Ducted Fans and Thrust Augmentation

[kool kitty89]

In the recent topics where jet engine development has come up in discussion, there hasn't been much mention of turbofans (or any form of high speed ducted fans) beyond noting their efficiency in the speed ranges actually useful for WWII aircraft.

During some of the pulse jet and rocket engine discussions, I linked this reference to a 1932 publication of NACA studies on thrust augmentation for jet (including rocket) propulsion: file:///home/chris/Desktop/jets/pdf%20links/Rocket%20Science%20Books:%20Thrust%20Augmentors%20for%20Jet%20Propulsion.html

The benefits of adding more cool/low speed air to a rocket or jet exhaust stream apply to pretty much any sort of jet/rocket engine and overlap with some of the NACA duct research applied to engine cowlings.


And while there were a great deal of advanced, exotic, and unusual turbine engine developments progressing in Germany, there didn't seem to be much of any simple or conservative attempts at applying bypass fans to augment thrust and improve efficiency in the subsonic and transonic speed ranges actually useful for the time, particularly with the practical limits on temperature and pressure in the hot section of those engines.

There was the overly complex 109-007 turbofan design, but the only example I'm aware of that's remotely simple or practical for short term development appears to be Ohain's HeS 10 design, adding a ducted bypass fan to the HeS 8, apparently in a single-spool arrangement. (Heinkel had also been developing a couple ducted fan arrangements using Hirth air cooled piston engines, and while those may not have ended up being very useful, the testing may have aided adaptation to a turbofan)

P1010101.jpg Photo by Quinto_Sertorio | Photobucket

In terms of single-spool designs (even single turbine ones) use of reduction gearing to better optimize for the fan stage would probably be beneficial. Beyond that the use of independent 'free wheeling' low pressure turbine driven, rear-mounted fan modules (ducted or open rotor) similar to what was implemented on the Metrovick F.3 (and later open-rotor F.5) as well as Whittle's thrust augmenter for the W.2/700 intended to power the Miles M.52.

It seems like adapting some of the first generation turbojets to turbofans would have been simpler and more effective than many of the second generation designs (especially in Germany) as well as possibly even simpler than several of the first generation attempts. (a fair amount of the unattractive elements of Ohain's HeS 3 and 6 would have been mitigated by turbofan adaptations and, indeed wouldn't be too different from some small turbofan engine designs employed on -or proposed for use on- drones and cruise missiles: Patent US7055306 - Combined stage single shaft turbofan engine - Google Patents )


With the delays in the Jumo 004 and BMW 003, it also seems like parallel turbofan developments might have even caught up with the turbojet counterparts.

Given the limitations placed on German engines, use of rear mounted fan modules might have been the most practical option on the whole given the potential for use of low pressure turbines with lower stresses and the modular configuration allowing independent manufacturing and maintenance/overhaul. (particularly important given the vastly varying TBO for different major engine components)

 

[Shortround6]

Trouble for the Germans is that all of these schemes need added turbine stages and it was the shortage of high temperature alloys (for turbine blades) that was one of the major stumbling blocks. The air cooled blades at the time were a work around. doubling, tripling, quadrupling the number of turbine blades needed is hardly going to increase engine production, whatever the benefits would be.

 

[Koopernic]

One of the problems for the German armaments industry was an extreme shortage of labour and machining tools. This drove them down a certain path that were a if not a blind end but suboptimal. Consider the production of turbine blades: those of the Jumo 004B1 were of forged tinidur, this of the Jumo 004B4 were cold drawn in ten punching steps from sheet metal. Some latter blades were made of cromidur which was just sheet stock folded and welded at the trailing edge. The sicromal blades of the BMW003 were made in a similar way.

Compare this to the blades used in British engine which seem to have been caste and then precisely machined with elaborate fir tree roots. I suspect that had the German alloys been fabricated with the British methods and design they would have been more reliable.

