THE STORY OF Germany’s jet engines is inseparable from that of the aircraft they were intended for. Those which actually flew operationally under jet power were the experimental Heinkel He 178 and He 280, the Messerschmitt Me 262, the Arado Ar 234 and the diminutive Heinkel He 162 Volksjäger. Experimental aircraft to fly were the Horten tail-less Ho 229 and the forward-swept Junkers Ju 287 testbed. In addition to these there was a diverse range of proposals for future jet aircraft and some were actively under development by the end of the war, most notably the Messerschmitt P.1101.
German interest in turbojets can be traced back to the early 1930s, when Hans von Ohain began his research at the University of Göttingen. While von Ohain was aware of the work of the British jet pioneer, Frank Whittle, he did not appear to have had any comprehensive information on Whittle’s designs and the two engineers are generally credited with producing their engines concurrently. By 1935 von Ohain had been able to demonstrate the principle of his turbojet and the following year he attracted the interest of the Heinkel company for whom he produced the Heinkel-Hirth HeS 1 engine. (The Hirth company had been founded by Hellmuth Hirth in 1920 as a manufacturer of engine components. Following his death as the result of an aircraft accident in 1938 the RLM nationalised the company, and in 1941 it was merged with Heinkel who used the Hirth facilities for von Ohain’s work on jet engines.)
The HeS 1 featured a centrifugal compressor, annular combustion chamber and radial inflow turbine, and it was fuelled by hydrogen. Producing a static thrust of around 551lb (250kg) the HeS 1 was successfully bench-tested in 1937 and it led directly to the development of the first turbojet flight engine, the HeS 3. This was basically a tidied up and more compact version of the HeS 1, which had also been converted to burn liquid fuel instead of hydrogen.
The Heinkel company carried out this work on jet engines of its own volition, but in 1938 officials at the RLM instigated two lines of development that would ultimately come together in the world’s first operational combat turbojet, the Messerschmitt Me 262. Hans Mauch and Helmut Schlep of the RLM power-plant development division had initiated an official jet engine programme, while Hans Antz of the airframe development department started a complementary programme for jet and rocket-powered airframes. Both of these were visionary steps taken at a time when the Luftwaffe was demonstrating its aerial supremacy through conventional piston-power, and in official circles the jet initiatives were greeted with very little enthusiasm. With the beginning of the Blitzkrieg matters were so clearly going Germany’s way that few in authority could envisage a time when the Fatherland would need such aircraft to defend itself. Indeed, apart from Heinkel the other aircraft companies were not that interested either. With development of a jet engine already well in hand, Heinkel was ahead of its competitors, Junkers and BMW, and in the summer of 1939 an HeS 3B was mounted within the prototype Heinkel He 178. Apart from the motive power this was a fairly conventional-looking aircraft, quite small with a metal fuselage, high-mounted straight wings and a retractable undercarriage including rear drag wheel (although in the trials the front wheels remained fixed in the down position). At the nose there was a gaping hole, the intake for the single HeS 3B engine which was contained within the fuselage. On 27 August 1939 the He 178 took to the air with Heinkel’s test pilot Erich Warsitz at the controls of world’s first turbojet flight. In general the He 178 performed well, attaining speeds of up to 375mph (598km/h), but it was let down by its short combat endurance of only about ten minutes or so. This may explain why a demonstration of the new aircraft in front of RLM officials in November 1939 failed to impress.
Kassel’s Koenigstrasse rendered almost unrecognisable by the Allied bombing raids. (NARA)
The Heinkel He 178 made the world’s first jet-powered flight in August 1939 but failed to impress officials from the RLM.
Its successor, the twin-engined He 280, was airborne two years later but its introduction was hampered by delays with the HeS 8 turbojet. (USAF)
Undaunted, Heinkel pushed on with its successor; a twin-engined jet fighter this time, at first designated as the He 180, although this was later changed to the He 280. This design had a more slender fuselage, the engines were mounted on the low wings, and at the back of the fuselage was a dihedral or twin tail. To provide clearance for backwash from the engines the He 280 had a retractable tricycle undercarriage. It was also the first aircraft to be equipped with an innovative compressed-air powered ejector seat. Although the He 280 prototype airframe was ready by the summer of 1941, aside from some gliding tests the first jet-powered flight was held up by delays in the completion of the more powerful HeS 8 turbojet, and it wasn’t until 3 April 1941 that test-pilot Fritz Schäfer finally flew it under jet power. Unfortunately for Ernst Heinkel the hold-ups with the engines would prove costly as he was now in direct competition with his old rival Willy Messerschmitt.
