By William Pearce

In 1936, Siegfried and Walter Günter began design work on the Heinkel He 119, an experimental, unarmed, high-speed light bomber and reconnaissance aircraft. The engine for the He 119 was buried in the fuselage, and the Günter brothers quickly realized that no engine available was capable of providing the desired power in excess of 2,300 hp (1,691 kW). Heinkel requested proposals from Germany’s leading aircraft engine manufacturers. Daimler-Benz responded with a plan to construct a doppelmotor (double engine) by coupling two DB 601 V-12 engines to create the 24-cylinder DB 606. Combining two engines as a single unit was seen as a quick way to double engine power without spending years to develop a new powerplant.

Development of the DB 601 was started in the mid-1930s and based on the DB 600. The main differences between the engines were that the DB 600 used a carburetor and geared supercharger, whereas the DB 601 used fuel injection and a variable speed supercharger. The DB 601 was an inverted, liquid-cooled engine with two banks of six cylinders. Its single-piece Silumin-Gamma (aluminum alloy) crankcase was closed out by a cover affixed to its top side. The six-throw crankshaft was supported by seven main bearings, and each main bearing was secured by four bolts and one transverse bolt that passed through the crankcase. The crankshaft turned counterclockwise. Fork-and-blade connecting rods were used, with the forked rods serving cylinders on the right side of the engine (when viewed from the rear). The connecting rods ran on roller bearings, but the blade rod had an additional plain bearing between it and the roller bearing.

The two cylinder blocks were made from Silumin (aluminum-silicon alloy) and attached to the bottom of the crankcase at a 60 degree angle. Each cylinder block consisted of six cylinders with integral cylinder heads. The dry cylinder liners (barrels) were made of chrome steel and were screwed and shrunk into the upper cylinder block. Threaded liner skirts protruded into the crankcase toward the crankshaft. A locking ring screwed onto each liner skirt and drew and secured the entire cylinder block to the crankcase. The locking ring had “teeth” around its outer edge and was tightened by a special pinion tool that was held secure in the crankcase and rotated the ring.

Each cylinder had two spark plugs mounted on its outer side and a fuel injector mounted on its inner side. The Bosch fuel injection pumps were mounted in the Vee between the cylinder banks. Two intake valves on the inner side of the cylinder brought in air. The combustion gasses were expelled through two sodium-cooled exhaust valves on the outer side of the cylinder. All four valves per cylinder were actuated via rockers by a single overhead (technically underhead) camshaft, which was driven by a vertical shaft at the rear of the engine.

The DB 601’s propeller shaft was driven clockwise via spur gears through a gear reduction housing mounted to the front of the engine. The gear reduction was made so that a gun or cannon could be mounted behind the engine and fire through the Vee between the cylinder banks and out the propeller’s hub. Mounted to the rear of the engine was an accessory section that provided the drives for the magnetos, generator, starter, fuel and oil pumps, and the transversely mounted supercharger.

The supercharger was mounted on the left side of the engine and driven from the crankshaft via a variable speed fluid coupling. In simple terms, two oil pumps supplied oil that flowed through the supercharger coupling. One pump continuously supplied the amount of oil needed for the supercharger to operate at its lowest (sea level) speed. The second pump was barometrically controlled and gradually supplied more oil as the aircraft’s altitude increased. At the engine’s critical altitude, the second pump was supplying the maximum amount of oil, and the supercharger was at its maximum speed. There was always some degree of slip in the coupling, but it was minimal (a few percent) at full speed. The variable speed of the supercharger created a gradual power curve rather than the saw-tooth power delivery of two- or three-speed superchargers. Air from the supercharger flowed through an intake manifold that looped in the Vee between the cylinder banks.

To form the DB 606, two DB 601 engines were mounted side-by-side at an included angle of 44 degrees and joined by a common propeller gear reduction. In this configuration, the engine banks formed an inverted W, and the inner cylinder banks were only eight degrees from vertical. The right and left engine sections were respectively referred to as the “W-Motor” (or DB 601 W) and the “X-Motor” (or DB 601 X). The exhaust ports for both inner cylinder banks were positioned in the narrow space between the two engine sections. DB 606 differed from the DB 601 by using the new propeller gear reduction and a modified accessory drive. The two engine sections drove a single propeller, and no gun or cannon could be fitted to fire through the propeller hub. Bolted between the two engine sections and near their rear was a mount for suspending the back of the DB 606 to the aircraft. The left and right engine sections remained separate with the exception of the gear reduction and the rear mount.

The new gear reduction housing combined the output from the two engine sections and fed it into a single propeller shaft, which typically had an extension that was approximately 44 in (1.11 m) long. The combining gear allowed the manual decoupling and recoupling of an engine section. Recoupling could only be accomplished when the engine sections were operating at the same RPM. In addition, an engine section would be automatically decoupled if its speed dropped suddenly compared to the other engine section. The coupling of each engine was accomplished by dogs (often referred to as claws in German literature) on a flange splined to the crankshaft that engaged dogs on a coupler that drove a spur gear in the reduction housing. To disengage an engine section, a lever for that engine section on the gear reduction housing had to be pulled forward. This would pull the coupler toward the propeller and disengage it from the crankshaft. The coupler would still be connected to the gears in the reduction housing. The levers on the engine were linked to levers in the cockpit, and the individual engine sections were started one at a time.

Different combining gear reductions enabled the propeller of the DB 606 to turn either clockwise or counterclockwise without changing the counterclockwise rotation of the engines’ crankshafts. The propeller of the DB 606 A turned clockwise. A 33-tooth gear on each of the two crankshafts meshed with an 80-tooth gear on the propeller shaft to create a .4125 reduction. The combining gear on the DB 606 B incorporated idler gears in the lower housing that enabled the propeller to turn counterclockwise. The idler gears increased the engine’s weight by approximately 88 lb (40 kg). For the DB 606 B, a 31-tooth gear on each of the two crankshafts meshed with 39-tooth idler gears, which engaged the 75-tooth gear on the propeller shaft to create a .4133 reduction. With two fewer gears, the combining gear reduction housing on the DB 606 A was initially smaller with angled corners when compared to that of the DB 606 B. However, to simplify production, later DB 606 A engines used the same larger, more rounded housing as the DB 606 B.

The supercharger and accessory section of the right DB 606 (W-Motor) engine section was basically the same as that used on the DB 601. The supercharger and accessory section of the left DB 606 (X-Motor) engine section was a mirror image of the left section so that the supercharger was on the right side of the engine.

The Daimler-Benz DB 606 A/B had a 5.91 in (150 mm) bore, a 6.30 in (160 mm) stroke, and a total displacement of 4,141 cu in (67.86 L). The engine was 6 ft 10 in (2.08 m) long without an extension shaft, 5 ft 4 in (1.63 m) wide, and 3 ft 6 in (1.06 m) tall. The dry weight of the DB 606 A was 3,263 lb (1,480 kg), and the dry weight of the DB 606 B was 3,373 lb (1,530 kg). Initially, 1,175 hp (864 kW) DB 601 Aa engines were used to create the DB 606 A/B. The supercharger on the DB 601 Aa ran full speed at an altitude of 13,123 ft (4,000 m), and the engine had a compression ratio of 6.9 to 1. For takeoff and emergency power at 2,500 rpm and 20.6 psi (1.42 bar) of boost, the early DB 606 A/B V-series (Versuch, experimental) produced 2,350 hp (1,728 kW) at sea level and 2,200 hp (1,618 kW) at 12,139 ft (3,700 m). For climb and combat power at 2,400 rpm and 19.1 psi (1.32 bar) of boost, the engine produced 2,090 hp (1,537 kW) at sea level and 2,100 hp (1,545 kW) at 13,451 ft (4,100 m). For maximum continuous power at 2,300 rpm and 16.9 psi (1.17 bar) of boost, the engine produced 1,900 hp (1,397 kW) at sea level and 1,760 hp (1,294 kW) at 14,764 ft (4,500 m).

Because it was based on an existing engine, the DB 606 was developed quickly. The engine made its first flight in the He 119 in June 1937 with Gerhard Nitschke at the controls. The single DB 606 was installed in the He 119’s fuselage and drove the 14 ft 1 in (4.30 m) diameter, four-blade propeller via a long extension shaft. DB 606 V1 through V4 powered the four He 119 aircraft that were built, and the engine proved to be reliable in that airframe. One He 119 did crash on 16 December 1937 after a faulty fuel transfer valve caused the engine to quit.

Heinkel also selected the DB 606 to power its new long-range heavy bomber design, which was submitted to the RLM (Reichsluftfahrtministerium, or Germany Air Ministry) in response to their Bomber A specification. The RLM ordered construction of a prototype on 2 June 1937, and the aircraft was soon designated as the Heinkel He 177 Greif (Griffon). Like with the He 119, the He 177 was designed by Siegfried and Walter Günter, although Walter was killed in a car accident on 21 September 1937. As changes in the design requirements mounted, particularly with RLM’s insistence that the He 177 be capable of dive bombing, Siegfried was forced to alter the aircraft and make compromises to its design.

