The Heinkel HeS 40 (HeS – Heinkel Strahltriebwerk) was an experimental constant-volume jet engine designed by Adoph Müller’s team at Heinkel starting some time in 1940 or 41. It was based on the mechanical layout of the HeS 30, but replaced the conventional flame cans with oversized ones including large poppet valves that sealed off the chambers during firing. Constant-volume combustion, similar to the Otto cycle used in most piston engines, is considerably more fuel efficient than the constant-pressure combustion used in a typical jet engine.

The design was based on the HeS 30 not only to make parts more readily available as well as to make direct comparisons between the two easier. The main changes were to reduce the compression ratio of the compressor to about 2:1 (from 2.8:1), and add the new combustion chambers. The new chambers were considerably larger than the originals, forcing a reduction in the number from ten to six burners. The valve stems projected forward into streamlined fairings in the intake area behind the compressor.

The operational cycle of the engine is somewhat similar to a conventional six-cylinder engine. Slightly compressed air, similar to an automobile equipped with a turbocharger, was channeled into the cylinders in turn, closed off with the poppet valves, and then burned. By the time the combustion was complete the pressure in the flame cans would be much higher, although the actual compression ratio is not specified. The hot gas then blew through a turbine to extract power, instead of forcing a piston to move. Although there would be some loss of charge during the burning period, and thus the design would be less efficient than the true Otto cycle, it would nevertheless be somewhat more efficient than a traditional jet engine, at the cost of some complexity.

It appears the HeS 40 was never built, and remained a paper design. Nevertheless, work on the design was ended by 1942, by which point the HeS 30 was making good progress.

The HeS 30 (HeS – Heinkel Strahltriebwerk) was an early jet engine, originally designed by Adolf Müller at Junkers, but eventually built and tested at Heinkel. The first Jumo 109-009 was running by the summer of 1938, but the project and its staff were switched to Heinkel after Junkers’ engine division discovered that jet engine development work was being undertaken in secret under the auspices of the company’s airframe division.

The engine division of Junkers had already begun to move into turbojet development, and in the summer of 1931 it received a German air ministry contract for the new Jumo 1009-004. This was designed for a thrust of 1543 lb / 700 kg at a speed of 559 mph / 900 kph, and was schemed round an eight-stage axial compressor, six combustors, a single stage turbine designed with the aid of AEG’s turbine expertise and provision for afterburning.

It was possibly the best of the “Class I” engines, a class that included the more famous BMW 003 and Junkers Jumo 004, but work on the design was stopped by the Reichluftfahrtministerium (RLM) as they felt the Heinkel team should put all their efforts into other designs.

The HeS 30 was designed before the RLM introduced standardized naming for their engine projects. It was assigned the official name 109-006, and it was sometimes called the HeS 006 as a short form. Development ended just as these names were being introduced, so “HeS 30” naming is much more common.

Herbert Wagner started engine developments at Junkers in 1936, placing Adolf Müller in overall charge of the project. In 1938 Junkers purchased Junkers Motoren (Jumo), formerly a separate company. In October 1939, under pressure from the RLM, Junkers moved all their engine work to Jumo’s Dessau factories from their main plants at Magdeburg. Müller would have ended up in a subordinate role after the move, but decided to leave instead. He and about half of the original Junkers team were scooped up by Ernst Heinkel and moved to his Rostock campus, where Hans von Ohain was working on the Heinkel HeS 3 engine.

The first unit ran in November 1940, but was beset with so many technical problems that it was early 1942 before anything approaching reliability was attained; a Jumo 109-004A was flown for the first time under a Messerschmitt Bf 110 in March 1942, and other A-series engines were flown in prototypes of the Me 262.
In the production-configured B-series the quantity of strategic materials was halved, largely through replacement of castings by sheet metal construction. This reduced weight and the number of man-hours involved in construction.

Of all of the designs Müller brought with him, the HeS 30 was simplest and easiest to build. Müller had already built a test engine while still at Junkers, however it was only able to run at about half its designed RPM, which limited compression and required a continuous supply of external compressed air. The design was abandoned when Müller left, the Jumo team’s simpler design being used instead. Müller promised Heinkel he could have the engine up and running on a testbed within one year of completing the move, a promise he was ultimately unable to keep.