THE ME 262
Unlike Heinkel, Messerschmitt’s involvement had come about in direct response to the RLM’s airframe development programme which called for a jet aircraft capable of flying for at least one hour and at a speed of 528mph (850km/h). Chief of development at Messerschmitt was Robert Lusser, and heading up the design team on the jet aircraft was Dr Woldemar Voigt. Under the project designation P.1065 – later to become the Me 262 – they had submitted their design proposal to the RLM in June 1939. This was for a twin-engined aircraft with engines mounted within the wings, and with the ministry’s approval a wooden mock-up had been completed by March 1940.
Messerschmitt had no direct involvement in the design of the jet engines and the RLM awarded contracts to both Junkers Motoren (otherwise known as Jumo) and BMW to develop a suitable turbojet capable of producing a static thrust of 1,496lb (680kg). This was despite the fact that the Heinkel company was already making progress with its von Ohain designs. Because Messerschmitt’s airframe programme was ahead of engine development the engineers revised the positioning of the engine pods, no longer embedding them within the wings but slinging them underneath to allow for repositioning if required. This would also make it far easier to replace them; an important factor given the relatively short time period between major services. As it turned out the BMW 109-003s did prove to be heavier than anticipated and accordingly the wings of the Me 262 were swept back by an angle of 18.5° to properly position the aircraft’s centre of gravity. This did not mean that the Me 262 had a true swept-back wing as explained in the previous chapter. (The RLM used the same ‘109’ prefix for the turbojets as already allocated to rocket engines, but for the sake of clarity this prefix is generally not used when referring to the turbojets.)
Diagram of the Heinkel-Hirth HeS-011 advanced turbojet with a three-stage axial compressor and, below, a cutaway of the real thing. The HeS-011 was not ready to enter production before the war ended. (USAF)
At its Stassfurt facility the Fedden Mission came across a quantity of smashed BMW turbojet engines and components.
For its turbojet BMW concentrated on a counter-rotating compressor design at first with the intention of eliminating torque, but this was later abandoned in favour of the simpler axial-flow engine which became the BMW 003. Running for the first time in August 1940, its output was a meagre 331lb (150kg) – disappointingly only half the level that had been expected. The 003 was first tested in flight mounted beneath a Messerschmitt Bf 110, although this aircraft flew using its piston engines and the jet was only fired up for a limited duration. Further technical delays on the 003 meant that the prototype Me 262, the V1, had to begin its testing programme fitted with a Jumo 210 piston engine in the nose. Once the BMW 003 engines were ready the Me 262 V1 first flew under jet-thrust in November 1941, but turbine failures occurred on both engines shortly after take-off. It was a major setback for BMW and it took them a further two years to raise the output of their engines to 1,760lb (800kg). Consequently, by the time the 003 entered any sort of production run in 1944 the Jumo 004 engines were already powering the Me 262s.
BMW 003 axial-flow turbojet on display in the Luftwaffenmuseum, Germany. (MisterBee1966)
Under the guidance of Dr Anselm Franz the Jumo 004 utilised an axial compressor design which allowed a continuous straight flow through the engine. The 004 ran on three types of fuel: J-2, a synthetic fuel produced from coal; diesel oil; or aviation gasoline, although the latter was not ideal because of its extremely high rate of consumption. The first of the Jumo turbojets, the 004A, had begun bench-runs in November 1940 although it was not until 15 March 1942 that it was ready to be test flown beneath the Messerschmitt Bf 110. The Jumo engines were subsequently fitted to the Me 262 V3 prototype for that aircraft’s first true jet flight, which took place on 18 July 1942. The RLM immediately placed an order for eighty of the engines and by October that same year the designs for the production version, the 109-004B, were completed. The first of the production engines were delivered early the following year and successfully test flown on Me 262 V1 on 2 March 1943. Unfortunately a series of engine vibration problems held up full production until early 1944 and this had a knock-on effect delaying the introduction of the Me 262 into service.
