Junkers Jumo 004

The Jumo 004 was the world's first turbojet engine in production and operational use, and the first successful axial compressor jet engine ever built. Some 8,000 units were manufactured by Junkers in Germany during late World War II, powering the operational Messerschmitt Me 262 fighter jet and the Arado Ar 234 reconnaissance / bomber jet, along with prototypes including the Horten Ho 229 aircraft. Variants of the engine were produced in Eastern Europe for years following the war.

Design and development
The feasibility of jet propulsion had been demonstrated in Germany in early 1937 by Hans von Ohain working with the Heinkel company. Most of the Reich Air Ministry (RLM) remained uninterested, but Helmut Schelp and Hans Mauch saw the potential of the concept and encouraged Germany's aero engine manufacturers to begin their own programmes of jet engine development. The companies remained skeptical and little new development was carried out.

In 1939 Schelp and Mauch visited the companies to check up on progress. Otto Mader, head of Junkers Motoren (Jumo), stated that even if the concept was useful, he had no one to work on it. Schelp responded by stating that Dr Anselm Franz, then in charge of Junkers' turbo- and supercharger development, would be perfect for the job. Franz started his development team later that year, and the project was given the RLM designation 109-004 (the 109- prefix, assigned by the RLM was common to all reaction engine projects in WW II Germany, and was also used for German WW II rocket engine designs for manned aircraft).

Franz opted for a design that was at once conservative and revolutionary. His design differed from von Ohain's in that he utilised a new type of compressor which allowed a continuous, straight flow of air through the engine (an axial compressor), recently developed by the Aerodynamische Versuchsanstalt (AVA - Aerodynamic Research Institute) at Göttingen. The axial-flow compressor not only had excellent performance, about 78% efficient in "real world" conditions, but it also had a smaller cross-section, important for high-speed aircraft.

On the other hand, he aimed to produce an engine that was far below its theoretical potential, in the interests of expediting development and simplifying production. One major decision was to opt for a simple combustion area using six "flame cans", instead of the more efficient single annular can. For the same reasons, he collaborated heavily on the development of the engine's turbine with Allgemeine Elektrizitäts-Gesellschaft (AEG - General Electric Company) in Berlin, and instead of building development engines, opted to begin work immediately on the prototype of an engine that could be put straight into production. Franz's conservative approach came under question from the RLM, but was vindicated when even given the developmental problems that it was to face, the 004 entered production and service well ahead of its more technologically advanced competitor, the BMW 003.

Technical description and testing


The first prototype 004A, which was constructed to run on diesel fuel, was first tested in October 1940, though without an exhaust nozzle. It was benchtested at the end of January 1941 to a top thrust of 430 kgf, and work continued to increase the output, the RLM contract having set a minimum of 600 kgf thrust.

Vibration problems with the compressor blades delayed the program at this point, until a new stator design by Max Bentele solved the problem. The original alloy compressor blades were replaced with steel ones and with the new stators in place the engine developed 5.9 kN in August, and passed a 10-hour endurance run at 9.8 kN in December. The first flight test took place on March 15 1942, when a 004A was carried aloft by a Messerschmitt Bf 110 to run up the engine in flight.

On July 18, one of the prototype Messerschmitt Me 262s flew for the first time under jet power from its 004 engines, and the 004 was ordered into production by the RLM to the extent of 80 engines.

The initial 004A engines built to power the Me 262 prototypes had been built without restrictions on materials, and they used scarce raw materials such as nickel, cobalt, and molybdenum in quantities which were unacceptable in production. Franz realized that the Jumo 004 would have to be redesigned to incorporate a minimum of these strategic materials, and this was accomplished. All the hot metal parts, including the combustion chamber, were changed to mild steel protected by an aluminum coating, and the hollow turbine blades were produced from folded and welded Cromadur alloy (12% chromium, 18% manganese, and 70% iron) developed by Krupp, and cooled by compressed air "bled" from the compressor. The engine's operational lifespan was shortened, but on the plus side it became easier to construct.

The first production model of the 004B weighed 220 lb less than the 004A, and in 1943 had passed several 100 hour tests, with a time between overhauls of 50 hours being achieved.

