GE YF120

[Sundog]

YF23 PAV2 never flew after the decision. PAV1 taxied several times to maintain it's airworthiness, systems wise. After the decision, there was zero budget left, the program was in the red, Northrop wise. We did not invest any more of outr own money since it was to no real benefit since the program was over. The idea of the YF23 going mach 3 is silly. The airframe materials and the inlet design were at about M2.5, briefly. The YF 23 did out supercruise the YF22 with both engines. The PW YF22 had problems super cruising within the Supersonic corridor. They put it in a shallow dive, then leveled off, to get there. The YF22 did go M2 on the GE engines, using afterburner. The YF23 PAV2 flew it's max super cruise speed in the M1.7 range on the GE engines. The PW version did the M1.43 or so as I remember, which outran the YF22 PW by a decent margin. Once both teams were flying you really couldn't hide much info. Things were mostly out in the open.

Thanks, I had read that they got the chance to do that but it was apparently erroneous. I thought it was in AvWk, but never found it, so I should have known better. It was probably from Combat Aircraft (Hardly a reliable source). I know you were there when this all went down, so it's good to hear it from a reliable source.

Yeah, I wondered that about the inlet. Regarding the inlet change for the production version, to the half shock cone inlet, do you know if that was for better performance (Pressure recovery), LO, or just to make the inlet more robust? I've been trying to find out if Northrop was planning to suck the boundary layer off from in front of the inlet with the shock cone just as they did with the trapezoidal design, or if they were expecting the shock cone to act like the inlet on the F11F tiger that was tested with the boundary layer bump that eventually ended up on the F-35? i.e. - diverterless.

Lastly, did you ever get to see either PAV-1 or PAV-2 fly?

 

[Cjc]

Theoretically being the key part there, most people seem to take it for granted they the f-23 had better speed and range demonstrated during the flight trials, but that was only with the f-120, which the f-22 matched when it flow with the same engines.

Dem/Val was all about demonstrating what each contracter thought was important - there weren't fixed points of comparison to be met - and thus none to properly compare. Both F-22 and what would have been F-23 evolved quite a bit going into the EDM phase. Now unless you have extra information proving Northrop wildly inaccurate on how they justified their claims based off their test points flown in Dem/Val we don't really have a basis to discredit what has leaked out which is that the F-23 would have been faster.

How much faster and if it would have been practical in combat is another story. Remember the F-15 could do M2.5 but realistically, once loaded with fuel, weapons and with its engines not in a selfdestruct power setting M1.8 was about it. Carrying internal weapons helps but I doubt the F-22 or the F-23 would for that matter spend much time at or close to their absolute max speed.

I have seen very little claiming the F-23 to have this huge range advantage but it stands to reason that with a higher cruise speed and an equal power setting for most economical fuel burn from the engine it would have longer legs compared to something with a lower cruise speed.

The book i recommended earlier says the difference in supercurese would have been less then 0.1 mach. Also seriously? Everyone that talks about the yf-23 mentions how it "would have been better then the f-22 in the Pacific" do to its better range, all of 20-30 extra km according to Advanced Tactical Fighter to F-22 Raptor: Origins of the 21st Century Air Dominance Fighter. wich frankly makes sense, the planes weren't difent enough to create a several hundred km difference.

 

[BLACK_MAMBA]

The book i recommended earlier says the difference in supercurese would have been less then 0.1 mach. Also seriously? Everyone that talks about the yf-23 mentions how it "would have been better then the f-22 in the Pacific" do to its better range, all of 20-30 extra km according to Advanced Tactical Fighter to F-22 Raptor: Origins of the 21st Century Air Dominance Fighter. wich frankly makes sense, the planes weren't difent enough to create a several hundred km difference.

And what is their source for these claims? Pentagon documentation from Dem/Val downselect or similar? Single sources are hardly a good reference point when even engineers involved in the ATF program disagree with the statements - see Cancerman comments.