The world’s first operational jet squadron, Erprobungskommando 262, or Ekdo 262, was formed in December 1944. Based at Lager-Lechfeld in Bavaria, the squadron was an experimental proving detachment for the Me 262 and would also be used to train a core of jet pilots. Ekdo 262 was equipped with the Me 262A-1a, commonly known as the Schwalbe (‘swallow’), which was built in both fighter and fighter/bomber versions. It was the most common type produced and was equipped with four short-barrelled MK108 30mm cannon embedded within the nose. The Me 262 soon proved to be an incredible aircraft. Legendary German fighter ace Adolf Galland described the experience of his first taste of jet flight as if being ‘pushed by angels’. The Allied bomber crews quickly came to recognise its distinctive shark-like snout and slightly swept wings and they marvelled at its incredible speed; up to 560mph (900km/h) in level flight. By the end of the war around 1,430 of the Me 262s had been built, but despite its undoubted superiority it was a case of too little too late to turn the fortunes of war in Germany’s favour.
A member of Fedden’s team examines a damaged compressor ring from a BMW 018 turbojet at Eisenach.
Drawing of BMW’s experimental 018 turbojet.
Full production of BMW’s 3306 turbojet was said to be still a year away by the war’s conclusion.
THE ARADO Ar 234
The second jet to enter service with the Luftwaffe was the Arado Ar 234, otherwise known as the Blitz (derived from Blitz-Bomber meaning ‘lightning bomber’, although strictly speaking this means that the Blitz name should only be applied to the ‘Schnellbomber’ Ar 234B variant). This twin-engined jet had begun life in 1940 in answer to an RLM specification for a high-speed reconnaissance aircraft with a range of 1,340 miles (2,156km). At that point it was unclear which of the two main jet engines, the Jumo 004 or BMW 003, would become available in time, but in the event it was to be the 004 once again. In appearance the Ar 234 was a high-wing design with an engine pod slung under each wing and it had a long slender fuselage with a rounded front canopy. To keep the aircraft below the RLM’s target weight of 17,600lb (8,000kg) the engineers at Arado had fitted early examples with a jettisonable three-wheeled landing trolley and triple landing skids, one skid under the fuselage and another under each engine pod.
Delays with the development of reliable turbojet engines held up the first flight and it wasn’t until 15 June 1943 that AR 234 V1 took off from the Rheine airfield equipped with a pair of Jumo 004B-0 engines.
The sixth prototype, V6, was powered by four BMW 003A-1s instead of the two Jumos and these were mounted in individual nacelles, whereas the V8 prototype had two pairs of 003A-1s installed within twinned nacelles. These two prototypes were the world’s first four-engined jets and the V8 became the first to fly, on 1 February 1944, followed by the V6 just a couple of months later on 8 April. However, it wasn’t all plain sailing for the Ar 234. The last of the A-series prototypes fitted with two 004B-1 turbojets, the V7, suffered a port engine fire during flight testing and Arado’s chief test pilot, Flugkapitän Selle, was killed as he attempted to land the aircraft. After this incident the AR 234 was fitted with a conventional retractable undercarriage. By then the RLM had come to recognise the aircraft’s potential as a fast bomber and the V9, the prototype Ar 234B, first took to the air on 10 March 1944.
As photographed by the Fedden Mission, bomb damaged Jumo 004 engines scattered around the factory yard at Magdeburg.
The V9 was equipped with cabin pressurisation and an ejector seat. On the V10 prototype bomb racks were installed beneath the engine nacelles as the restricted space within the aircraft’s slender fuselage was mostly occupied by fuel tanks.
In subsequent flight trials V15 and V17 were fitted with just two BMW 003A-1 turbojets to investigate problems with the engine’s thrust control, but these engines proved to be difficult to restart in flight after a flame-out, and the first pre-production versions of the Ar 234B-0 left the production lines at Alt-Lonnewitz with the Jumo 004 engines installed.
In the meantime, several of the prototype aircraft had entered active service in a reconnaissance role. Flying at a height of around 29,000ft (9,100m), the V7 became the first German jet to fly over the UK. Apart from engine flame-outs and the requirement for frequent engine overhauls, operationally the Ar 234s performed well and were generally much liked by their crews. There was an issue with excessively long take-off runs but this was addressed by fitting rocket-assisted take-off (RATO) units (see Chapter 9).
THE HEINKEL He 162
The last of the German jet aircraft to enter service was the diminutive Heinkel He 162 which carried a single BMW 003 engine mounted on its back. It was designed in response to a competition launched by the RLM in August 1944 to create a Volksjäger, a people’s fighter, which could tackle the Allied bomber formations. (The He 162 is also covered in greater detail in Chapter 9.)