Each of the He 177’s wings had one DB 606 engine installed fairly deep and immediately forward of the main landing gear. Each main gear consisted of two legs, with the inboard leg retracting toward the wing root and the outboard leg retracting toward the wing tip. Because of the cramped installation of the engine and landing gear, there was no firewall behind the DB 606. Room was at such a premium that right-angle fittings were used for connections behind the engine. Originally, surface cooling had been planned, but this was switched to annular radiators installed in the engine nacelle just before the engine. The DB 606’s extension shaft led from the engine, through the radiators, and to the He 177’s four-blade propeller, which was 14 ft 9 in (4.5 m) in diameter.

At least 800 He 177 aircraft had been ordered before the prototype made its first flight on 20 November 1939, piloted by Carl Francke. For reference, the He 177 prototype flew with engines V5 and V6, indicating just how few DB 606s had been produced up to that point. In December 1940, DB 606 A/B-1 engines uprated to 2,700 hp (1,986 kW) were installed in He 177 V6. The uprated DB 606 A/B-1 used two 1,350 hp (993 kW) DB 601 E engines. The supercharger of the uprated DB 601 E ran full speed at an altitude of 15,748 ft (4,800 m).

For takeoff and emergency power at 2,700 rpm and 20.9 psi (1.44 bar) of boost, the DB 606 A/B-1 produced 2,700 hp (1,986 kW) at sea level and 2,650 hp (1,949 kW) at 15,748 ft (4,800 m). For climb and combat power at 2,500 rpm and 19.1 psi (1.32 bar) of boost, the engine produced 2,400 hp (1,765 kW) both at sea level and at 16,076 ft (4,900 m). For maximum continuous power at 2,300 rpm and 16.9 psi (1.17 bar) of boost, the engine produced 2,000 hp (1,471 kW) at sea level and 2,075 hp (1,526 kW) at 16,732 ft (5,100 m).

Starting around 1940, Daimler-Benz used a lower compression ratio in the right (non-supercharger side) cylinder bank. This was due to the crankshaft’s rotation flinging extra oil toward the right cylinders. Some of the oil would get past the piston rings and into the combustion chamber. The presence of this oil increased the possibility of detonation (knock) in the cylinder. The compression ratio was decreased slightly to increase the cylinder’s knock resistance. Because the inner cylinder banks of the doppelmotoren were nearly vertical, they captured more oil than the outer cylinder banks. The inner banks also ran hotter because of their tight installation. The extra oil and the heat both increased the possibility of detonation in the inner cylinder banks. As a result, the inner cylinder banks of the doppelmotoren had a slightly lower compression ratio than that of the outer cylinder banks. For the DB 606 A/B-1, the outer (supercharger side) cylinder banks had a compression ratio of 7.2 to 1, and the inner (non-supercharger side) cylinder banks had a compression ratio of 7.0 to 1.

The DB 606 engine and its installation in the He 177 proved to be disastrous. As doppelmotor production picked up, vibration issues were discovered with the two engine sections, and the combining gear required much more development than had been anticipated. There were also issues with failures of the engine couplings. A major DB 606 issue was with its oil circulation at high altitudes. The oil would foam, leading to inadequate lubrication and the subsequent failure of bearings and seizing of pistons. Some of these failures would be catastrophic, with parts (connecting rods) breaking through the crankcase.

But it was the engine installation that caused the biggest issues. The annular radiators provided inadequate cooling, resulting in the engines running hot. The exhaust between the two inner cylinder banks ran so hot that any fuel or oil that dripped down from leaking fittings or during a catastrophic engine failure was ignited. Weeping fittings and seeping seals (partly caused by material shortages and substitutions during the war) were a constant issue, as the leaked fluid would pool and eventually be ignited by the hot exhausts’ radiant heat. Through lack of a firewall, fires in the engine nacelle would spread to the main gear and ignite any leaking hydraulic fluid. In addition, the hot exhaust being expelled just forward of the extended main gear was enough to ignite any hydraulic oil that had leaked.

Any fire in the wing spread quickly and spelled disaster for the aircraft and its crew. With the crew siting well forward of the engines, fires often went unnoticed until severe damage had occurred. Despite the best efforts of maintenance crews, the DB 606 engines needed constant attention and proved very difficult to service. Engine fires occurred with such regularity that crews referred to the He 177 as the Luftwaffenfeuerzeug, or Luftwaffe’s cigarette lighter. To resolve the engine issues, suggestions were made to extend the engine nacelle, install a firewall, reroute lines to prevent the pooling of fluids under the engine, and redesign the exhaust system. Such changes were ignored at first because they would delay He 177 production, which had already been rushed. However, the aircraft was also experiencing a number of structural issues unrelated to the engines that made modifications necessary.

Toward the end of 1942, the He 177 underwent a redesign as the A-3 variant. This aircraft would do away with the troublesome DB 606 engines and replace them with DB 610s. The DB 610 was a doppelmotor consisting of two 1,475 hp (1,085 kW) DB 605 A engines. The DB 605 was a development of the DB 601 that operated at a higher RPM, had an increased bore, and had a higher compression ratio. The DB 605/610 used plain bearings for the connecting rods rather than the roller bearings used on the DB 601/606.

The DB 610 kept the same engine section naming convention as the earlier doppelmotor, with the “W-Motor” (or DB 605 W) as the right section and the “X-Motor” (or DB 605 X) as the left section. The supercharger ran full speed at an altitude of 18,701 ft (5,700 m). The compression ratio of the outer (supercharger side) cylinder banks was 7.5 to 1, and the compression ratio of the inner (non-supercharger side) cylinder banks was 7.3 to 1.

The Daimler-Benz DB 610 A/B had a 6.06 in (154 mm) bore, a 6.30 in (160 mm) stroke, and a total displacement of 4,365 cu in (71.53 L). For takeoff and emergency power at 2,800 rpm and 20.9 psi (1.42 bar) of boost, the engine produced 2,950 hp (2,170 kW) at sea level and 2,700 hp (1,986 kW) at 18,701 ft (5,700 m). For climb and combat power at 2,600 rpm and 19.1 psi (1.32 bar) of boost, the engine produced 2,620 hp (1,927 kW) at sea level and 2,500 hp (1,839 kW) at 19,029 ft (5,800 m). For maximum continuous power at 2,300 rpm and 16.9 psi (1.17 bar) of boost, the engine produced 2,150 hp (1,581 kW) at sea level and 2,160 hp (1,589 kW) at 18,045 ft (5,500 m). The DB 610 was the same size as the DB 606: 6 ft 10 in (2.08 m) long, 5 ft 4 in (1.63 m) wide, and 3 ft 6 in (1.06 m) tall; however, it was around 130 lb (60 kg) heavier. The dry weight of the DB 610 A was 3,395 lb (1,540 kg), and the dry weight of the DB 610 B was 3,483 lb (1,580 kg).

The DB 610 installation on the He 177 A-3 was extended 200 mm (7.9 in) forward, and a firewall was incorporated behind the engine. On 22 March 1943, the DB 610 made its first flight in an He 177 (V19, VF+QA). Although reliability had been improved, engine fires still occurred, and the DB 610 suffered from the same engine coupling failures that had been experienced with the DB 606. In May 1942, Hermann Göring, commander of the Luftwaffe, made the following comment in reference to the He 177 and DB 606: “I have never been so furious as when I saw this engine. …Nobody mentioned this hocus-pocus with two welded-together engines to me at all.” By early 1944, plans were in motion to build He 177 with four separate engines, a suggestion that Heinkel had discussed back in late 1938 and proposed in mid-1939. Further production and development of the He 177 was abandoned on 1 July 1944. Once the Allies had landed on the continent, German aircraft production was focused on defensive fighters and attackers.

The Daimler-Benz doppelmotoren were also installed in the Junkers Ju 288 bomber. As issues with its intended 24-cylinder Junkers Jumo 222 inline radial engine created a short supply, the DB 606 was substituted in Ju 288 prototypes. A DB 606 engine was installed on each wing in a form-fitting nacelle with an annular radiator at its front. Like with the He 177, the extension shaft connected the engine to the propeller. The DB 606-powered Ju 288 V11 made its first flight in July 1942. Three additional Ju 288s were powered by DB 606 engines. A switch to the DB 610 was made for the Ju 288 V103, which was first flown in the spring of 1943. Five additional Ju 288s were powered with DB 610 engines. The doppelmotor installation in the Ju 288 did not result in the frequent engine fires experienced with the He 177. The DB 610 was planned for later Ju 288 C and D variants, but the aircraft were cancelled.