Edit: Also remember that we can only compare the YF-22 and 23. The F-22 had another 6 or so years worth of development before it first flew after ATF. Lots of things can change in that time. Lockheed didn't perfect the YF-22 due to their major redesign shortly before Dem/Val so they had lots to gain. Northrop possibly less so, but we won't ever know if their EDM info publically available is exactly what would have flown.

 

[icyplanetnhc (Steve)]

The authors extensively interviewed those involved in the program and incorporated their accounts in quoted passages, including from Sherman Mullin, who was the program manager of the F-22 during Dem/Val and director of Skunk Works. Col. Piccirillo himself was the ATF SPO director.

The F-23 EMD DWGs (DP231 and DP232 in Paul Metz’s book) that have been released only represent the “starting point” to any EMD and production vehicle that would have flown. This was especially the case with the DP232 with the F120 engines, since it had a slight notch in the back that would need to be cleaned up to match the trailing edge seen in DP231 with the F119. DP232 is also a bit longer, and given this dimensional difference, perhaps the F120 is a slightly larger engine than the F119?

It’s certainly reasonable to say that the YF-23 is simply much more refined and arguably more mature than the YF-22, as the YF-23 is a continual refinement of a configuration that has remained the same as Northrop’s Dem/Val RFP proposal, DP110; the F-22's performance improved substantially as it was further refined throughout the rest of Dem/Val and into EMD. With thrust reversers deleted, the EMD F-23 preserved the YF-23’s area ruling by smoothing over the “bread loafs” nacelle and filling in the volume in between to maintain the proper cross section distribution. While contours on the EMD F-23 design were smoother than on the YF-23, the EMD design’s overall volume did increase, which would allow for increased fuel capacity and range, while the F-22 did become much more refined but lost volume compared to the YF-22, which improved drag characteristics to allow it to better compete with the F-23 in terms of speed, but perhaps at the cost of decreased fuel capacity.

 

[Cancerman]

The book i recommended earlier says the difference in supercurese would have been less then 0.1 mach. Also seriously? Everyone that talks about the yf-23 mentions how it "would have been better then the f-22 in the Pacific" do to its better range, all of 20-30 extra km according to Advanced Tactical Fighter to F-22 Raptor: Origins of the 21st Century Air Dominance Fighter. wich frankly makes sense, the planes weren't difent enough to create a several hundred km difference.

And what is their source for these claims? Pentagon documentation from Dem/Val downselect or similar? Single sources are hardly a good reference point when even engineers involved in the ATF program disagree with the statements - see Cancerman comments.

Edit: Also remember that we can only compare the YF-22 and 23. The F-22 had another 6 or so years worth of development before it first flew after ATF. Lots of things can change in that time. Lockheed didn't perfect the YF-22 due to their major redesign shortly before Dem/Val so they had lots to gain. Northrop possibly less so, but we won't ever know if their EDM info publically available is exactly what would have flown.

The supercruise issues were basically the PAVs, since there is only one production aircraft, the F22. A lot changed from the YF22 to the F22. There are (were) things that didn't make either ship, as the clock was ticking and there were many delays for both companies. The PW F-119 was delivered late, as an example. Lockheed had a major redesign. etc. The ships that flew were what could be built and demonstrated at the time. Yes, I saw them fly at Edwards. Both teams did an amazing job under the crazy changes to the requirements which were occurring almost weekly. We (Northrop) had a new inlet that we wanted to add but the production schedule would not allow it. It helped from an RCS spec, as well as smoothed out the flow to the face of the enging helping high mach performance. Yes, there were design changes planned for the boundry air system which also helped the RCS as well. The "Breadloafs" were to be reduced and engine placement were to be moved closer together for better thrust management and single engine performance. The V tails were to be reduced in size a bit as the tail was almost too powerful. With that much tail, TV was completely unneeded. The bottom of the speed envelope was the only place it helped, a place you never flew at except landing on a carrier. Everyone knows we were stretching the forward section for a separate AIM-9 missile bay. This structural change also strengthened the ship as well. Radome was also changed for better radar performance. Custom landing gear and brakes to solve the off the shelf F15, F18 problems. As you see, if we had the 6 years of redesigning the F23 that the F22 had, things could have been sweet. Lockheed took the problem laden YF22 and turned it into a world class fighter. Nobody talks about the YF22 flight test disasters the program experienced. When Paul Metz flew PAV1 at it's max supercruise near the end of the program, he had a celebration with the support crew as we knew what speeds each team had achieved, he said it wa the fastest ATF in the world. This was before PAV2 did it's thing, which of course, they classified. Customer, not Northrop. I asked Paul Metz when he was going to produce the book (story) on the F22. He said he had tried, and Lockheed would not allow him to do a book as they had him under an NDA and would not release the info, pictures, etc, needed to do a great book like he did with the YF23A. Northrop withheld very little, mostly some performance data, which he had, just was not cleared to use. I'm not sure how many know this, but Lockheed almost lost the PW YF22 on it's first flight. They had some close calls. They crash landed one on the Edwards runway in a thrust vectoring gear down/up touch and go, P.I.O. software issue. They still ended up building an aircraft that set the standard for a 5th gen fighter which nobody has matched. The country is better for it.