From Fedden’s perspective the advent of the jet age threatened to undermine the dominance of the piston-driven aero engines, especially his beloved air-cooled radials which had been the cornerstone of his own career. But technological evolution could not be ignored and, as he observed in his pre-mission briefings, jet propulsion was obviously going to be the subject for ‘the most intensive study’:
We know of two gas turbine units being made in Germany, Jumo 004, used in the Me 262, which is in service in fairly large numbers, and the BMW 003, which is just coming into service in the Arado 234. The Daimler-Benz and Heinkel-Hirth companies are also believed to be producing the BMW unit. Heinkel-Hirth built their own design of unit two or three years ago, but this has presumably proved unsuccessful so far. Brown-Boveri are also making some form of gas turbine. [A German subsidiary of the Swiss company which made electric motors for locomotives and developed steam and gas turbines to power ships and, during the war, some U-boats.] It is anticipated that present German policy is to make jet propelled fighters the sole instrument of their first line fighter strength, and recent experiences by our bomber crews tend to confirm this.
This extract reveals that his pre-mission information on specific jet projects was still very sketchy at the time, which was only to be expected as examples of the aircraft had yet to fall into Allied hands for closer examination. An insight into the level of Allied data on the jets can be gauged from a confidential report on Jet Propelled Aircraft which was issued by the US Army Headquarters 115th Anti-Aircraft Artillery Group, in March 1945. Clearly it was essential that the anti-aircraft units were able to correctly identify the various new aircraft in order to deal with them and safely distinguish friend from foe. In the introduction to the report Colonel Peter S. Peca emphasised the significance of the German jets:
Me 262 V3 photographed at Leipheim prior to the first flight on 18 July 1942. Note the tail-dragger wheel arrangement raising the nose up into the air.
The 10hp Riedel two-stroke starter motor concealed within the intake cone of the Jumo 004. The ring of the starter handle protrudes on the right. (JC)
A steady rise in operational flights of German jet propelled aircraft over the western front has been noted since the first of this year. It is possible that as the war continues, the German jet propelled aircraft will become a serious menace to both air and ground operations. Due to the high speed and reduced noise level of jet aircraft, you will find them more difficult to detect, engage and destroy than the conventional type aircraft. You must be continually alert; learn to recognise these plane’s in a split second; utilise completely your AAAIS and other warning; and use your fire control equipment properly.
The last sentence was underlined for emphasis. Presumably the reference to the ‘reduced noise level’ really meant the reduced time in which to identify the fast-moving jets.
The report went on to provide information and, in most cases, reasonably detailed drawings, plan views and silhouettes of the key aircraft; the Messerschmitt 262, Arado 234, Messerschmitt 163A (it does point out that this was a short-duration rocket-powered aircraft for the ‘home-defence’ of specific sites) which were all listed as being both ‘fighter and fighter-bomber aircraft’, and the Heinkel 280 ‘fighter’. According to the document a P-47 Group had reported an encounter with an He 280 over the Rhine. However, this would have been impossible as only nine prototypes had been built and after the project was cancelled by Milch in March 1943 these were used only for experimental purposes. In all probability, the sighting over the Rhine was a misidentification of an Me 262.
THE HEINKEL He 343
Under the heading ‘Experimental’ several aircraft were listed, the He 343, Horten 3 and Dornier 325, but here the report strayed into the realms of speculation. The Heinkel 343 was described as a multi-seat fighter with a mid wing, the leading edge swept back and the trailing edge tapered. It was of all-metal construction, with the exception of the control surfaces, and it had a conventional tailplane mounted fairly high. ‘Thought to be a four-jet aircraft, but twin-jet units may be prototypes. Prisoner of war interrogations have definitely confirmed the existence of this aircraft.’ In truth the German POWs had been leading their interrogators astray and there was zero chance that the anti-aircraft gunners would see one of these. The He 343 had begun development in early 1944 as a type of enlarged Ar 234, and at least parts for the prototype, if not the whole aircraft, were in production when the project was cancelled at the end of 1944 due to the Emergency Fighter Programme. Post-war, the He 343 was the basis for the Illyushin IL-22, the first of the Soviet jet bombers when it flew in July 1947. The Il-22 is said to have led on to the twin-engined Il-28 of 1948.
THE HORTEN Ho 3
The Horten 3 was described by Fedden as ‘reportedly a rocket-propelled version of the unmanned aircraft also assumed to be in existence as a jet-propelled type’. Again a mishmash of half-facts. The Horten III was an unmanned glider built in 1938, while it was the Ho 229 (or Go 229) prototype H.IX V2 which had first flown with two Jumo 004s in February 1945. This flying wing was not combat-ready by the time the war ended.