Post war, a DB 610 was used in the French SNCAC NC.3021 Belphégor high altitude research aircraft. The large single-engine aircraft had an annular radiator positioned in front of the DB 610 engine. The NC.3021 was first flown on 6 June 1946. Issues servicing the DB 610 were encountered, and the aircraft required much maintenance. SNCAC went bankrupt in mid-1949, and no other funds were provided for the aircraft. The NC.3021 was withdrawn from testing in 1950 and scrapped.

Development of the DB 613, a third doppelmotor, had a lower priority than that of the DB 606 and DB 610. With what appeared to be the successful creation of the DB 606, Daimler-Benz decided to apply the same doppelmotor concept to the DB 603 engine. The DB 603 was based on and built like the DB 601, but it had an enlarged bore and an elongated stroke. Compared to the DB 601, the DB 603 had slightly decreased supercharging at takeoff power but an increased compression ratio. The compression ratio of the outer (supercharger side) cylinder banks was 7.3 to 1, and the compression ratio of the inner (non-supercharger side) cylinder banks was 7.5 to 1.

Around 1940, the DB 613 was created by combining two 1,750 hp (1,287 kW) DB 603 G engines. The combining gear housing on the DB 613 was different that those used on the DB 606 and DB 610. The DB 613’s housing was asymmetric with an accessory drive from the W-Motor (right engine). The Daimler-Benz DB 613 A/B had a 6.38 in (162 mm) bore, a 7.09 in (180 mm) stroke, and a total displacement of 5,434 cu in (89.04 L). For takeoff and emergency power at 2,700 rpm and 21.5 psi (1.48 bar) of boost, the engine produced 3,600 hp (2,648 kW) at sea level and 3,100 hp (2,280 kW) at 22,966 ft (7,000 m). For climb and combat power at 2,500 rpm and 19.8 psi (1.37 bar) of boost, the engine produced 3,150 hp (2,317 kW) at sea level and 2,860 hp (2,104 kW) at 23,293 ft (7,100 m). For maximum continuous power at 2,300 rpm and 18.4 psi (1.27 bar) of boost, the engine produced 2,790 hp (2,052 kW) at sea level and 2,650 hp (1,949 kW) at 21,982 ft (6,700 m). The DB 613 was 7 ft 3 in (2.22 m) long without its extension shaft, 5 ft 10 in (1.77 m) wide, and 3 ft 9 in (1.14 m) tall. The dry weight of the DB 613 A was 4,321 lb (1,960 kg), and the dry weight of the DB 613 B was 4,409 lb (2,000 kg). The DB 613 was proposed for the Heinkel He 177 A-7 variant, but the aircraft was not produced, and the engine never progressed beyond the prototype stage. It is not believed that the DB 613 was ever flight tested.

C/D variants of each engine were planned, but it is not clear if they were ever built beyond prototype examples. Development of the C/D variants seemed to start toward the end of 1942. In general, the C/D variants produced more power, had increased critical altitudes, and were planned for 100 octane fuel. The DB 606 C/D produced 2,600 hp (1,912 kW) for takeoff and had a critical altitude of 19,029 ft (5,800 m).

The DB 610 C/D was based on the DB 605 D and had a compression ratio of 8.5 to 1 for the outer (supercharger side) cylinder banks and 8.3 to 1 for the inner (non-supercharger side) cylinder banks. For takeoff and emergency power at 2,800 rpm and 20.6 psi (1.42 bar) of boost, the DB 610 C/D produced 2,870 hp (2,111 kW) at sea level and 2,560 hp (1,883 kW) at 24,934 ft (7,600 m). For climb and combat power at 2,600 rpm and 19.1 psi (1.32 bar) of boost, the engine produced 2,550 hp (1,876 kW) at sea level and 2,400 hp (1,765 kW) at 24,278 ft (7,400 m). For maximum continuous power at 2,300 rpm and 17.6 psi (1.22 bar) of boost, the engine produced 2,100 hp (1,545 kW) at sea level and 2,050 hp (1,508 kW) at 22,966 ft (7,000 m). The dry weight of the DB 610 C was 3,461 lb (1,570 kg), and the dry weight of the DB 610 B was 3,538 lb (1,605 kg).

The DB 613 C/D had the same compression ratio increase as the DB 610 C/D. For takeoff and emergency power at 2,900 rpm and 20.9 psi (1.44 bar) of boost, the DB 613 C/D produced 4,000 hp (2,942 kW) at sea level and 3,600 hp (2,648 kW) at 19,685 ft (6,000 m). For climb and combat power at 2,700 rpm and 19.1 psi (1.32 bar) of boost, the engine produced 3,500 hp (2,574 kW) at sea level and 3,280 hp (2,412 kW) at 19,685 ft (6,000 m). For maximum continuous power at 2,300 rpm and 17.6 psi (1.22 bar) of boost, the DB 613 C/D produced 2,850 hp (2,096 kW) at sea level and 2,860 hp (2,104 kW) at 17,388 ft (5,300 m).

The Daimler-Benz doppelmotoren represented a quick way to double an engine’s output without quite doubling the drag of installation. While the engines worked well in the He 119 and Ju 288, the engine package failed to work reliably in the He 177, which was the main application. A total of approximately 1,916 doppelmotoren were produced: 820 (544 by some sources) DB 606s, 1,070 (1,346 by some sources) DB 610s, and 26 DB 613s. The engines tested in a Junkers Ju 52 transport and powered four He 119s, 915 (1,135 by some sources) He 177s, ten Ju 288s, and one SNCAC NC.3021.

An early DB 606 A is displayed at the Technik Museum in Sinsheim, Germany. DB 610 engines are on display in Germany at the Deutsches Museum in Munich and the Luftfahrttechnisches Museum (Aviation Museum) in Rechlin; and in the United Kingdom at the Royal Air Force Cosford Museum in Shropshire and the Science Museum at Wroughton. Reportedly, a DB 610 is in France, but its location and condition have not been found. The Smithsonian National Air and Space Museum has in storage a DB 610 engine in a complete He 177 nacelle. A DB 610 combining gear reduction housing is on display at the Muzeum Lotnictwa Polskiego (Polish Aviation Museum) in Krakow, Poland. No DB 613 engines are known to have survived.

Note: The figures in this article listed as hp (horsepower) are actually PS (Pferdestärke, metric horsepower). The kW figures are converted from the PS value.

Sources:
– Jane’s All the World’s Aircraft 1945-46 by Leonard Bridgman (1946)
– Flugmotoren und Strahltriebwerke by Kyrill von Gersdorff, et. al. (2007)
– The Secret Horsepower Race by Calum E. Douglas (2020)
– Heinkel He 177 Greif by J. Richard Smith and Eddie J. Creek (2008)
– Junkers Ju 288/388/488 by Karl-Heinz Regnat (2004)
– Major Piston Engines of World War II by Victor Bingham (1998)
– DB 606 A-B Baureihe 0 u. 1 Motoren-Handbuch by Technisches Amt (November 1942)
– Ersatzteilliste für Mercedes-Benz-Flugmotor Baumust DB 606 A-B by Daimler Benz (December 1941)
– DB 610 A-B Baureihe 0 u. 1 Motoren-Handbuch by Technisches Amt (November 1942)
– Betriebs und Wartungsvorschrift zum Mercedes-Benz Flugmotor DB 601 A u. B by Daimler Benz (October 1940)
– Motorhandbuch zum Mercedes-Benz-Flugmotor DB 603 A Baureihe 0, 1 und 2 by Daimler Benz (November 1942)

By William Pearce

Around 1936, the RLM (Reichsluftfahrtministerium, or Germany Air Ministry) sought the design of an 1,800 hp (1,342 kW) engine for the next generation of bomber aircraft. Otto Mader, the head of the Junkers Flugzeug- und Motorenwerke AG (Junkers Aircraft and Motor Works) research institute in Dessau, considered various V and H configurations for such an engine. However, each configuration had various drawbacks. Mader discussed the engine requirements with Ferdinand Brandner and tasked him with the project. Brandner was an experienced engine and railway engineer that had recently joined Junkers.

Brandner and his team set a goal of 2,000 hp (1,491 kW) and immediately began designing a completely new engine. The engine originally carried the manufacturer’s designation P2001, and Junkers submitted a proposal that outlined the 1,900 hp (1,417 kW) engine to the RLM on 4 December 1936. On 4 May 1937, the RLM placed an order for a prototype P2001 engine, which would officially become the Junkers Jumo 222 on 4 April 1938. Brandner and his team continued to work on the engine design, which was finalized on 4 June 1937. Nearly from the start, the Jumo 222 was intended to power the improved development of the Junkers Ju 88 bomber, which originally carrier the manufacturer’s designation EF 73 (Entwicklungs-Flugzeug 73, Development Aircraft 73). Later, the aircraft would become the Ju 288 bomber.

