PAK-FA, T-50: News #4

GarryB wrote:

The gas turbine performance bound by Carnot, means cooler fuel/air intake means higher efficient and performance. -> see Charge Air Coolers on cars/trucks (CAC)

Rubbish.

Cool air going into the intake means denser air, which means more are for thrust, but colder fuel does not help at all.

The point is to heat the air using burning fuel... hot fuel means it burns more efficiently... the whole point is expansion via heat...

The starting condition in the engine is the temperature/volume of the air fuel mixture,and the end is the combusted temperature/volume.

So it helps, but the air:fuel mass ratio higher than ten, means the fuel temperature difference has negligible effect on the performance.

So the warm fuel decrease the performance by same negligible small number, the cold is increase.

With external tanks

So the warm fuel decrease the performance by same negligible small number, the cold is increase.

Wrong... it is not the increase in temperature of the fuel from storage to use that generates more thrust, but merely the temperature the engine raises the burnt fuel to by combustion.

Actively cooling the fuel before putting it in the engine might actually reduce the engines performance rather than improve it.

The MiG-25 uses an alcohol spray into the air intakes to increase thrust at high altitude where the air is very thin.

It does not actively cool its stored fuel supply... you would think they would bother doing that if they could if it would increase thrust. It would not.

The ambient temperature at mach 2.8 is quite hot but at the altitude it has to operate to fly that fast the normal air temperature outside the aircraft is about minus 60 degrees C, so any part of the aircraft not friction heated by the air stream will actually be very very cold anyway.

GarryB wrote:

So the warm fuel decrease the performance by same negligible small number, the cold is increase.

Wrong... it is not the increase in temperature of the fuel from storage to use that generates more thrust, but merely the temperature the engine raises the burnt fuel to by combustion.

Actively cooling the fuel before putting it in the engine might actually reduce the engines performance rather than improve it.

The MiG-25 uses an alcohol spray into the air intakes to increase thrust at high altitude where the air is very thin.

It does not actively cool its stored fuel supply... you would think they would bother doing that if they could if it would increase thrust. It would not.

The ambient temperature at mach 2.8 is quite hot but at the altitude it has to operate to fly that fast the normal air temperature outside the aircraft is about minus 60 degrees C, so any part of the aircraft not friction heated by the air stream will actually be very very cold anyway.

https://en.wikipedia.org/wiki/Carnot_cycle

I think the key here is this comment:

The behaviour of a Carnot engine or refrigerator is best understood by using...

A Carnot engine is a theoretical model for the efficiency of a refrigerator...

Not really relevant for a jet engine... they are not really the same thing.

Of course using cold fuel will make a fridge more efficient... or more accurately using cold refrigerator cooling fluid makes a fridge colder...

It is just basic common sense that a jet engine uses fuel to heat air. Air is sucked in and compressed and fuel is added and burned... the hotter the combustion the more energetic the thrust flow... if you heat the fuel to 1 million degrees then the exhaust thrust would be rather more than with super cold fuel because the thrust comes from the heat generated by the fuel burning in air.

GarryB wrote:I think the key here is this comment:

The behaviour of a Carnot engine or refrigerator is best understood by using...

A Carnot engine is a theoretical model for the efficiency of a refrigerator...

Not really relevant for a jet engine... they are not really the same thing.

Of course using cold fuel will make a fridge more efficient... or more accurately using cold refrigerator cooling fluid makes a fridge colder...

It is just basic common sense that a jet engine uses fuel to heat air. Air is sucked in and compressed and fuel is added and burned... the hotter the combustion the more energetic the thrust flow... if you heat the fuel to 1 million degrees then the exhaust thrust would be rather more than with super cold fuel because the thrust comes from the heat generated by the fuel burning in air.

https://en.wikipedia.org/wiki/Carnot_Cycle

A system undergoing a Carnot cycle is called a Carnot heat engine, although such a "perfect" engine is only a theoretical construct and cannot be built in practice.
The Carnot heat-engine cycle described is a totally reversible cycle. That is, all the processes that comprise it can be reversed, in which case it becomes the Carnot refrigeration cycle.
It can be seen from the above diagram, that for any cycle operating between temperatures T H {\displaystyle T_{H}} T_{H} and T C {\displaystyle T_{C}} T_{C}, none can exceed the efficiency of a Carnot cycle.




