PRC VCE and low bypass engines (fighters, tactical jets, UAVs, others)

[latenlazy]

Sorry but this seems like total nonsense to me. Compression ratio is not something you struggle to achieve, it is a design point you choose.
The challenge and the technical sophistication is in the size of the package you require to achieve your chosen OPR, i.e. how many stages you require, and how efficiently you do it, and how big and heavy that total compressor is. Those are interesting metrics, but the ORP alone is basically meaningless because it is a strategically picked design point.
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Maybe you mean compressor efficiency, which is something much harder to achieve than the ratio, but even that argument does not hold up.

When I look at generational leaps in compressor efficiency, they seem to contribute roughly 1/3rd the thrust which the generational leap in turbine inlet temperature contributes.

This intuitively makes sense to me given how close compressor efficiency is near the theoretical maximum, while turbine inlet temperatures seem significantly below the theoretical maximum of about 2000°C, the peak temperature achievable with the fuel.

Compression ratio is not something you struggle to achieve if you're okay with just adding more stages. To get higher compression ratios with fewer stages you need greater compression efficiency. The point here is that for the same engine size and other mechanical constraints, you need higher efficiency to reach higher ratios, but if you can achieve higher ratios given the same physical constraints for the engine, you can push performance without a greater fuel mix.

Since no engine can withstand that temperature, the combustion gas is diluted with excess air to cool it down. This excess air has to go through the compressor, which eats a lot of mechanical energy the turbine has to extract.
If we could raise the allowable TIT to 2200°C, we would need zero excess air for cooling, thus raising TIT is a massive thrust increase because it also cuts down on the energy we need to dump into the compressor to get that cooling air into the engine in the first place.
Forcing that air through the engine core without combusting it is a high parasitic load compared to the thrust it delivers. If we instead combusted this cooling air as well, or pushed it with a fan around the engine, the efficiency would go up dramatically.

You gotta take a step back and first ask how it is that you are increasing the TIT. There are only two ways. One is more stoichiometrically efficient.

Specific thrust is not a performance result, it is a design choice you pick for the speed regime you want your plane to operate in.
We had Mach 3 planes in the 1950's, they had terrible thrust to weight ratios and terrible fuel economy but high specific thrust for the high speed regime they were designed to perform in.
It is not a metric to judge the sophistication of the engine technology, but merely the design speed of the plane the engine was build for.

Higher specific thrust is a performance result if you are physically constrained by other parameters like say how big you want your engine to be.

On the contrary, the opposite is the case. The compressor outlet temperature is always higher than we want, it reduces performance significantly.
The limiting factor is the turbine inlet temperature, so you can only raise the temperature until you hit that.
The hotter your compressor outlet temperature is, the less fuel you can inject into it to generate work.
For this reason high efficiency industrial gas turbines, which do not have the same size and weight constraints of a plane engine, use intercoolers to cool the air after compression to increase efficiency.
The compressor does not do useful work by heating the air, on the contrary, it prevents useful work from being done.

Firstly, you cannot get effective combustion without air compression, so in fact the compressor performance is *essential* to the meaningful work you do. Secondly, a sizable portion of your *thrust* (remember what we care about ultimately is the force of the exhaust mass for an engine) is coming purely from air compression and re-expansion, aka increasing the pressure on your air stream from the inlet and then letting it expand at the outlet. That's how a fan works. If that weren't fundamentally true a high bypass engine couldn't be efficient.

But third and most importantly, if the TIT is the hard design constraint, are you getting more work per unit of air mass out of a hotter compressor outlet on a lower fuel mix or a cooler compressor outlet on a higher fuel mix? Combustion does not extract the same energy at all starting temperatures and pressures. You have to factor in combustion efficiency envelopes too. Stoichiometric efficiency is not a static fixed value at all temperature and pressure values. That's the part you guys aren't thinking about.

I do not want to seem offensive but you seem to have a fundamental misunderstanding when it comes to the thermodynamic fundamentals.

And I do not want to seem offensive but it seems you have missed some thermodynamic fundamentals.

