Supercruising turbofan engines

[F119Doctor]

For a given airframe, what is the difference between an engine that can supercruise and one that can't? Is it purely a difference of dry thrust? Or would the engines exhibit other differences as well.

To supercruise, an engine must have the lowest bypass ratio possible to keep the exhaust jet's velocity high; a turbojet is preferable to a turbofan. After all, a jet can fly only as fast as the velocity of its exhaust jet and the bypass air slows the exhaust jet's velocity down.

Correct - to supercruise, an engine must have a high exhaust velocity, at the supercruise engine inlet conditions. And a clean airframe with low supersonic drag. Both are required.

High exhaust velocity means a high nozzle pressure ratio going thru an optimized convergent / divergent exhaust nozzle. Nozzle pressure ratio is engine pressure ratio (exhaust pressure / engine inlet pressure) x ram pressure (engine inlet pressure / ambient pressure). Higher exhaust temperature also increases thrust, but it only goes up with the square root of the absolute temperature increase (same relationship as speed vs Mach number).

To get a high EPR at Mil power with a turbofan engine, you need a high pressure ratio Fan section, and a core module big and powerful enough to drive that high pressure Fan. The F100-220 fan pressure ratio is around 3:1, and the supercruise engines are significantly higher than that. At the typical quoted supercruise condition of 40K ft, 1.5Mn, the inlet temperature of the engine is approximately 100F. Most engines reach their rotor speed and turbine temperature limits in the 60-70F range. As the inlet gets hotter, these engines lose airflow, EPR, and thrust. At 100F inlet supercruise, both the Fan/LPT and Core have have the design margin to turn faster and hotter than at 60F inlet, just to keep making the same thrust (airflow and EPR). This points to having both a robust high pressure ratio fan, and probably an even larger core module. Thus, your design points to a low bypass turbofan - i.e. the F119.

more like a leaky turbojet, certainly not in the bypass range of a TF30 even, so not truly a turbofan but rather a very optimized turbojet for supercruise..

The F119 is a low bypass turbofan. Why turbofan instead of turbojet? A turbojet has a very hot outer skin in the main combustor, turbine, and AB areas, requiring heat shielding (weight) and a significant amount of secondary airflow (i.e. drag) to keep the airframe cool. The low bypass design of the F119 enables the structural fan ducts to be the heatshield, making it a "self cooled turbojet". The bypass air is also used extensively to cool the exhaust and nozzle components.

The other part of that thrust equation in the inlet ram recovery. At 40K, 1.5Mn the engine inlet pressure is around 10 psia, approximately 3.5 times the ambient pressure. Multiply that by the supercruise EPR, and you have a nozzle pressure ratio greater than 10:1. The convergent section of the nozzle accelerates the flow to Mach 1 using about 2:1 pressure ratio, leaving 5:1 or greater to be expanded in the divergent nozzle for supersonic flow. If you have a typical fixed ratio convergent / divergent nozzle, it will not have a large enough divergent section to efficiently utilize that available nozzle pressure ratio - i.e. under expanded flow. You could make the divergent section larger, but that adds weight, drag, and overexpansion concerns at lower speeds. The F119 engine, since it has full control of the 2D nozzle divergent flaps for thrust vectoring, can also position the divergent flap angles to optimize the supersonic expansion over a wide range of flight conditions and power settings.

There's something that's bugged me for years. Based on the Dash-1s (F-16 I believe) the -229s exhaust is about a thousand degrees hotter, and much higher in velocity, than that of the -129. Yet (apparently) the -129 performs better at high velocity. Any idea why this might be the case?

The -229 has a lower bypass ratio and higher EPR than the -129 to get equivalent thrust with lower mass flow of 248 pps vs 275 pps. It run a little hotter than the -129, but nothing like 1000 degrees. They may measure turbine temp differently ( F100 measure at the low turbine inlet - FTIT), not sure about -129. But they both use the same engine temp gauge that reads somewhere in the 900 - 1000C range at Mil power.

The higher EPR and exhaust temp of the -229 should help with supercruise type performance, but apparently the higher mass flow gives the -129 an advantage in supersonic AB thrust. I don’t have enough information about the -129 to make any definitive analysis.

 

[F119Doctor]

But, as PaulMM notes, a higher performing core can bring the fan and engine pressure ratio back to the same level as before, and exhaust velocity increase is roughly proportional to engine pressure ratio (EPR) increase.