Likewise the use of impulse compressors which were easier to manufacture compared to the lighter more efficient reaction types. The lower efficiency could only be compensated with higher temperatures in the turbine, or lower thrust etc.

Junkers was producing a Jumo 004 in less than 700 hours. That's stunning. I believe it would have become reliable even with the 'mass produced' characteristics they used as refinements came in. I suspect the British were spending almost as much on a turbine as the Germans were on the whole engine.

 

[Koopernic]

In the recent topics where jet engine development has come up in discussion, there hasn't been much mention of turbofans (or any form of high speed ducted fans) beyond noting their efficiency in the speed ranges actually useful for WWII aircraft.

During some of the pulse jet and rocket engine discussions, I linked this reference to a 1932 publication of NACA studies on thrust augmentation for jet (including rocket) propulsion: file:///home/chris/Desktop/jets/pdf%20links/Rocket%20Science%20Books:%20Thrust%20Augmentors%20for%20Jet%20Propulsion.html

The benefits of adding more cool/low speed air to a rocket or jet exhaust stream apply to pretty much any sort of jet/rocket engine and overlap with some of the NACA duct research applied to engine cowlings.


And while there were a great deal of advanced, exotic, and unusual turbine engine developments progressing in Germany, there didn't seem to be much of any simple or conservative attempts at applying bypass fans to augment thrust and improve efficiency in the subsonic and transonic speed ranges actually useful for the time, particularly with the practical limits on temperature and pressure in the hot section of those engines.

There was the overly complex 109-007 turbofan design, but the only example I'm aware of that's remotely simple or practical for short term development appears to be Ohain's HeS 10 design, adding a ducted bypass fan to the HeS 8, apparently in a single-spool arrangement. (Heinkel had also been developing a couple ducted fan arrangements using Hirth air cooled piston engines, and while those may not have ended up being very useful, the testing may have aided adaptation to a turbofan)

P1010101.jpg Photo by Quinto_Sertorio | Photobucket

In terms of single-spool designs (even single turbine ones) use of reduction gearing to better optimize for the fan stage would probably be beneficial. Beyond that the use of independent 'free wheeling' low pressure turbine driven, rear-mounted fan modules (ducted or open rotor) similar to what was implemented on the Metrovick F.3 (and later open-rotor F.5) as well as Whittle's thrust augmenter for the W.2/700 intended to power the Miles M.52.

It seems like adapting some of the first generation turbojets to turbofans would have been simpler and more effective than many of the second generation designs (especially in Germany) as well as possibly even simpler than several of the first generation attempts. (a fair amount of the unattractive elements of Ohain's HeS 3 and 6 would have been mitigated by turbofan adaptations and, indeed wouldn't be too different from some small turbofan engine designs employed on -or proposed for use on- drones and cruise missiles: Patent US7055306 - Combined stage single shaft turbofan engine - Google Patents )


With the delays in the Jumo 004 and BMW 003, it also seems like parallel turbofan developments might have even caught up with the turbojet counterparts.

Given the limitations placed on German engines, use of rear mounted fan modules might have been the most practical option on the whole given the potential for use of low pressure turbines with lower stresses and the modular configuration allowing independent manufacturing and maintenance/overhaul. (particularly important given the vastly varying TBO for different major engine components)

Below is the Jumo 004B (Weight 740kg thrust 880Kg) compared to the HeS 008A weight 380kg thrust 600kg. Some sources claim the HeS 008 actually achieved 720kg even 780 but even at its 600kg rating it had a better T/W ratio.

This is a German centrifugal engine, it flew in a Heinkel He 280 when the Me 262 still needed a piston engine.

The type of turbofan you speak of I believe is called a 'bleed air turbojet'.

One of the interesting lines of work the Germans were pursuing was water cooled turbine blades, the war came to a close before an engine could be built but individual blades were tested. They also did work on ceramic turbine blades. After the war, about 1948 the Siemens company built a power plant gas turbine that ran of blast furnace gas. Siemens received the persomission from the allied commission that was running Germany at this time to build such an device.

