Su-57 intakes, supercruise performance and 2nd stage engine

[tequilashooter]

No one is saying the 2nd version Su-57 meets the NGAD requirements thats either the PAK-DP or supposedly mig-41 tasks. Sources had suggested a near hyper-sonic interceptor which suggest their research in pulse detonation engines. and others stating 3 stream cycle engines. Stealth is considered rather a matter of opinion at this point, since none are willing to share classified information of either aircraft's for their chosen choices. I am sure Russian industry sources themselves will state they made their better option.

 

[icyplanetnhc (Steve)]

My own measurement with blueprints. To be honest, I am not sure anymore whether the exact value was 20 or 30%, but it was quite significant in any case. I would assume max thrust at very high altitude does require to maximize the airflow to the engine. I also assume there might be an element of growth reserve, specially if three streams engines are developed for the Su-57, which seems indeed in the plans from what we know about both the aircraft's and the izd. 30's development roadmap. These would greatly mitigate any possible spillage issues the oversized intakes may create with the first stage engine.

No, generally maximum Mach does not occur at service ceiling, as any cursory glance of various 1g envelopes would show. Furthermore, airflow to the engines is generally not a limiting factor in these conditions (where you actually have excess airflow that creates spillage drag), but rather drag, or in the case of aircraft like the F-22 and Su-57, materials. Sukhoi’s own patents and documentations even stated a reduction in maximum Mach to around Mach 2 in order to increase the amount of composite materials used.

I don’t dismiss the Su-57 as irrelevant, but I’m frankly seeing rather bold proclamations of the aircraft’s individual superiority (5.5 generation) based on components that are still under development, or even based on supposition or extrapolation, such as the supposed variable cycle nature of the izdeliye 30. I’ve not seen such claims from trustworthy sources such as Piotr Butowski (who makes very accurate predictions and even describes the number of fan and compressor stages of the izdeliye 30) or SPF’s very own flateric.

To be frank your own arguments are rather lacking in technical merit or understanding.

 

[LMFS]

No, generally maximum Mach does not occur at service ceiling, as any cursory glance of various 1g envelopes would show.

Who is talking about max Mach? To be clear, I am talking about supercruising conditions.

Furthermore, airflow to the engines is generally not a limiting factor in these conditions (where you actually have excess airflow that creates spillage drag), but rather drag, or in the case of aircraft like the F-22 and Su-57, materials.

Excess airflow at high altitude? Are you sure? Every diagram I see shows the thrust at altitude as a low fraction of that at sea level, even with ram compression and all. Clearly you want to fly as high as possible to reduce drag as much as possible and add range to your missiles, and get your lift from the increased speed. The oversized intakes of the Su-57 tell me quite a different story from what you are saying, what is your explanation to such big capture area? Why do they use intake ramps, when they are only useful starting from a speed they cannot surpass? To put it bluntly, are they cretins, increasing spillage and adding complexity for the fun of it?

You clearly know very well what you are talking about, but you are not very willing to answer certain questions or go into the thorny issues I am rising. That is OK, but I think we could come to interesting conclusions if you would.

Sukhoi’s own patents and documentations even stated a reduction in maximum Mach to around Mach 2 in order to increase the amount of composite materials used.

Can you point out to those documents? I have not seen them. But I have seen Sukhoi referring to the plane as generation 5+

I don’t dismiss the Su-57 as irrelevant, but I’m frankly seeing rather bold proclamations of the aircraft’s individual superiority (5.5 generation) based on components that are still under development, or even based on supposition or extrapolation, such as the supposed variable cycle nature of the izdeliye 30. I’ve not seen such claims from trustworthy sources such as Piotr Butowski (who makes very accurate predictions and even describes the number of fan and compressor stages of the izdeliye 30) or SPF’s very own flateric.

Well, forget about the VCE issue if you find it disturbing, an engine with the world's highest specific thrust and the SFC of a relatively high BPR one is not a superior propulsive solution? When they talk about specific weight 1/3 lower than AL-41F-1S, that means to me TWR ca. 13, is that 4.5G? Marchukov indeed talks about the engine as gen 5+, BTW.

What I find very bold is to call Su-57 4.5G as some disingenuous sources do, based on talking points instead of facts. For instance as noted, the fact that USAF has slated F-22 for retirement and replacement by NGAD, indicating the already mentioned features (speed, range, payload) as weighting heavily on that decision. Such features are already present or imminent in the Su-57, but simply ignored by the Western block in their assesments. First it was argued that the Indo-Pacific theater demanded long range fighters, but then it was noted that there would probably be two versions, one for that theater and one with shorter range for the European theater. How is a high end, mature 5G fighter like the F-22 rendered obsolete by a threat of the level of 4.5G fighters like Su-35 and Su-57? Clearly the narrative has holes, quite big ones. Not to talk about the narrative related to the fast iterations in design, which deserves one analysis in its own.

If you think about the time, it is only logical that a plane that appears 15 years after F-22 belongs to a later and more mature understanding of things and uses more advanced technologies, from design methods to materials to avionics to propulsion. I cannot see how this is dismissive of the F-22, the problem is that US for some reason did not really maintained the plane up to date. As to considering the izd. 30 as part of the package, well, it has been in development for 10 years, in flight testing for almost 4 and is slated for immediate start of state tests and induction into service (between 2023 and 2025). It is closer than Block 4 for the F-35 for instance, which is the one that US military is using already in their war games. But the other features like sensor suite, EW, intellectual support and airframe advantages are already there and absent in the F-22. I don't see why saying it is 5.5G is ludicrous, while saying that supposedly 6G NGAD has already flying prototypes raises no eyebrows...

@tequilashooter
Remember there are no official values for the Su-57, you depend on very detailed and accurate calculations to know the range of a plane. Range on internal fuel in Russia means in subsonic, optimal cruise conditions.

 

[haavarla]

As for Max durability or Max time on station, ALL flight are subsonic. Most likely not under 40.000ft eighter.
If you want to do a mission at the deck lvl or at supersonic cruise, sure. But it wont be max range.

 

[icyplanetnhc (Steve)]

Excess airflow at high altitude? Are you sure? Every diagram I see shows the thrust at altitude as a low fraction of that at sea level, even with ram compression and all. Clearly you want to fly as high as possible to reduce drag as much as possible and add range to your missiles, and get your lift from the increased speed. The oversized intakes of the Su-57 tell me quite a different story from what you are saying, what is your explanation to such big capture area? Why do they use intake ramps, when they are only useful starting from a speed they cannot surpass? To put it bluntly, are they cretins, increasing spillage and adding complexity for the fun of it?

Inlet sizing for an external compression inlet for a turbofan engine, which only “consumes” as much air as it needs and the rest is bled out, tends to be driven by maximum thrust conditions, not cruise. These could be low speed/takeoff, maximum Mach, or even a powered climb profile.

I’ve also never suggested that they are “cretins”, and in any case, design choices that may have marginal value certainly don’t have to originate from those that would fit that epithet.

Can you point out to those documents? I have not seen them. But I have seen Sukhoi referring to the plane as generation 5+

From SPF’s very own flateric, a Sukhoi paper from around 2006, when the shape of the Su-57 was largely finalized.

https://www.secretprojects.co.uk/threads/sukhoi-su-57-t-50-pak-fa-flight-testing-and-development-part-ii-2012-current.15626/page-8#post-195251

Front-line fighters have their own critical performance requirements for the use of composites in the design. In this case, the share of composites applications is most affected by the maximum airspeed - more precisely, a Mach number.

The largest share of composites application with a maximum weight effect can be realized in the design of the front-line fighter aircraft with a maximum flight speed of 2.1 M, as the range of temperatures of airframe skin surface lay within the range of operating temperatures of the existing composites.

However, in this case, there are areas with increased operating temperature, where the use of composites is practically impossible, like nacelles of powerplant based on augmented turbofans, especially in the afterburners area. Due to the large thrust-weight ratio of modern front-line fighter and dimension of modern powerplants the mass fraction of metallic nacelles in the airframe is quite large.

The absence of metal armor and less stringent requirements for repair in the field can significantly extend the use of composites in the construction of the front-line fighter in comparison to attack aircrafts, so composites can be widely used in the fuselage, wing and empennage.

Domestic front-line fighters are intended for operations from non-prepated runways, and that limits the application of composites
in the lower and side surfaces of the mid-fuselage and aft-fuselage, as well as lower part of horizontal tail. But forward fuselage, in contrast to attack aircrafts, can be built mainly from composites.