Key to the engine’s working cycle was an axial compressor of then-unique construction. Most German engines of the era had the stators do all of the actual compression, with the rotors speeding up the air for them to compress. In the HeS 30, the rotor and stators shared compression about 50-50, a design originally provided by Rudolph Friedrich of Junkers. Overall the engine had a five-stage compressor providing air at a compression ratio of 3:1 to ten flame cans, which powered a single-stage turbine. The turbine was also unique for the era, using a set of guide vanes that were adjustable for various operating speeds. Like most German axial engines, the engine also included a variable-geometry exhaust cone to lower back pressure when starting, and an electric starter motor.

Due to the move, it took considerable time for the team to restart work on the design, and even though three experimental engines were ordered as the 109-006 in 1939, it was not until May 1942 the first engine actually ran. In addition to problems with the move, the compressor turned out to provide more mass flow than initially suspected, forcing a redesign of the turbine. To add to the problems, Müller and Heinkel had an argument in May that eventually led to Müller resigning.

Work on the engine continued, and by October it was running at full speed. Of all of the early engines, the HeS 30 was by far the best design. It produced a thrust of 860 kg (1,895 lb), almost equidistant between the BMW 003’s 800 kg (1,780 lb) and the Jumo 004’s higher 900 kg (1,980 lb), but weighed only 390 kg (860 lb), providing a much better power-to-weight ratio than the dry weights of either the 003 at 562 kg (1,240 lb) or the 004 at 720 kg (1,585 lb). The HeS also had better specific fuel consumption and was also smaller in cross-section.

Helmut Schelp, in charge of engine development at the RLM, refused to give Heinkel a production contract, an event Hans von Ohain claims brought Ernst Heinkel near tears. Schelp noted that while the design was excellent, BMW and Jumo were so far ahead they simply did not need another “Class I” engine – something that would prove ironic in another two years when both of those engines were still not operational. It also appears he had some misgivings about the compressor arrangement, but if this was the case it was never official. He also cancelled von Ohain’s Heinkel HeS 8 at the same time.
Instead of yet another Class I engine, Schelp asked Heinkel to continue work on a Class II engine of about 1,300 kg thrust, which would be needed for reasonably sized single-engine fighters, and as a useful addition to twin-engine bombers. Thus work on the HeS 30 and HeS 008 ended, and Heinkel turned, grudgingly, to the Heinkel HeS 011, which would not enter production before the war ended. The remains of Müller’s team were then moved to the Heinkel-Hirth plants to work on the new engine.

Starting some time in 1940 or ’41, the basic mechanical layout of the HeS 30 was also used on an experimental constant-volume engine known as the Heinkel HeS 40.

The Jumo 109-004B-1 first ran in May 1943, and the use of blades of improved shape in the first two stages of the compressor helped to increase thrust to 1984 lb / 900 kg. This variant was quite rapidly cleared for production, the first units being delivered in March 1944 to power the Me 262A. Further development resulted in the Jumo 109-004B-4, with hollow rather than solid turbine blades, which entered production in December 1944, it was about to be supplanted by the 2315 lb / 1050 kg thrust Jumo 109-004D-4 as the war ended.

Specifications
Type: Turbojet
Length: 2.72m
Diameter: 0.62m
Dry weight: 390kg (860lb)
Compressor: Axial 5-stages
Combustors: 10 Cannular chambers
Turbine: Axial 1 stage
Fuel type: Gasoline
Oil system: pressure scavenge return
Maximum thrust: 860kp (1,896lb)
Overall pressure ratio: 3:1 Pressure ratio

The Heinkel HeS 8 (prefix being an abbreviation for “Heinkel Strahltriebwerk”-Heinkel Jet Engine) was an early jet engine designed by Hans von Ohain while working at Heinkel, and first run in September 1940. It was the first jet engine to be financially supported by the RLM, bearing the official name 109-001.

The HeS 8 was intended to power the Heinkel He 280 twin-engine fighter, although both Heinkel and von Ohain preferred the axial HeS 30. A lengthy gestation period meant it was finally becoming ready for production at about the same time as the Junkers Jumo 004 and BMW 003. In 1942 work was ended on the HeS 8 and HeS 30, and Heinkel was ordered to move on to the larger Heinkel HeS 011 instead. The He 280 was left engineless, and was eventually abandoned.