The engine used a Jumo 004 compressor, a vertical ceramic combustion chamber, a 5 stage turbine blade with water cooling of stainless steel. The turbine nozzles (turbine 'stator') used ceramic blades also from German wartime research. It ran successfully though Siemens eventually switched to nimonic when that alloy became available.

Water cooled turbine blades were planned for aircraft engines. There is a glight global in which Roy Fedden interviews BMW's chief of design and he mentions this.

A 'bleed air' turbojet with a single fan would provide a considerable flow in the duct which could be used to cool the steam or hot water generated in the turbine thereby also generating thrust. Water cooled turbine blades work well. The coaxial rotary union is hardly a problem and any tendancy to leakage can be over come by pressure balancing from the compressor air. The other object of calcification can be overcome by using distilled water and a decalcification procedure say every 500 hours.

The von Ohain turbojets of course had a centrifugal compressor ahead of which lay ahead inductor fan which provided a little compression but greatly calmed the airflow. Making this a turbofan involves only increasing the size of the inductor fan slightly.

(I think cancelling the He 280 with its original HeS 008 engine was foolish. It was better than the He 162 and might actually have made production by 1944 prior to the invasion)

Note in regards to the Daimler Benz DB007 turbofan. There was nothing wrong with the concept, GE produced a rear drive fan engine derived from the J79 known as the CJ805-23.

The idea of the DB007 was to pass much of the bypass air not around the engine but through the inside over the turbine blade roots, thus cooling this critical area. The idea of a geared compressor is also good and the problems of small gearboxes are not severed as alignment can be maintained. A gearbox allows close matching of turbine and compressor and apart from aiding efficiency should also greatly accelerate development as simply changing gear ratios can produce a "match". The contra rotating stator looks a little over the top but even it had its purpose. I think it would have been a success and it would also have been very silent due to the calming effect of the duct and the airflow speed reduction from duct/turbine gas mixing. (A true whispering death?)

 

[kool kitty89]

Trouble for the Germans is that all of these schemes need added turbine stages and it was the shortage of high temperature alloys (for turbine blades) that was one of the major stumbling blocks. The air cooled blades at the time were a work around. doubling, tripling, quadrupling the number of turbine blades needed is hardly going to increase engine production, whatever the benefits would be.

True, but for an engine of a given thrust value, would a turbofan really require MORE scarce materials? Plus, the lower pressure, lower power auxiliary turbine stages driving fan(s) could use looser tolerances on thermal and mechanical stress than the primary turbine. (and potentially use the fan blades/hub/disc as heat sinks -though probably add to weight as hollow/sheet steel blades would be more likely than aluminum -which could warp under heat and load)

Aside from that, extracting more power from the main turbine and using a single spool bypass/bleed fan arrangement (or reduction geared fan -using much lower ratio and torque than a turboprop would) would avoid added material use entirely.

And in either case, use of fans to increase mass flow and thrust would avoid added aerodynamic stresses and sheer size/material required for larger combustion chambers for similar thrust levels on pure jet engines. (or possibly allow use of less efficient, lower pressure, lower inlet temperature compressor/turbine arrangements while still having acceptable fuel consumption)


In the case of the HeS 8, an added axial compressor stage was used in later prototypes, but using that turbine power to instead drive a larger intake impeller with bypass bleed might have been more useful. (granted, an axial turbine would also have used much less material for the power extracted than the radial one and possibly easier to air-cool, but that's a separate issue) And beyond that, there's many issues to reducing limited resource requirements for engines ... including machined parts requiring high speed cutting steel as well as alloy components themselves. (contemporary german piston engines were hardly light on all of the above)

Compare this to the blades used in British engine which seem to have been caste and then precisely machined with elaborate fir tree roots. I suspect that had the German alloys been fabricated with the British methods and design they would have been more reliable.