 

[ZeEa5KPul]

Compressing air is doing work to generate potential energy for the core stream. The core stream starts as static air at the inlet with *no* energy imparted to it. The turbine drives the compressor yes, but the compressor also drives the turbine. That’s why it’s called a *cycle*. Outside combustion the turbine isn’t recovering any energy that isn’t being pushed first by the compressor, but even the combustion isn’t happening without the compressor compressing air to ignite first. So it is in fact the compressor driving the turbine cycle not the other way around. This is also why you need an auxiliary engine starter to start the turbine cycle. The turbine isn’t starting itself because you need to run the compressor first to drive the turbine.

This doesn't address the key point I'm making: the only process in the cycle in which the net enthalpy of the working fluid increases is the heat addition at the combustor. There aren't two ways about this, either this is true or jet engines are perpetual motion machines. You can look at it any way you like, the turbine drives the compressor or the compressor drives the turbine, the net enthalpy argument remains true.

No, higher inlet gas temp into the combustor means you have more added heat to combustion. Higher added heat means you need less fuel to get to the same amount of stoichiometric combustion. Higher starting heat is more stoichiometrically efficient. There is no hard upper limit constraining energy extraction from combustion outside the stoichiometric limit of the fuel to oxygen chemical interaction. Yes you are pushing your material’s heat tolerance limits with higher inlet gas from the compressor, which can be a soft limit to the heat budget, *but you’re doing that regardless if you’re increasing TIT*. The kind of TIT increase you’re referring to is from higher fuel mix to burn hotter, but pushing higher fuel mix to burn hotter is less efficient than higher inlet gas temp to burn a lower fuel mix more efficiently! And either way, if your core stream has more mechanical energy imparted to it from the compressor you need less fuel burn for the same core stream energy and thus gas velocity.

Let's ignore chemical combustion entirely and just consider abstract heat addition.
The gas enters the engine inlet at E = 0 enthalpy.
The compressor does work W on the gas, raising its enthalpy to E = 0 + W = W. Since we have mad OPR, this W is huge.
The gas enters the combustor, where heat H is added. Enthalpy is now E = W + H.
Now the gas enters the turbine. Here's the crucial point, for this to not be a perpetual motion machine, the work the compressor did to pressurize the gas must be drawn from the working fluid's enthalpy by the turbine. The turbine draw U must be at least W. Therefore, the enthalpy is now E = W + H - U <= H.
This enthalpy E which is now at most H is all the energy the nozzle has to generate thrust.

Because the heat addition process in Brayton cycle engines is isobaric, the specific heat is c_p (the specific heat capacity at constant pressure) * delta T, where delta T is TIT - CET (compressor exit temperature). For high specific thrust, we want this difference to be as large as possible, which is why TIT is treated as the One Ring of engine parameters. It allows everything; in this argument higher OPR while maintaining the same temperature delta.

 

[latenlazy]

This doesn't address the key point I'm making: the only process in the cycle in which the net enthalpy of the working fluid increases is the heat addition at the combustor. There aren't two ways about this, either this is true or jet engines are perpetual motion machines. You can look at it any way you like, the turbine drives the compressor or the compressor drives the turbine, the net enthalpy argument remains true.

Okay, then I will put it in more simple terms. You are not only extracting work from heat, but also work from pressure.

Let's ignore chemical combustion entirely and just consider abstract heat addition.
The gas enters the compressor inlet at E = 0 enthalpy.
The compressor does work W on the gas, raising its enthalpy to E = 0 + W = W. Since we have mad OPR, this W is huge.
The gas enters the combustor, where heat H is added. Enthalpy is now E = W + H.
Now the gas enters the turbine. Here's the crucial point, for this to not be a perpetual motion machine, the work the compressor did to pressurize the gas must be drawn from the working fluid's enthalpy by the turbine. The turbine draw U must be at least W. Therefore, the enthalpy is now E = W + H - U <= H.
This enthalpy E which is now at most H is all the energy the nozzle has to generate thrust.

Because the heat addition process in Brayton cycle engines is isobaric, the specific heat is c_p (the specific heat capacity at constant pressure) * delta T, where delta T is TIT - CET (compressor exit temperature). For high specific thrust, we want this difference to be as large as possible, which is why TIT is treated as the One Ring of engine parameters. It allows everything; in this argument higher OPR while maintaining the same temperature delta.

You are forgetting that the mechanical work we are trying to analyze is thrust, not just energy. Thrust is force of the airstream exhaust. Force of an airstream is defined by pressure. You are applying the Brayton cycle equation in abstract while overlooking the actual work you're trying to do.