"Same level as it was before" as in a turbojet? Are you saying a high performing turbofan core can achieve exhaust velocities of a similarly sized turbojet?

The real challenge is to maintain airflow and EPR as the inlet temperature increases with increased Mn. As the inlet temperature increases, the rotor speed and internal temperatures of the engine have to keep increasing to maintain that flow. Once you reach the rotor and temperature limits of the engine, the performance of the engine decreases, either airflow, EPR, or both.

So as airspeed increases, the temperature (and as a result, pressure) in the inlet increases, which reduces EPR? Or am I reading this wrong? Admittedly my understanding of how pressure and airspeed differences affect jet engine (specifically gas turbine jets) thrust performance is lacking, I understand the basic theory of operation of turbine engines and that thrust is a product of mass flow and velocity, and now I'm understanding pressure as well affects thrust output. So tell me, does an afterburner increase thrust primarily through increasing velocity or pressure in the exhaust outlet?

As inlet temperature increases, the compressor aerodynamically slows down, pumping less air. To maintain the same airflow, it has to turn faster as the inlet temp goes up. Once you reach the structural speed limit or turbine temp limit, you cannot increase the compressor speed any further, so airflow and EPR decrease with any further increase of inlet temperature.

As the aircraft speed increases the inlet pressure recovery increases along with the temperature. This inlet compression is multiplied by the engine EPR to provide the nozzle pressure ratio. The rate of EPR lapse with increasing speed determines if the nozzle pressure ratio is high enough to generate forward thrust at that flight condition.

AB operation primarily increases thrust by increasing the nozzle exit velocity by increasing the speed of sound in the nozzle due to the increased exhaust air temperature. There may be a minor thrust component due to excess pressure at the nozzle exit (acting over the area of the nozzle exit) if the nozzle provides incomplete expansion of the available nozzle pressure ratio, but this is an inefficient way to generate thrust vs increasing the velocity with complete expansion.

 

[Yankee_Aviator]

As the aircraft speed increases the inlet pressure recovery increases along with the temperature. This inlet compression is multiplied by the engine EPR to provide the nozzle pressure ratio.

Is this why turbojet engines produce more thrust at higher speeds?

 

[F119Doctor]

As the aircraft speed increases the inlet pressure recovery increases along with the temperature. This inlet compression is multiplied by the engine EPR to provide the nozzle pressure ratio.

Is this why turbojet engines produce more thrust at higher speeds?

Two things are possible:

1. Turbofans have a relatively small hard working core (high compression, high turbine inlet temperature for good thermal efficiency), then extract more power in the low turbine to turn the relatively larger fan. This hard working core probably is running closer to its limits, so it has less margin as the inlet temperatures go up. And the cooler exhaust flow (both from the cool bypass and the larger energy extraction from the hot flow) has a lower velocity even with the same EPR and NPR. So the thrust lapse with increasing speed is typically greater for a turbofan. But not necessarily, seeing that the F119 is a very low bypass turbofan with great supercruise capabilities.

2. With a turbojet, especially a single shaft engine, supersonic thrust can be maximized by opening the exhaust nozzle to maintain rotor speed and airflow once you reach your turbine temp limits. This reduces EPR, but the increasing ram recovery can offset this loss to maintain or even increase NPR. This is especially effective in AB, where the airflow and high exhaust temperature keeps the exhaust velocity high even before the NPR nozzle expansion.

 

[H_K]

Of the widely used Western fighter engines today, which would be most appropriate for supercruise at say M1.2-1.3? (Thinking of a lightweight fighter application)

F100? F414? EJ200? M88?

 

[Yankee_Aviator]

As the aircraft speed increases the inlet pressure recovery increases along with the temperature. This inlet compression is multiplied by the engine EPR to provide the nozzle pressure ratio.

Is this why turbojet engines produce more thrust at higher speeds?

Two things are possible:

1. Turbofans have a relatively small hard working core (high compression, high turbine inlet temperature for good thermal efficiency), then extract more power in the low turbine to turn the relatively larger fan. This hard working core probably is running closer to its limits, so it has less margin as the inlet temperatures go up. And the cooler exhaust flow (both from the cool bypass and the larger energy extraction from the hot flow) has a lower velocity even with the same EPR and NPR. So the thrust lapse with increasing speed is typically greater for a turbofan. But not necessarily, seeing that the F119 is a very low bypass turbofan with great supercruise capabilities.

