design competition organised just before the start of World War II to develop a second-generation high-speed bomber for the Luftwaffe. The new designs would be a direct successor to the Schnellbomber philosophy of the Dornier Do 17 and Junkers Ju 88, relying on high speed as its primary defence. But the Bomber B would also be a much larger and more capable platform, with range and payload figures far greater than the Schnellbombers, besting even the largest conventional designs then under consideration. The winning design was intended to form the backbone of the Luftwaffe bomber force, replacing the wide collection of semi-specialized designs then in service. 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outright, so when the project eventually failed to deliver a working design the Luftwaffe was left with hopelessly outdated aircraft.

Background

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 production capacity of both nations was considered too small to equip a fleet of such designs. 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.

Large twins

One way to address this issue would be to build larger engines. An engine in the 2,000 to 2,500 hp range, a twin-engine aircraft would have considerably more surplus 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 bringing together two V-12 engines' "quartet" of cylinder banks onto a single crankcase, to produce a working larger design led to all sorts of additional problems. Development of the Avro Manchester pressed ahead, but Handley Page was asked to adapt their HP.56 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 from its original designer, Roy Chadwick. The resulting Avro Lancaster and Handley Page Halifax designs formed the backbone of RAF Bomber Commands efforts for the rest of the war.

The troublesome DB 610 "welded-together engine" – the DB 606A/B powerplants were similar in configuration, with DB 610s used to substitute for the failed Jumo 222 powerplants. Outer engine mount forgings not present on this restored example.

In Germany, the original Bomber A design program in the summer of 1936 had led to the Heinkel He 177A, powered by two Daimler-Benz DB 606 "power system" 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. The DB 606's twinned-up pair of DB 601 component engines, however, were arranged in an inverted W-block layout using the twin crankcases of the component engines instead of the X-crankcase/cylinder layout. The Daimler-Benz firm was also working on a single-crankcase 24-cylinder design solution in parallel with the DB 606 design, creating the an X-engine design of their own from 1939 through September 1942. Like the Vulture, DB engineers found the DB 606, weighing in at a massive 1.5 tonnes, simply did not work well, particularly when the airframe mounting them possessed a deficient design that prevented adequate maintenance access and ventilation.

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 Luftwaffe never placed any serious efforts into a separate four-engine version until the 1943-44 timeframe.

High-output engines

Simultaneously with the early development of the "coupled" engines, Daimer-Benz's began work on a 1,500 kW class design using a single crankcase. The result was the twenty-four cylinder Daimler-Benz DB 604, with four banks of six cylinders each. Possessing essentially the same displacement of 46.5 litres (2830 in³) as the initial version of the Junkers Jumo 222, 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.

BMW worked on what was essentially an enlarged version of its highly successful BMW 801 design from the Focke-Wulf FW 190. This led to the BMW 802 in 1943, an eighteen-cylinder air-cooled radial, and the even larger BMW 803 28-cylinder liquid-cooled radial. Both proved dismal failures in testing, which led to the company's engineering being redirected to place all efforts on improving the 801 to develop it to its full potential. Only the BMW 801F radial development, through its use of features coming from the 801E subtype, was able to substantially exceed the over-1,500 kW output level — the F-version was tested at a top output level of some 1,765 kW (2,400 PS) of take-off power.

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 the 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 sleeve valved 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. As it was meant to have the desirable combination of extended range, larger payload and better performance, whatever design won the Bomber B competition would replace all existing bombers in service.

Different designs

Focke-Wulf Fw 191A

The Jumo 222 engine, on which so much depended concerning the Bomber B project

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 to power nearly all its flight accessories, that replaced hydraulics wherever possible. The Fw 191 thus 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. For comparative purposes, regarding the engines, the nearly-equal displacement Wright Twin Cyclone radial engine was successfully powering the American B-25 Mitchell twin-engined medium bomber with some 1,270 kW (1,700 hp) apiece of output, even with the B-25 having only a top airspeed of some 440 km/h (273 mph) at a takeoff weight topping out at 15.9 tonnes (35,000 lb).

The Junkers Motorenwerke's first Jumo 222A/B development engines did not arrive until October 1941, and some eleven months later the DB 604 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, partly due to its complex method to stow its oleo strut units during retraction.

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 as replacements for the mainline variants of the He 177A 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 — four months before Arado would be able to commence license-built construction of the He 177B-5 — the Luftwaffe's attention turned to fighter production.