How do square nozzles reduce IR?

Unread postPosted: 23 Jan 2019, 10:07
by collimatrix
I was curious if anyone knew, and is allowed to discuss, how exactly the square nozzles reduce the IR plume of the engines on the F-22. Do the big chevrons generate vortices that stir cool air into the plume, like on the LOAN? Or is it something else? I note that the flat nozzles on the F-117 and B-2 have no obvious vortex-generating features.

Re: How do square nozzles reduce IR?

Unread postPosted: 23 Jan 2019, 14:25
by hornetfinn
AFAIK, they flatten the exhaust plume which make the plume wider and shorter. They also probably generate vortices that mix the cool air with the plume. I think big thing is that engine hot parts are really well shielded from most viewing angles.

Interesting patent: https://patents.google.com/patent/US6000635A/en
and interesting article relating to this: http://www.jmargolin.com/laser/ref7_AWS ... -IR-bw.pdf

Re: How do square nozzles reduce IR?

Unread postPosted: 24 Jan 2019, 13:28
by collimatrix
Thanks!

I wonder if LOAN makes flat nozzles obsolete.

Re: How do square nozzles reduce IR?

Unread postPosted: 27 Jan 2019, 06:24
by charlielima223
collimatrix wrote:Thanks!

I wonder if LOAN makes flat nozzles obsolete.


From my understanding the LOAN was the advantage of being easier to maintain and offers lighter structural weight over the flat serrated nozzles the F-22 uses. However this is a bit of an apples to oranges comparison in some cases. Obviously the LOAN isn't optimized for TVC unlike that of the F-22 which blended TVC along with radar and some IR low observable features. I would think that it is improvement over the conventional "turkey feathers"

Re: How do square nozzles reduce IR?

Unread postPosted: 27 Jan 2019, 06:42
by charlielima223
I would think that LM and Pratt&Whitney engineers also added some active IR suppressing measures like introducing ambient air into the exhaust stream

Image

note the gap of separation in the early YF-119 engine test/prototype model
Image

I've also heard or read somewhere that the F-22 has a longer exhaust pipe possibly where cooler ambient air is introduced into the exhaust.

Re: How do square nozzles reduce IR?

Unread postPosted: 27 Jan 2019, 09:25
by collimatrix
In theory, the larger a nozzle is the lower an IR signature it should have. The closer the nozzle can get to ideal isentropic expansion, the cooler the exhaust gas should be. Excess heat in the exhaust plume is, after all, energy that isn't being used to make thrust. I'm sure this runs into all sorts of practical limitations.

Re: How do square nozzles reduce IR?

Unread postPosted: 27 Jan 2019, 09:52
by SpudmanWP
charlielima223 wrote:I would think that LM and Pratt&Whitney engineers also added some active IR suppressing measures like introducing ambient air into the exhaust stream
That's pretty much what a 3-stream engine is.

Re: How do square nozzles reduce IR?

Unread postPosted: 12 Jan 2020, 09:53
by zero-one
https://theaviationgeekclub.com/heres-w ... s-dont/amp

a circular nozzle would compromise the stealth of the F-22 from many rearward angles. Yes, the F-35 has a circular nozzle, and yes, it does compromise the stealth of the F-35 from some rear hemisphere angles. The philosophy with the F-35 design was to concentrate on forward hemisphere stealth and not to spend so much on rearward stealth. The F-35 still has reasonable rear hemisphere stealth, but it cannot come close to the F-22 in that regard.”

The radar in fact will see the F-35 nozzle as a curved continuous surface which will generate specular reflections. “In a very narrow band of frequencies it is stealthy from the rear, but nowhere near as wide a range of frequencies as the F-22,” Knight explains.


Interesting...
More at the jump.

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 03:17
by Corsair1963
zero-one wrote:https://theaviationgeekclub.com/heres-why-the-f-22-uses-square-exhaust-nozzles-and-f-35-su-57-and-chinese-stealth-fighters-dont/amp

a circular nozzle would compromise the stealth of the F-22 from many rearward angles. Yes, the F-35 has a circular nozzle, and yes, it does compromise the stealth of the F-35 from some rear hemisphere angles. The philosophy with the F-35 design was to concentrate on forward hemisphere stealth and not to spend so much on rearward stealth. The F-35 still has reasonable rear hemisphere stealth, but it cannot come close to the F-22 in that regard.”

The radar in fact will see the F-35 nozzle as a curved continuous surface which will generate specular reflections. “In a very narrow band of frequencies it is stealthy from the rear, but nowhere near as wide a range of frequencies as the F-22,” Knight explains.