Later in 1943 a series of engines suffered vibration problems which dragged on. Eventually, in December, blade-vibration specialist Max Bentele was once again brought in during a meeting at the RLM headquarters, and the problem was solved by raising the blades' natural frequency by increasing their taper, shortening them by 1 millimeter, and reducing the operating speed of the engine from 9,000 to 8,700 rpm.

It was not until early 1944 that full production could finally begin. These setbacks were the principal factor delaying the Luftwaffe's introduction of the Me 262 into squadron service.

Given the lower-quality steels used in the 004B, these engines typically only had a service life of some 10-25 hours, perhaps twice this in the hands of a skilled pilot. Another shortcoming of the engine, common to all early turbojets, was its sluggish throttle response. Worse, it was fairly easy to inject too much fuel into the engine by throttling up too quickly, allowing heat to build up before the cooling air could remove it. This led to softening of the turbine blades, and was a major cause for engine failures. Nevertheless, it made jet power for combat aircraft a reality for the first time.

The exhaust area of the 004 featured a variable geometry nozzle, which had a special restrictive body nicknamed the Zwiebel (German for onion, due to its shape when seen from the side) which had roughly 40 cm (16 inch) of fore-and-aft travel to vary the jet exhaust's cross-sectional area for thrust control, as the active part of a pioneering "divergent-convergent" nozzle format.

One interesting feature of the 004 was the starter system, which consisted of a Riedel 10 hp 2-stroke motorcycle engine hidden in the intake, and essentially functioned as a pioneering example of an APU for starting a jet engine. A hole in the extreme nose of the intake diverter body contained a pull-handle for the cable which "turned-over" the piston engine, which in turn spun up the turbine. Two small gasoline tanks were fitted within the upper perimeter of the annular intake's sheet metal housing for fueling the Riedel two-stroke mechanical APU unit.

The Jumo 004 could run on three types of fuel:
 * J-2, its standard fuel, a synthetic fuel produced from coal.
 * Diesel oil.
 * Aviation gasoline; not considered desirable due to its high rate of consumption.



Postwar production
Following World War II, Jumo 004s were built in small numbers by Malešice in Czechoslovakia, designated M-04, to power the Avia S-92 which was itself a copy of the Me 262. Jumo 004 copies were also built in the Soviet Union as the RD-10 engine, where they powered the Yakovlev Yak-15 as well as many prototype jet fighters.

In France, captured 004s powered the Sud-Ouest SO 6000 Triton and the Arsenal VG-70.

Variants
A number of more advanced versions were in development at the end of the war. The 004C included an afterburner for increased thrust, but was not built. The 004D improved fuel efficiency with a two-stage fuel injector, and introduced a new throttle control that avoided dumping too much fuel into the engine during throttle-ups. The 004D had passed testing and was ready to enter production in place of the 004B, when the war ended. The 004E was a 004D model with an improved exhaust area for better altitude performance.

A much more advanced model based on the same basic systems was also under development as the Jumo 012. The 012 was based on a "two-spool" system, in which two turbines, spinning at different speeds, drove two separate sections of the compressor for more efficiency. In a jet engine the compressor typically uses up about 60% of all the power generated, so any improvements can have a dramatic effect on fuel use. Plans were also underway to use the 012's basic concept in an engine outwardly identical to the 004, known as the 004H, which improved specific fuel consumption from the 004B's 1.39 kg/(daN*h) to a respectable 1.20 kg/(daN*h), a decrease of about 15%.

Variants table
Layout: ax=axial flow compressor stages, in=individual combustion chambers, tu=turbine stages.

Applications
Apart from the Me 262 and Arado Ar 234, the engine was used to power the experimental Junkers Ju 287, and prototypes of the Horten Ho 229 and Heinkel He 280. There were plans to install the Jumo 004D variant in the Heinkel He 162 in its proposed A-8 version, as well as the Focke-Wulf Ta 183, Henschel Hs 132 and Blohm & Voss P.188 then under development.

Survivors
A number of examples of the Jumo 004 turbojet exist in aviation museums in North America and Europe, specifically at the Smithsonian's National Air and Space Museum, the National Museum of the U.S. Air Force, at the New England Air Museum, Bradley International Airport, Windsor Locks, CT; and in Europe in such museums like the RAF Museum in the UK, and Munich's Deutsches Museum, as well in preserved examples of the Me 262A jet fighters in several aviation museums.