Heinkel HeS 011

The Heinkel HeS 011 or Heinkel-Hirth 109-011 (HeS - Heinkel Strahltriebwerk) was an advanced World War II jet engine built by Heinkel-Hirth. It featured a unique compressor arrangement, combining a three-stage axial compressor with a "diagonal" stage similar to a centrifugal compressor directly forward of the three-stage axial compressor, along with a low-compression impeller in the intake, just ahead of the diagonal stage to smooth out airflow. Much like many of the projected piston-engined late-war German combat aircraft designs meant to be powered with the experimental Junkers Jumo 222 multibank powerplant, many of the German jet-powered aircraft designs at the end of the war were based around the HeS 011, but like the Jumo 222, the HeS 011 engine was not ready for production before the war ended in Europe.

Design and development
Starting in 1936, Junkers started a jet engine development project under the direction of Wagner and Müller, who worked on axial compressor designs. By 1940 they had progressed to the point of having a semi-working prototype, which could not run under its own power and required an external supply of compressed air.

Meanwhile, Hans Mauch, in charge of engine development at the RLM, decided that all engine development should take place at existing engine companies. In keeping with this new policy, he forced Junkers to divest itself of their internal engine teams. Müller and half of the existing Junkers team decamped and were happily accepted by Ernst Heinkel, who had started German jet development when he set up a lab for Hans von Ohain in 1937. The two teams worked on their designs in parallel for some time, von Ohain's as the HeS 8 (or 109-001), and the Junkers team as the HeS 30 (109-006). Heinkel's efforts were later re-organized at Hirth Motoren.

Helmut Schelp, who had taken over from Mauch, felt that the BMW 003 and Junkers Jumo 004 would reach production at about the same power levels long before either would be ready, and cancelled both of the Heinkel projects. He had outlined a new development plan, with three engine classes, the 003 and 004 were "Class 1" engines of under 1000 kg thrust suitable for small fighters, but only really useful in twin-engine designs. Schelp was much more interested developing a "Class II" engine of 1000–2000 kg, larger designs able to power a full-sized fighter design with a single engine. Schelp was also interested in seeing one of his own pet projects, the diagonal compressor, adopted. Schelp had earlier convinced Heinkel to put some effort into another pet project of his, a twin-compressor single-turbine turboprop, but had given up on this and instead offered Heinkel his new concept as a consolation prize.

In some ways, the HeS 011 can be considered a combination of the two teams' designs, a three-stage axial compressor from Müller's team, combined with a single-stage centrifugal compressor from von Ohain's, the two driven by a single two-stage turbine. The engine operated at somewhat higher thrust levels, about 2,700 lbf (12 kN), as opposed to about 1,750 to 2,000 lbf (7.8 to 8.8 kN) thrust for the 003 and 004 respectively. The 011 shared two features with the Jumo 004, with an engine-mounted Reidel two-stroke engine functioning as an APU to get the central shaft turning during engine startup, but mounted above the intake orifice within a Heinkel-crafted prefabricated sheet-metal intake passage instead of inside the intake diverter as the 004 had done, and also had a variable geometry exhaust nozzle, with a restrictive body of differing aerodynamic shape to the 004's Zwiebel (onion) unit, that likewise traveled fore and aft in the nozzle to vary the thrust. Plans were also made for a turboprop version, the HeS 021, but the workload at Heinkel was so high that this project was later given to Daimler-Benz to complete.

Prototypes were available in 1944, and tested using a Heinkel He 111 bomber, mounting the engine on the external hardpoints under the fuselage. Over the next year, practically all German aircraft designers based their projects on the 011, very much as had been done only a year or two previously with projected piston-engined designs, such as those of the twin-engined Bomber B program, widely based on the equally experimental Junkers Jumo 222 twenty-four cylinder powerplant. Like the nearly three hundred experimental examples built of the complex Jumo 222 piston engine, the HeS 011 turbojet never entered production, with only 19 prototypes built in total. One of these was mounted in the Messerschmitt Me P.1101 that was taken to the United States, forming the basis of the Bell X-5.

Two museum-preserved examples of the HeS 011 engine still exist in the United States, at the National Museum of the U.S. Air Force in Dayton, Ohio, and at the EAA AirVenture Museum in Oshkosh, Wisconsin.