Interesting...
More at the jump.


Sounds like more speculation than fact to me....

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 05:08
by disconnectedradical
Corsair1963 wrote:Sounds like more speculation than fact to me....


Seems very reasonable to me. The smaller serrations on F135 nozzle compare to big one on F119 would be less effective at lower frequencies because of Rayleigh scattering, it's physics. How big of practical difference that makes, we don't know, but it's still a difference.

But even F-22 nozzle isn't as stealthy as you can get. The nozzle on Lockheed/Boeing/GD A/F-X design is even stealthier though thrust is a bit less. And YF-23 arrangement is also stealthier.

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 05:49
by Corsair1963
disconnectedradical wrote:
Corsair1963 wrote:Sounds like more speculation than fact to me....


Seems very reasonable to me. The smaller serrations on F135 nozzle compare to big one on F119 would be less effective at lower frequencies because of Rayleigh scattering, it's physics. How big of practical difference that makes, we don't know, but it's still a difference.

But even F-22 nozzle isn't as stealthy as you can get. The nozzle on Lockheed/Boeing/GD A/F-X design is even stealthier though thrust is a bit less. And YF-23 arrangement is also stealthier.



That is like saying the F-22 looks more stealthy than the F-35.... :|

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 06:05
by disconnectedradical
Corsair1963 wrote:
disconnectedradical wrote:
Corsair1963 wrote:Sounds like more speculation than fact to me....


Seems very reasonable to me. The smaller serrations on F135 nozzle compare to big one on F119 would be less effective at lower frequencies because of Rayleigh scattering, it's physics. How big of practical difference that makes, we don't know, but it's still a difference.

But even F-22 nozzle isn't as stealthy as you can get. The nozzle on Lockheed/Boeing/GD A/F-X design is even stealthier though thrust is a bit less. And YF-23 arrangement is also stealthier.



That is like saying the F-22 looks more stealthy than the F-35.... :|


There is difference between just going by looks and knowing how dimensions and size affect interaction with different wavelengths. It's physics, if you can't see beyond that then I can't help you.

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 07:15
by Corsair1963
disconnectedradical wrote:

That is like saying the F-22 looks more stealthy than the F-35.... :|


There is difference between just going by looks and knowing how dimensions and size affect interaction with different wavelengths. It's physics, if you can't see beyond that then I can't help you.



WOW, I didn't realize you had the data in regards to the F-22's and F-35's IFR Nozzle Signature. To make that call....

Please, enlighten us.... :|

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 08:34
by disconnectedradical
Corsair1963 wrote:
disconnectedradical wrote:

That is like saying the F-22 looks more stealthy than the F-35.... :|


There is difference between just going by looks and knowing how dimensions and size affect interaction with different wavelengths. It's physics, if you can't see beyond that then I can't help you.



WOW, I didn't realize you had the data in regards to the F-22's and F-35's IFR Nozzle Signature. To make that call....

Please, enlighten us.... :|


Let me try again. The nozzle serrations on F135 is much smaller than the two large serrations on F119 nozzles. This means bigger serration length on F119 nozzles and also less circular profile. At lower frequencies and longer wavelengths especially when it's longer than the serration of F135 nozzle, the nozzle will appear circular instead of serrated, that's physics, and because of Rayleigh scattering when edge length becomes similar length as wavelength, F135 nozzle will not be as good as F119 nozzle at lower frequencies. At higher frequencies they're probably about equal in stealth. This is basic geometry and scattering.

Now, how much practical difference this makes, no one knows outside of classified information. Since both nozzles are pretty hidden by fuselage and tails, it may not matter that much.

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 09:22
by Corsair1963
"disconnectedradical"

Let me try again. The nozzle serrations on F135 is much smaller than the two large serrations on F119 nozzles. This means bigger serration length on F119 nozzles and also less circular profile. At lower frequencies and lower wavelengths especially when it's longer than the serration of F135 nozzle, the nozzle will appear circular instead of serrated, that's physics, and because of Rayleigh scattering when edge length becomes similar length as wavelength, F135 nozzle will not be as good as F119 nozzle at lower frequencies. At higher frequencies they're probably about equal in stealth. This is basic geometry and scattering.

Now, how much practical difference this makes, no one knows. Since both nozzles are pretty hidden by fuselage and tails, it may not matter that much.


Get's back to my point about the F-22 looking stealthier than the F-35. Honestly, much we don't know to speculate one way or another. At least without much more information. Than currently available in the public domain.