THE DORNIER Do 325
Last on the list was the Dornier 325, described as ‘a suspected jet-propelled aircraft in experimental stage’. Clearly a reference to the double piston-engined Do 335 which was very fast but not a jet.
And what of the Allies own jets? Several were under development and even in production before the cessation of hostilities, but none saw active service over in the European theatre. Three Allied jets were included in the report to avoid possible misidentification as hostile aircraft. These were the Bell P-59A, the Curtiss P-60A Shooting Star and the British Gloster Meteor.
With events moving so rapidly in the latter stages of the war it was a race for the Allies to bring their data on the German jets up to date. Fedden was well aware that there was much to learn when his mission team reached Germany, as he revealed in the following handwritten caveat in the margins of his personal copy of the pre-mission briefing notes:
I would recommend that we do not accept too readily the idea that Germany is basically behind on this vital subject, because knowing something about the technicians who have been working on this development, and the length of time they have been on the job, I would be inclined to weigh this up very carefully and consider whether we have fully appreciated all of the contributing factors.
Although the Fedden team would examine various aspects of piston engine development in Germany (see Chapter 5), the terms of reference outlined in the introduction to the Final Mission Report also gave prominence to the investigation of gas turbines and jet engines under the following terms:
To endeavour to make a broad review of their relative development in Germany in comparison with this country, and to ascertain the cost of manufacture and general technique of jet engines, as compared with piston engines in Germany.
Clearly, even as the world stood at the threshold of the jet age it would take time for the more entrenched aeronautical community to come to terms with the impending paradigm shift from piston to jet.
Once in Germany Fedden’s team of experts targeted the three principle players in jet development and production: Junkers, BMW and Heinkel-Hirth. The first on the Mission’s itinerary was the Junkers works at Magdeburg.
The Zwiebel, or ‘onion’, located at the rear of the Jumo 004 could be moved forward or aft to alter the exhaust area and hence the thrust. (JC)
An abandoned Me 262A found intact in April 1945 by advancing US 9th Army troops at an airfield near Stendal. (NARA)
THE JUNKERS WORKS, MAGDEBURG
On Saturday 16 June they had divided into two groups after having been billeted overnight at the American Intelligence Centre, Camp Dentine, 15 miles from Kassel. One aeroplane flew north-east to Oschersleben and that group then went by Jeep on to Magdeburg, while the others were flown to Göttingen to continue investigations there.
Located on the River Elbe, Magdeburg is now the largest city in the Saxony-Anhalt region within unified Germany, but in the summer of 1945 it was inside the territory already allocated to the Russians, along with the Junkers main plant at Dessau. Consequently there was precious little time left to the Americans and British before it was handed over. The Junkers works at Magdeburg had been used for the manufacture and overhaul of the Jumo 004 jet engines. Fedden found that although the works did contain several piston engine test-beds there was no evidence of any experimental or development work having been undertaken on the jets. In general the works had been extensively damaged in the Allied bombing raids and the mission photographs show a number of bomb-damaged Jumo 004 engines strewn about in the outside areas.
Dr Franz, Junkers’ chief technician at Magdeburg, was not available to meet the British delegation and instead the former production director, Otto Hartkopf, dealt with them in his capacity as acting works manager. Hartkopf began by explaining that all of the drawings relating to the jets had already been removed by the Allies. Hartkopf said that over 5,000 of the engines had been produced overall (although the actual figure is now thought to be nearer the 8,000 mark). ?As well as the Magdeburg site a series of additional factories had been planned in order to raise manufacturing output up to an ambitious figure of 5,000 engines per month. This programme was well in hand by the time the war ended and more than 1,500 Jumo 004s were being delivered each month.
The production series Jumo 004 turbojet weighed 1,543lb (700kg) and produced 1,984lb (900kg) of thrust at 8,700rpm. It consisted of an eight-stage axial flow compressor, multiple combustion chambers built up from sheet steel and axially positioned around the body of the engine, and a single-stage turbine incorporating hollow blades. The engine had two interesting features. The exhaust had a variable geometry nozzle which could be adjusted by the movement of a restrictive body nicknamed the Zwiebel (‘onion’). This varied the jet exhaust cross-sectional area for thrust control. And concealed within the intake cone there was a 10hp Reidel two-stroke engine which was used as a starter engine, the ring of its starter handle protruding at the front. The compressor casing, of cast magnesium, was split axially into two halves, each one having bolted to it the half sections of the stator assemblies. (Fedden comments that this is one aspect of the design that did not appear to have been executed very satisfactorily.) The first four stators consisted of profiled alloy blades located in their mounting rims by welds, while the last five stators were pressed sheet blades which were located on the rims by bending over and welding. The compressor blades were also of alloy and dovetailed into slots on the compressor discs and locked into position by small axial screws, half in the blade root and half in the disc. The compressor discs were, in turn, assembled onto a steel shaft and fastened by twelve radial set screws.
