|Bomber B project|
|The Junkers Ju 288 V2 (Second prototype Ju 288)|
|Project for||Second-generation high-speed bomber|
|Issued by||Reich Air Ministry|
|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 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. Bomber B would also be a much larger and more capable aircraft, with range and payload 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 optimistic that more modest projects were generally cancelled; when the project failed the Luftwaffe was left with hopelessly outdated aircraft.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.
Engines for such designs was a great problem; mid-30s 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. 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), 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 with such designs. The United States, confident in its ability to produce aviation engines in any needed quantity, opted for four-engine designs with massed defensive firepower, as seen in the Boeing B-17 Flying Fortress.
In Germany, most bomber designs were adapted from pre-war designs, many of them passenger aircraft or dual-use designs. The first specialist bomber aircraft was the Junkers Ju 88, which had limited range and payload, forcing the Luftwaffe to maintain the Heinkel He 111 for other missions. A shortage of both types forced the early-war Luftwaffe to improvise with a collection of aircraft, a problem no one in the Luftwaffe was at all happy with. The earlier Ural bomber program that had been championed by Luftwaffe General Walther Wever but which had failed to produce any practical Allied-style "heavy bombers". Wever's death on June 3, 1936 prompted the issuance of the RLM's "Bomber A" heavy bomber design specification on the day he died, to inspire development of a new heavy bomber with much better range and payload than the Ural Bomber prototypes, the Dornier Do 19 and the Junkers Ju 89 would ever be able to provide. The winning design, given its RLM airframe number on November 5, 1937 was the Heinkel He 177.
This program was just in the process of selecting the winning design when the first German large capacity engines began 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 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 larger bomb load, 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.
Apparently excited by the possibilities of an aircraft with the payload and range of the He 177 combined with even higher performance than the Ju 88, 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 bombers in service.
With engines in the 2,000 to 2,500 hp range, twin-engined aircraft would have considerably more surplus power, allowing for much greater payloads. In theory, the more powerful engine would take no longer to produce than a 1,000 hp design, it would simply be larger. By the late 1930s, engines of this sort of power began to be seriously considered and the British and Germans drew up bomber designs based on them.
In the UK, Avro and Handley Page drew up proposals for a large bomber based on two Rolls-Royce Vulture engines. The Vulture was a quartet of six-cylinder-long cylinder blocks connected to a common crankcase and crankshaft, to make a larger displacement X-block design. As the bomber matured, problems with the Vulture became evident. Contrary to hopes, bringing together two V-12 engines' "quartet" of cylinder banks onto one crankcase for a larger design led to all sorts of 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 revision by its designer, Roy Chadwick. The Avro Lancaster and Handley Page Halifax designs formed the backbone of RAF Bomber Command for the rest of the war.
In Germany, the Bomber A program in mid-1936 had led to the Heinkel He 177 A, powered by two Daimler-Benz DB 606 "power system" engines. The 606 was a late-1930s attempt to use two Daimler-Benz DB 601 powerplants mated to a common gear reduction case to arrive at a 24-cylinder powerplant like the Vulture but using twin crankcases for the demanding German Heinkel He 119 high-speed reconnaissance design and the Messerschmitt Me 261 long range aircraft. The pair of DB 601 component engines of the DB 606 were arranged in an inverted W-block layout, keeping each component engine as a discrete, "all up" engine as much as possible (except for the "final drive" to the propeller shaft) instead of the X-crankcase/cylinder layout. Daimler-Benz was also working on a single-crankcase 24-cylinder design in parallel with the DB 606, creating the Daimler-Benz DB 604 as 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 apiece was mediocre, particularly when the airframe mounting them possessed an engine accommodation design that prevented adequate maintenance access and ventilation.
Production of the He 177A continued and in service with deficient powerplant installation, engine nacelle internal design and maintenance access, it was plagued by engine failures, overheating and fires while airborne, earning it the nickname "Flaming Coffin" by its crews. Unlike the British and Ernst Heinkel, who complained in November 1938 over what Reichsmarschall Hermann Göring was calling welded-together engines by August 1942, the Luftwaffe was partly hesitant of the need for a truly "four-engined" Heinkel He 177B for level bombing, due to the November 1937-imposed dive bombing requirement on the existing He 177A, until Goering cancelled that requirement on September 15, 1942.
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 53.7-litre displacement BMW 802 in 1943, an eighteen-cylinder air-cooled radial, and the even larger, 83.5 litre displacement BMW 803 28-cylinder liquid-cooled radial. As both the 802 and 803 proved to be dismal failures in testing, their development only went so far as to prioritize the 802 for "completion and prototype construction as a secondary issue", while the 803 only received attention to "its design and development". This state of affairs at BMW led to the company's engineering staff 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.
Arado, Dornier, Focke-Wulf and Junkers all responded with designs and Henschel later added the Hs 130. 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. The Fw 191 was based around an all-electric platform to power nearly all its flight accessories, in place of hydraulics. The Fw 191 thus earned the nickname of Das Fliegende Kraftwerk (the flying power station). 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 considered terribly important--it was felt that the Junkers design would work anyway.
Prototype airframes of the Ju 288 and Fw 191 designs were ready mid-1940 but neither the Jumo 222 nor the DB 604 were ready. 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 were seriously underpowered. For comparative purposes, the nearly-equal displacement Wright Twin Cyclone radial engine was 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 take off weight of 15.9 tonnes (35,000 lb).
The first Jumo 222A/B development engines did not arrive until October 1941 and some eleven months later the DB 604 project was cancelled. By May 1942, in desperation, it was suggested that the Daimler-Benz DB 606 be used instead, even though it was considerably larger and heavier (1.5 tonnes) and was well known to have serious problems. 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 Ju 288 was showing a continual tendency to break its main landing gear on touchdown, partly due to its undercarriage problems caused by its complex method of stowing the oleo struts during retraction.
The RLM had no designs 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, 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 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' 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 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 the war in Europe, with no combat-ready designs to show for the effort, meant that the 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 developed in the mid-late 30s, started to become hopelessly outdated.
With the failure of Bomber B, four engine versions of the He 177, which had first been officially considered as early as October 1941 with the "He 177H" paper-only derivative, the ancestor of the Heinkel He 274 high-altitude design project, were considered as replacements for the mainline variants of the He 177A through most of 1943. The trio of completed DB 603-powered He 177B prototypes would start their flight tests by the end of 1943. Production of the B-series He 177s by Arado Flugzeugwerke, the prime subcontractor for Heinkel's heavy bombers, was never undertaken as the Arado firm had priority for its jet-powered Arado Ar 234 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 began the Emergency Fighter Program.
The Ju 288's landing gear was most innovative in its design. A Y-shaped bearer was mounted in the engine nacelle with its upper arms hinged. At the bottom end of this bearer was the shock absorber leg, which was likewise hinged. Two double-brake wheels, with (metric) size 1015 × 380 tires, were mounted on the cross-axle. During the retraction cycle a folding strut was raised by a hydraulic jack. The bottom part of the folding strut drew the Y-bearer upwards. Functioning via a lever-and-gear arrangement, a pushrod positioned parallel to the Y-bearer acted upon another gear segment mounted to the oleo leg's hinge pin and rotated it about this as the Y-bearer was drawn upwards.