Or to put it another way, is your design goal to design an efficient higher performance propulsion system or an efficient high performance heater?

 

[Nx4eu]

Alleged specific thrust figures for the WS-19 are from the attached snippet below. Forgot it was in an auxiliary leak and not the spec sheet itself. Your calculation is probably off because 1) not all inlet air is going to be used for thrust since also bleed air that intake mass flow figures have to account for 2) We don’t know what mass flow is at the intermediate thrust setting.

View attachment 172139

The 120-130daN/kg/s specific thrust figure is for a TIT of 1950-2100K, which then is completely normal and within my expectation. For example the F119 again, has a estimated specific wet thrust of about 128daN/kg/s, for a TIT of 1922K. Despite the higher maximum 2100K, the engine still retains a much higher bypass ratio, thus its ST won't be much higher. Purely based off the data given from that 15 year old chart though, it would suggest an engine not optimized for supercruise.

 

[gwel]

Latenlazy, you are trying to invent new thermodynamics to make your arguments make sense.
If high compressor outlet temp was good and contributing to work, we would inject zero fuel and merely add compressor stages to magically heat the air mechanically.

This would be, as ZeEa5KPul rightfully points out, a perpetual motion machine.

You're completely ignoring thermodynamic fundamentals. I see no basis for a discussion until you rectify your understanding in that area.

 

[latenlazy]

Latenlazy, you are trying to invent new thermodynamics to make your arguments make sense.
If high compressor outlet temp was good and contributing to work, we would inject zero fuel and merely add compressor stages to magically heat the air mechanically.

This would be, as ZeEa5KPul rightfully points out, a perpetual motion machine.

You're completely ignoring thermodynamic fundamentals. I see no basis for a discussion until you rectify your understanding in that area.

I'm not "inventing" anything and I'm also not describing a perpetual motion machine. I didn't say anything about running a high compressor outlet temp without fuel or combustion. If that's what you took away from my reply you are either strawmanning or you need to work on your reading comprehension. Do you not know what the concept of stoichiometric combustion efficiency is? If you don't then why are you grandstanding about thermodynamic fundamentals? Why are you opining about combustion powered systems without an understanding of combustion?

I don't need to rectify anything. You guys have just confused the focus of mechanical analysis here. What we care about with engines is net thrust, not net energy. For a plane engine what you are trying to do is to draw out energy from *both your mechanical compression and your combustion* to generate thrust, so overall pressure is actually pretty nonnegotiable in the analysis. Remember, the form of the work we are looking at for a reaction mass engine is the mass expelled, and the way you are getting force from the mass expelled in an air breathing jet engine is the pressure difference between your inlet and outlet gas. If you only try to apply a textbook explanation of a Brayton cycle without focusing on the right units you're engineering for you'll get the wrong picture. You should take your edginess down a notch and calm down.

 

[Nx4eu]

Specific thrust is not a performance result, it is a design choice you pick for the speed regime you want your plane to operate in.
We had Mach 3 planes in the 1950's, they had terrible thrust to weight ratios and terrible fuel economy but high specific thrust for the high speed regime they were designed to perform in.
It is not a metric to judge the sophistication of the engine technology, but merely the design speed of the plane the engine was build for.

Yes thank you for specifying. I just personally expected WS-19 to be a super cruise optimized engine with a high specific thrust, maybe we'll just have to wait for WS-15. But it seems that instead it's intended to be a more fuel efficient engine, with a high T/W ratio and an average specific thrust. Unless it turns out that they managed to hit the 1950-2100K figure mentioned above. in which I will retract my statements.

I will let you guys carry on to argue about the fundamentals of a jet engine. To the best of my knowledge and in the simplest terms,
higher TIT -> higher specific thrust, with all else being equal.

 

[latenlazy]

Yes thank you for specifying. I just personally expected WS-19 to be a super cruise optimized engine with a high specific thrust, maybe we'll just have to wait for WS-15. But it seems that instead it's intended to be a more fuel efficient engine, with a high T/W ratio and an average specific thrust. Unless it turns out that they managed to hit the 1950-2100K figure mentioned above. in which I will retract my statements.

I will let you guys carry on to argue about the fundamentals of a jet engine.