 

[overscan (PaulMM)]

Air Force people from the LWF program have indicated General Dynamics worked very collaboratively and openly, no doors barred and bad news shared, while Northrop corporate attitude was less open and more "we are the experts, trust us" (with some exceptions at the individual level).

I'm not sure if the parallel is valid but in both LWF and ATF Northrop also settled on a basic configuration at an earlier stage that the competitor(s) and then refined it in multiple iterations. In LWF, this resulted in a excellent aircraft that didn't quite meet the requirement as well as GD's design did.

If we go back to the YF120 - it seems clear the engine was immature compared to the YF119 during PAV flight testing. F119 was the less risky choice.

 

[BDF]

The book i recommended earlier says the difference in supercurese would have been less then 0.1 mach. Also seriously? Everyone that talks about the yf-23 mentions how it "would have been better then the f-22 in the Pacific" do to its better range, all of 20-30 extra km according to Advanced Tactical Fighter to F-22 Raptor: Origins of the 21st Century Air Dominance Fighter. wich frankly makes sense, the planes weren't difent enough to create a several hundred km difference.

And what is their source for these claims? Pentagon documentation from Dem/Val downselect or similar? Single sources are hardly a good reference point when even engineers involved in the ATF program disagree with the statements - see Cancerman comments.

Edit: Also remember that we can only compare the YF-22 and 23. The F-22 had another 6 or so years worth of development before it first flew after ATF. Lots of things can change in that time. Lockheed didn't perfect the YF-22 due to their major redesign shortly before Dem/Val so they had lots to gain. Northrop possibly less so, but we won't ever know if their EDM info publically available is exactly what would have flown.

Actually it's a really good book. The three authors were directly involved in the ATF program. That being said they're obviously restricted to open source documentation. I do think the F-23A would've had more range only because it actually added fuel from the DemVal configuration to the production configuration. I made 3d models of the fuel tank groups using the high def plans from Aerospace Projects Review and found that the production jet had about 24% more volume. Of course there will be ullage and unusable fuel losses so it may not be 24% more per se but its clear that its a lot more fuel. That being said its not going to make a huge difference for the pacific which frankly needs double the range of the Raptor.

I know it's been said that the F-22 lost fuel during EMD but I can't seem to find any evidence of it and it seems to me that its unlikely that it did. For instance it has often been reported that the prototypes had around 24-25Klbs of fuel yet Metz's book says that the YF-23 had "a little more than a F-15C with a center line tank". I.e. about 18,000lbs. This suggests to me that neither prototypes at anything close to the ~25Klbs although perhaps that was the goal. It would be interesting to stick 3d models into CFD software and see how they compare at various Mach/Altitudes.

One more interesting point about Advanced Tactical Fighter to the F-22 Raptor is the authors stated that in Northrop's final submission to the Airforce, the F-23A was actually lighter than the F-22A. This seems difficult to grasp give how much larger the F-23A is dimensionally. It was stated because of the structural requirements for the 4- tail and thrust vectoring approach of the F-22 which cause it's empty weight to be higher.