Jumo had been experimenting with various construction methods for its compressor blades, although it is not clear whether this was an attempt to solve particular problems or simply to improve the production process. In early models the turbine blades had been of solid steel, but on later versions the blades were hollow, formed from sheet metal and welded along the join line on the tapering edge. Their roots were formed to fit over rhomboid-shaped studs on the turbine wheel to which they were pinned and brazed.
BMW’S WORKS AT EISENACH AND STASSFURT
To investigate production of BMW’s 003 jet engines the Fedden Mission travelled to two separate locations, Eisenach in the foothills of the Thuringia Forest, and Stassfurt, which is between Nordhausen and Dessau. Both of these establishments were within the soon-to-be Russian zone. On Monday 18 June, the mission had split into two once again, with one party travelling southwards by road to the BMW jet engine works at Eisenach. BMW’s jet engine research had originally been undertaken in Berlin, under the control of Mr Bruckmann and his assistant Dr H. Oestrich. Since 1942 Bruckmann had taken charge of the company’s piston engines, and it was in that capacity that the Fedden Mission met with him in Munich (see Chapter 5) but he was also an enthusiastic advocate of the jet engine for high-speed aircraft. In particular he foresaw a potential application for turbine engines to drive propellers, in other words turbo prop engines. Fedden reports that Bruckmann was of the opinion that they would eventually come into widespread use for long-range aircraft once the development of suitable heat exchangers had been brought to fruition:
He stated that it would be a relatively simple matter to design a 5,000hp piston engine now, but that a propeller turbine engine would probably take five years’ development to arrive at an equivalent fuel consumption.
Oestrich, who had taken over responsibility from Bruckmann for the BMW jet division, had previously been a ‘piston man’ at Spandau in Berlin, the former Siemens works which were absorbed by BMW in 1938. He had started working on jets in 1939 and as with the other German engineers he had chosen the axial flow compressor design because of its smaller diameter.
At Eisenach the mission also interrogated Dr Schaaf, the managing director, as well as two directors listed as Dr Fattler and Dr Stoffregen. They said that around 11,000 men had been employed at Eisenach, with 4,500 of them working in a camouflaged factory located within the side of the hill, producing the Type 132 nine-cylinder piston engines and parts for the 801 fourteen-cylinder engine. The remainder worked in the town itself at the factory that had produced motorcycles and aero engine components up until 1944. It was then that BMW had been instructed to concentrate on the jet engines with 003 production planned at Eisenach, Spandau, Nordhausen and later Prague, to meet the overly ambitious target output of about 5,000 to 6,000 engines per month. In the event, because the BMW 003 lagged behind the Jumo 004, many more Junkers engines were completed by the end of the war than the BMW 003s.
The Lockheed P-80A Shooting Star first flew in 1944, but the Americans had no jets in service by the end of the war. This example was photographed at the Ames Aeronautical Laboratory, Moffett Field, in 1946. (Nasa)
Several models of the BMW 003 were produced although they were basically variants of the 003/A1. The main difference between it and the Jumo 004 is that the BMW engine used an annular combustion chamber containing sixteen individual burners instead of separate chambers. The BMW 003 was approximately 4in (10cm) smaller in diameter – an important factor for a high-speed aircraft – and despite its lower thrust the engine’s reduced weight and diameter resulted in similar aircraft speeds.
The BMW 003’s compressor casing was not split and there were other details on the compressor parts that differed from the Jumo 004. According to Fedden the 003s were generally better executed in detail. The stator blades – stationary fans between each pair of rotors which realigned the gas flow to more effectively direct it towards the blades of the next rotor – were all dural alloy pressings inserted into magnesium rings with provision for expansion, the blades being bent over and spot-welded to the inner ring but still free to expand in the outer ring. The compressor rotors were also alloy, mounted in annular grooves in magnesium rotor wheels and pinned in position. The rotor wheels were assembled on the shaft with the stator rings loosely in position before the whole assembly was inserted into the cast magnesium compressor casing. In the initial versions the rotor blades had a tendency to fracture after only a few hours of running; most probably the result of bending fatigue caused by a resonance effect in the wake of the three bearing supports. Despite efforts to cure this by increasing the number of supports to four, the only reliable solution was to considerably stiffen the blades of the first stage.