2. With a turbojet, especially a single shaft engine, supersonic thrust can be maximized by opening the exhaust nozzle to maintain rotor speed and airflow once you reach your turbine temp limits. This reduces EPR, but the increasing ram recovery can offset this loss to maintain or even increase NPR. This is especially effective in AB, where the airflow and high exhaust temperature keeps the exhaust velocity high even before the NPR nozzle expansion.

Can you explain the difference between EPR and NPR? Also I still don't quite understand ram recovery, does higher airspeed (thus higher velocity ram air) increase pressure in the engine inlet? Would this pressure at any point be above ambient or would it still be below ambient pressure, just higher than at 0 airspeed? Or am I completely misunderstanding the term?

 

[overscan (PaulMM)]

Of the widely used Western fighter engines today, which would be most appropriate for supercruise at say M1.2-1.3? (Thinking of a lightweight fighter application)

F100? F414? EJ200? M88?

F414 has the lowest bypass ratio at 0.25 to 1, so in theory should be better assuming the two European engines don't have specific cycle/technical optimisation for supercruise. F100 would be last I think.

 

[Trident]

Of the widely used Western fighter engines today, which would be most appropriate for supercruise at say M1.2-1.3? (Thinking of a lightweight fighter application)

F100? F414? EJ200? M88?

If the F100 you have in mind is the -229, it should hold its own (<0.4 BPR IIRC). Cycle-wise there's not much in it between these engines, highest OPR and oldest tech could maybe see the F100 hit a temperature limit before the others. The EJ200 has a relatively high BPR among this selection, but might compensate with its class-leading fan pressure ratio. Possibly the weakest candidate is the M88 actually, due to its convergent ejector nozzle (all the others have con/di nozzles with superior supersonic performance). But broadly speaking they should be pretty close.

 

[Yankee_Aviator]

Of the widely used Western fighter engines today, which would be most appropriate for supercruise at say M1.2-1.3? (Thinking of a lightweight fighter application)

F100? F414? EJ200? M88?

If the F100 you have in mind is the -229, it should hold its own (<0.4 BPR IIRC). Cycle-wise there's not much in it between these engines, highest OPR and oldest tech could maybe see the F100 hit a temperature limit before the others. The EJ200 has a relatively high BPR among this selection, but might compensate with its class-leading fan pressure ratio. Possibly the weakest candidate is the M88 actually, due to its convergent ejector nozzle (all the others have con/di nozzles with superior supersonic performance). But broadly speaking they should be pretty close.

Ironically, the Rafale can supercruise

 

[F119Doctor]

As the aircraft speed increases the inlet pressure recovery increases along with the temperature. This inlet compression is multiplied by the engine EPR to provide the nozzle pressure ratio.

Is this why turbojet engines produce more thrust at higher speeds?

Two things are possible:

1. Turbofans have a relatively small hard working core (high compression, high turbine inlet temperature for good thermal efficiency), then extract more power in the low turbine to turn the relatively larger fan. This hard working core probably is running closer to its limits, so it has less margin as the inlet temperatures go up. And the cooler exhaust flow (both from the cool bypass and the larger energy extraction from the hot flow) has a lower velocity even with the same EPR and NPR. So the thrust lapse with increasing speed is typically greater for a turbofan. But not necessarily, seeing that the F119 is a very low bypass turbofan with great supercruise capabilities.

2. With a turbojet, especially a single shaft engine, supersonic thrust can be maximized by opening the exhaust nozzle to maintain rotor speed and airflow once you reach your turbine temp limits. This reduces EPR, but the increasing ram recovery can offset this loss to maintain or even increase NPR. This is especially effective in AB, where the airflow and high exhaust temperature keeps the exhaust velocity high even before the NPR nozzle expansion.

Can you explain the difference between EPR and NPR? Also I still don't quite understand ram recovery, does higher airspeed (thus higher velocity ram air) increase pressure in the engine inlet? Would this pressure at any point be above ambient or would it still be below ambient pressure, just higher than at 0 airspeed? Or am I completely misunderstanding the term?