Erosion problems with the leading edges of the wings and tail of the front-line fighters are as valid as the for the ground attack aircrafts. The difference in the sweep angle of the leading edges of the wing and tail is compensated by an increase of the maximum flight speed at sea level.

For front-line fighter with a maximum speed of flight M=2.1-2.35, there are additional restrictions on the use of existing composites in the area of air intakes and air duct of power plants due to deceleration of airflow to subsonic speeds with heating.

For supersonic interceptor with a maximum flight speed of M=2.35 use of existing composites in load-bearing structures is almost impossible, because their level of heat resistance does not cover the interceptor's operating temperature range. Here composites
can be used successfully only in airframe parts, protected from aerodynamic heating, such as equipment bays, and crew cabin.

Development of the new composites with high level of mechanical and thermal stability and higher performance than the metal alloys is a complex scientific and technical challenge, since the strength of composites more determined by the strength of the polymer matrix and the strength of its connection to the reinforcing filler, the mechanical properties of all polymers with increasing temperature faster fall than mechanical properties of the
metal alloys. Possible progress in heat resistance composite materials is associated with the use of metallic, ceramic and carbon matrix, but their mechanical properties close to those of metal alloys, price is excessively high, and manufacturability is lowered.

 

[LMFS]

Inlet sizing for an external compression inlet for a turbofan engine, which only “consumes” as much air as it needs and the rest is bled out, tends to be driven by maximum thrust conditions, not cruise. These could be low speed/takeoff, maximum Mach, or even a powered climb profile.

Makes sense for a commercial airplane, but supercruising aircraft very specifically need the highest military thrust at altitude.

From SPF’s very own flateric, a Sukhoi paper from around 2006, when the shape of the Su-57 was largely finalized.

https://www.secretprojects.co.uk/th...part-ii-2012-current.15626/page-8#post-195251

Thanks for the link!

 

[icyplanetnhc (Steve)]

Makes sense for a commercial airplane, but supercruising aircraft very specifically need the highest military thrust at altitude.

No, this applies even for supersonic aircraft. Quoting Raymer:

The inlet must be sized to provide enough air at the worst-case condition, when the engine demands a lot of air. This sets the capture area. Most of the time the engine demands less air than an inlet with this capture area would like to provide (i.e., mass flow ratio is less than 1.0).

When the mass flow ratio is less than 1.0, the excess air must either be spilled before the air enters the inlet or bypassed around the engine via a duct that dumps it overboard (Fig. 13.5) or into an ejector-type engine nozzle.

Inlet capture area tends to be driven by maximum thrust conditions, not cruise. At low speed or takeoff conditions, auxiliary inlets can provide additional air flow, but these wouldn't really work in supersonic conditions.

 

[LMFS]

No, this applies even for supersonic aircraft. Quoting Raymer:

The inlet must be sized to provide enough air at the worst-case condition, when the engine demands a lot of air. This sets the capture area. Most of the time the engine demands less air than an inlet with this capture area would like to provide (i.e., mass flow ratio is less than 1.0).

When the mass flow ratio is less than 1.0, the excess air must either be spilled before the air enters the inlet or bypassed around the engine via a duct that dumps it overboard (Fig. 13.5) or into an ejector-type engine nozzle.

Inlet capture area tends to be driven by maximum thrust conditions, not cruise. At low speed or takeoff conditions, auxiliary inlets can provide additional air flow, but these wouldn't really work in supersonic conditions.

Yes, thanks for the quote.
See how he speaks about worst case condition, not maximum thrust . In supercruisers, that worst case condition or max airflow demand is clearly high speed cruising at high altitude. The lack of thrust in very rarefied air (the faster you go, the higher you fly) is the constraint that drives the need for a low BPR engine in the F-22, despite the high fuel consumption, that is what drives the need for a new engine in the the Su-57, that is what is behind the VCEs attempted for the ATF and MFI programs. The factors you can work on to improve that thrust are essentially specific thrust of the engine and airflow through intake size / efficiency.

 

[icyplanetnhc (Steve)]

Yes, thanks for the quote.
See how he speaks about worst case condition, not maximum thrust . In supercruisers, that worst case condition or max airflow demand is clearly high speed cruising at high altitude. The lack of thrust in very rarefied air (the faster you go, the higher you fly) is the constraint that drives the need for a low BPR engine in the F-22, despite the high fuel consumption, that is what drives the need for a new engine in the the Su-57, that is what is behind the VCEs attempted for the ATF and MFI programs. The factors you can work on to improve that thrust are essentially specific thrust of the engine and airflow through intake size / efficiency.

No, worst case conditions are conditions that demand maximum mass flow that requires a large capture area, which is not cruise conditions. Furthermore, for supercruising, the increase in dynamic thrust is generally achieved through greater exhaust exhaust velocity and specific thrust; a larger inlet would provide greater mass flow rather than an increase in specific thrust (which is achieved through lower bypass ratio and greater temperatures). Specific thrust is an intensive property that’s independent of mass; increase in mass flow is generally beneficial in subsonic conditions.

 

[LMFS]

No, worst case conditions are conditions that demand maximum mass flow, which is not cruise conditions.

Subsonic cruising on a low thrust setting is not what supercruising planes do. They need all their mil thrust to attain the highest speed possible. This is obviously not trivial and demands specific design on the engine and its integration on the plane. The increase of thrust can be obtained by any combination of the two strategies mentioned, be it increasing the mass of the air propelled or its speed. Of two engines with the same specific thrust, the one with highest overall thrust will be the one with the more airflow and conversely. On the other hand, the spillage doors on the Su-57 are necessarily going to be used first at low altitude than at max mil @60 kft, whith air density 10 times lower than at sea level and the engine spinning as fast as possible.

 

[icyplanetnhc (Steve)]

No, worst case conditions are conditions that demand maximum mass flow, which is not cruise conditions.

Subsonic cruising on a low thrust setting is not what supercruising planes do. They need all their mil thrust to attain the highest speed possible. This is obviously not trivial and demands specific design on the engine and its integration on the plane. The increase of thrust can be obtained by any combination of the two strategies mentioned, be it increasing the mass of the air propelled or its speed. Of two engines with the same specific thrust, the one with highest overall thrust will be the one with the more airflow and conversely. On the other hand, the spillage doors on the Su-57 are necessarily going to be used first at low altitude than at max mil @60 kft, whith air density 10 times lower than at sea level and the engine spinning as fast as possible.

Sigh…

Again, supercruising conditions are usually not going to be conditions that drives inlet sizing, as that is not the condition that demands maximum air mass flow. Attached are some engine performance figures from the Navy for the F110-GE-400 on the F-14. Note that at 35,000 ft, the mass flow is about the same at M=0.8 and M=1.6, despite the speed difference. At M=1.6 the capture area of the inlet would provide airflow in excess of what the engine needs and the excess air would be spilled out. Note that the capture area doesn't represent the actual airflow to the engine, due to the air bending from the oblique shocks and the inlet likely operating at somewhat subcritical mass flow ratio for better stability of the system.

My point is that it’s faulty to assume superior supercruising performance based on the inlet capture area. You need to take into consideration engine airflow, maximum turbine inlet and outlet temperatures, and so on.

 

[LMFS]

Sigh…

Again, supercruising conditions are usually not going to be conditions that drives inlet sizing, as that is not the condition that demands maximum air mass flow. Attached are some engine performance figures from the Navy for the F110-GE-400 on the F-14. Note that at 35,000 ft, the mass flow is about the same at M=0.8 and M=1.6, despite the speed difference. At M=1.6 the capture area of the inlet would provide airflow in excess of what the engine needs and the excess air would be spilled out.

My point is that it’s faulty to assume superior supercruising performance based on the inlet capture area.

That is a very good table. But I am afraid that , for some reason and despite your obvious knowledge of the issue, you are mixing AB and mil operation, obviously the engine is spinning at very similar speeds in both cases, but generating a lot more thrust with the afterburner on. The plane represented is not a supercruiser and the data given for 1.6 M is under AB only, so the inlet design assumptions you are making about spillage at high speed do not necessarily apply to designs like F-22 or Su-57.

It all revolves around quite simple questions that I would kindly ask you to answer:
> airframe aside, does mil thrust determine the speed of a supercruising plane?
> for the same specific thrust, does an increase in mass flow increase overall thrust?