Originally the compressor had six stages but this was subsequently increased to seven, with the intention of taking it to eight in later models. The BMW 003 used hollow turbine stator blades with air-cooling for the blades bled from the fourth stage of the compressor, apparently consuming about 5 per cent of its capacity in the process. The turbine blades were dovetailed into shaped groves within the turbine wheel and secured by end plates. In the 003/A2 series the number of blades was reduced to enable further strengthening of the root fixings.
Arado Ar 234 jettisoning the launch trolley, with landing skids extended beneath the fuselage and engine pods. Despite the weight penalty this system was replaced by a retractable undercarriage.
Messerschmitt’s 1944 drawings of the Me 262A-1a.
The BMW 003 turbojets underwent several improvements during development. The models AO, A, A2, E and E2 were the production types and featured only minor changes, while the 003/C and D versions incorporated major modifications to increase thrust from 1,768lb (800kg) to 1,990lb (900kg) and finally to 2,762lb (1,250kg), the latter incorporating eight compressor stages and a two-stage turbine.
The Fedden Mission learnt that an experimental version of the 003/A had been produced incorporating a special non-expendable rocket motor fitted in the rear end to provide an additional 2,762lb (1,250kg) of thrust for take-off or rapid climb as well as dash on-demand at critical moments in combat. This was not a separate or jettisonable unit like the RATO assistors, but was a permanent fixture fed by fuel pumps. The duration of these rockets was determined by the capacity of the tanks carrying the special fuels (see Chapter 9). One Me 262 fitted with this unit is claimed to have climbed to a height of 40,000ft (12,000m) in just three minutes.
Fedden states that the mission found the quality of the work at Eisenach to be excellent. But the long delays in development meant that the 003 lost out to the simpler Jumo 004 on the main jet types, the Me 262 and the Ar 234. Apart from a handful of tests on various Me 262 prototypes and experimental aircraft such as the Horten Ho 229, the only production aircraft to fly with the BMW 003 were the He 162 and some four-engined versions of the Ar 234. It is estimated that around 500 of the engines were built, mostly 003-A1s and a few of the A2s. However, production at the Eisenach factory had been severely disrupted by the Allied bombing raids.
The day after the Eisenach trip the mission inspected an underground BMW jet engine plant at Stassfurt. This had been set up in 1944 within a salt mine 0.75 miles (400m) underground and was to have been used for the machining operations on the BMW 003 and, possibly, final engine assembly as well. Mr Stoffergen explained that 1,700 machine tools had been installed on the site and around 2,000 people were employed there, but by the time of Fedden’s visit widespread looting had left the underground factory in a state of utter chaos. He did manage to obtain some information on BMW’s experimental 018 engine which was generally along the lines of the 003 but with a twelve-stage axial flow compressor and a three-stage turbine giving around 7,770lb (3,500kg) of thrust. Development of the 018 had commenced in 1940, although a complete engine had not been produced and the workers had destroyed the remains of an 018 compressor and a number of jet engines before the place was overrun by the Allies. Fedden did examine some examples of the compressor blade forgings and also a blade from the third stage of the turbine.
THE HEINKEL-HIRTH WORKS, KOLBERMOOR
On the afternoon of Thursday 21 June members of the Fedden mission visited the third of the main jet companies. They travelled on from the BMW rocket development department in Munich south-eastwards to Kolbermoor, near the Austrian border, to the Heinkel-Hirth works.
The HeS 011 is generally considered to be one of the most advanced of the German turbojets produced during the Second World War, but, although it was intended for a number of proposed aircraft, it was not ready for mass production and only nineteen examples were finished by the fall of Germany.
During their investigations the Fedden team paid particular attention to the production of the turbojets and the comparative costs of piston versus jet engine. Lengthy interrogations were conducted with key personnel: Mr Schaaf, managing director of BMW, Mr Dorls, the planning engineer who had been with the company for over ten years and was closely involved in piston engine production, and also Mr Hartkopf, the works manager at the Junkers works who had extensive experience with both piston and jet engine production. From information supplied by Dorls they compiled a table which clearly demonstrates the difference in material costs between the types of engines:
type 213 piston engine
type 801 piston engine
These figures for raw materials showed that the jets could be manufactured for approximately half, if not a third, of the cost of a piston engine. The process of constructing the jet engines was also much more straightforward, as Fedden stated:
The mission was impressed with the simplicity and straightforward methods of production, especially of the BMW jet engines. Relatively speaking, there are no very special machining operations or expensive tooling required. The Mission saw few special machines for turbine manufacture except for the seven spindle indexing machines for cutting the slots in the compressor hubs, made by the Magdeburg Junkers production plant which was previously a large machine tool manufacturing concern.