Key points:
-carnot is a theoretical maximum efficiency for any heat engine/refrigerator.
-carnot is fully reversible. If not then it is possible to make perpetuate mobile.
-carnot efficiency depends ONLY on the heat difference between the cold and hot side.
-jet engine is a thermal engine, bound by carnot.

Bollocks.

A nuclear powered scramjet engine can operate at enormous temperatures and cannot be reversed to make cold air come out the exhaust...

Singular_Transform wrote:
https://en.wikipedia.org/wiki/Carnot_Cycle

A system undergoing a Carnot cycle is called a Carnot heat engine, although such a "perfect" engine is only a theoretical construct and cannot be built in practice.
The Carnot heat-engine cycle described is a totally reversible cycle. That is, all the processes that comprise it can be reversed, in which case it becomes the Carnot refrigeration cycle.
It can be seen from the above diagram, that for any cycle operating between temperatures T H {\displaystyle T_{H}} T_{H} and T C {\displaystyle T_{C}} T_{C}, none can exceed the efficiency of a Carnot cycle.

<img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/a93048fb3f32bfd98058850c487546c878173b47">

Key points:
-carnot is a theoretical maximum efficiency for any heat engine/refrigerator.
-carnot is fully reversible. If not then it is possible to make perpetuate mobile.
-carnot efficiency depends ONLY on the heat difference between the cold and hot side.
-jet engine is a thermal engine, bound by carnot.

The formula you’ve quoted is for Carnot Efficiency, which expresses the theoretical limit for the efficiency of a ideal heat engine that operates between TH (temperature of heat input, eg flame combustion temperature in a jet engine) and TC (temperature of waste heat stream, eg that of the jet wash as it exits the engine).  It doesn’t give the actual efficiency of a real-world engine in terms of work done per unit of fuel/energy consumed (as this is determined by the effectiveness of the physical engine design), but is the maximum possible within this temperature range.

The key item here is that Carnot Efficiency increases proportionally with TH.   Preheating the air/fuel mixture will cause a relative increase in the flame temperature within the combustion zone of the engine.  For a given exhaust temperature TC, a hotter combustion zone TH allows for a greater volumetric expansion , and therefore, more volume flow and greater thrust.  

In short, higher fuel/air temperatures will INCREASE the efficiency of a heat engine, while cold fuel/air will REDUCE efficiency.

As an aside, engine efficiency improvements are made by minimising the temperature of the engine exhaust by maximising the degree of cooling by volumetric expansion in the engine expander sections.  Bypass turbofans use this principle by mixing fresh air (bypassed around the combustion chamber and hi-pressure sections) with the exhaust, ie using the waste heat to achieve additional expansion and therefore thrust.  Each successive expansion stage requires an additional stage of larger diameter, so given that jet engine weight and length are design constraints, its clear that this optimisation can only be taken so far.  Modern jet engines are about as efficient as they will ever be, and improvements going forward will be incremental.

GarryB wrote:Bollocks.

A nuclear powered scramjet engine can operate at enormous temperatures and cannot be reversed to make cold air come out the exhaust...

He's confusing the issue by mentioning "reversability"...  The ideal Carnot Cycle is for a conceptualised heat engine, where heat is converted to mechanical work.  The cycle can be "reversed" in a reverse engine where mechanical work is turned into heat.  An example of this is a refrigeration system or "heat pump" where mechanical work (ie from an electric motor) is used to cycle a working fluid and pull heat from one environment (eg the fridge interior) and dump it at a higher temperature into another environment (ie the room via the radiator at the rear of the unit). Pumping heat out of the fridge interior results in a drop in temperature, hence the phrase "heat pump".

The concept of Reverse Carnot Cycle clearly doesn't apply to a jet engine.

Big_Gazza wrote:
The formula you’ve quoted is for Carnot Efficiency, which expresses the theoretical limit for the efficiency of a ideal heat engine that operates between TH (temperature of heat input, eg flame combustion temperature in a jet engine) and TC (temperature of waste heat stream, eg that of the jet wash as it exits the engine).  It doesn’t give the actual efficiency of a real-world engine in terms of work done per unit of fuel/energy consumed (as this is determined by the effectiveness of the physical engine design), but is the maximum possible within this temperature range.

