The Junkers Jumo 004 was the first axial-flow turbojet.

    After the demonstration of the HeS-3B centrifugal jet designed by Hans von Ohain in 1937 proved that turbojets was a viable technology, the German Ministry of Aviation (Reichsluftfahrtministerium or RLM) approached engine makers to produce turbojets of their own design. Junkers already had a turbojet program at the same time as Heinkel, but had abandoned the project. Junkers Motoren (Jumo), Daimler-Benz, Bramo and BMW began developing turbojet engines and in the end only Junkers and BMW engines would actually go into production.

    Development progressed slowly as the compressor and turbines of jet engines are very complicated with delicate components operating under high physical forces. Metal temperatures within an engine can reach as high as 1,800° F (982° C) and there was a shortage of precious alloys such as nickel, cobalt and molybdenum. Today, titanium is used for compressor blades and high percentages of cobalt, nickel and chromium alloys for exhaust turbines, to withstand the extreme temperatures and stresses produced by jet engines.² Although the metallurgy was understood at the time, the alloys just weren't available in sufficient quantities. These alloys were scarce and mild steel (SAE 1010) was substituted in series production engines.

    The engine utilized a two-stroke engine as a starter which was built into the nose-cone of the engine that developed 10hp at 6,000 rpm. The starting engine had its own electric starter, plus a pull cord for emergency starts.

    The Junkers Jumo 004 engine had a single eight-stage compressor with enameled steel stator blades. On early engines, the first two stages of stator blades were aluminum, with the remainder being mild steel. On the later engines, the stator blades were converted to all steel. The first two stages had 27 rotor blades and 38 in the rest. The rotor blades were all stamped aluminum with machined roots fitting into pyramid shaped slots in the rotor disk.

    An annular combustion chamber would have been preferred, but to reduce complexity, six separate combustion-cans were installed. A short flame was necessary and the primary air was introduced into the chamber with a swirl and fuel was injected against the airflow to provide complete burning of the fuel/air mixture. The combustion chambers were made from mild sheet steel with an aluminum coating to protect against oxidation.³

    The turbine and stator blades on the 004A were of solid material, but the production blades on the 004B were hollow. Two types of hollow blades were produced; a Krupp alloy Tindur, deep drawn blade of 30% nickel, and a nickel free Cromadur blade, made of folded sheet metal that was welded at the trailing edge. The Cromadur blade was somewhat more reliable and easier to produce and used less than five lbs. of chromium. The turbine blades were cooled by bleed-air from the 4th and last stage of the compressor. Cooling air was also bled off between the compressor and combustion chambers.

    Since the heat resistance of these alloys was below what was required, the average life span of production engines was 25 hours. Overhauls were required after 10 hours.⁴ If the alloys had been available as required, this would have extended engine life. Engines were tested up to 150 hours in actual flight tests and up to 500 hours on the test stand.⁵

    Initial production began in 1942 with an order for eighty 004A engines, but these were to be used for testing only. They were not suitable for flight because they were too heavy (850 kg/1,870 lbs) and they used high amounts of strategic materials, such as nickel, cobalt, molybdenum, etc. On the 004B, almost all the hot metal parts were changed to mild steel with an aluminum coating. Weight on the 004B was reduced to 1,650 lbf (7.40 kN) and the strategic materials used was less than half of the A series. The 004B was also a modular design for ease of production.

    The 004B-1 produced a thrust of 2,000 lbf (8.90 kN) and ran primarily on diesel fuel. Gasoline was used for the two-stroke starter motor and for starting the turbojet. A combination of gasoline and diesel was used after starting and then to all diesel fuel.⁶ Deliveries began in June 1943, but turbine failures were encountered due to resonance vibrations at full speed. The problem was solved by increasing the natural frequency of the blade, (increasing the blade taper) and reducing the speed from 9,000 rpm to 8,700 rpm. Full production was not achieved until early 1944, which delayed the introduction of the Messerschmitt Me 262.

    The model 004E was the first turbojet equipped with an afterburner. Extra fuel was injected just before the turbine stator vanes and the exhaust temperature increased to 1,600° F (870° C) and thrust increased from 2,200 lbf (9.80 kN) to 2,640 lbf (11.75 kN). It was scheduled to go into production in 1945.

    More than 5,000 Jumo 004 engines were produced before the end of the war. After the war, Dr. Anselm Franz would work for Lycoming and developed the T53 and T55 turboshaft engines.