The Junkers Jumo 222 was a liquid-cooled inline radial. The engine had six cylinder banks placed radially around the crankcase at 60 degree angles, with the left and right banks horizontal. Each of the six cylinder banks had four cylinders, giving the engine a total of 24 cylinders. The outer points of the six cylinder banks formed a hexagon, making the Jumo 222 one of the rare hexagonal engines, like the Curtiss H-1640 Chieftain, the Wright H-2120, the SNCM 137, and the Dobrynin series of aircraft engines.

The two-piece aluminum crankcase was split horizontally below the left and right cylinder banks. The top four cylinder banks were on the upper crankcase half, and the bottom two cylinder banks were on the lower crankcase half. The two crankcase halves were joined by 11 studs on each side and four long studs that extended from the center main bearing. The one-piece balanced crankshaft was supported in the upper crankcase half by five plain main bearings. Each main bearing cap was secured by four vertical studs and one very long stud that passed transversely through the entire crankcase. Mounted on each of the crankshaft’s four throws was a rather typical radial-engine master rod with five articulating rods. The connecting rod was split, with three articulating rods attached to the bottom and the master rod flanked by two articulated rods on the top. The two pieces of the connecting rod were joined by four bolts. Reportedly, the master connecting rods for cylinder rows 1 and 4 (front and rear) were located in bank 5 (7 o’clock position), and cylinder rows 2 and 3 (middle two) were located in bank 6 (5 o’clock position). However, a drawing of the Jumo 222 depicts a master rod in one of the upper cylinder banks. It appears the drawing shows the configuration used in early engines, and the master rods were relocated to the lower cylinder banks in later variants. The flat-top aluminum pistons were rather short with two compression rings and two oil rings, all located above the piston pin. The engine’s compression ratio was 6.5 to 1.

The steel cylinder barrels were installed through the crankcase and sealed at their lower end by three rubber rings. Near the bottom of each barrel was a flange with four long studs that extended up through the cylinder head. The tightening of these studs drew the barrel up into the cylinder head and sealed it with a tapered aluminum ring to the combustion chamber. The combustion chamber was wedge-shaped with the exhaust valve on the short side. The cylinder head of each bank was a single aluminum casting secured to the crankcase by 10 studs. At the top of each cylinder were two intake valves and one sodium-cooled exhaust valve. A fuel injector was positioned between the two intake valves, and a spark plug was positioned between each of the two intake valves and the common exhaust valve. The valves for each cylinder bank were actuated via rockers by a single overhead camshaft. The rear of each camshaft was driven from the crankshaft by a series of spur gears. The camshafts turned clockwise for cylinder banks 1, 3, and 5 and counterclockwise for cylinder banks 2, 4, and 6.

Attached to the front of the crankcase was a planetary gear reduction with the propeller shaft positioned at the center of the engine. While the crankshaft turned clockwise, different gear reduction housings could be used to turn the propeller in either direction. The propeller of the Jumo 222 A, C, E and G models turned counterclockwise at .368 crankshaft speed. The counterclockwise gear reduction used a fixed planetary carrier with the propeller shaft driven from the free outer ring gear. The propeller of the Jumo 222 B, D, F and H models turned clockwise at .364 crankshaft speed. The propeller shaft of the clockwise gear reduction was driven from the free planetary carrier that rotated against the fixed outer ring gear. The clockwise gear reduction on the Jumo 222 B, D, F and H models weighed approximately 66 lb (30 kg) additional. A short gear reduction housing was available, but the extended version was most common. An inertia starter was mounted to the crankcase above the gear reduction housing, and the coolant pump was mounted below the gear reduction housing.

Attached to the rear of the crankcase was an accessory housing followed by the single-stage, two-speed supercharger. The supercharger impeller was 12.8 in (325 mm) in diameter and turned at 6.70 and 9.16 times crankshaft speed in low and high gears. It provided 8.8 lb (.61 bar) of boost for takeoff. Each of the three outlets from the supercharger fed an intake pipe that branched into two manifolds. These manifolds were positioned between the cylinder Vees at the 4, 8, and 12 o’clock positions, and each fed once cylinder bank. An eight-cylinder fuel injection pump was also positioned between the intake manifolds in each of these cylinder Vees. Individual exhaust stacks were fitted to the cylinder heads between the bank Vees at the 2, 6, and 10 o’clock positions. Engine mounting pads were located on the crankcase between the bank Vees at the 2 and 10 o’clock positions.

When viewed from the rear, the cylinder banks were numbered counterclockwise starting at the right horizontal bank at the 3 o’clock position, which was bank 1. Bank 2 was at 1 o’clock, bank 3 was at 11 o’clock, and so on. The front cylinder of each bank was No 1, and the rear cylinder was No 4. Each of the upper two cylinder banks had a magneto mounted to its rear. Each magneto fired all the cylinders for three banks with no redundancy. If a magneto failed, one entire side of the engine would not fire. Cylinders in opposite banks fired simultaneously. The firing order changed during the engine’s development. The following firing order is specific to the Jumo 222 E/F but may be applicable to other engine models. The Jumo 222 E/F’s firing order was as follows: Bank 2 Cylinder 1 & Bank 5 Cylinder 2, B1C1 & B4C2, B6C4 & B3C3, B2C3 & B5C4, B1C2 & B4C1, B6C2 & B3C1, B2C4 & B5C3, B1C4 & B4C3, B6C1 & B3C2, B2C2 & B5C1, B1C3 & B4C4, and B6C3 & B3C4.

The Junkers Jumo 222 A/B-1 had a 5.31 in (135 mm) bore and stroke. The engine had a total displacement of 2,830 cu in (46.38 L). The Jumo 222 A/B-1 initially produced 2,000 hp (1,491 kW) at 3,200 rpm. At the expense of reliability, further development eventually pushed its maximum power at 3,200 rpm to 2,500 hp (1,417 kW) for takeoff and 2,200 hp (1,641 kW) at 16,404 ft (5,000 m). Climbing power at 2,900 rpm was 2,260 hp (1,685 kW) at sea level and 2,090 hp (1,559 kW) at 16,404 ft. Cruising power at 2,700 rpm was 1,900 hp (1,617 kW) at sea level and 1,700 hp (1,268 kW) at 17,060 ft (5,200 m). The engine’s fuel consumption at cruise power was .477 lb/hp/hr (290 g/kW/h) at sea level. The Jumo 222 A-1 weighed 2,690 lb (1,220 kg), and the Jumo 222 B-1 weighed 2,745 lb (1,245 kg). The engine had a diameter of 3 ft 10 in (1.16 m) and was 7 ft 5 in (2.25 m) long.

In early 1938, RLM requested that the Jumo 222’s output be increased to 2,000 hp (1,491 kW). Since the engine was designed from the start for 2,000 hp (1,491 kW), this request did not present any issues, but it foreshadowed what was to come. A single-cylinder test engine was first run in March 1938, followed by one complete row of six cylinders in June 1938. On 24 April 1939, a complete Jumo 222 A/B-1 was run for the first time and taken up to 3,000 rpm. The engine was disassembled and inspected after the test and showed no signs of wear or issues.

In May 1939, Junkers submitted to the RLM design proposals for the Jumo 222-powered EF 73 (Ju 88 development) bomber aircraft. Incidentally, EF 74 was the same basic aircraft but powered by Jumo 224 engines. Encouraged by Junkers’ proposal, the RLM issued specifications in July 1939 for a new medium bomber capable of high-speeds. Originally known as Kampfflugzeug B (Warplane B), the aircraft proposal was eventually renamed Bomber B. The Bomber B specification requested an aircraft that could carry a 2,000 kg (4,410 lb) bomb load 3,600 km (2,237 mi) and have a top speed of 600 km/h (373 mph). For alternatives to the Jumo 222, the RLM requested engine designs from BMW and Daimler-Benz. The Junkers Bomber B proposal became the Ju 288, and other entrants included the Arado E.240, Focke-Wulf Fw 191, Dornier Do 317, and later, Henschel Hs 130C. The additional engine proposals were the BMW 802 18-cylinder radial and the Daimler-Benz DB 604 X-24.

The Ju 288 was selected for production, although prototypes of the Fw 191, Do 317, and Hs 130C would also be built. The Ju 288 and the Fw 191 were to be powered by the Jumo 222, which was ultimately selected over the other engines. The Jumo 222 was also planned for a future development of the Do 317. When war officially broke out on 1 September 1939, the Ju 288 was perceived as an aircraft needed for a decisive victory. It and the 2,000 hp (1,491 kW) Jumo 222 A/B-1 were given a high priority. At the time, three complete Jumo 222 A/B-1 engines were running on test stands. During 1939, Junkers had formed the Otto-Mader-Werke at Dessau to focus on engine design and development. This division was run by Mader and worked on the Jumo 222 and Jumo 004 (turbojet) engines.