 

[icyplanetnhc (Steve)]

That is a very good table. But I am afraid that , for some reason and despite your obvious knowledge of the issue, you are mixing AB and mil operation, obviously the engine is spinning at very similar speeds in both cases, but generating a lot more thrust with the afterburner on. The plane represented is not a supercruiser and the data given for 1.6 M is under AB only, so the inlet design assumptions you are making about spillage at high speed do not necessarily apply to designs like F-22 or Su-57.

It all revolves around quite simple questions that I would kindly ask you to answer:
> airframe aside, does mil thrust determine the speed of a supercruising plane?
> for the same specific thrust, does an increase in mass flow increase overall thrust?

The operation of the inlet does not care if the engine is operating with or without afterburner, which is downstream of the turbines and generally doesn't affect airflow requirements as the engine would be operating at maximum RPM.

I'm not sure what your confusion is here. The forces that determine speed of a supercruising airplane are not different from any other airplane. You have steady speed when your thrust and drag are at equilibrium. An airplane designed for supercruise generally has better supersonic drag characteristics (area ruling, packaging, interference drag) and engines that provide better military thrust and good dynamic thrust at higher Mach numbers. Specific thrust is defined as thrust per unit of air mass flowrate, but it's not a static, fixed value, nor is it something that you would always want to prioritize. Again, a turbine engine has a maximum airflow, and any excess air has to be bled off.

 

[LMFS]

The operation of the inlet does not care if the engine is operating with or without afterburner, which is downstream of the turbines and generally doesn't affect airflow requirements as the engine would be operating at maximum RPM.

Exactly, that is why those two different points in the table that you mention do not represent a plane cruising at different speeds and hence levels of thrust, since they don't demand different airflow...

I'm not sure what your confusion is here.

I have no confusion. I am just saying that for a plane whose goal is to be as fast as possible without engaging AB, cruising is INDEED a design point for the intakes.

An airplane designed for supercruise generally has better supersonic drag characteristics (area ruling, packaging, interference drag) and engines that provide better military thrust

That is what I am talking about, but instead of answering my two simple questions you come with other arguments.

The fact is that Su-57, in order to be a competitive supercuiser needs new engines with higher military thrust in cruise conditions, not a new airframe. And that engine will depend both on its specific thrust and mass flow to achieve the performance improvement required, hence the intakes are a relevant element of the propulsive design to be considered. You claim the cruising conditions are not the design point of the intake, but I am demonstrating that they are indeed a flight condition where:
- Max possible mil thrust is demanded and hence the highest combination of mass flow and specific thrust
- Air density is 10 times lower than at ground level (design point of an airliner due to the take off)

The Su-57 is already STOL with the first stage engines and you have seen above it taking off as if weightless, without even bothering in deploying flaps. So that is not the requirement. If you have a more demanding design point than supercruising, I am all ears to hear the justification.

 

[icyplanetnhc (Steve)]

Exactly, that is why those two different points in the table that you mention do not represent a plane cruising at different speeds and hence levels of thrust, since they don't demand different airflow...

The table shows that at 35,000 ft, airflow at M=0.8 is close to the maximum airflow of the engine. In fact, the somewhat lower airflow of at M=1.6 is likely more due to the turbine discharge temperature limit. You seem to be focusing on terminology; it doesn’t matter of the engine is using afterburner or not. The situation where, at M=1.6 the capture area of the inlet would provide airflow in excess of what the engine needs and the excess air would be spilled out, would be no different if the engine is using intermediate thrust or afterburner. The inlet principles don’t change regardless of whether or not the aircraft is using afterburner.

Also note that at 35,000 ft and M=0.8, the airflows for intermediate thrust and with afterburner are the same.

You claim the cruising conditions are not the design point of the intake, but I am demonstrating that they are indeed a flight condition where:

I said that capture area isn’t driven by cruising conditions. There is a lot that goes into inlet design aside from capture area. You were using capture area as evidence of superior supercruise capability, when that condition isn’t what drives inlet capture area. You’re welcome to verify this with publications from Raymer or Nicolai.

 

[LMFS]

I said that capture area isn’t driven by cruising conditions. There is a lot that goes into inlet design aside from capture area. You were using capture area as evidence of superior supercruise capability, when that condition isn’t what drives inlet capture area. You’re welcome to verify this with publications from Raymer or Nicolai.

You said so, but you did not provide reason or evidence or any other design point which is more demanding for a supercruising aircraft, just that there are many factors and refer me to books. Essentially you are not proving anything.

 

[icyplanetnhc (Steve)]

The table shows that at 35,000 ft, airflow at M=0.8 is close to the maximum airflow of the engine.

I am sorry, but there is no way to make progress if you keep dodging my questions. But if you insist on the table, explain please why the thrust in intermediate at 35 kft is one fourth of the static thrust at sea level, despite the advantage in mass flow.

I don’t see how I’m dodging your questions. Thrust is not determined just by air mass flow, but also the ambient pressure and temperature (which varies with altitude), and their ratios with stagnation properties.

http://www.stengel.mycpanel.princeton.edu/MAE331Lecture7.pdf

Note the effect of ambient temperature and pressure on thrust for a turbojet; similar principles apply to turbofans.

I have given you examples of what drives inlet capture area size, such as takeoff, or maximum Mach. These are conditions that would demand maximum airflow that requires large capture area. Given all the information, if I've "proven" nothing, then you've proven even less.

 

[LMFS]

Thrust is not determined just by air mass flow, but also the ambient pressure and temperature (which varies with altitude), and their ratios with stagnation properties.
http://www.stengel.mycpanel.princeton.edu/MAE331Lecture7.pdf
Note the effect of ambient temperature and pressure on thrust for a turbojet; similar principles apply to turbofans.

That is a nice lecture indeed

I have given you examples of what drives inlet capture area size, such as takeoff, or maximum Mach. These are conditions that would demand maximum airflow that requires large capture area.

This case is max Mach, only with AB off (no difference for mass flow because of that), because the whole point here is to have a cruising speed as high as possible. I really don't get why this is a point of contention.

This is not peace time cruising, it is a combat condition where the plane tries to fly as high and fast as possible to gain a kinematic advantage for itself and its weapons without needing to switch on the AB.

 

[icyplanetnhc (Steve)]

This case is max Mach, only with AB off (no difference for mass flow because of that), because the whole point here is to have a cruising speed as high as possible. I really don't get why this is a point of contention.

It’s unlikely maximum Mach is what the inlet is sized to; the inlet design is external compression, and with the density increase from the shock compression (even a normal shock at M=1.5 increases density by a factor of 1.862, and isentropic compression to the same Mach number is even higher) mass flow is generally not the limiting factor for fighter engine at high Mach numbers. Again, at higher Mach numbers, specific thrust (exhaust velocity, generally) becomes the limiting factor, not mass flow.

From the inlet’s perspective, maximum Mach does not matter if it’s with afterburner or not. If capture area is driven by maximum Mach, it factors more into the corrected airflow and mass flow ratio in mixed (external and internal) compression inlets; this generally wouldn’t be a driving factor for supercruise.

http://www.icas.org/ICAS_ARCHIVE/ICAS1996/ICAS-96-7.4.2.pdf

Normally supersonic intakes are sized for the cruise condition, but with emphasis on the maximum acceleration during the take-off from sea level and subsonic climbs. A supersonic intake has to cope with the engine airflow demand over the required flight envelope, see figure 8 on page 11. All supersonic aircraft have to fly at subsonic speed, therefore the intake must be able to handle flow at subsonic speed, this includes take-off from sea level, subsonic climb, descent, land, and taxi. In addition to sizing the intake for engine demand allowance should be made for boundary layer control, environmental control and the engine cooling system.
An intake sized for subsonic operation will be too large for the cruise condition and therefore will generate excess drag.

I'm not sure why you're so insistent on using capture area to argue for superior supercruise performance. There are plenty of merits to the Su-57 design and the airplane represents a set of requirements that differ from other stealth fighters, so I find this insistent need to try to argue for its superiority in every aspect strange.

 

[LMFS]

It’s unlikely maximum Mach is what the inlet is sized to

It remains you making a supposition and applying common knowledge from your field, but with the caveat that high flying supercruising fighters are not the topic of those documents. I guess only topic specific articles or actual calculations can offer a more conclusive take and this is not the easiest topic, but it it is what it is and if I have the time I will do th required research and calculations to settle it.

mass flow is generally not the limiting factor for fighter engine at high Mach numbers. Again, at higher Mach numbers, specific thrust (exhaust velocity, generally) becomes the limiting factor, not mass flow.