Cost and hours of labor required for british engines was a good deal higher, and the turbines were unreliable prior to adopting very high nickle/chromium alloys not unlike nichrome. Whittle had trouble with combustion stability and turbine integrity that Jumo seems to have addressed much earlier, and Ohain/Heinkel avoided entirely by adopting hydrogen for early tests followed by fuel vaporization systems and a much more tolerant radial turbine. (though from what I've read, that turbine suffered more with the combustion arrangement and axial guide vanes used on the HeS 8 with problematic tip stresses and overheating, generally issues culminating from the compromises used to make the HeS 8 more compact than the preceding HeS 3 and 6 -I do seriously wonder if lengthening the combustion chamber would have alleviated some of the problems without increasing diameter -though weight would necessarily increase) The HeS 8 visibly has a smaller and more contorted combustion chamber than the relatively 'straight through' ones of the HeS 3 and 6. (with the diffusor folded forward, but combustion occuring straight back with flame jets flowing into an open annular chamber, combining with bleed air, and then flowing into the radially arranged turbine stators)

Do note it's also HeS 8 or 109-001, and not 008.

Likewise the use of impulse compressors which were easier to manufacture compared to the lighter more efficient reaction types. The lower efficiency could only be compensated with higher temperatures in the turbine, or lower thrust etc.

This is another benefit to a turbofan arrangement. Applying those same manufacturing methods would increase thrust and improve fuel consumption without requiring heavy use of precision machine tools, highly skilled labor, and high speed steel. (granted, Heinkel's HeS 30 from Wagner/Muller's team had advantages in compact size, mechanical simplicity and material usage as well -precision machined compressor stages, but only 5 of them and of smaller diameter than the 004 or 003; the use of the variable turbine guide vanes may have improved turbine wear/life as well)

Ohain's centrifugal/radial engines were even less efficient and made compromises for ease of construction and expedience in development as well as material limitations. (including the compression ratio of 2.7~2.8:1)

Honestly, combining the best of the simple and effective aspects of Wagner/Muller and Ohain's engine designs may have yielded more practical early development as well. (mating Ohain's compressor arrangement with the flame can and turbine designs being developed for the HeS 30 may have worked very well AND eased manufacturing -Ohain's compressor rotor used formed sheet aluminum metal blades fastened to a steel hub rather than using large forged and/or machined aluminum blocks as Whittle based designs -and many superchargers- used ... and the later diagonal compressor on the HeS 011)

The radial turbine was good for expedience though, and may have paid off better if not for the combustion chamber difficulties on the HeS 8.

 

[kool kitty89]

One of the interesting lines of work the Germans were pursuing was water cooled turbine blades, the war came to a close before an engine could be built but individual blades were tested. They also did work on ceramic turbine blades. After the war, about 1948 the Siemens company built a power plant gas turbine that ran of blast furnace gas. Siemens received the persomission from the allied commission that was running Germany at this time to build such an device.

The engine used a Jumo 004 compressor, a vertical ceramic combustion chamber, a 5 stage turbine blade with water cooling of stainless steel. The turbine nozzles (turbine 'stator') used ceramic blades also from German wartime research. It ran successfully though Siemens eventually switched to nimonic when that alloy became available.

Whittle had been pursuing water cooled turbine bearing/hubs from early in his experiments at powerjets, not blade cooling, granted, but still significant (perhaps more so on a radial turbine). He switched to air cooling on the W.1A, I believe, using a machined aluminum impeller/heat sink at the rotor hub exposed to the engine's intake air between the flame cans. With a more contained/annular arrangement like Ohain used, bleed air forced between the compressor and turbine rotors may have been more feasible. (possibly with cooling slots or pores cut into the turbine hub as well, possibly including channels that penetrate to the exhaust side of the turbine -somewhat akin to the hollow blades or perhaps blade root/hub cooling slots on the 004)

A 'bleed air' turbojet with a single fan would provide a considerable flow in the duct which could be used to cool the steam or hot water generated in the turbine thereby also generating thrust. Water cooled turbine blades work well.