In March 1940, the Jumo 222 A/B-1 achieved 2,000 hp (1,491 kW) for the first time, but some difficulties were encountered at this higher output with inadequate lubrication and connecting rod issues. Modifications were made to resolve the deficiencies, and the revised engine was running in August 1940. For flight testing, the Jumo 222 was installed in the center position of a Junkers Ju 52 trimotor transport and made its first flight on 3 November 1940. However, the Jumo 222 was not ready to be installed in the Ju 288, and the aircraft made its first flight on 29 November 1940 powered by 14-cylinder BMW 801 radial engines.

In April 1941, the Jumo 222 A/B-1 completed a 100-hour type test at 2,000 hp (1,491 kW), running at 2,860 rpm. Some of the issues during the test included spark plug damage after 60 hours, a leaking injection pump controller at 75 hours, and a camshaft bearing block failure after 88 hours. When the engine was dismantled after the test, coolant and fuel leaks were discovered, but they were not considered serious. Based on the overall positive results of the 100-hour test, the RLM ordered the Jumo 222 into production on 30 April 1941. The engine would be built at the new Flugmotorenwerke Ostmark (Aircraft Engine Factory in annexed Austria) plant under construction in Wiener Neudorf, Austria, with production expected to start on 30 August 1942. A monthly output of 1,000 engines was forecasted.

Starting in mid-1940, the RLM began to alter requirements for the Ju 288. A fourth crew member, additional equipment, and airframe changes resulted in the aircraft’s weight increasing to the point that 2,000 hp (1,491 kW) was no longer sufficient for the Ju 288 to achieve its originally-specified performance. Around mid-1941, the RLM requested that the Jumo 222 produce 2,500 hp (1,864 kW) for the Ju 288. Junkers had foreseen this request and began developing the Jumo 222 A/B-2 in 1940 to reliably produce 2,500 hp (1,864 kW) and resolve issues encountered with the early engines.

The Jumo 222 A/B-2’s cylinder bore was increased .20 in (5 mm) to 5.51 in (140 mm), while its stroke remained unchanged at 5.31 in (135 mm). This change increased the Jumo 222 A/B-2’s displacement by 214 cu in (3.50 L) to 3,044 cu in (49.88 L). The H-beam articulated connecting rods of the early engines were replaced with an I-bean articulated connecting rod design. The engine’s compression ratio may have been raised to 6.735 to 1, and valve diameters may have been altered slightly. The Jumo 222 A/B-2 had a balance pipe between the intake manifolds of adjacent cylinder banks. Engine speed was limited to 2,900 rpm in an attempt to increase its reliability. The Jumo 222 A/B-2’s maximum power at 2,900 rpm was 2,500 hp (1,864 kW) for takeoff and 2,490 hp (1,857 kW) at 16,404 ft (5,000 m). Climbing power at 2,700 rpm was 2,250 hp (1,678 kW) at sea level and 2,050 hp (1,529 kW) at 16,404 ft (5,000 m). Cruising power at 2,500 rpm was 1,900 hp (1,417 kW) at sea level and 1,750 hp (1,305 kW) at 16,404 ft (5,000 m). The engine’s fuel consumption at cruise power was .449 lb/hp/hr (273 g/KW/h) at sea level.

The Jumo 222 A/B-2 was first run in mid-1941 and was taken briefly to 3,000 hp (2,237 kW) by overboosting to 11.5 psi (.79 bar) in October 1941. However, the increased bore size created a harmonic resonance within the engine. With three Ju 52s serving as Jumo 222 test beds and a number of other engines on test stands, the entire project began to encounter significant issues. Connecting rod bearings were still a problem as was corrosion of the engine’s internal components. Despite the issues, the Jumo 222A/B-1-powered Ju 288 V5 made its maiden flight on 8 October 1941. Brandner had managed to talk his way onto the aircraft for the flight, which was completed without issue. For the Ju 288, the Jumo 222 turned a four-blade Junkers VS 7 propeller that was a 13 ft 1 in (4.0 m) in diameter. An annular radiator was positioned in the cowling, and experiments were conducted on Ju 288 V5 using a ducted spinner to deliver cooling air to the radiator.

As the manufacturing plant in Austria neared completion in late October 1941, it was clear that the Jumo 222 was not going to be ready for production. A decision was made to manufacture the Daimler-Benz DB 603 at the plant with production starting in March 1942. On 24 December 1941, the RLM cancelled the Jumo 222 for the Ju 288. The decision was based on the engine’s then-current takeoff rating of only 2,000 hp (1,491 kW), its ongoing issues, and its operational readiness not being sufficient for the Ju 288’s planned production schedule. The Ju 288 would be powered by DB 610 (two coupled DB 605s) engines, and Junkers would focus on developing the Jumo 213 inverted V-12. Work on the Jumo 222 would continue, but the engine was no longer a priority. Brandner stated that, at the time, various Jumo 222 engines had completed 20 100-hour test runs, and many at Junkers felt that the engine was basically ready for production. However, further issues with the connecting rod bearings caused a developmental delay that extended from January to March 1942.

The connecting rod bearing failures took a long time to resolve with experimentation of different bearing materials and lubrication techniques. Ultimately, a new connecting rod design was employed, the antimony alloy bearing material was replaced with a tin alloy, and the synthetic engine oil used was switched to a natural oil with an increased sulfur content. Due to tin shortages, antimony had been substituted early in the engine’s development.

the Jumo 222 A/B-3 was developed to cure the vibration and harmonic issues of the A/B-2 and with an improved supercharger to maintain power up to 20,997 ft (6,400 m). Along with a revised gear train, the engine incorporated all other revisions to improve reliability. The Jumo 222 A/B-3’s maximum power at 3,000 rpm was 2,500 hp (1,864 kW) for takeoff and 2,410 hp (1,797 kW) at 9,186 ft (2,800 m). Climbing power at 2,700 rpm was 2,250 hp (1,678 kW) at sea level and 1,980 hp (1,476 kW) at 20,997 ft (6,400 m). Cruising power at 2,500 rpm was 1,860 hp (1,387 kW) at sea level and 1,640 hp (1,223 kW) at 20,997 ft (6,400 m). The engine’s fuel consumption at cruise power was .463 lb/hp/hr (282 g/Kw/h) at sea level.

The Jumo 222 A/B-3 was developed quickly, and was first run in late 1941. Like the Jumo 222 A/B-2, it was briefly tested to 3,000 hp (2,237 kW) by overboosting to 11.5 psi (.79 bar) on 26 May 1942. The RLM became interested in the Jumo 222 A/B-3 and ordered it into production on 5 August 1942. Production would be undertaken at a plant in Prague in German-occupied Czechoslovakia. Optimistically, the first Jumo 222 A/B-3s were expected in October 1944 with a peak production of 1,500 engines per month achieved in September 1945. Running at 2,500 hp (1,864 kW), the Jumo 222 A/B-3 completed a 50-hour test on 9 December 1942 and a 100-hour test on 11 March 1943. The engine was tested in Ju 52s and Ju 288 aircraft, but the production plans were never realized.

The Fw 191 made its first flight in early 1942 and used BMW 801s. It was not until December 1942 that the Fw 191 V6 (third aircraft built) flew with Jumo 222 engines. With engine issues and constant changes to the underperforming bomber aircraft, the Bomber B program was cancelled in June 1943. Germany was short on resources, which were better utilized in the production of fighter aircraft rather than building troubled experimental bombers with problematic engines.

The Jumo 222 C/D was conceived in 1941 to produce 2,500–3,000 hp (1,864–2,237 kW) for high-altitude operations. The Jumo 222 C/D was designed and built with its bore increased to 5.71 in (145 mm) and its stroke increased to 5.51 in (140 mm). This gave the Jumo 222 C/D a total displacement of 3,386 cu in (55.48 L). The engine produced 3,000 hp (2,237 kW) at 3,000 rpm and could maintain much of that power up to 32,808 ft (10,000 m) thanks to an improved supercharger. Some reports indicate the Jumo 222 C/D was first run in mid-1942, but it was never given priority or considered for production until 1945, when a 3,000 hp (2,237 kW) engine was desperately needed. Apparently, two Jumo 222 C/D engines were completed, but the deteriorating war conditions shifted priorities and prevented them from being tested.

In late 1943, the Jumo 222 E/F was developed from the A/B-3 series with a 5.51 in (140 mm) bore, 5.31 in (135 mm) stroke, and 3,044 cu in (49.88 L) displacement. The engine was equipped with a two-stage, two-speed supercharger. While the primary stage of the supercharger was mechanically driven, the auxiliary stage used an infinitely variable fluid coupling. An air-to-water aftercooler was incorporated on each of the three intake pipes between the supercharger and where the pipe branched into the two intake manifolds. Coolant for the aftercooler system was circulated by a separate pump. Reports indicate that the Jumo 222 E/F had sodium-cooled intake valves and a 6.75 to 1 compression ratio.