There are compression losses too and your article points precisely to that and to the need of a variable intake for maximum performance.

As to the limiting factor, again, with the same specific thrust, the higher mass flow provides more thrust. You say the Su-57 intake is designed not for max cruising but for other undefined requirements, but still this is not so easy to determine and besides the PAK-FA does have additional intakes for low speed and/or high AoA operation. In any case, it is clear that the engine of the Su-57 has much more air available than that of the F-22.

Correct me if I am wrong, but the speed adds ram compression quadratically and removes thrust roughly linearly (more with turbofans, much less with turbojets), but the altitude takes density away in an exponential manner and the intake losses apply on top of all that. Why are you sure that the first effect is dominant at a given flight condition? Density at 60 kft is roughly ten times less than at sea level, more than the effect of ram compression at M = 2. You seem to disregard supersonic maneouvering too, which is a requirement for a fighter aircraft and which will restrict the airflow to the intake. Again, a requirement mentioned in the article to accommodate for a wide range of flight regimes is a variable intake.

From the inlet’s perspective, maximum Mach does not matter if it’s with afterburner or not.

Yes, this is what I said too.

I'm not sure why you're so insistent on using capture area to argue for superior supercruise performance. There are plenty of merits to the Su-57 design and the airplane represents a set of requirements that differ from other stealth fighters, so I find this insistent need to try to argue for its superiority in every aspect strange.

I made the point that the propulsive design of the Su-57 is more advanced and capable than that of the F-22 and you attacked it from the intake approach, but IMHO you still did not prove that the bigger intake in the Su-57 does not contribute and I therefore continue to dispute your position. I am just trying to bring a technical argument to its logical outcome.

In turn I find it strange that, in light of a bigger capture area, more effective variable intakes, a shorter diffuser without S duct, a similarly sized more modern engine with higher specific thrust and almost necessarily higher mass flow (18 vs ca. 16 tf max thrust), more fuel to burn at max mil, more lifting area, lift-generating trimming surfaces ideal for supersonic flight and more efficient nozzles, you dispute the fact that the Su-57 is indeed clearly ahead in propulsive design. Based on those elements, it is basically a fact.

The question is not to defend some absolute lack of shortcomings of the plane (that is never the case), it is to address groundless attacks that are done very lightly and that are an insult to intelligence, specifically when, like in propulsion, it is not one aspect where the Su-57 is ahead, is practically all of them, and not by a small margin. When people state dismissive oversimplifications about the F-35 which are outright false I do the same, I am as stubborn as that

 

[icyplanetnhc (Steve)]

It remains you making a supposition and applying common knowledge from your field, but with the caveat that high flying supercruising fighters are not the topic of those documents.

There is no reason that somehow supercruising fighters aren’t held to the same principles. Principles of supersonic flow don’t change between aircraft and aircraft. Frankly, it is you who is trying to make a “supposition” that supercruising fighters don't have to follow these inlet principles.

There are compression losses too and your article points precisely to that and to the need of a variable intake for maximum performance.

As to the limiting factor, again, with the same specific thrust, the higher mass flow provides more thrust. You say the Su-57 intake is designed not for max cruising but for other undefined requirements, but still this is not so easy to determine and besides the PAK-FA does have additional intakes for low speed and/or high AoA operation. In any case, it is clear that the engine of the Su-57 has much more air available than that of the F-22.

Again, specific thrust is not some static value, it varies across the flight envelope. It's not as simple as designing around higher mass flow; mass flow will be driven by what the engine needs and can accept, not the other way around. Inlet capture area is just one area that goes into the inlet design, and I never said that it isn't designed for supercruise. I'm skeptical that capture area is necessarily driven by maximum Mach at military power. For instance, the F-22's aerodynamics and inlet likely have a design point of Mach 1.5 for supercruise, even though it can reach about Mach 1.8 in military power; a variable geometry inlet has some more flexibility around its design point, but even this inlet is an external compression inlet where the ramp varies the throat area (contrast this to the F-15 or SR-71 inlet that can vary both the capture area and throat area).

This table from Leland Nicolai's aircraft design book, page 410, for turbine engine inlet sizing shows the airflow and capture area demand of the F-15's F100-PW-100 engine at different points in the envelope. Here, A_inf_E (the fourth column) represents the demand capture area, while A_C (the fifth column) is the actual capture area.

​	​	engine and secondary airflow​	A_inf_E​	A_inf_E/A_C​
Mach​	Altitude​	(lbm/s)​	(ft2)​	​
0.25​	2,000​	205​	10.9​	1.76​
0.5​	4,000​	220​	5.9​	0.95​
0.75​	5,000​	245​	4.53​	0.73​
1.0​	30,000​	130​	4.56​	0.74​
1.2​	30,000​	154​	4.53​	0.734​
1.4​	30,000​	183​	4.62​	0.749​
1.6​	30,000​	217​	4.76​	0.77​
1.8​	30,000​	257​	5.01​	0.81​
2.0​	30,000​	300​	5.26​	0.853​
2.3​	30,000​	388​	5.92​	0.96​

Here you can see that despite the variable in airflow, the demand capture area is the largest at low speed. The demand capture area in supersonic flight wouldn't overtake the low speed demand until well past Mach 2, which is beyond supercruising at this point. Furthermore, as you increase airspeed, engine required airflow increases, but as you increase altitude, the engine's required airflow decreases. For instance, at 45,000 ft, with the F100-PW-100, the required airflow at around Mach 1.9 is only about 150 lbm/s (Nicolai, Figure 14.8g page 377).

A supercruising engine won't have the exact same curves, but will follow the same trends; they both turbine engines and optimizing for supercruise won't somehow make it follow different principles.

Correct me if I am wrong, but the speed adds ram compression quadratically and removes thrust roughly linearly (more with turbofans, much less with turbojets), but the altitude takes density away in an exponential manner and the intake losses apply on top of all that. Why are you sure that the first effect is dominant at a given flight condition? Density at 60 kft is roughly ten times less than at sea level, more than the effect of ram compression at M = 2. You seem to disregard supersonic maneouvering too, which is a requirement for a fighter aircraft and which will restrict the airflow to the intake. Again, a requirement mentioned in the article to accommodate for a wide range of flight regimes is a variable intake.

I don't think you quite understand compression. In incompressible fluids, dynamic pressure increases with the square of the velocity, but that is not how it works in compressible supersonic flow; you'll need to use oblique and normal shock tables/equations, and then isentropic equations in the subsonic section (for approximation). There's a reason that dynamic pressure calculations for anything higher than low subsonic take Mach number into account.

I made the point that the propulsive design of the Su-57 is more advanced and capable than that of the F-22 and you attacked it from the intake approach, but IMHO you still did not prove that the bigger intake in the Su-57 does not contribute and I therefore continue to dispute your position. I am just trying to bring a technical argument to its logical outcome.

In turn I find it strange that, in light of a bigger capture area, more effective variable intakes, a shorter diffuser without S duct, a similarly sized more modern engine with higher specific thrust and almost necessarily higher mass flow (18 vs ca. 16 tf max thrust), more fuel to burn at max mil, more lifting area, lift-generating trimming surfaces ideal for supersonic flight and more efficient nozzles, you dispute the fact that the Su-57 is indeed clearly ahead in propulsive design. Based on those elements, it is basically a fact.

For how emphatic you're making your arguments, your technical understanding is honestly quite questionable; what I see is you simply listing a litany of qualities to enthusiastically declare superiority without fully understanding true purpose and functionality, or any consideration of the system as a whole. You're disputing without an understanding of the technical arguments...

Frankly, you need to prove that larger inlet capture area automatically translates to superior supercruise; capture area is but one factor in supercruise performance. Larger capture area can provide greater airflow if the engine demands it, but it also increases weight and drag. You're using a larger capture area as proof of something that requires far more information. Advanced is also rather subjective here. If the propulsion system is mainly beneficial at conditions beyond the limits of the airplane from other factors (such as materials), can we really say it's more advanced? I'm not saying it's the right or wrong choice, but there are many tradeoffs and aspects to consider before declaring something as unconditionally superior. Not everything Sukhoi (or Lockheed, Northrop, any designer) does should be treated as infallible or beyond reproach.