The key item here is that Carnot Efficiency increases proportionally with TH.   Preheating the air/fuel mixture will cause a relative increase in the flame temperature within the combustion zone of the engine.  For a given exhaust temperature TC, a hotter combustion zone TH allows for a greater volumetric expansion , and therefore, more volume flow and greater thrust.  

In short, higher fuel/air temperatures will INCREASE the efficiency of a heat engine, while cold fuel/air will REDUCE efficiency.

As an aside, engine efficiency improvements are made by minimising the temperature of the engine exhaust by maximising the degree of cooling by volumetric expansion in the engine expander sections.  Bypass turbofans use this principle by mixing fresh air (bypassed around the combustion chamber and hi-pressure sections) with the exhaust, ie using the waste heat to achieve additional expansion and therefore thrust.  Each successive expansion stage requires an additional stage of larger diameter, so given that jet engine weight and length are design constraints, its clear that this optimisation can only be taken so far.  Modern jet engines are about as efficient as they will ever be, and improvements going forward will be incremental.

Ahoy, comrade : )

Without saying you are wrong, jut going through the logical chain.
The combustion chamber temperature is not constrained by the available energy, but by the materials.

The Kerosene Stoichiometric adiabatic flame temperature is 2093 Celsius from 20 Celsius.

Means that if you compress it prior of combustion then the temperature of the suction will increase to 550, and the exhaust temperature will increase even more.

So the engine is not constrained by the chamber temperature, but by the heat resistance of the materials.

If the combustion chamber maximum temperature is 1600 Celsius , then the combustion products expand from 550 Celsius to 1600 Celsius .

The compressor end stage pressure and temperature given for an ambient temperature, if you want to increase the efficiency then the pressure needs to be increased, but that will increase the end temperature as well.
Means the best way to increase the pressure if you can cool the compressed air.

Big_Gazza wrote:

GarryB wrote:Bollocks.

A nuclear powered scramjet engine can operate at enormous temperatures and cannot be reversed to make cold air come out the exhaust...

He's confusing the issue by mentioning "reversability"...  The ideal Carnot Cycle is for a conceptualised heat engine, where heat is converted to mechanical work.  The cycle can be "reversed" in a reverse engine where mechanical work is turned into heat.  An example of this is a refrigeration system or "heat pump" where mechanical work (ie from an electric motor) is used to cycle a working fluid and pull heat from one environment (eg the fridge interior) and dump it at a higher temperature into another environment (ie the room via the radiator at the rear of the unit). Pumping heat out of the fridge interior results in a drop in temperature, hence the phrase "heat pump".

The concept of Reverse Carnot Cycle clearly doesn't apply to a jet engine.

Without thinking a bit about it, in this case you transform the heat to mechanical work.

Actually, in the case of a nuclear power plant the normal working is the reactor generating H heat, the generators creating E work, and the environment get W waste heat. H=E+W.

Reverse should be you use the generator as motor, drive the steam turbine as compressor, and cool the reactor to say 0 Celsius.
In that case H+E=W.

So, the ramjet reactor is pretty much the same.But the reverse working is actually moving forward the ramjet (or air) and cooling the reactor of it .

The jet engine on its own a compressor + turbine.

theoretically a compressor is reverse working turbine.

Singular_Transform wrote:

Big_Gazza wrote:
The formula you’ve quoted is for Carnot Efficiency, which expresses the theoretical limit for the efficiency of a ideal heat engine that operates between TH (temperature of heat input, eg flame combustion temperature in a jet engine) and TC (temperature of waste heat stream, eg that of the jet wash as it exits the engine).  It doesn’t give the actual efficiency of a real-world engine in terms of work done per unit of fuel/energy consumed (as this is determined by the effectiveness of the physical engine design), but is the maximum possible within this temperature range.

The key item here is that Carnot Efficiency increases proportionally with TH.   Preheating the air/fuel mixture will cause a relative increase in the flame temperature within the combustion zone of the engine.  For a given exhaust temperature TC, a hotter combustion zone TH allows for a greater volumetric expansion , and therefore, more volume flow and greater thrust.  

In short, higher fuel/air temperatures will INCREASE the efficiency of a heat engine, while cold fuel/air will REDUCE efficiency.