The Jumo 222 E/F’s maximum power at 3,000 rpm was 2,500 hp (1,864 kW) for takeoff and 1,930 hp (1,439 kW) at 29,528 ft (9,000 m). Climbing power at 2,700 rpm was 2,220 hp (1,655 kW) at sea level and 1,680 hp (1,253 kW) at 36,000 ft. Cruising power at 2,500 rpm was 1,840 hp (1,372 kW) at sea level and 1,400 hp (1,044 kW) at 34,689 ft (11,000 m). The engine’s fuel consumption at cruise power was .454 lb/hp/hr (276 g/kW/h) at sea level. At 42,651 ft (13,000 m), an output of 1,710 hp (1,275 kW) at 2,900 rpm was possible with GM 1 (Göring Mischung 1 / Göring Mixture 1) nitrous oxide injection. The addition of MW 50 (Methanol-Wasser 50), a 50-50 mixture of methanol and water injected into the induction system, further boosted performance by approximately 400 hp (298 kW) up to the engine’s critical altitude. The engine was 8 ft 2 in (2.50 m) long. The Jumo 222 E weighed 3,009 lb (1,365 kg), and the Jumo 222 F weighed 3,075 lb (1,395 kg). First run in 1944, the engine initially received a high priority. However, development and plans for mass production of the Jumo 222 E/F were halted in mid-1944 to focus resources on the Jägernotprogramm (Emergency Fighter Program).

Reports indicate that the Jumo 222 E/F was flown in Ju 288 V9, and presumably it was also tested in a Ju 52. Heinkel He 219 V16 was planned to test Jumo 222 A/B-3 engines, but Jumo E/F engines were used instead when the aircraft made its first flight on 23 July 1944. Estimates indicated that the Jumo E/F-powered He 219 would be capable of 414 mph (666 km/h) at 32,808 ft (10,000 m) and 435 mph (700 km/h) with MW 50 injection at 26,247 ft (8,000 m). With the shift in priorities, He 219 V16 made less than 20 flights, and the project was abandoned by January 1945. Six Jumo 222 E/F engines were finished by war’s end, and another four were partially completed.

In early 1944, the Jumo 222 G/H (sometimes referred to as the Jumo 222 Turbo) was developed from the A/B-3 series with a 5.51 in (140 mm) bore, 5.31 in (135 mm) stroke, and 3,044 cu in (49.88 L) displacement. The engine incorporated a turbocharger and intercoolers. Running at 3,200 rpm, the Jumo G/H produced 2,400 hp (1,790 kW) for takeoff and 2,070 hp (1,544 kW) at 40,354 ft (12,300 m). At 2,900 rpm, the engine produced 1,970 hp (1,469 kW) at 41,339 ft (12,600 m). A single Jumo 222 B-2 was used as the G/H test engine and made 22 runs before the end of the war, but it was not installed in any test aircraft.

On 28 April 1944, the Otto-Mader-Werke at Dessau, which was developing the Jumo aircraft engines, was heavily damaged by an Allied bombing raid. As a result, the Jumo 222 program was relocated to Oberursel near Frankfurt. These events caused major delays with all tests and engine work then in progress.

Various versions of the Jumo 222 were flown in approximately 11 aircraft: three Ju 52 test beds, six Ju 288s (V5, V6, V8, V9, V12, and V14), one Fw 191 (V6), and one He 219 (V16). Jumo 222 engines were also planned for the Heinkel He 219B and C and the Hütter Hü 211 heavy fighters. Engines were not ready for the He 219B and C airframes, and the two Hü 211 prototypes were destroyed while under construction during an Allied bombing raid in December 1944. Some sources state that Jumo 222 engines were fitted to a four-engine Heinkel He 177 (V101), as the burned out remains of this aircraft were found at Cheb in Czechoslovakia. However, examination of the aircraft reveals the engine’s exhaust stacks were in the standard four and eight o’clock positions for a Daimler-Benz DB 603 engine rather than the 2, 6, and 10 o’clock positions for the Jumo 222. The Jumo 222 was proposed for numerous other aircraft designs ranging from fighters, like the Focke-Wulf Ta 152, to bombers, like the Heinkel He 177. However, none of these plans came to fruition. A total of 289 Jumo 222 engines were built.

The Jumo 225 was conceived back in 1937 as a development of the basic Jumo 222. The Jumo 225 was a 36-cylinder engine with six banks of six cylinders. With the original 5.31 in (135 mm) bore and stroke, the Jumo 225 displaced 4,245 cu in (69.57 L). The engine was forecasted to produce 3,500–4,000 hp (2,610–2,983 kW) at 3,000 rpm and was 8 ft 10 in (2.69 m) long. The Jumo 225 was never built.

While the Jumo 222 was not trouble-free, its development progressed as well as could be hoped for considering it was a new engine design, the repeated changes to engine requirements and design, and that the ongoing war resulted in material shortages. Some contend that the changing Ju 288 and Jumo 222 requirements were intentionally made to cause the aircraft and engine to fail.

Heinrich Koppenberg was the managing director of Junkers, the only German company producing both aircraft and aircraft engines. Koppenberg had become a powerful man who worked himself into various positions that gave him control over many strategic resources. Erhard Milch was the Air Inspector General of the Luftwaffe and in charge of aircraft production. He had gained increasing control over aircraft procurement in Germany. Milch felt that Koppenberg and Junkers would have an aircraft production monopoly and economically ruin other companies if the current Ju 288, Jumo 222, and other company projects were successful. New large-scale Junkers production orders meant that resources at other companies would be allotted to produce Junkers products under license rather than develop their own. Some contend that Milch began to alter the official requirements just as they were about to be met by Junkers. After Ernst Udet, head of the T-Amt (Technisches Amt, Technical Office of the RLM), committed suicide on 17 November 1941, Milch took his place. Milch now had the power to dictate programs for the Luftwaffe. Acting as the RLM’s authority, Milch continued to change project requirements, which left Junkers to perpetually chase the goal. Koppenberg was imprisoned in April 1942 when Junkers repeatedly failed to achieve what the RLM asked of them. While the above may be true, it is also true that the Jumo 222 had its own design issues. Brandner felt the engine was “developed to death” with its numerous displacement changes and constant design revisions.

In Spring 1944, Japan and Germany entered negotiations for Japan to purchase production rights for the Jumo 222. An agreement was reached in September 1944 that included drawings, sample parts, and the assistance of Brandner in exchange for 10 million Reichsmarks. The trip was to be made via submarine, and the departure date was set for mid-January. However, Brandner was shifted to resolve issues with the Jumo 004 turbojet in December 1944 and never went to Japan. It is not clear if Jumo 222 parts and plans were ever sent.

Brandner was captured by the Soviets at the end of the war and was interned in the USSR until 1953. While there, he worked on the M-222 engine design, which was essentially a reconstruction of the Jumo 222. Although the Soviets had captured five examples of the Jumo 222, the M-222 engine was never built. Among other projects, Brandner led a team that developed the 12,000 hp (8,948 kW) Kuznetsov NK-12 turboprop engine that powered the Tupolev Tu-95 Bear and other aircraft.

In addition to the Soviets, the United States and the British captured a number of Jumo 222 engines at the end of the war. A Jumo 222 E was built up by the United States Army Air Force at Wright Field with the intent to test the engine’s performance. While the engine was mostly complete by the end of 1946, other priorities took precedence, and the captured Jumo 222 E was never tested. Most likely, this engine was returned to Germany in 1978. It is now on display at the Deutsches Museum in Munich and is the only Jumo 222 known to exist.

Sources:
– “The Junkers Jumo 222” by Kimble D. McCutcheon, Torque Meter Volume 6, Number 3 (Summer 2007)
– Junkers Flugtriebwerke by Reinhard Müller (2006)
– Flugmotoren und Strahltriebwerke by Kyrill von Gersdorff, et. al. (2007)
– Ein Leben Zwischen Fronten by Ferdinand Brandner (1973)
– Junkers Aircraft & Engines 1913–1945 by Antony Kay (2004)
– Jane’s All the World’s Aircraft 1945/46 by Leonard Bridgman (1946)
– Junkers Ju 288/388/488 by Karl-Heinz Regnat (2004)
– Heinkel He 219 by R. Francis Ferguson (2020)
– Dornier Do 217-317-417 An Operational Record by Manfred Griehl (1991)
– http://www.enginehistory.org/Piston/Junkers/Jumo222/Jumo222.shtml
– https://ww2aircraft.net/forum/threads/jumo-222-whats-the-truth.39301/
– https://digital.deutsches-museum.de/item/1978-17/

By William Pearce

In the mid-1930s, the British Air Ministry predicted the need for 2,000 hp (1,491 kW) engines to power new aircraft expected to enter service in the early 1940s. Rolls-Royce responded to this anticipated need with a 24-cylinder, liquid-cooled aircraft engine of an X-configuration, known as the Vulture. Initially, the Vulture design was based on utilizing four six-cylinder banks of the V-12 Kestrel engine. As the Vulture design developed, many changes were incorporated that shifted away from the Kestrel, and the Vulture ultimately had no parts in common with the Kestrel.