I don't understand why you think I'm somehow attacking the Su-57's supercruise capability. It's clear that supercruise is one of the main design drivers of the aircraft. I wouldn't declare it as outright superior to all competitors in all parts of the envelope, and since you're demanding proof, you frankly haven't presented an argument with rigorous technical merit (it would be difficult to do so in any case because of lack of important data that would be necessary for calculations). For what it's worth, with the right engines I think the Su-57 can be excellent at supercruise, and likely has more range than the F-22.

 

[LMFS]

There is no reason that somehow supercruising fighters aren’t held to the same principles. Principles of supersonic flow don’t change between aircraft and aircraft. Frankly, it is you who is trying to make a “supposition” that supercruising fighters don't have to follow these inlet principles.

Of course they are held to the same principles, but the numerical values used as reference have a context in which they are valid, the flight regime and associated demands of the supercruisers are different. See more below.

I'm skeptical that capture area is necessarily driven by maximum Mach at military power. For instance, the F-22's aerodynamics and inlet likely have a design point of Mach 1.5 for supercruise, even though it can reach about Mach 1.8 in military power;

This is not quite clear to me: if the design point is 1.5 M, how does it reach 1.8? There would be not enough air for the engine...

This table from Leland Nicolai's aircraft design book, page 410, for turbine engine inlet sizing shows the airflow demand of the F-15's F100-PW-100 engine at different points in the envelope. Here, A_inf_E (the fourth column) represents the demand capture area, while A_C (the fifth column) is the actual capture area.

The airflow in the third column is the demand or the actual?
The plane is in steady horizontal flight?
I assume the 5th column represents the relationship between the theoretical demand capture area divided by the actual one, is that correct?

Here you can see that despite the variable in airflow, the demand capture area is the largest at low speed. The demand capture area in supersonic flight wouldn't overtake the low speed demand until well past Mach 2, which is beyond supercruising at this point. Furthermore, as you increase airspeed, engine required airflow increases, but as you increase altitude, the engine's required airflow decreases. For instance, at 45,000 ft, with the F100-PW-100, the required airflow at around Mach 1.9 is only about 150 lbm/s (Nicolai, Figure 14.8g page 377).

There are a few comments and questions here:
> Starting at low speed, the actual intake provides less airflow than what the engine could take (1.76 more demand than actual airflow?) In this case the plane does not seem to be designed with that demand in consideration. In the Su-57, additional intakes exist that should need to be considered
> @30 kft, as the speed starts increasing, the engine is already working in AB and as it spools faster, the airflow demand increases and the use of the capture area is almost maxed, up to 0.96 at 2.3 M. This is interesting, because the increase in speed at the same altitude should help the intake, but the effect we see is is not enough (the added compression due to speed does not compensate the additional airflow demand?). This is quite significant and I was not even counting on that. Actually it seems to strongly support my point that the capture area indeed is an issue, and is not only driven by altitude but also by speed. So the bigger intakes of the Su-57 would help not only at extremely high altitudes, but at way lower ones already.
> In this example the engine must be using AB, if those speeds were to be attained on dry settings, the airflow would need to be substantially higher, correct? So the same intake design that would allow to attain 2.3 M in AB for the F-15 would not be enough for doing it on mil power, even if the engine was theoretically capable of delivering the necessary thrust. The comment you make about the demand in supersonic only overtaking subsonic one beyond 2 M strongly depends on this.
> As to the airflow reduction at higher altitude, I am assuming it refers to the demand for steady flight and is related to the reduced drag, does that make sense?
> What would be the factor of use of the intake in those conditions, considering the air is way less dense at that altitude?

I don't think you quite understand compression. In incompressible fluids, dynamic pressure increases with the square of the velocity, but that is not how it works in compressible supersonic flow; you'll need to use oblique and normal shock tables/equations, and then isentropic equations in the subsonic section. There's a reason that dynamic pressure calculations for anything higher than low subsonic take Mach number into account.

True, I was not aware of that and it explains my assumption that the main use of the bigger intake was the very high altitudes only. This is a complex topic indeed, thank you.

Frankly, you need to prove that larger inlet capture area automatically translates to superior supercruise; capture area is but one factor in supercruise performance. Larger capture area can provide greater airflow if the engine demands it, but it also increases weight and drag. You're using a larger capture area as proof of something that requires far more information. Advanced is also rather subjective here. If the propulsion system is mainly beneficial at conditions beyond the limits of the airplane from other factors (such as materials), can we really say it's more advanced? I'm not saying it's the right or wrong choice, but there are many tradeoffs and aspects to consider before declaring something as unconditionally superior. Not everything Sukhoi (or Lockheed, Northrop, any designer) does should be treated as infallible or beyond reproach.

I don't understand why you think I'm somehow attacking the Su-57's supercruise capability. It's clear that supercruise is one of the main design drivers of the aircraft. I wouldn't declare it as outright superior to all competitors in all parts of the envelope, and since you're demanding proof, you frankly haven't presented an argument with rigorous technical merit (it would be difficult to do so in any case because of lack of important data that would be necessary for calculations). For what it's worth, with the right engines I think the Su-57 can be excellent at supercruise, and likely has more range than the F-22.

Fair enough. Since I cannot provide true numerical values I am referring to the kind of information that is available and which is of qualitative nature. The elements that, to the best of my knowledge, determine the supersonic performance, are essentially maxed in the Su-57. The radar blocker will have an impact, the weight of the plane is unknown and by the numbers that I have seen, the cross sectional area of the plane should be very similar to that of the F-22, other than that I don't see elements that could strongly skew what seems a setup consciously designed to go beyond the limits of the F-22. I really think the effort put by Sukhoi in this regard is easy to perceive, as they seem to have been very systematic in addressing every aspect that could give them an advantage. But I agree that any platform as a whole and the compromises taken in different aspects are are very difficult topic to analyse and compare in fairness.

 

[icyplanetnhc (Steve)]

This is not quite clear to me: if the design point is 1.5 M, how does it reach 1.8? There would be not enough air for the engine...

Because supercruise isn’t flying at maximum military power, as that would still be very inefficient even without afterburner. You would want your supersonic cruise design point to be somewhat lower than that (around 80-90% or so) and try to maximize your efficiency there. Of course, the inlet also has to be able to handle off-design conditions. Inlet and engine airflow matching is quite a complicated process, and variable geometry will grant you more flexibility, but it still has its limits, especially if it’s only external compression with fixed capture area.

The airflow in the third column is the demand or the actual?
The plane is in steady horizontal flight?
I assume the 5th column represents the relationship between the theoretical demand capture area divided by the actual one, is that correct?

The third column is the demand from the engine as well as airflow demand from oil cooling, nozzle cooling, and so on. Your other assumptions are correct, although steady flight is not really relevant from the engine’s perspective.

This is interesting, because the increase in speed at the same altitude should help the intake, but the effect we see is is not enough (the added compression due to speed does not compensate the additional airflow demand?). This is quite significant and I was not even counting on that. Actually it seems to strongly support my point that the capture area indeed is an issue, and is not only driven by altitude but also by speed. So the bigger intakes of the Su-57 would help not only at extremely high altitudes, but at way lower ones already.

There is much more that goes into the airflow for the engine than just the mass. You’ll also need to take into consideration pressure and temperature. Pressure decreases steadily with altitude, while temperature decreases with altitude from sea level until the tropopause, 36,000 ft, above which it remains constant until about 66,000 ft when it increases again. With increasing speed and altitude, your may reach your turbine temperature limits before your airflow limits. Again, all these factors are why you can’t just look at the capture area and declare superiority in supercruise. A larger capture area can provide greater airflow, but it also increases weight and drag at Mach numbers like 1.5 where you’d be spending most of your time, so there’s a tradeoff for how large you want that area to be.

In this example the engine must be using AB, if those speeds were to be attained on dry settings, the airflow would need to be substantially higher, correct?

No. These numbers are all at maximum power, but again required airflow generally doesn’t depend on afterburner as it is downstream of the turbines. So the problem for military power here is not merely air mass flow, but exhaust velocity, nozzle pressure ratio, and other factors. An engine that tries to reach that thrust in military power settings can’t simply do that just with more airflow; more important is exhaust velocity, which enables better dynamic thrust at higher Mach numbers.

> As to the airflow reduction at higher altitude, I am assuming it refers to the demand for steady flight and is related to the reduced drag, does that make sense?
> What would be the factor of use of the intake in those conditions, considering the air is way less dense at that altitude?

There are many factors that goes into this. At higher altitudes and Mach numbers, you may run into temperature limits. The ambient pressure also continues to drop, which affects engine performance; there’s much more than just air density at altitude to consider.