 

[icyplanetnhc (Steve)]

Also remember that we can only compare the YF-22 and 23. The F-22 had another 6 or so years worth of development before it first flew after ATF. Lots of things can change in that time. Lockheed didn't perfect the YF-22 due to their major redesign shortly before Dem/Val so they had lots to gain. Northrop possibly less so, but we won't ever know if their EDM info publically available is exactly what would have flown.

To clarify on this point, the complete redesign of the Lockheed/Boeing/GD ATF began in summer 1987, which ended up in the configuration changing from swept trapezoidal to diamond-like delta wings; in many ways it resembles a 4-tailed evolution of GD’s ATF design. The YF-22 design, Configuration 1131, was frozen in February 1988 and the unfrozen to remove thrust reversers and frozen again into 1132 in May 1988, while the EMD submission, Configuration 638, came in late 1990. This configuration would largely be representative of the F-22 EMD/production design, Configuration 645, with the external lines largely unchanged.

So in a way, the F-22 can trace some of its roots to GD’s design as well as Lockheed’s; GD’s design emphasized supercruise and maneuver but struggled with all-aspect VLO due to the large single vertical tail, and the redesign starting in summer 1987 solved this by having four tails, while the aft fuselage would need to be carefully reshaped and slimmed down for area ruling and supersonic wave drag (not quite finished yet the YF-22, compared to how much less voluminous the F-22’s aft section is). In any case, the external lines did not change much from EMD submission in 1990 to the actual F-22 that flew in 1997.

Of course, this is not to say that Northrop/McDD would have followed the same kind of path with the F-23 if it was selected, although I’d imagine because the design is a gradual evolution of the same configuration that Northrop had submitted for Dem/Val in 1986, the F-23 EMD DWGs are likely a pretty good representation of what would have flown had it been selected, with models and an isometric perspective cutaway sketch in Paul Metz’s book giving us glimpses of what could have flown. Metz’s book also shows the design evolution from Dem/Val RFP (DP110), to YF-23 (DP117K) to F-23 EMD (DP231/232). Given the cleaner trailing edge lines, I’m inclined to say that DP231 with the F119s is more mature than DP232 with F120s.

Back to the F120, Aronstein, et al. did say that the EMD design would have had YF120 thrust levels with better temperature margins. I’d also expect that the throttle input lag as reported by Paul Metz would be addressed.

 

[F-2]

Northrop withheld very little, mostly some performance data, which he had, just was not cleared to use.

Well that’s something I hope eventually sees the light of day. A wonder what the thrust curves of a variable cycle engine look like?

 

[SCAT-15F]

FWIW, the first published supercruise numbers (1991) for the ATF competition were as follows: (via Popular Science & AWST). Later numbers are higher in some cases, but we might never know the definitive numbers (or the F-23's performance with the F-119 production version, which is significantly more powerful than the YF-120)

YF-22 with PW........mach 1.43
YF-23 with PW........mach 1.48
YF-22 with GE.........mach 1.58
YF-23 with GE.........mach 1.64

 

[icyplanetnhc (Steve)]

FWIW, the first published supercruise numbers (1991) for the ATF competition were as follows: (via Popular Science & AWST). Later numbers are higher in some cases, but we might never know the definitive numbers (or the F-23's performance with the F-119 production version, which is significantly more powerful than the YF-120)

YF-22 with PW........mach 1.43
YF-23 with PW........mach 1.48
YF-22 with GE.........mach 1.58
YF-23 with GE.........mach 1.64

No, the production F119 is significantly more powerful than the YF119, and comparable to the YF120. In late 1987, when the ATF’s 50,000 lb takeoff goal did not appear to be achievable, engine thrust requirement increased from 30,000 lbf to 35,000 lbf thrust class. P&W and GE responded by modifying their designs, including 15% larger (P&W) and 12% larger (GE) fan, respectively. GE incorporated the larger fan into the YF120 but P&W kept the original small fan on the YF119, which is why the PAVs with the YF119 had less thrust and lower performance. The production F119 has the larger fan and is significantly more powerful than its demonstrator, and largely matched the YF120.