The Rolls-Royce Vulture was designed by Albert George Elliott, and its development was started in September 1935. The engine’s two-piece aluminum crankcase was split horizontally at the crankshaft’s centerline. Each crankcase half had two surfaces for mounting cylinder banks with an included angle of 90 degrees. The two crankcase halves were attached by 28 cross bolts and a series of smaller bolts along the parting flange. The cross bolts were tightened against the cylinder bank mounting surface and staggered to allow clearance for the cross bolts from the adjoining bank. Each side of the crankcase had two engine mounting pads. The single, hollow, six-throw crankshaft was secured between the two crankcase halves and supported by seven main bearings.

Each of the four monobloc cylinder banks was made of aluminum with an integral cylinder head. Steel liners were inserted for the six cylinders of each bank. Each cylinder bank was secured to the crankcase by 26 long studs that passed through to the top of the bank. The cylinder spacing was wider than that of the Kestrel to accommodate wider connecting rod bearings and to enable a future increase in bore diameter. Each cylinder had two intake valves and two sodium-cooled exhaust valves. The valves for each cylinder bank were actuated by a single overhead camshaft that was driven via bevel gears and by a vertical shaft from the gear reduction at the front of the engine.

The Vulture’s connecting rod consisted of a master rod extending at a 45-degree angle from a square big end, with three articulating rods extending from the other corners of the big end. Initially, the connecting rod’s big end cap had a hinged joined on one side and was secured to the crankshaft with two bolts on the opposite side. Although different versions were tried, this configuration proved problematic and was replaced by omitting the hinge and using four bolts (two long bolts on one side and two short bolts on the other) to secure the cap to the connecting rod around the crankpin. The mating surfaces of the big end had corresponding serrations to ensure a secure fit. Incidentally, this was the same type of big end employed on the connecting rods of the Rolls-Royce Exe, the development of which had slightly proceeded that of the Vulture. When viewed from the rear of the engine, the upper right cylinder bank was designated as the ‘A’ bank, and the designations proceeded counterclockwise. The master rod served the ‘D’ bank, which was the lower right.

At the front of the engine, a spur gear on the crankshaft engaged four compound layshafts, the opposite side of which drove the propeller shaft. This compound gear reduction resulted in the propeller turning .350 times crankshaft speed and being mounted on the engine’s centerline. Viewed from the rear, the crankshaft and propeller both rotated counterclockwise. A bevel gear on the back side of each compound layshaft drove the vertical shaft for the respective cylinder bank’s camshaft. A spur gear on the rear of the crankshaft supplied power to various accessory drives and to the two-speed, single stage supercharger. The supercharger’s impeller turned at 5.464 and 7.286 times crankshaft speed in low and high gears. A coolant pump was mounted by each side of the supercharger. The engine’s compression ratio was 6.0 to 1.

Air was drawn through the two-barrel SU carburetor and fed into the supercharger. The air/fuel mixture exited the supercharger via two outlets that respectively fed an upper or lower manifold. Each manifold was respectively positioned between the upper or lower cylinder banks. The manifold had three outlets on each side. The three outlets were connected to another manifold that was attached directly to and extended the length of the cylinder bank. The incoming charge for each cylinder was ignited by two spark plugs, one positioned in the intake side of the cylinder and the other on the exhaust side. This meant that access to the top, bottom, left, and right sides of the engine was needed to replace the spark plugs. The task was further complicated by the intake manifolds on the top and bottom and the exhaust manifolds and engine mounts on the left and right sides of the engine. Needless to say, the 24-cylinder Vulture was not a favorite with ground crews. The spark plugs were originally fired by a battery-powered coil ignition system, which was replaced by two magnetos and distributors driven from the gear reduction. The exhaust ports were on the left and right sides of the engine. A mixture of 70 percent water and 30 percent ethylene glycol was used to cool the engine.

The Vulture had a 5.00 in (127 mm) bore and a 5.50 in (140 mm) stroke. The engine’s total displacement was 2,591 cu in (42.47 L), and it had a takeoff rating of 1,800 hp (1,342 kW) at 3,200 rpm with 6 psi (.41 bar) of boost. At 3,000 rpm with 6 psi (.41 bar) of boost, the Vulture had a maximum rating of 1,845 hp (1,312 kW) at 5,000 ft (1,524 m) and 1,710 hp (1,223 kW) at 15,000 ft (4,572 m). At 2,850 rpm with 6 psi (.41 bar) of boost, the Vulture had an international rating of 1,780 hp (1,327 kW) at 4,000 ft (1,219 m) and 1,660 hp (1,237 kW) at 13,500 ft (4,115 m) and a maximum climb rating of 1,760 hp (1,312 kW) at 5,000 ft (1,524 m) and 1,640 hp (1,223 kW) at 15,000 ft (4,572 m). At 2,600 rpm with 5 psi (.34 bar) of boost, the engine had a maximum cruise rating of 1,540 hp (1,148 kW) in low gear and 1,460 hp (1,089 kW) in high gear. The Vulture was 87.2 in long, 35.8 in wide, and 42.3 in tall. The engine weighed 2,450 lb.

Preliminary testing of the Vulture engine included building an X-4 engine, and running this engine revealed the issues with the early two-bolt connecting rod design. Stresses on the bolts caused their failure, and the four-bolt connecting rod was developed. Another issue was insufficient lubrication of main bearings. The first complete 24-cylinder Vulture was run on 1 September 1937, the second in January 1938, and the third in May 1938. By November 1938, Vulture test engines had accumulated 1,150 hours of operation. Issues with the coil ignition system came to light while testing the complete engines, resulting in a switch to magnetos. In 1938, the Vulture produced 1,750 hp (1,305 kW) while on test.

Vulture engine development spanned from Mark I to Mark V. The Vulture I entered limited production and were mainly developmental engines. Refinements were incorporated into the Vulture II, which was intended for use in multi-engine aircraft. The Vulture II had a detached, five-drive, auxiliary gearbox that was driven from the engine by a flexible shaft. The flexible shaft connected to a right-angle drive mounted vertically behind the A (upper right when viewed from the rear) cylinder bank. The Vulture II was first run in September 1938. No descriptive information has been found regarding the Vulture III. The Vulture IV was nearly identical to the Vulture II but intended for single-engine aircraft. The Vulture IV had an engine-mounted three-drive auxiliary gearbox and different accessories.

The Air Ministry authorized engine production on 23 March 1939, anticipating a need for 1,560 Vultures, and true engine production started in January 1940. Issues with the Vulture necessitated a drop in its maximum speed to 3,000 rpm, but boost was increased to 9 psi (.62 bar) to maintain the engine’s takeoff rating of 1,800 hp (1,342 kW).

Development of the Vulture V followed that of the Vulture IV and featured additional supercharging, with an impeller that turned at 6.018 and 8.111 times crankshaft speed in low and high gears. For takeoff, the engine had a rating of 1,995 hp (1,488 kW) at 3,000 rpm with 9 psi (.62 bar) of boost. Military power at the same rpm and boost was 2,035 hp (1,517 kW) at 5,000 ft (1,524 m) and 1,840 hp (1,372 kW) at 20,250 ft (6,172 m). At 2,650 rpm and with 7 psi (.48 bar) of boost, the Vulture V had a cruise rating of 1,650 hp (1,230 kW) at 3,500 ft (1,067 m) and 1,525 hp (1,137 kW) at 17,500 ft (5,334 m).

The Hawker Henley light-bomber prototype (K5115) was converted with a Vulture engine to serve as a testbed. A ventral scoop was added to the aircraft’s bomb bay that housed the radiator and oil cooler. The cowling was modified for the Vulture with its four rows of exhaust stacks, and a scoop for the carburetor was added just forward of the cockpit. The Vulture-powered Henley was first flown on 17 April 1939, and the Vulture passed a type-test with an 1,800 hp (1,342 kW) takeoff rating in August 1939. A second Henley (L3302) was converted to a Vulture testbed and made its first Vulture-powered flight on 3 May 1940. The Vulture engine was intended for a number of aircraft under development, four of which were flown.

The Avro 679 Manchester medium bomber used two Vulture I engines and was ordered in mid-1937, before the aircraft’s design was finalized. Eventually, orders for some 700 examples were placed. The Manchester prototype (L7246) made its first flight on 24 (some sources state 25) July 1939. When Vulture II engines became available, they were used in the Manchester, and the type entered service in November 1940.