 

[LMFS]

The third column is the demand from the engine as well as airflow demand from oil cooling, nozzle cooling, and so on.

So this demand is met at all points but the slowest speed in the table. Interestingly @ 0.5 M, 4kft and 2.3 M, 30 kft the demand of the intake is similar (0.95 vs 0.96). It seems in subsonic, the increase in speed indeed improves the airflow. From M=1 to 1.6 roughly, the intake is under relatively steady demand, at higher speeds the demand raises quickly. Maybe because the pressure recovery of that particular design starts not working that good?

With increasing speed and altitude, your may reach your turbine temperature limits before your airflow limits. Again, all these factors are why you can’t just look at the capture area and declare superiority in supercruise. A larger capture area can provide greater airflow, but it also increases weight and drag at Mach numbers like 1.5 where you’d be spending most of your time, so there’s a tradeoff for how large you want that area to be.

But remember we talk about mil power, the turbine temperature should not be as high as that suffered at the higher speeds attained with AB, so should be below the tolerance of the engine. I wonder what is the mil cruising speed goal Sukhoi established for the plane, when they equipped it with such complex and big intake. Why to do that, if you expect to fly 1.5 - 1.7 M, to name a figure the F-22 already manages well with the fixed inlet?

No. These numbers are all at maximum power, but again required airflow generally doesn’t depend on afterburner as it is downstream of the turbines.

You mean, the aircraft is accelerating? I have difficulty understanding that the engine is demanding so different mass flows at the same power setting...

On the other hand, I know the airflow is not affected by the AB, but the thrust indeed is. So, if you want to reach in mil power the same speed reached in the table with AB, that same engine would need (if it could handle it) substantially increased airflow. Because the AB should provide a very fast gas jet that is most effective at high speeds, compared to a turbofan in mil power. And at that point, the capture area demand would already be well above the subsonic demand at 0.5 M, 4 kft and well above 1.

So the problem for military power here is not merely air mass flow, but exhaust velocity, nozzle pressure ratio, and other factors. An engine that tries to reach that thrust in military power settings can’t simply do that just with more airflow; more important is exhaust velocity, which enables better dynamic thrust at higher Mach numbers.

Yes I know it is very complex, but I am making the supposition that the variable we are modifying is the mass flow only, to illustrate one of the dependencies that I am interested in exploring, I know an effective supercruising engine should try to increase the exhaust velocity rather than trying to have a huge, slow mass flow. But in the case you had a F119 and a F119 v2 with same exhaust velocity and rest of parameters, but higher overall size and mass flow in the second, that one would generate more thrust. Wouldn't it be supported to assume izd. 30 requires a higher mass flow vs. F119, when the max thrust is ca. 2 tf more? The reference we have to the thrust level of both engines is a bit indirect, but reasonably solid.

There are many factors that goes into this. At higher altitudes and Mach numbers, you may run into temperature limits. The ambient pressure also continues to drop, which affects engine performance; there’s much more than just air density at altitude to consider.

Pressure and density vary very similarly with altitude from wat I have seen. If the engine can handle the temperature, would the intake be under higher demand at higher altitude for the same amount of thrust? I assume clearly yes, and that would be a second factor complementing the first (speed) pointing to the convenience of increasing capture area.

 

[overscan (PaulMM)]

I'm not being facetious but it seems like only one side of this discussion really understands the topic.

if you want to reach in mil power the same speed reached in the table with AB, that same engine would need (if it could handle it) substantially increased airflow

That's not how it works, you can't just force extra air in to make the engine generate more thrust. You need a differently designed engine.

Supercruise-optimised engines have a larger core and smaller bypass ducts, so that they generate a greater proportion of thrust in dry thrust.

Alternatively, variable cycle so the amount of air going to the core and bypass can be varied.

 

[icyplanetnhc (Steve)]

But remember we talk about mil power, the turbine temperature should not be as high as that suffered at the higher speeds attained with AB, so should be below the tolerance of the engine.

No, afterburner is downstream of the turbines, so turbine temperatures are no different in full military power or in afterburner. As for Sukhoi's rationale for the inlets, it may be a way for Sukhoi to maximize the capabilities of the AL-41F1 in the Su-57 while providing growth margin for future variants in the long term. I think it's important to remember that this engine was never actually meant to be "interim", and when the aircraft was designed in the 2000s, it was expected that a sizable fleet of Su-57 would be using the AL-41F1; Sukhoi's T-50 submission for the PAK FA competition had the izdeliye 117 (AL-41F1). See Piotr Butowski's book on the Su-57.

You mean, the aircraft is accelerating? I have difficulty understanding that the engine is demanding so different mass flows at the same power setting...

At the same power setting (i.e. zone 5 afterburner), the engine may demand different airflow due to different limits being reached first depending on the conditions, whether it's rotor speed, turbine temperatures, airflow, etc. This is not at all straightforward and can be difficult to predict. Thrust is dynamic, and the rated thrust at you see is often uninstalled, sea level thrust.

So, if you want to reach in mil power the same speed reached in the table with AB, that same engine would need (if it could handle it) substantially increased airflow. Because the AB should provide a very fast gas jet that is most effective at high speeds, compared to a turbofan in mil power. And at that point, the capture area demand would already be well above the subsonic demand at 0.5 M, 4 kft and well above 1.

No, if afterburner is required to reach a certain speed, then the same engine in military power can't reach the same speed, even if you try to increase airflow. Especially at higher Mach numbers, airflow isn't the limitation, exhaust velocity is. Greater mass flow is of no use if the exhaust velocity isn't high enough.

Wouldn't it be supported to assume izd. 30 requires a higher mass flow vs. F119, when the max thrust is ca. 2 tf more? The reference we have to the thrust level of both engines is a bit indirect, but reasonably solid.

It may, or may not. Greater mass flow isn't something you would arbitrarily want, and there are various tradeoffs that we don't have the data to draw conclusions for. And in any case I don't know why you would be comparing maximum thrust when supercruise involves military power. Furthermore, comparing maximum thrust isn't helpful since it doesn't provide information about dynamic thrust at different speeds and altitudes. I'll also note that lower bypass ratio can result in a smaller thrust increase from military power to afterburner, as there is less unconsumed oxygen for the afterburner combustion.

 

[LMFS]

I'm not being facetious but it seems like only one side of this discussion really understands the topic.

I take absolutely no offence that some guy that links in his profile to AIAA student branch at UCLA knows more about such a thorny issue like inlet design than some random guy in the internet like me. That he wants to actively contribute with myself proving my point is a different story, and he is fully entitled to not doing so BTW. I appreciate (a lot) the information and links he is providing regardless, this is a topic I wanted to research for a long while but it is not easy to do 100% on your own with very limited time.

That's not how it works, you can't just force extra air in to make the engine generate more thrust. You need a differently designed engine.

Supercruise-optimised engines have a larger core and smaller bypass ducts, so that they generate a greater proportion of thrust in dry thrust.

Alternatively, variable cycle so the amount of air going to the core and bypass can be varied.

Maybe I am not explaining myself properly, the considerations you make are common knowledge even to me and not what I am referring too. The trend in the table above is that increasing speed places an increasingly high demand on the intake. In that particular case at 2.3 M, the capture area is already almost fully used up, and that engine is not supercruising and therefore is generating thrust with AB, so it uses less airflow for that generated thrust that it would be necessary for generating it if AB was off. That is true, no matter what other complications and considerations you want to introduce in the discussion. I know no single current fighter engine where AB cannot add some thrust without increasing airflow. And that proves that the airflow demand on a supercruising plane @1.5 - 2.0 M is higher than it would be for a plane flying at that speed using AB and that the table above is understating that demand.

No, afterburner is downstream of the turbines, so turbine temperatures are no different in full military power or in afterburner.

They are, if the speed is higher because AB is on...

As for Sukhoi's rationale for the inlets, it may be a way for Sukhoi to maximize the capabilities of the AL-41F1 in the Su-57 while providing growth margin for future variants in the long term. I think it's important to remember that this engine was never actually meant to be "interim", and when the aircraft was designed in the 2000s, it was expected that a sizable fleet of Su-57 would be using the AL-41F1; Sukhoi's T-50 submission for the PAK FA competition had the izdeliye 117 (AL-41F1). See Piotr Butowski's book on the Su-57.