Other specialist tools included a Deckel copying machine for the forming of blades which was seen at Stassfurt, and a treble balancing machine. Otherwise all turning work was very simple and the hollow blade and other sheet metal work on the engines was tooled on plant previously used in the production of motor car body panels. Despite earlier comments, in general Fedden considered the standard of workmanship at both BMW and Junkers to be of a reasonable standard ‘but quite ordinary and commercial, and definitely of a lower order than that of current British piston engines’. Once again Fedden harks back to the piston engine as the only point of reference with which he was familiar. But as the German production engineers explained, one of the big advantages is that they had been able to employ a ‘lower grade’ of labour in manufacturing the jets. In many cases this can usually be taken as a euphemism for enforced or slave labour. The mission was also able to make a comparison of labour times, including manufacture of individual components, assembly and shipping, plus testing. For the BMW 801 fourteen-cylinder radial engine this came to 1,400 hours per unit, while for the jet it was only 375 hours; a considerable saving.
One of the biggest engineering issues in producing the jets had been the acute shortage of important strategic materials. The early pre-production engines, in particular the Jumo 004s, had been built without restrictions on the use of materials such as nickel and cobalt. For large-scale production the hot metal parts were changed to mild steel protected by a coating of aluminium, or hollow turbine blades produced from folded and welded Cromadur alloy which had been developed by Krupp. As a consequence the engines had become cheaper to manufacture but their operational lifespan was shortened:
It appeared from the comparative performance of German and British engines, that the performance of the German types had suffered from the necessity of conserving alloys, which led to the development of cooled blades. Apart from the loss of performance due to bleeding air from the compressor, the use of the variable exit nozzle may well lead to turbulence losses in the jet. In an attempt to restore the situation the German designers appear to have sacrificed efficiency so as to achieve a low pressure drop across the chamber and this may have aggravated their troubles in connection with unstable combustion under altitude conditions.
One big disadvantage with the jet engines had proven to be the short periods between overhauls. At Junkers the Mission was informed that about 300 engines had been back for repairs, some of them being overhauled more than once. The engineers admitted that the engines were lasting for between thirty and fifty hours between overhauls – some sources suggest even less at only ten to twenty hours for the Jumo 004 – and that thrust would start to fall off after this period. It was a similar situation with the BMW 003 which, the engineers claimed, required attention every fifty hours. Set against this they were very satisfied with the short turnaround times. The usual practice was to replace any damaged blades and rebalance the rotors, although the combustion chamber, tail cone, automatic governor and starter also required attention. In general the combustion chambers needed servicing every twenty to fifty hours and had a life of around 200 hours. But it was the turbine blades that bore the brunt of the damage, mostly caused by foreign matter entering the engine or by abuse on the part of the pilots resulting in the overheating of the turbine blades. Replacing the blades was a straightforward task which, Fedden was informed, could be handled by women and slave labour. In terms of costs, Fedden was surprised by the low wages for even the skilled workers: ‘These wages appear to be in the order of a half of those in the Coventry area.’
It is interesting to note that the engine life and time between services could be significantly affected by the way they were handled. It was suggested that a skilled pilot could coax up to twice the endurance time out of them. This was because the early jets had a sluggish throttle response and in the hands of an impatient or inexperienced pilot it was a common mistake to open up the throttle too quickly, thereby injecting too much fuel into the engine before it had been able to speed up. The result was a build up of heat before the cooling fans could remove it, leading to a softening of the turbine blades and possibly damaging the combustion chambers. To counter this, the engineers had been working on a delayed-action control to limit the time of acceleration from idling speed to maximum rpm.
The final and sobering comment on the German jets comes from the pages of the Mission Report:
Series production of jet engines in large quantities was undoubtedly in a more advanced state in Germany than in Britain and the USA, and had the war continued and had their factories not been overrun, they would have been producing several thousand jet engines per month by this autumn . By the middle of 1946, the output would have been at a rate of 100,000 jet engines per annum, at least.