As an aside, engine efficiency improvements are made by minimising the temperature of the engine exhaust by maximising the degree of cooling by volumetric expansion in the engine expander sections.  Bypass turbofans use this principle by mixing fresh air (bypassed around the combustion chamber and hi-pressure sections) with the exhaust, ie using the waste heat to achieve additional expansion and therefore thrust.  Each successive expansion stage requires an additional stage of larger diameter, so given that jet engine weight and length are design constraints, its clear that this optimisation can only be taken so far.  Modern jet engines are about as efficient as they will ever be, and improvements going forward will be incremental.

Ahoy, comrade : )

Without saying you are wrong, jut going through the logical chain.
The combustion chamber temperature is not constrained by the available energy, but by the materials.

The Kerosene Stoichiometric adiabatic flame temperature is 2093 Celsius from 20 Celsius.

Means that if you compress it prior of combustion then the temperature of the suction will increase to 550, and the exhaust temperature will increase even more.

So the engine is not constrained by the chamber temperature, but by the heat resistance of the materials.

If the combustion chamber maximum temperature is 1600 Celsius , then the combustion products expand from 550 Celsius to 1600 Celsius .

The compressor end stage pressure and temperature given for an ambient temperature, if you want to increase the efficiency then the pressure needs to be increased, but that will increase the end temperature as well.
Means the best way to increase the pressure if you can cool the compressed air.

Fuck this, I'm losing the will to live....

Thermodynamic efficiency of a Carnot Cycle engine is increased if the fuel/air is preheated prior to ignition. This is an established fact and is not up for debate.

Cooling the fuel/air mixture will lower the efficiency of the engine, but will raise the ouput power by increasing the mass of useable fuel per volume of fuel/air mixture. More energy released in combustion, more expansion, more volumetric flowrate of combustion gases, ie more power.

You seem to be confusing the two relationships. Now, for the love of all that is Good and Holy, lets stop this pseudo-intellectual wanking....

Gazza's an engineer (if I recall correctly) Singular_Transform... I'd probably let this go mate, he's got the bigger guns.

Big_Gazza wrote:

Fuck this, I'm losing the will to live....

Thermodynamic efficiency of a Carnot Cycle engine is increased if the fuel/air is preheated prior to ignition. This is an established fact and is not up for debate.

Cooling the fuel/air mixture will lower the efficiency of the engine, but will raise the ouput power by increasing the mass of useable fuel per volume of fuel/air mixture. More energy released in combustion, more expansion, more volumetric flowrate of combustion gases, ie more power.

You seem to be confusing the two relationships. Now, for the love of all that is Good and Holy, lets stop this pseudo-intellectual wanking....

C'mon,you haven't read that I wrote, or you don't understand it.
I haven't said that your logic is bad.

My original statement about that the hot fuel introduction into the combustion process was bad, and ambient external temperature vs combustion chamber that matter, not the air intake vs. combustion temperature.

I have to mention that the pre-heating of the intake by the exhaust helps only if the engine has low efficiency.

The pre-heating of the fuel/air doesn't help with the gas turbine, because of the above problems.

But in the case of say ramjet or rocket engine it can help to keep the efficiency high by cooling the combustion chamber.

OminousSpudd wrote:Gazza's an engineer (if I recall correctly) Singular_Transform... I'd probably let this go mate, he's got the bigger guns.

There is no engineer/doctor or soldier on an internet forum, only nickname.

Singular_Transform wrote:

OminousSpudd wrote:Gazza's an engineer (if I recall correctly) Singular_Transform... I'd probably let this go mate, he's got the bigger guns.

There is no engineer/doctor or soldier on an internet forum, only nickname.

Well, I would argue about that. Just take a look at this topic: https://www.russiadefence.net/t3680-military-service-of-members

There are actually plenty of members here who got military service behind their backs. Plus there are ones who have some degrees in engineering. Basic knowledge isn't everything, and it is very good that (basically all) members of this forum can share their personal knowledge and experience with others (this is actually one of the main principles of any internet forum), and this forum is about military stuff, so members with military/engineering knowledge are vital for it.

Benya wrote:
Well, I would argue about that. Just take a look at this topic: https://www.russiadefence.net/t3680-military-service-of-members

There are actually plenty of members here who got military service behind their backs. Plus there are ones who have some degrees in engineering. Basic knowledge isn't everything, and it is very good that (basically all) members of this forum can share their personal knowledge and experience with others (this is actually one of the main principles of any internet forum), and this forum is about military stuff, so members with military/engineering knowledge are vital for it.