That at least is one take on the issue. It is a possibility, and I am thinking in future 3 stream engines where airflow would need to be very high and that should be also better handling spillage as a reasonable possibility. Still izd. 30 was started relatively early on (early 2010's), since then there have been modifications on the airframe, if that huge intake size was going to be a handicap for 10 or 15 years I don't think it would have made sense to keep them that big, similar changes are done on other planes (i.e. F-16) when different engines are used and they don't seem to be the end of the world. Still, interested to understand what you mean by maximizing the capabilities of the AL-41, since I don't think anyone out there would be making intakes wilfully small not to use their engines as optimally as possible.

At the same power setting (i.e. zone 5 afterburner), the engine may demand different airflow due to different limits being reached first depending on the conditions, whether it's rotor speed, turbine temperatures, airflow, etc. This is not at all straightforward and can be difficult to predict. Thrust is dynamic, and the rated thrust at you see is often uninstalled, sea level thrust.

I admit I don't understand it either. RPM should be max in all cases, turbine temperature should be progressively increasing due to the increasing speed, and still the airflow grows and grows. That's why I referred to the increasing pressure recovery losses as an possible explanation...

No, if afterburner is required to reach a certain speed, then the same engine in military power can't reach the same speed, even if you try to increase airflow. Especially at higher Mach numbers, airflow isn't the limitation, exhaust velocity is. Greater mass flow is of no use if the exhaust velocity isn't high enough.

Sure, therefore I am talking about a bigger engine with more airflow capacity and the same exhaust velocity, and saying it would need more air. I referred to izd. 30 as that engine, once we have decently reliable information about its max thrust. What would be your static mass flow estimation for a modern engine with those 18 tf thrust? I don't know what values do you handle or take for reasonable for F119, but taking 16 tf, that would be already 12% more thrust in max settings and the airflow would need to match that from what I understand.

It may, or may not. Greater mass flow isn't something you would arbitrarily want, and there are various tradeoffs that we don't have the data to draw conclusions for. And in any case I don't know why you would be comparing maximum thrust when supercruise involves military power. Furthermore, comparing maximum thrust isn't helpful since it doesn't provide information about dynamic thrust at different speeds and altitudes. I'll also note that lower bypass ratio can result in a smaller thrust increase from military power to afterburner, as there is less unconsumed oxygen for the afterburner combustion.

I compare max thrust because I understand that it gives a good estimation of the mass flow of the engine. As you say, lower BPR means less AB boost, because the limiting factor is the oxygen that goes through the engine without being used at the core. So in the end and excluding what would seem to my limited knowledge as second order effects, AB equalizes the differences in layout and BPR and allows us to know what is the rough comparison in mass flows for two engines.

As to your second comment, we know from the lead designer that izd. 30 is claimed to have the highest specific thrust of any comparable engine, so I take F119 as lower bound reference in that regard. It should be then both an engine with higher exhaust velocity and higher mass flow and hence ideal for supercruising.

 

[icyplanetnhc (Steve)]

That he wants to actively contribute with myself proving my point is a different story

Frankly, this discussion has not proven your point.

In that particular case at 2.3 M, the capture area is already almost fully used up, and that engine is not supercruising and therefore is generating thrust with AB, so it uses less airflow for that generated thrust that it would be necessary for generating it if AB was off. That is true, no matter what other complications and considerations you want to introduce in the discussion.

Again, no. If you are at Mach 2.3 in afterburner, and then you pull out of afterburner and are on military power, your thrust will decrease and you’ll start decelerating, but the engine’s airflow and turbine temperature will remain the same at that condition. Remember that the afterburner is downstream of the turbine. If you want to generate the same thrust without afterburner, you can’t simply just increase airflow, if your nozzle pressure ratio and exhaust velocity aren’t sufficient. You’ll literally need a differently designed engine for the task, and the airflow is just one factor, not something you design the whole system around.

I know no single current fighter engine where AB cannot add some thrust without increasing airflow. And that proves that the airflow demand on a supercruising plane @1.5 - 2.0 M is higher than it would be for a plane flying at that speed using AB and that the table above is understating that demand.

No. At a given airspeed, an engine will demand the same airflow with or without afterburner. Exit mass flow is increased slightly because of fuel injection from the afterburner, but the airflow demand at that condition remains the same, and you’ll just accelerate because of excess thrust. The table is not relevant to your assertion here at all, and in fact shows that there are other conditions that drives the capture area, such as low altitude subsonic or takeoff.

They are, if the speed is higher because AB is on...

No. You’re changing the conditions. Again, at a given airspeed; the afterburner doesn’t change your turbine temperatures. An engine that can produce the same amount of thrust at military power as another engine in afterburner would have entirely different characteristics, airflow may or may not be greater.

Still izd. 30 was started relatively early on (early 2010's), since then there have been modifications on the airframe, if that huge intake size was going to be a handicap for 10 or 15 years I don't think it would have made sense to keep them that big, similar changes are done on other planes (i.e. F-16) when different engines are used and they don't seem to be the end of the world. Still, interested to understand what you mean by maximizing the capabilities of the AL-41, since I don't think anyone out there would be making intakes wilfully small not to use their engines as optimally as possible.

Again, you don’t simply take an airflow and design your engine and propulsion system around it. A bigger inlet may not even be necessary for better supersonic performance. The F-16/79 had the J79 engine which has lower airflow than the F100, but while subsonic performance was worse, the supersonic performance was actually better. Engine and inlet matching fits into an overall system, you don’t just design one around the other.

I admit I don't understand it either. RPM should be max in all cases, turbine temperature should be progressively increasing due to the increasing speed, and still the airflow grows and grows. That's why I referred to the increasing pressure recovery losses as an possible explanation...

Turbine and rotor temperatures depend on speed, altitude, and compression. Every engine will behave differently, you’ll need to know the characteristics of the specific engine to know the airflow requirements, what limits are reached first, etc. It’s definitely not as simple as looking at an inlet capture area to make determinations.

Sure, therefore I am talking about a bigger engine with more airflow capacity and the same exhaust velocity, and saying it would need more air. I referred to izd. 30 as that engine, once we have decently reliable information about its max thrust.

Is that actually the case with the izdeliye 30, or are you just assigning attributes that matches what you want to believe? Additional mass flow may not even be necessary for improved supersonic performance, see the example with the F-16/79, and there are conditions other than supersonic cruise that drives inlet capture area. Also compare the J79-powered F-4 Phantoms with the Spey-powered ones; the latter’s greater airflow enabled better efficiency and subsonic acceleration, but worse high altitude and supersonic performance.

In any case, the mass flow and specific thrust of the production F119 and F135 have never been disclosed, as far as I know, so I would not put much stock into statements claiming the best attribute of any known engine.

 

[LMFS]

@icyplanetnhc

My point is that the increasing mil thrust requirement of a higher supercruising performance is coherent with the increased specific thrust and air flow we see in the combo Izd. 30 / Su-57, and that other explanations for those features are simply weak or non existing. You seem to demand a numerical demonstration of the compared performance, which none of us will have for any of those two planes. I did not prove my point beyond any reasonable questioning (that was already known from the beginning, since we don't have the actual, complete data), but there is abundant evidence supporting my understanding. You focused more in the methodological weaknesses of my claims, but not in the actual, reasonable evidence I was providing. I can perfectly understand your position, and still think that I am on the right track. We will hopefully get to know not too far from now.

Above you are repeating to me issues that were already clear (AB and airflow, or AB and turbine temp for instance) and misunderstanding my point. I rest my case, I said what I had to say and the data in the discussion is enough evidence of the strong dependence between speed - airflow demand. It would be great to go further but I think I will have to do this on my own as I progressively acquire the theoretical tools needed to do it properly. In any case I thank you for the information and patience.

 

[overscan (PaulMM)]

Overall pressure ratio (OPR) improvement is not tied increased mass flow, you can't simply assume one will cause the other.

Keeping in mind that thrust is the product of mass flow and exhaust velocity, as well as pressure differences between the inlet and exit, higher OPR can increase the stagnation properties of the gas, which in turn can result in more work being extracted by the turbine and greater exhaust velocity from higher nozzle pressure ratio, depending on how the system as a whole is designed. I don't know why you're fixated on mass flow, and to be frank it's looking like circular reasoning at this point.

How would you increase mass flow through an engine of the same diameter if you don't improve the compression and thermal characteristics?

BTW I am addressing both exhaust velocity and mass flow, being both the main manageable factors of thrust as you say. It is obvious that improving any of those two is of immediate use in a supercruising engine.