 

[SCAT-15F]

The aerodynamic mods to the F-22A alone can't possibly fully account for the increase in supercruise speed from mach 1.58 with the YF120 to mach 1.82 with the F119-100. The F119-100 has to be more powerful than the YF120 (dry, at least) which would jive with the 37,500 lb-plus thrust figure given some time back. Dry thrust, in particular, has to be higher to account for the speed increase. There is simply no way around that if the current mach 1.82 number is to be credible.

 

[F119Doctor]

There is more at play than just increasing dry thrust. Higher exhaust velocity (higher specific impulse) and the rotor speed and turbine temperature margin necessary to maintain that that dry thrust at the elevated inlet temperatures at M1.5+ are just as important for usable thrust at those flight conditions.

 

[Waterballoon]

There is more at play than just increasing dry thrust. Higher exhaust velocity (higher specific impulse) and the rotor speed and turbine temperature margin necessary to maintain that that dry thrust at the elevated inlet temperatures at M1.5+ are just as important for usable thrust at those flight conditions.

Are 3rd gen SCTB necessary to achieve this? Or are 2nd gen ones sufficient granted they have the same amount of cooling available?

 

[SCAT-15F]

There is more at play than just increasing dry thrust. Higher exhaust velocity (higher specific impulse) and the rotor speed and turbine temperature margin necessary to maintain that that dry thrust at the elevated inlet temperatures at M1.5+ are just as important for usable thrust at those flight conditions.

Understood.
I've always assumed that the YF120 would have higher velocity dry because its effectively a pure turbojet with no bypass air. That being said, I'm also assuming the F119-100 has to have significantly more dry thrust than the YF120 to compensate for the lower velocity bypass air being mixed with the high velocity core exhaust and still allow the F-22A to hit a significantly higher supercruise speed than the YF-22 could with the YF120.

 

[F119Doctor]

Understood.
I've always assumed that the YF120 would have higher velocity dry because its effectively a pure turbojet with no bypass air. That being said, I'm also assuming the F119-100 has to have significantly more dry thrust than the YF120 to compensate for the lower velocity bypass air being mixed with the high velocity core exhaust and still allow the F-22A to hit a significantly higher supercruise speed than the YF-22 could with the YF120.

In a mixed flow turbofan, there is no low velocity bypass flow and high velocity core flow. Both flows have the same pressure and are mixed in the augmentor duct, with the pressure of the combined flow being turned into one exhaust velocity thru the nozzle. There may still be a temperature gradient between the inner core flow and the outer duct flow going to the nozzle at Mil power, but this is pretty much squeezed out passing thru the nozzle.

The YF120 was a low bypass turbofan in supercruise mode with a three stage fan (final stage core mounted) and a 4 stage high compressor. In the cruise mode, part of the fan 2nd stage discharge was bypassed around the 3rd stage, adding air to the 3rd stage bypass, effectively increasing the total bypass flow. While it was effective in supercruise thrust, it was very thirsty in fuel flow.

 

[SCAT-15F]

In a mixed flow turbofan, there is no low velocity bypass flow and high velocity core flow. Both flows have the same pressure and are mixed in the augmentor duct, with the pressure of the combined flow being turned into one exhaust velocity thru the nozzle. There may still be a temperature gradient between the inner core flow and the outer duct flow going to the nozzle at Mil power, but this is pretty much squeezed out passing thru the nozzle.

The YF120 was a low bypass turbofan in supercruise mode with a three stage fan (final stage core mounted) and a 4 stage high compressor. In the cruise mode, part of the fan 2nd stage discharge was bypassed around the 3rd stage, adding air to the 3rd stage bypass, effectively increasing the total bypass flow. While it was effective in supercruise thrust, it was very thirsty in fuel flow.

Interesting, that's more complex than I first thought. Thanks for the explanation!

 

[F119Doctor]

Are 3rd gen SCTB necessary to achieve this? Or are 2nd gen ones sufficient granted they have the same amount of cooling available?