You don't care about the person ,who /what position/ what training he has, the only that counts is what he write.

Singular_Transform wrote:

You don't care about the person ,who /what position/ what training he has, the only that counts is what he write.

Not the person himself, but rather what he writes, but that is strongly based on his background and knowledge. But let's conclude this, let's not fill this thread with off-topic stuff.

I have no engineering background but I do know that diesel needs to be preheated before the engine will even start.

Preheating avgas is only possible if you don't reach flashpoint till it gets inside the engine.

Cryogenic fuels are sometimes pumped through the rocket exhaust nozzles on some designs because it helps cool the rocket nozzles and it also preheats the fuel... the hotter you can get hydrogen the faster it moves in the exhaust stream... = more powerful thrust.

Is it just me or does the side bays look a little small?

Since nobody addressed this: the outer bays are for 1* short range AA missile only so yes quite small.

I don't think there are any actual images of the outer bays open to confirm how big they actually are but there is this pic which shows its not actually as small as it looks at first, possibly indicates even a K-77 might fit.


And someone made this CG showing maybe a really tight fit (but probably not)

GarryB wrote:I have no engineering background but I do know that diesel needs to be preheated before the engine will even start.

Preheating avgas is only possible if you don't reach flashpoint till it gets inside the engine.

Cryogenic fuels are sometimes pumped through the rocket exhaust nozzles on some designs because it helps cool the rocket nozzles and it also preheats the fuel... the hotter you can get hydrogen the faster it moves in the exhaust stream... = more powerful thrust.

The glow plug needed for cold start of the diesel engine, and the preheating is to avoid the cloging of the diesel.

https://en.wikipedia.org/wiki/Glowplug
A glowplug (alternatively spelled as glow plug or glow-plug) is a heating device used to aid starting diesel engines. In cold weather, high speed diesel engines can be difficult to start because the mass of the cylinder block and cylinder head absorb the heat of compression, preventing ignition (which relies on that heat). Pre-chambered engines use small electric heaters (glowplugs) inside the pre-chambers.

In the case of the internal combustion engines the target is to cool the incomming airflow.

Before the CAC they used fuel for pre-cooling.
https://en.wikipedia.org/wiki/Intercooler
Intercoolers increase the efficiency of the induction system by reducing induction air heat created by the supercharger or turbocharger and promoting more thorough combustion. This removes the heat of compression (i.e., the temperature rise) that occurs in any gas when its pressure is raised or its unit mass per unit volume (density) is increased.
...
Intercoolers also eliminate the need for using the wasteful method of lowering intake charge temperature by the injection of excess fuel into the cylinders' air induction chambers, to cool the intake air charge, prior to its flowing into the cylinders.
...
The inter prefix in the device name originates from historic compressor designs. In the past, aircraft engines were built with charge air coolers that were installed between multiple stages of forced induction


The purpose of the fuel pre-heating is to cool the wall of the thrust chamber.

From efficiency standpoint it will give good case the same as without cooling.

hoom wrote:

Is it just me or does the side bays look a little small?

Since nobody addressed this: the outer bays are for 1* short range AA missile only so yes quite small.

I don't think there are any actual images of the outer bays open to confirm how big they actually are but there is this pic which shows its not actually as small as it looks at first, possibly indicates even a K-77 might fit.


And someone made this CG showing maybe a really tight fit (but probably not)

It seems like extreme.

IT cut into the wing at the highest load points.

I can't found any picture showing open these doors.

I don't think that the internal space of it can portrude into the wing.
It is more likelz just on the bottom of the wing.

While carrying long range AAMs is a great advantage, its still a shame that the T-50 cant carry 6 R-77s like the F-22 can use 6 AMRAAMs.

KomissarBojanchev wrote:While carrying  long range AAMs is a great advantage, its still a shame that the T-50 cant carry 6 R-77s like the F-22 can use 6 AMRAAMs.

What? Why don't you try counting hardpoints. Key point here - internal weapons bay too.

miketheterrible wrote:

KomissarBojanchev wrote:While carrying  long range AAMs is a great advantage, its still a shame that the T-50 cant carry 6 R-77s like the F-22 can use 6 AMRAAMs.

What?  Why don't you try counting hardpoints.  Key point here - internal weapons bay too.

The T-50 has only 4 internal hardpoints for heavy weapons. The other 2 are only for short range AAMs.