Which, is not to say your wrong. Just that there maybe variables that we're unaware of.... :|

Respectfully

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 10:05
by hornetfinn
This document has already been posted here but it has some interesting things to say about F-35 nozzle design:
https://www.lockheedmartin.com/content/ ... erview.pdf

Before the development of the F-35 low observable axisymmetric nozzle (LOAN), signature demands typically
drove nozzles to fixed, structurally integrated affairs (e.g., F-117). They had to have very high aspect ratio designs
(e.g., F-117) or highly capable but heavy two-dimensional systems (e.g., F-22), as illustrated in Image 1. Departing
from what was then the state of the art, industry and CRAD efforts developed multiple nozzle configurations to create
a LOAN for the F-35. The F135 engine with a LOAN balanced the requirements of LO and efficient aeromechanical
performance. This resulted in a lightweight configuration with reduced radar cross-section.


I'd say that for F-22 the square nozzles were the only sensible design at the time as LOAN nozzle was developed slightly after F-22 was designed. For F-35 the square nozzle would've been too heavy, expensive and probably couldn't be designed for STOVL easily enough. It might well be (I'd say almost certainly) that F-22 nozzle is stealthier at lower radar frequencies (like VHF and UHF). But I bet F-35 nozzle is pretty stealthy even in low frequencies. I think there might not be much difference in higher frequencies (probably L-band and up). However I wonder how much of a problem that would in real life. There would need to be such a low frequency radar almost directly behind F-35. It would also mean that the pilot is either incompetent or the F-35 systems didn't generate good enough SA for the pilot to use flight path that didn't expose the rear to the radar.

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 10:10
by zero-one
The information of the link is said to be from a Pilot and "aviation expert" who answers questions in Quora, by the name of Mark Knight.

So I decided to dig him up. His answers in Quora are mainly aviation related and they are pretty accurate for your typical F-16.net reader.

Here are some
Why doesn’t the F-22 have roll thrust vectoring?

The F-22 does not use thrust vectoring to improve roll rates because it was not deemed to be worth the cost of developing the flight control software.

The F-22 has an excellent rate of roll in all parts of the flight regime where the US Air Force require it. Simulations before construction suggested this, and testing of the YF-22 confirmed it; the F-22A has excellent rates of roll. Adding an even faster rate using thrust vectoring would add nothing to production costs but developing the control software further would have a development cost. The US Air Force did not want to pay for it so that was never done.

The exhaust nozzles on the F-22 are locked to the same angle by the aircraft’s system software and the US Air Force has obviously seen no reason to pay for that to change.



If an adversary figures out how to defeat stealth technology completely, will it make the F-35 obsolete?

No it won’t make it obsolete.

The US Air Force is already planning how to upgrade the F-35 to operate when its radar stealth has been defeated, and indeed it expects this to happen against some potential adversaries by 2025 or 2030 at the latest.

The biggest advantage that the F-35 will still retain once its stealth is defeated is information. Pilots in an F-35 attack wing will still be able to see everything that any sensors on any of the F-35s in the group can see. If you are an F-35 pilot this means that you will have a complete picture of what is happening in the air and on the ground for hundreds of miles. That means you are still likely to see enemies before they see you, even without the radar stealth being effective.

In air combat if you see the enemy first you can shoot first, and that usually means you win......(the actual answer is far longer, his is the 3rd answer on the link below)
https://www.quora.com/If-an-adversary-f ... 5-obsolete


Point is, he seems to be at the least knowledgeable about the topics and would fit nicely here.

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 10:57
by Corsair1963
"zero-one"

If an adversary figures out how to defeat stealth technology completely, will it make the F-35 obsolete?

[i]No it won’t make it obsolete.

The US Air Force is already planning how to upgrade the F-35 to operate when its radar stealth has been defeated, and indeed it expects this to happen against some potential adversaries by 2025 or 2030 at the latest.

The biggest advantage that the F-35 will still retain once its stealth is defeated is information. Pilots in an F-35 attack wing will still be able to see everything that any sensors on any of the F-35s in the group can see. If you are an F-35 pilot this means that you will have a complete picture of what is happening in the air and on the ground for hundreds of miles. That means you are still likely to see enemies before they see you, even without the radar stealth being effective.



Absurd all future 5th and 6th Generation Fighters will incorporate a high level of stealth. So, are we to believe the USAF expects stealth to be defeated againt some potential adversaries between 2025 - 2030!