This reminds me of another issue: bleed/bypass air from a fan could be in part ducted over the external hot sections of the engine and help prevent overheating of the combustion chamber and exhaust as well as acting as a buffer/heat shield around the engine itself. (even with a rear mounted fan, ducts could be arranged to pull cool air though those hot sections before hitting the fan blades -another reason steel could be more practical than aluminum)

The von Ohain turbojets of course had a centrifugal compressor ahead of which lay ahead inductor fan which provided a little compression but greatly calmed the airflow. Making this a turbofan involves only increasing the size of the inductor fan slightly.

Indeed, though the question of efficiency as diameter increases without employing reduction gearing might get tricky, and particularly maintaining subsonic tip-speed. (so you'd at least want to be little or no wider than the centrifugal impeller diameter)

(I think cancelling the He 280 with its original HeS 008 engine was foolish. It was better than the He 162 and might actually have made production by 1944 prior to the invasion)

Agreed, though I still think Ohain and Heinkel missed other opportunities with the early engine designs and others combining the experience from the HeS 8 and 30 (ohain's small diameter and axial diffusor are impressive in their own right, if a problematic compromise for the short term), but canceling the existing HeS 8 (AND HeS 30!!!) was very short sighted, forcing the development of the HeS 011 even more so.

An more precisely on-topic: a turbofan development of the existing HeS 30 (not scaled up, same compact 5-stage design) very well may have met the class 2 thrust range on its own. And given the rotational speeds and mass flow involved, adapting the hot section of the HeS 30 to a compressor based on the HeS 8's might have been relatively practical as well. (including possible axial stages following the centrifugal one -the diagonal/mixed flow impeller was just a bad design, even if it had been aerodynamically efficient -which in hind sight it was not- it was heavy and VERY costly to manufacture, far worse than the reaction axial rotors or especially Ohain's composite sheet metal bladed centrifugal impeller -and forged aluminum inducer ... possibly adaptable to being stamped akin to the 004's compressor blades)

The idea of a geared compressor is also good and the problems of small gearboxes are not severed as alignment can be maintained. A gearbox allows close matching of turbine and compressor and apart from aiding efficiency should also greatly accelerate development as simply changing gear ratios can produce a "match". The contra rotating stator looks a little over the top but even it had its purpose. I think it would have been a success and it would also have been very silent due to the calming effect of the duct and the airflow speed reduction from duct/turbine gas mixing. (A true whispering death?)

Mounting the reduction gearbox at the front of the engine may have simplified things somewhat as well, rather than placing it directly between the compressor and turbine, plus optimize solely for the larger fan stage and bear stresses and loads only associated with that fan. (possibly integrated into the gear system used for the accessories drive, granted that also making it integral rather than modular somewhat like the supercharger on the V-1710)

And on the stators, it's interesting to note that Griffith's augmentor used contra-rotating turbine+fan rotors with no stators at all (only inlet guide vanes) in a rather simple and elegant arrangement that also avoided fan torque. (granted, a single fan stage rotating in the opposite direction as the engine's rotor may have been plenty practical in minimizing torque as well)

And for that matter, using the reduction gearing to reverse the direction of the fan relative to the main rotor might also be attractive both for torque and counter-swirl of the air impacting the centrifugal impeller's intake.

 

[Thos9]

a turbofan development of the existing HeS 30 (not scaled up, same compact 5-stage design) very well may have met the class 2 thrust range on its own.

There was such a development - the HeS 30A. Compared with the He S30, length was 2680 (2715)mm, diameter 890(600)mm, weight 470 (380)kg, speed 13000 (same) rpm, thrust 1050 (750 - 820 reached) kg, compression ratio 3.2 (same). One built. (Data from Nowarra, Heinkel und seine Flugzeuge, page 137.)