Specific thrust isn't determined by the TIT. Force per unit mass is captured directly in the pressure differential between inlet and outlet. Heat after combustion is not the only contributor to that pressure difference.

 

[gwel]

I'm not "inventing" anything and I'm also not describing a perpetual motion machine. I didn't say anything about running a high compressor outlet temp without fuel or combustion. If that's what you took away from my reply you are either strawmannirg or you need to work on your reading comprehension.

It is the logical conclusion when one adopts the logic you laid out.

Do you not know what the concept of stoichiometric combustion efficiency is? If you don't then why are you grandstanding about thermodynamic fundamentals?

A higher compressor outlet temperature leaves less headroom for injecting fuel and further raising the temperature before hitting the TIT limit, which increases the stoichiometric ratio, which is already way higher than optimal due to the requirement of more cooling air volume.
Theoretically hotter T3 would enable faster ignition but combustion efficiency of modern engines is already 99%.

For a plane engine what you are trying to do is to draw out energy from *both your mechanical compression and your combustion*

The mechanical compression is a parasitic load, it consumes energy, it does not generate any.
Ideally you want to reduce that to zero.
Your claim that mechanical compression produces useful work violates the first law of thermodynamics.

the way you are getting force from the mass expelled in an air breathing jet engine is the pressure difference between your inlet and outlet gas.

Okay that's great, let's reduce the outlet size to zero, we get infinite pressure, so infinite differential, thus infinite thrust.
You see the fallacy in your logic? You reduce complex interactions too much by focussing on one singular factor, and this oversimplification then leads to wrong conclusions.
In this case you focus on pressure - ignoring mass.
Pressure is merely a tool to create exhaust velocity.
Thrust is momentum change, not pressure change.

Again, civilian engines have 2x the OPR of fighter jet ones. They are vastly more efficient, but they're limited in flight regime. They are unsuitable for supersonic flight where ram compression provides so much compression that the compressor only needs to do half the work.

 

[latenlazy]

It is the logical conclusion when one adopts the logic you laid out.

Only if you don't know how to read. Just admit you don't know anything about combustion dynamics or how propulsion systems work.

A higher compressor outlet temperature leaves less headroom for injecting fuel and further raising the temperature before hitting the TIT limit, which increases the stoichiometric ratio, which is already way higher than optimal due to the requirement of more cooling air volume.
Theoretically hotter T3 would enable faster ignition but combustion efficiency of modern engines is already 99%.

If you want maximum headroom for injecting fuel then don't do any compression at all. Your *propulsion* system is going to work *great* that way amiright? "Combustion efficiency of modern engines is already 99%" is the kind of simplified reddit number that would get you laughed at in an engineering design room. The simplified numbers they will market at you in a spec sheet will rarely reflect the reality in dynamic operating conditions. Putting it another way, do you think you're getting 99% combustion efficiency when the RD-93 is pumping out a smoke plume? The real world ain't marketing flyers and textbooks.

The mechanical compression is a parasitic load, it consumes energy, it does not generate any.
Ideally you want to reduce that to zero.
Your claim that mechanical compression produces useful work violates the first law of thermodynamics.

Okay then, don't do any mechanical compression and see how much heat and propulsion you get out of your engine.

Is me pumping my legs to move around violating the first law of thermodynamics?

Okay that's great, let's reduce the outlet size to zero, we get infinite pressure, so infinite differential, thus infinite thrust.
You see the fallacy in your logic? You reduce complex interactions too much by focussing on one singular factor, and this oversimplification then leads to wrong conclusions.

Reducing outlet size to 0 doesn't give you infinite pressure at outlet because you've removed the outlet. What are you talking about? Have you considered maybe that a system with an inlet and no outlet doesn’t outlet anything so outlet pressure is actually 0? Do you not know how to do physics without depending on formal abstractions?

In this case you focus on pressure - ignoring mass.
Pressure is merely a tool to create exhaust velocity.
Thrust is momentum change, not pressure change.

Again, civilian engines have 2x the OPR of fighter jet ones. They are vastly more efficient, but they're limited in flight regime. They are unsuitable for supersonic flight where ram compression provides so much compression that the compressor only needs to do half the work.

You cannot move air mass without a pressure difference. How much force that air mass moves with is directly defined by the pressure. Where did you learn your physics buddy? Please do not tell me you learned it by reading about it on the internet..