Centrifugal vs axial compressors in WW2 jets

[kool kitty89]

Continued from: http://www.ww2aircraft.net/forum/av...e-missiles-43188-post1200913.html#post1200913

The He280 was intended to have the HeS8 as it's powerplants...and the ones tested and demonstrated with the HeS8 performed to all expectations. It was the HeS30 that was supposed to replace the HeS8

This is one of the most informative articles on Ohain's and Heinkel's Jet engine developments that I've found online. ASME DC | Proceedings | GT1999 | Volume 1: Aircraft Engine; Marine; Turbomachinery; Microturbines and Small Turbomachinery | Pioneering Turbojet Developments of Dr. Hans von Ohain â€" From the HeS 1 to the HeS 011

Ohain's biography (Elegance in Flight) is very informative as well, but in pure technical terms and as a historical digest of events (plus some specific details not present in the biography) it's extremely useful.

When the initial follow-on projects for a jet fighter and jet engines to power it were started in late 1939, it was initially hoped that Adolf Muller's design (claimed to be in an advanced stage of development) would be ready to fly quite soon. This quickly proved to not be the case with major delays in Wagner/Muller's team moving and unpacking from Junkers as well as Muller's exaggerated claims being exposed for what they were. So, before the end of 1939, Ohain started work on a back-up design, the HeS 8, adopting a combustion chamber embedded between the compressor and turbine as well as an axial diffuser and axial flow turbine stators and give vanes in order to keep the diameter within the minimum constraints of the He 280 (or He 180 as it was initially named). Then there's Heinkel and Udet's agreement on acquiring Hirth on the condition the He 280 flew under jet power by spring of 1941.

My point being that at that time, late 1939, the HeS 3/6 and He 178 project having just wound down and the He 180/280 still on the drawing board, the very easily could have taken the conservative approach and compromised to modify the He 280's design to practically mount the existing HeS 6 engines rather than banking on some new unproven developments (be it Wagner's team's axial designs or Ohain's HeS 8 with several fairly radical or at least untried features). The HeS 6 was managing performance in 1939 that the HeS 8 wouldn't until 1942 (and even the HeS 3 was managing performance on par with what the troubled HeS 8 managed in 1941). The He 280 could have flown in 1940 and with at least 18% more thrust (and engines without cowlings due to fuel leaks) and potential an engine that could have matured much closer to being production ready by that point or at least have a greater number of spares. (and move work over to Hirth around a year sooner).

A scaled up HeS 6 derivative (something closer to the Goblin's thrust ... a Class II RLM engine if you will) would have been worth pursuing as well. That and potentially axial turbines based on Hirth's and Wagners teams work. (and possibly combustion chamber developments progressing with Wagner's team as well -particularly the HeS 30's flame cans)

The RLM was actually impressed with the He280 and intended to acquire at least 300 units BUT the RLM also got involved with the engine development and insisted on development of the HeS011 instead. It was this meddling that spelled the doom of the He280 as the HeS011 was having serious development problems and cost a great deal of time when the HeS8 was nearly "bug" free and the HeS30 development was coming along behind the HeS8.

Late 1942 when the HeS 8 and HeS 30 were canceled, I'm talking about missed opportunities due to decisions made within Heinkel 3 years earlier. The HeS 8 was never nearly bug free and never produced the anticipated thrust (didn't even exceed the HeS 6's 1939 thrust levels until after the axial compressor stage was added in 1942). Problems with the diffusor, combustion chamber, and turbine (including several problems that were never as severe on the HeS 3 or 6) had not been solved by the time of cancellation and progress had been very sluggish. Given the rate of progress the HeS 30 was making once the team fully settled in (possibly more so after Muller left), it may have been production ready before the HeS 8 ever had its problems worked out.