The Vickers Type 284 Warwick medium bomber was originally ordered in October 1935, but a change for the first prototype (K8178) to be powered by two Vulture I engines rather than the Bristol Hercules occurred in January 1937. K8178 made its first flight on 13 August 1939, and Vulture II engines were installed in November 1940.

Two prototypes of the Hawker Tornado fighter were ordered in December 1938. The first prototype (P5219) was powered by a Vulture II engine and made its first flight on 6 October 1939. Production contracts were issued in November 1939, with the Vulture V selected as the intended powerplant. The second prototype (P5224) used a Vulture V engine and made its first flight on 7 December 1940.

The Blackburn B-20 flying boat (V8914) was ordered in 1936 and made its first flight on 26 March 1940. The experimental aircraft was powered by two Vulture II engines and featured an extendable hull and retractable wing floats. The aircraft was lost on 7 April 1940 after aileron flutter was experienced during a high-speed test flight.

In March 1941 the improved Vulture II was type tested with a takeoff rating of 2,010 hp (1,499 kW) at 3,000 rpm with 9 psi (.62 bar) of boost. At the same rpm and boost, the engine’s military power rating was 1,845 hp (1,376 kW) at 5,000 ft (1,524 m) and 1,710 hp (1,275 kW) at 15,000 ft (4,572 m). At 2,850 rpm and with 6 psi (.41 bar) of boost, the Vulture II had a normal rating of 1,780 hp (1,327 kW) at 4,000 ft (1,219 m) and 1,660 hp (1,238 kW) at 13,500 ft (4,115 m). However, the Vultures in service were taking a turn for the worse.

The Manchester’s rush into production and subsequent rush into service meant that a number of deficiencies with the airframe and serious issues with the Vulture engine were not discovered until it was too late. The engines proved to be unreliable and prone to failure. As a result, all Manchesters were grounded numerous times. Manchesters with a failed Vulture were often unable to maintain height on one engine, and about 75 percent of the time, the aircraft crashed before an emergency landing could be executed at a suitable location. A contributing factor to the Vulture’s issues was that the Battle of Britain forced Rolls-Royce to focus on the Merlin engine, which delayed Vulture development.

Some engine failures were attributed to cooling issues. One of the coolant pumps would cavitate, halting the flow of coolant to that side of the engine. The affected cylinder banks would subsequently overheat, and the engine would seize; an engine fire resulted on a number of occasions. To fix the issue, a balance tube was installed which connected the inlet of the pumps to equalize pressure between the two. The crankshaft main bearings were also prone to failure. Numerous issues resulted in the failed bearings: over-heating due to the already mentioned coolant issues, poor lubrication, ineffective bearing material, and a slight misalignment of the two crankcase halves. The Vulture’s lubrication system was reworked to prevent aeration, and a new LA4-type bearing material was adopted. The misalignment issue was solved by including locating dowels through which cross bolts passed. A dowel was positioned on each side of the main bearings between the crankcase halves. The most vexing issue was the random failure of bolts securing the connecting rod cap. This typically created cascading failures that resulted in the sudden and catastrophic loss of the engine. The issue was traced to brittle bolts, and new measures were implemented to ensure they were tightened to the new, lower toque standard to prevent excessive strain and stretching. The connecting rod was also modified slightly. In addition, the Vulture’s maximum speed was reduced again to 2,850 rpm to minimize the risk of failure. The last of these changes were detailed by Rolls-Royce under Vulture Modification No. 44. By August 1941, engines with these changes were installed in some Manchesters, and the Vulture began to reliably make it 120-hours between major inspections. In addition, Manchesters were now able to make it to an airfield on a single engine more often than not. Eventually, the time between inspections was raised to 180 hours, and the engine’s maximum takeoff speed was increased to 3,000 rpm. However, another issue with Vulture engines came to light in late 1941. Exhaust manifolds were cracking and failing, resulting in a jet of hot gasses flowing against the engine, cowling, or other internal components. The failed manifolds caused engine failures or airframe damage or both. A new manifold was designed, and all of the older units were replaced in December 1941.

Even though the main problems with the Vulture were mostly resolved, engines continued to encounter various random issues, including failures, overheating, lack of power, and excessive fuel consumption. Overall, there was little faith in the Vulture engine. The Manchester itself continued to have issues, and production was halted in November 1941. Of the 202 aircraft built, approximately 33 (16.3 percent) crashed or were struck off charge due to engine failures or fires. This number does not include aircraft that were repaired after an engine failure, nor does it include the six or so aircraft lost due to propeller issues (some of which precipitated an engine failure). Tragically, also not included are the numerous Manchesters that crashed after one engine was knocked out from battle damaged only to have the “good’ engine fail after it was overstressed trying to keep the underpowered aircraft aloft. The Manchester was withdrawn from operations in mid-1942 and served in various secondary roles through 1943, when all examples were scrapped.

The Manchester was redesign to use four Merlin engines and became the Lancaster (originally Manchester III), one of the greatest World War II bombers. Production Warwicks were fitted with either Pratt & Whitney R-2800 or Bristol Centaurus engines. While around 1,760 Tornados were ordered at one point, only three Vulture-powered examples were built, and the Napier Sabre-powered Typhoon took over in place of the Tornado.

By September 1942, a Vulture engine with a contra-rotating gear reduction was installed in the sole-production Tornado (R7936). The engine and aircraft were used to test Rotol and de Havilland contra-rotating propellers. Some sources report that one Vulture engine was built with its bore increased by .4 in (10 mm) to 5.4 in (137 mm), the same as the Merlin. This increased the engine’s displacement by 432 cu in (7.08 L) to 3,023 cu in (49.54 L). However, no further information on these engines has been found.

From as early as August 1939, Rolls-Royce wanted to cancel Vulture development so that the company could focus its resources on other engines, mainly the Merlin and Griffon. However, the Air Ministry felt that it needed the Vulture engine, so development continued. Vulture development was halted in October 1941, and production ended in March 1942, with 538 engines built. The Vulture was the only X-24 aircraft engine to enter production.

Rolls-Royce had designed a number of changes to be incorporated into the Vulture engine if production had continued. The connecting rod was redesigned with the three articulated rods attached to the bearing cap, and the cap was secured to the master connecting rod via four long bolts made from improved material. The cylinder banks were redesigned to incorporate a detachable cylinder head. A lighter planetary gear reduction for the propeller would have replaced the four compound layshafts. The two-speed supercharger was redesigned to include two-stage supercharging to improve the engine’s performance at higher altitudes.

Only a small number of Vulture engines survive, and most were recovered from Manchester wrecks. Two recovered Manchester engines (engine 1 and engine 2) are held by the Luchtoorlogmuseum (Aerial Warfare Museum) Fort Veldhuis in Heemskerk, near Amsterdam in the Netherlands. A Vulture engine from the B-20, consisting mainly of the crankshaft, connecting rods, and cylinder barrels, is displayed in the Dumfries and Galloway Aviation Museum in Scotland. Three engines are part of the Royal Air Force Museum’s collection, and all are believed to have been recovered from Manchester wrecks. One of these engines is on loan to the Rolls-Royce Heritage Trust and is displayed at the Hucknall Flight Test Museum.

Note: Many sources state that the Vulture I used an updraft carburetor, and the Vulture II and later variants used a downdraft carburetor. However, the only aircraft that appears to have had an updraft carburetor was the first Tornado prototype, which reportedly flew with a Vulture II. Early Manchesters that reportedly flew with Vulture Is appear to have downdraft carburetors. In my opinion, the most logical explanation, although still questionable, is that all Vultures had downdraft carburetors and that the early installation in the Tornado prototype that incorporated the carburetor inlet with the belly scoop was an attempt to maximize the pilot’s forward vision and minimize the number of external protuberances.

Sources:
– Major Piston Engines of World War II by Victor Bingham (1998)
– The Avro Manchester: The Legend Behind the Lancaster by Robert Kirby (2015)
– Rolls-Royce Piston Aero Engines – a designer remembers RRHT 16 by A. A. Rubbra (1990)
– Rolls-Royce Vulture II and IV Description: Air Publication 1801A Volume I, via the Aircraft Engine Historical Society (December 1940)
– Rolls-Royce Aero Engines by Bill Gunston (1989)
– Aircraft Engines of the World 1945 by Paul H. Wilkinson (1945)
– Hawker Aircraft since 1920 by Francis K. Mason (1991)
– Avro Aircraft since 1908 by A. J. Jackson (1990)
– Vickers Aircraft since 1908 by C. F. Andrews and E. B. Morgan (1988)
– Blackburn Aircraft since 1909 by A. J. Jackson (1989)
– https://www.key.aero/forum/historic-aviation/62280-rolls-royce-vulture-survivors