The primary improvement to Izdeliye 30 is the raising of the turbine entry temperature from 1745K to 2000-2100K. This puts it close to the F135 class (2200-2300K)

I don't know what the source for that temperature is?

The points I made about the izd. 30 are all sourced at the designer, it is a supercruising engine with higher claimed specific thrust than F119 or F135 and that is hardly compatible with the high BPR you are referring, unless it is so much ahead of them in TIT that it can compensate a big difference in bypass ratio.

Saturn (Lyulka) have given the temperature figures. No dry thrust figures for Izdeliye 30 are available, so conclusions on bypass ratio are purely speculation. Better fuel economy is claimed, but in cruise, dry, reheat? All three?

Higher temperatures should reduce SFC across the board. If its truly optimised for supercruise, then bypass ratio would be 0.3 or close to it. Afterburning thrust is irrelevant for supercruise.

I'd expect it to be lower bypass ratio than AL-31F, which means for the same external physical diameter it has a proportionately larger core, which will help increase dry thrust. Low bypass ratio engines get a smaller thrust boost from afterburning however, so a high afterburning thrust says nothing much about bypass ratio - the best clue to that is the relative thrust of military to afterburning.

There's not enough data points to do more than speculate though.

 

[icyplanetnhc (Steve)]

Even maximum thrust figures are rather murky, and as far as I know, no official thrust figures were actually given. Piotr Butowski has repeatedly reported Saturn stating that the izdeliye 30 will be 16-17% “more effective” than the AL-41F1 without specifying what that exactly means. If “more effective” refers to maximum augmented thrust, then it would produce about 166 kN, but again this is simply a static thrust value.

How would you increase mass flow through an engine of the same diameter if you don't improve the compression and thermal characteristics?

BTW I am addressing both exhaust velocity and mass flow, being both the main manageable factors of thrust as you say. It is obvious that improving any of those two is of immediate use in a supercruising engine.

Compare the OPR and airflow for similar weight and size class engines like the F404, RB199, Snecma M88, or EJ200 and you'll see that OPR itself isn't directly correlated to mass flow. It all depends on the specific engine's design points. For an engine with similar dimensions as the AL-31F family, I would expect that the izdeliye 30 may have some increased airflow, but not hugely so. Mass flow itself is not something you specifically design an engine around.

 

[Anduriel]

Even maximum thrust figures are rather murky, and as far as I know, no official thrust figures were actually given. Piotr Butowski has repeatedly reported Saturn stating that the izdeliye 30 will be 16-17% “more effective” than the AL-41F1 without specifying what that exactly means. If “more effective” refers to maximum augmented thrust, then it would produce about 166 kN, but again this is simply a static thrust value.

I get 172 kN(147kN*1.17), which is ~17500kgf and quite close to 18tf thrust mark that is popular in speculation.

 

[icyplanetnhc (Steve)]

I get 172 kN(147kN*1.17), which is ~17500kgf and quite close to 18tf thrust mark that is popular in speculation.

As far as I know, the AL-41F1 has a maximum thrust of 14.5 metric tons (142 kN), with a special/emergency setting of 15 tons (147 kN). I don't think the latter is meant to be used normally.

 

[LMFS]

Saturn (Lyulka) have given the temperature figures.

Do you have the source? In any case, how do you explain those temperatures you mention plus a high BPR to produce a higher specific thrust than both F119 and F135?

No dry thrust figures for Izdeliye 30 are available, so conclusions on bypass ratio are purely speculation.

Why would such values be conclusive and statements about specific thrust would not?

Higher temperatures should reduce SFC across the board. If its truly optimised for supercruise, then bypass ratio would be 0.3 or close to it.

And same SFC of AL-31F? VCE starts making sense? Or the engine is technologically so much ahead of the F119? Honest question...

I'd expect it to be lower bypass ratio than AL-31F, which means for the same external physical diameter it has a proportionately larger core, which will help increase dry thrust. Low bypass ratio engines get a smaller thrust boost from afterburning however, so a high afterburning thrust says nothing much about bypass ratio - the best clue to that is the relative thrust of military to afterburning.

Yes, but we do not have that relationship mil to max. The only hint that we have in that regard is the statement about specific thrust, as said above.

There's not enough data points to do more than speculate though.

Not fruitless speculation IMHO, Marchukov has already given away many of the main features of the engine. But we will see.

Even maximum thrust figures are rather murky, and as far as I know, no official thrust figures were actually given. Piotr Butowski has repeatedly reported Saturn stating that the izdeliye 30 will be 16-17% “more effective” than the AL-41F1 without specifying what that exactly means. If “more effective” refers to maximum augmented thrust, then it would produce about 166 kN, but again this is simply a static thrust value.

I have not taken the 18 tf claim seriously until I got wind of a 2012 statement where Marchukov talked about specific weight of the engine being 1/3 lower than izd. 117S. That would be TWR of the engine around 13 and a max thrust of ca. 18 tf, if we assume the weight as 1400 kg (izd. 117 should be 1370 kg). I don't use Butowski's data, they might be right but they are very unspecific and unsourced.

Compare the OPR and airflow for similar weight and size class engines like the F404, RB199, Snecma M88, or EJ200 and you'll see that OPR itself isn't directly correlated to mass flow. It all depends on the specific engine's design points. For an engine with similar dimensions as the AL-31F family, I would expect that the izdeliye 30 may have some increased airflow, but not hugely so. Mass flow itself is not something you specifically design an engine around.

Of course, every engine uses a different set of parameters. But if you jump from one generation to the next, a series of technological parameters like TIT and OPR improve, because they are the direct ways of improving mass flow and exhaust velocity and hence improving thrust. I don't try to be 200% correct and exhaustively so in every single case, but this is a rule of thumb that I think is fully reasonable to use, specially if supported by other evidence.

About the second part, I also don't assume the mass flow in the izd. 30 is hugely bigger than AL-31F (what is huge BTW?), I, as apparently you too, assume they tried to improve in that part too, because it makes sense when you have less stages, better materials, 3D aero design and so on.

As far as I know, the AL-41F1 has a maximum thrust of 14.5 metric tons (142 kN), with a special/emergency setting of 15 tons (147 kN). I don't think the latter is meant to be used normally.

For izd. 117 no official thrust values have been given that I have seen. The best evidence I know are statements from Pogosyan about it producing 2.5 tf of thrust more than AL-31F and weighting 150 kg less. The official values for izd. 117S are known, though, as 14 tf max (normal) / 14.5 tf max (emergency mode).

 

[stealthflanker]

On the thrust figure of AL-41F1 (117) and Izd-30. This is from Saturn presentation. Scaling from that graph it's where the 17500 Kgf figure come from. Which i think reasonable.

The one with 14500 Kgf is as seen the 117S (AL-41F1S).

 

[overscan (PaulMM)]

I don't believe 117 weighs 150kg less than 117S. I think Izdeliye 30 probably does. We'll see.

 

[icyplanetnhc (Steve)]

The AL-41F1 (izdeliye 117) is not very different from the AL-41F1S (izdeliye 117S), with the biggest distinction being the latter’s own control unit, while the 117 is integrated into the Su-57’s flight control system. I would expect them to weigh nearly the same at roughly 1,600 kg. The Izdeliye 30 should weigh less, as it has fewer fan and compressor stages and more modern materials. I believe the target weight is about 1,450 kg.

 

[haavarla]

The AL-41F1 (izdeliye 117) is not very different from the AL-41F1S (izdeliye 117S), with the biggest distinction being the latter’s own control unit, while the 117 is integrated into the Su-57’s flight control system. I would expect them to weigh nearly the same at roughly 1,600 kg. The Izdeliye 30 should weigh less, as it has fewer fan and compressor stages and more modern materials. I believe the target weight is about 1,450 kg.

Unless the control unit weighed 150kg I suppose?

Doubt it. I read somewhere they used different alloys on some of the 117 stages. Hense less weight.

 

[LMFS]

Pogosyan said it weights 150 kg less than the AL-31F, not the AL-41F-1S. We can chose to take our opinion before his word if we want, of course...

 

[overscan (PaulMM)]

AL-31FP weighs 1520-1530kg. AL-41F1S weighs about 1604kg. Brochure figures. We have no definitive weight for 117, only a single comment from 2010 aimed specifically at rebutting Russian media criticisms of the engine choice. Sukhoi also claimed 30% lower weight for Izdeliye 30 - which is 1071kg, which is lighter than F414. We'll see about that I guess.