How do square nozzles reduce IR?

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hornetfinn

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Unread post13 Jan 2020, 10:05

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.
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zero-one

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Unread post13 Jan 2020, 10:10

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.
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Corsair1963

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Unread post13 Jan 2020, 10:57

"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:
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disconnectedradical

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Unread post13 Jan 2020, 11:30

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.
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zero-one

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Unread post13 Jan 2020, 11:36

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.
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hornetfinn

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Unread post13 Jan 2020, 12:25

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.
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