By the time the HeS 3 program wound down in 1939, it appears that von Ohain no longer favoured the centrifugal compressor for jet engines. He had been “sold” on the axial compressor as early as 1938, after a meeting with D. Encke of AVA, but continued with the centrifugal design in the HeS 3 because it was much easier to work with. It is likely he would have developed an axial design as a follow-on to the HeS 3, but it appears the RLM was interested in keeping him working on centrifugal designs as a backup in case the various axial designs ran into problems.

The main problem with the centrifugal compressor was the large cross-sectional area. von Ohain had been looking at solutions to this problem as soon as the HeS 1 design was winding down in 1937. His first attempt at the HeS 3 was to separate the compressor and turbine —which were back-to-back in the HeS 1— and place the combustion chambers between them. Various problems in this original HeS 3 design forced them to abandon this layout for an updated HeS 3b, but it appears von Ohain felt it was still the best solution, and he returned to it for the HeS 8. The result was an engine that was only slightly wider than the compressor disk, whereas earlier models had piping lying outside the compressor disk and were therefore somewhat larger.

Another problem with the original engine series was that the compressor was fairly sensitive to disturbances in the intake airflow. To address this, the HeS 8 added a low-pressure impeller in the intake in front of the main compressor. The impeller did not add much to the compression, but by increasing pressure on the compressor face the airflow was greatly stabilized. The 14-blade impeller and 19-blade compressor were both made of milled aluminum. The 14-blade turbine was made of steel, and uncooled, which suggests turbine burnout would be common. The various components were connected together on a common tubular power shaft, supported by three ball races. The combustion chamber consisted of two diffusers that slowed the airflow from the compressor, and then injected fuel through 128 nozzles arranged in two sets at different “depths”. Various accessories, including the starter, were grouped around the intake and did not add to the overall diameter.

Work progressed slowly, and by the time the first He 280 prototype was ready in September 1940 the engine was nowhere near ready for flight. The prototype then started glider testing while work on the engines and additional airframes continued. The engines were finally considered ready to go in early 1941, although at only 500 kg thrust instead of the planned 700 kg. The engines were later fitted and the He 280 first took to the air on 2 April 1941, although the cowlings had to be left off as the engine proved to leak fuel. Three days later the aircraft was demonstrated for a party of RLM officials, who were impressed, and full backing for Heinkel’s program was forthcoming.

Development of the engine stalled at this point, and by early 1942 the thrust had crept up to only 550 kg. An attempt to improve the design by adding a single axial compressor stage behind the centrifugal compressor was used from V15 (the 15th prototype) on, and new airflow routing in the compressor started on V16. It appears about 30 engines were completed in total, the later models with the various improvements generating about 600 kg of thrust. But by this point the various all-axial designs, including Heinkel’s own HeS 30, were progressing nicely. Helmut Schelp, in charge of jet development at the RLM, decided that the 003 and 004 were “good enough”, and cancelled all work on Heinkel’s existing designs. Instead he asked them to move onto a “Class II” engine design, which would evolve as the Heinkel HeS 011.

Several modifications of the basic HeS 8 design were also explored over the project’s lifetime. The HeS 9 appears to be a modification adding a second axial compressor stage, and replacing the full centrifugal stage with a new “diagonal compressor” that Schelp favoured. Little is known about this design other than the fact that RLM ordered ten of them, but none was built. It appears it was this layout that was used to develop the 011. Another modification, the HeS 10, placed a complete HeS 8 engine inside a larger nacelle, and expanded the intake impellor to be larger than the engine. The HeS 10 appears to have been the first example of what would today be called a turbofan engine. In order to extract more power from the exhaust to drive the fan, an additional single axial-stage turbine was added behind the HeS 8’s existing centrifugal one. The only real difference between the HeS 10 and a modern turbofan engine was that the fan was not powered independently of the core, although, given the separate axial turbine stage, this would not have been difficult to arrange.

Specifications

V16 on
Type: Turbojet
Length: 2.400 m
Diameter: 0.775 m
Dry weight: 380 kg
Maximum thrust: 700 kg at 13,500 RPM planned, 600 kg delivered
Overall pressure ratio: 2.7:1