The latest alloy Single Crystal Turbine Blades along with extremely detailed cooling design that has multi pass internal cooling passages for convection cooling combined with sophisticated shaped cooling holes for film cooling and the latest thermal barrier coatings are necessary for the airfoils to live at these extreme temperature conditions.

 

[icyplanetnhc (Steve)]

The aerodynamic mods to the F-22A alone can't possibly fully account for the increase in supercruise speed from mach 1.58 with the YF120 to mach 1.82 with the F119-100. The F119-100 has to be more powerful than the YF120 (dry, at least) which would jive with the 37,500 lb-plus thrust figure given some time back. Dry thrust, in particular, has to be higher to account for the speed increase. There is simply no way around that if the current mach 1.82 number is to be credible.

Actually, the F-22A is a significant aerodynamic improvement over the YF-22, which frankly isn't even a true prototype, but a barebones demonstrator that was quite immature (it's really more analogous to an XF type aircraft), if you look at the development timelines. It's worth remembering that the Lockheed ATF team completely discarded their Dem/Val design in July 1987 due to weight concerns and started over. The completely new configuration that the current F-22 evolved from was selected in late August 1987 (with fuselage and planform largely resembling General Dynamics’ Dem/Val submission), while the YF-22 was then frozen pretty soon after in May 1988, which was quite a fast turnaround time. Contrast this with the YF-23 which was much more refined in terms of its shape because it maintained the same configuration as Northrop's design since at least early 1986 that it gradually evolved from. In fact, what Northrop submitted for Dem/Val was very recognizably the YF-23, while the same is not the case for the YF-22. This is why the F-22 represents a larger evolution from the YF-22 than the F-23 over the YF-23, and once Lockheed had the chance to refine the F-22 design over the years through the remainder of Dem/Val and EMD, it was able to significantly improve its performance from YF-22 and catch up to the F-23; based on envelope charts, the F-22's standard day maximum supercruise is Mach 1.76 to 1.78, while Mach 1.82 is likely for cold day performance.

With regards to the engines, again the YF119 trailed behind the YF120 in thrust because when the takeoff gross weight was increased in 1987 that required an increase in engine thrust, both P&W and GE responded with design changes including a larger fan, but P&W didn't install that larger fan in the YF119 while GE did for the YF120. By all accounts, the EMD/production F119 catches up to the YF120 in static thrust rating, but given the variable cycle architecture of the YF120, it probably still has the edge in supersonic dry thrust, although it was allegedly also a thirstier engine. Apparently, for the EMD/production F120, GE would have made refinements that allowed it to achieve the same thrust levels with better temperature margins and perhaps TSFC improvements as well. That said, given that the F120 has two fewer compressor stages than the F119, the OPR would likely have been lower, which isn’t great for fuel efficiency even if subsonic bypass ratio is higher. Also, from a risk perspective, GE accrued considerably fewer testing hours than P&W, which may have factored into the Air Force’s decision.

 

[Scott Kenny]

The YF120 was a low bypass turbofan in supercruise mode with a three stage fan (final stage core mounted) and a 4 stage high compressor. In the cruise mode, part of the fan 2nd stage discharge was bypassed around the 3rd stage, adding air to the 3rd stage bypass, effectively increasing the total bypass flow. While it was effective in supercruise thrust, it was very thirsty in fuel flow.

That's quite the complex operation... And GE said they managed to do that basically with just the internal engine air pressure providing the ducting controls and actuation?

Spring loaded bypass doors?

With regards to the engines, again the YF119 trailed behind the YF120 in thrust because when the takeoff gross weight was increased in 1987 that required an increase in engine thrust, both P&W and GE responded with design changes including a larger fan, but P&W didn't install that larger fan in the YF119 while GE did for the YF120. By all accounts, the EMD/production F119 catches up to the YF120 in static thrust rating, but given the variable cycle architecture of the YF120, it probably still has the edge in supersonic dry thrust, although it was allegedly also a thirstier engine. Apparently, for the EMD/production F120, GE would have made refinements that allowed it to achieve the same thrust levels with better temperature margins and perhaps TSFC improvements as well. That said, given that the F120 has two fewer compressor stages than the F119, the OPR would likely have been lower, which isn’t great for fuel efficiency even if subsonic bypass ratio is higher. Also, from a risk perspective, GE accrued considerably fewer testing hours than P&W, which may have factored into the Air Force’s decision.