:doh:

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 11:30
by disconnectedradical
hornetfinn wrote:This document has already been posted here but it has some interesting things to say about F-35 nozzle design:
https://www.lockheedmartin.com/content/ ... erview.pdf

Before the development of the F-35 low observable axisymmetric nozzle (LOAN), signature demands typically
drove nozzles to fixed, structurally integrated affairs (e.g., F-117). They had to have very high aspect ratio designs
(e.g., F-117) or highly capable but heavy two-dimensional systems (e.g., F-22), as illustrated in Image 1. Departing
from what was then the state of the art, industry and CRAD efforts developed multiple nozzle configurations to create
a LOAN for the F-35. The F135 engine with a LOAN balanced the requirements of LO and efficient aeromechanical
performance. This resulted in a lightweight configuration with reduced radar cross-section.


I'd say that for F-22 the square nozzles were the only sensible design at the time as LOAN nozzle was developed slightly after F-22 was designed. For F-35 the square nozzle would've been too heavy, expensive and probably couldn't be designed for STOVL easily enough. It might well be (I'd say almost certainly) that F-22 nozzle is stealthier at lower radar frequencies (like VHF and UHF). But I bet F-35 nozzle is pretty stealthy even in low frequencies. I think there might not be much difference in higher frequencies (probably L-band and up). However I wonder how much of a problem that would in real life. There would need to be such a low frequency radar almost directly behind F-35. It would also mean that the pilot is either incompetent or the F-35 systems didn't generate good enough SA for the pilot to use flight path that didn't expose the rear to the radar.


No, UHF and VHF wavelength is too long, it’s a meter or longer so even F-22 will not be very stealthy against that. But L band is probably where F119 nozzle is stealthier than F135.

But again it might not make much practical difference since the nozzle is pretty well blocked by fuselage and tails.

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 11:36
by zero-one
Corsair1963 wrote:
Absurd all future 5th and 6th Generation Fighters will incorporate a high level of stealth. So, are we to believe the USAF expects stealth to be defeated againt some potential adversaries between 2025 - 2030!


He may have said that due to lines such as this from the US Airforce's 2030 air superiority flight plan,
The Air Force’s projected force structure in 2030 is not capable of fighting and winning
against this array of potential adversary capabilities.

https://www.af.mil/Portals/1/documents/ ... 20Plan.pdf

Personally, I believe the F-22 will continue it's dominance well into the 2030s but probably not to the margins that it enjoys today. The F-35 on the other hand will still be the best mud mover and considerably effective in the A-A role as well.
But I also think that without consistent upgrades, their level of dominance will decrease over time.

Re: How do square nozzles reduce IR?

Unread postPosted: 13 Jan 2020, 12:25
by hornetfinn
Corsair1963 wrote:
"zero-one"

If an adversary figures out how to defeat stealth technology completely, will it make the F-35 obsolete?

[i]No it won’t make it obsolete.

The US Air Force is already planning how to upgrade the F-35 to operate when its radar stealth has been defeated, and indeed it expects this to happen against some potential adversaries by 2025 or 2030 at the latest.

The biggest advantage that the F-35 will still retain once its stealth is defeated is information. Pilots in an F-35 attack wing will still be able to see everything that any sensors on any of the F-35s in the group can see. If you are an F-35 pilot this means that you will have a complete picture of what is happening in the air and on the ground for hundreds of miles. That means you are still likely to see enemies before they see you, even without the radar stealth being effective.



Absurd all future 5th and 6th Generation Fighters will incorporate a high level of stealth. So, are we to believe the USAF expects stealth to be defeated againt some potential adversaries between 2025 - 2030!

:doh:


My problem with these statements is that they tend to make it like stealth is just one attribute that can be made irrelevant. However any kind of VLO stealth is going to make the aircraft or other object much more difficult to detect than without those qualities. So even if some sensors in 2030 might well be able to detect and track stealth aircraft, they will stil be much less effective than against conventional aircraft. F-117 will always be more difficult to detect, track and engage using radar systems than any non-VLO aircraft unless the latter has really massive EW support.

I don't see any kind of realistic development in radar tech, which would make F-35 as easily detected by radar systems as say F-16. That would require radar system which is at least 1,000 times more powerful or sensitive than current radar systems.

I agree though that information is extremely important in every way and also to maximize stealth qualities. F-117 didn't have any kind of SA really and even then only one aircraft has ever been shot down. If it had even basic RWR, I doubt it would've been shot down. With the great SA F-35 has, it must be really difficult to detect and track as the F-35 will know when it's threatened and the pilot can act accordingly.