Engine inlet pressure varies with altitude and airspeed. At zero airspeed with the engine at full full power, the pressure at the engine face on an installed engine will be below ambient due to restriction thru the aircraft inlet (how much is design dependent).

As you increase altitude, the ambient pressure decreases, thrust goes down correspondingly.

As airspeed increases, ram recovery goes up, pressure at the engine face goes up, thrust goes up, everything else being equal (which it isn’t - temperature also is increasing). By Mn 0.2 - 0.3, engine face pressure is probably back to ambient. This pressure continues to increase above ambient with increasing speed. At 40K, 1.5 Mn, ambient is approximately 2.5 psi, -70F, but the conditions at the engine face is approx 12 psi, +100F.

If your engine EPR at that 40K, 1.5 Mn is 3.0 (for example), the pressure at the inlet of the nozzle would be 36 psi. The NPR would be 36 / 2.5 (ambient) = 14:1 NPR. The convergent nozzle uses approx 2:1 pressure ratio to accelerate the exhaust flow to Mach 1, leaving 7:1 available for supersonic expansion aft of the convergent throat of the nozzle.

Hope that helps your understanding of ram recovery, EPR, and NPR.

 

[LMFS]

Hats off to @F119Doctor for the contributions, much appreciated. Even when the engine phenomena and variables can be more or less known, to be able to but practical numbers and "dissect" complexity demands a professional with experience and that is quite relevant to understand how things actually work...

 

[Yankee_Aviator]

inlet of the nozzle

Just to clarify, this means the point in the engine immediately behind the exhaust nozzle, and NOT the inlet of the compressor, correct?

 

[Yankee_Aviator]

Hope that helps your understanding of ram recovery, EPR, and NPR.

Yes it does, thank you very much. And about turbofans, not necessarily high or low bypass, just fans in general, in addition to energy extracted from the exhaust flow by the LP turbines in order to drive the fan section, does the lower temperature of the bypass air in an engine such as the F119 actually contribute to reducing exhaust velocity?
I just want to throw it out there, I'm an A&P, very interested in turbine engines, just want to get a better understanding of how engine performance is affected by real world conditions

 

[F119Doctor]

inlet of the nozzle

Just to clarify, this means the point in the engine immediately behind the exhaust nozzle, and NOT the inlet of the compressor, correct?

No, immediately in front of the Nozzle (i.e. inside the augmentor duct).

To reduce confusion, it makes sense to refer to longitudinal stations thru the engine. For a typical military turbofan, P&W uses the following station identification.

Station
zero - Ambient
1 - aircraft inlet
2 - fan inlet
2.5 - fan discharge
3 - high compressor discharge
4 - combustor exit / turbine valve inlet
4.1 - high turbine blade entry.
4.5 - low turbine vane inlet (FTIT on an F100)
5 - Turbine exit
6 - Augmentor inlet
7 - nozzle inlet
8 - convergent throat
9 - nozzle divergent exit

EPR is Pt6 / Pt2
NPR is. Pt7/ Pt0
Pt means Total Pressure

 

[Yankee_Aviator]

immediately in front of the Nozzle

My bad, that was what I meant to say, the area within the engine immediately upstream of the exhaust nozzle. I said "behind" because I was thinking in terms of the direction of air flowing through the engine at the time I originally typed that

 

[BullpupRafale]

I am a layman, but it seems to me from reading posts here that SNECMA M53 would be ideal compromise for super cruising?

 

[F119Doctor]

I am a layman, but it seems to me from reading posts here that SNECMA M53 would be ideal compromise for super cruising?

The M53 is an unusual configuration, being single spool turbofan with fan mechanically fixed to the front of the high compressor. I don’t have any personal experience with the engine

Using Wikipedia specifications, it has similar dry thrust to the F100-220, with 200 pps airflow compared to the F100s 225 pps. I would estimate the EPR of the M53 around 3.2:1, given the higher exhaust temp (only 9.8:1 OPR vs F100 24:1) and the F100 EPR of 3:1.

This might be enough to enable low supercruise of 1.2-1.3 Mn, but it would probably be marginal at best. The M53 also has a convergent ejector nozzle, which is unlikely to be good for supersonic drag at Mil power. I doubt there is significant rotor speed and turbine temperature margin to maintain this airflow and EPR at any Mn higher than this.

But, this is just a wide ass guess on my part based on available specs.