Some of the changes to the CFM-56 have resulted in a lower number of compressor stages but increased fuel efficiency...

 

[F119Doctor]

The active controls for the variable bypass were variable vanes in front of the 3rd stage fan and at the inlet of the high compressor, independently operated, along with the variable exhaust nozzle. By opening the nozzle to lower fan pressure ratio and cambering the 3rd stage vane to unload the 3rd stage fan, the discharge pressure of the 2nd and 3rd stage was similar and the 2nd stage additional bypass doors would passively open, possibly lightly spring loaded open. For low bypass, closing the nozzle and loading the 3rd stage increases the 3rd stage discharge relative to the 2nd stage, driving the additional bypass doors closed.

In the high bypass, low fan pressure mode, the high compressor inlet temperature would be lower, increasing the corrected core rotor speed. The HPC inlet vane would move more axial, increasing the core airflow and pressure ratio, partially offsetting the effect of lower fan pressure ratio on OPR.

At the back end of the fan ducts, the GE variable bypass design had a Variable Bypass Injector VABI. This was another passive movable feature that help match the pressures of the bypass and core flow as they combined in the augmentor duct under the varying bypass conditions.

The problem with this design is for supercruise operation, you want a high pressure fan with a big lazy core to provide lots of speed and turbine temperature margin. But in high bypass mode, the core is still a big lazy (low OPR) machine instead the small hard working high OPR core that is best for SFC with the low pressure ratio high bypass flow fan.

 

[Scott Kenny]

The active controls for the variable bypass were variable vanes in front of the 3rd stage fan and at the inlet of the high compressor, independently operated, along with the variable exhaust nozzle. By opening the nozzle to lower fan pressure ratio and cambering the 3rd stage vane to unload the 3rd stage fan, the discharge pressure of the 2nd and 3rd stage was similar and the 2nd stage additional bypass doors would passively open, possibly lightly spring loaded open. For low bypass, closing the nozzle and loading the 3rd stage increases the 3rd stage discharge relative to the 2nd stage, driving the additional bypass doors closed.

In the high bypass, low fan pressure mode, the high compressor inlet temperature would be lower, increasing the corrected core rotor speed. The HPC inlet vane would move more axial, increasing the core airflow and pressure ratio, partially offsetting the effect of lower fan pressure ratio on OPR.

At the back end of the fan ducts, the GE variable bypass design had a Variable Bypass Injector VABI. This was another passive movable feature that help match the pressures of the bypass and core flow as they combined in the augmentor duct under the varying bypass conditions.

That is way too cool that they got all the bits to work passively! Passively equals simpler and lighter weight, with less things to break or go wrong, I love it!

The problem with this design is for supercruise operation, you want a high pressure fan with a big lazy core to provide lots of speed and turbine temperature margin. But in high bypass mode, the core is still a big lazy (low OPR) machine instead the small hard working high OPR core that is best for SFC with the low pressure ratio high bypass flow fan.

Bugger, that is a nasty problem...

So now you need a way to increase core OPR relatively passively without messing up the already working bits... Adaptive and/or 3 spool systems here we come!

 

[F119Doctor]

Bugger, that is a nasty problem...

So now you need a way to increase core OPR relatively passively without messing up the already working bits... Adaptive and/or 3 spool systems here we come!

I’ve seen some ideas on how the adaptive engines are going to do exactly that, raising OPR while making the core aerodynamically smaller.

 

[Scott Kenny]

I’ve seen some ideas on how the adaptive engines are going to do exactly that, raising OPR while making the core aerodynamically smaller.

My immediate mental image was 3 spools:

Fan spool operates as YF120.​

IP spool is basically a smaller version of the fan spool, works the same way, adds OPR feeding into the HP spool when in high bypass mode.​

HP spool is relatively low OPR for supercruise.​