Aerodynamic problems within the HeS 8 also meant most if not all versions produced were only managing a 2.7:1 compression ratio to the 2.8:1 of its predecessors. (and just over 3:1 for the HeS 30, 004, and 003)

Dumping the HeS 8 made sense, the HeS 30 did not, and pushing the HeS 11 made very little sense and was forced by Milch against the judgement of Heinkel and his engineering teams. IMO, the practical, sensible thing to do (as things stood in mid 1942) would be to continue development of the HeS 30, provide communication/collaboration and exchange of information between the Hirth engineers, Wagner's team, and Ohain's team (which was already the case to some degree), and start work on a class two engine either based directly on the HeS 30's design elements or a larger, single stage centrifugal design that focused on further refining the axial diffusor layout and combined it with the HeS 30's flame can and axial turbine layout. (the combusion chamber and turbine in the HeS 8 were major problems, and the compact diffuser and combustion routing also meant cramped turbine inlet routing -as it was a radial turbine as wide as the compressor impeller, an axial turbine would avoid this problem entirely and allow much greater benefit from the axial diffuser layout, and the axial diffuser would mate extremely well with the flame can arrangement of the HeS 30)

The radial turbine might have been more interesting if applied to Hirth's turbochargers. Those turbines excel in smaller applications and have far better wearing properties as well. (all modern automotive turbochargers use radial turbines)

 

[GrauGeist]

The point I'm making, is that the HeS8 was further along than any other systems and that the He280 was designed for the lighter, smaller engines so it did perform well as designed.

Now, had the He280 continued into production with the HeS8 with the intention to replace those engines with the HeS30 once it was production-worthy, then alot of valuable time would not have been lost.

Now bear in mind that this becomes a "what-if" because, like has been mentioned before, the RLM derailed the HeS30 for the HeS011 and thus the He280 became an opportunity lost.

 

[kool kitty89]

The point I'm making, is that the HeS8 was further along than any other systems and that the He280 was designed for the lighter, smaller engines so it did perform well as designed.

Now, had the He280 continued into production with the HeS8 with the intention to replace those engines with the HeS30 once it was production-worthy, then alot of valuable time would not have been lost.

Now bear in mind that this becomes a "what-if" because, like has been mentioned before, the RLM derailed the HeS30 for the HeS011 and thus the He280 became an opportunity lost.

No attempts were made to transition the HeS 8 to mass production quality or even make a small production run of it. They were all prototypes and focusing more on attempting to reach the target thrust than manage mass produceability.

They may have proceeded to reach mass production more easily than the 004B did, they likely would have had relatively low TBO times similar to the production 004B and may have required a larger amount of stainless steel. They would have had poorer specific fuel consumption than the 004B (though the lower weight and drag would have saved a lot on fuel efficiency on a complete aircraft performance basis -ie range likely would be significantly worse using the 004s).

From what I understand, Heinkel was using much less exotic alloys in the prototype engine construction than Jumo did on the 004A, much closer to the 004B but with greater portions of stainless and lesser use of mild steel. (percentage of overall weight would certainly have been more stainless steel, though I'm not certain overall material use would be higher) So it's more likely there'd have been a smoother transition from proto to production (at least as long as the mid-war stainless steel supplies held out -much better than the situation in 1944). Plus, vibration problems had consistently slowed development of the 004 prototypes and later B series, this had not been the case on Ohain's engines. (also not one of the major problems with Wagner/Muller's developments -though I believe it was for BMW's 003 development along with their combustion problems)

So yes, had the HeS 8 been pressed into mass production ASAP based on its prototypes of 1942 ... have the design frozen and rushed into service as the 004B bad been two years later, they might have gotten an adequately service ready He 280 into production during 1942. (there were a few other problems with the airframe itself -the twin-fin tail was determined to be less than optimal I believe, but not ones serious enough to totally bar it from being service ready, or at least used for pre-production evaluation A-0 models for operational testing) The ejection seat was not one of the problem areas of the aircraft, and had been used successfully more than once in testing, so that certainly would have been a positive point for pilot survivability.

Shifting to HeS 011 development was just a horrible decision all around.

 

[Juha]

A bit OT but when Jumo 004B-4 entered series production? Most of my sources say in Dec 1944 but Price in one of his newer articles (in International Air Power Review Vol 23)says that during Sept 44 production of the Jumo 004B-4 reached significant levels.