Stealth-related question

[Posbi]

How are the IR signatures of modern supposedly "stealthy" designs minimized? Are there ways and means to limit the exhaust heat signatures?

[primer]

The F-117 and the B-2 have their exhaust shielded from below and behind and if I recall correctly there is also a cold airflow mixed in to cool them down. The F-22 I don't know, but it doesn't have to use it's afterburner as much as modern fighters. The F-35 has a normal exhaust and hardly has IR shielding.

[Apocal]

Exhausts is focused into a narrower cone. Most fighters already do this, see the tails of the F-16 and F/A-18s for example. The F-22 is another example, although with some compromises to support thrust vectoring.

You can also divert it sideways, which, IIRC, the Apache does.

As far as I know, there are some IR-scatter materials used in the leading edges to take a little of the friction-induced IR signature off, but I doubt they are terribly effective at making it invisible in the IR spectrum.

[primer]

Originally Posted by Apocal

Exhausts is focused into a narrower cone. Most fighters already do this, see the tails of the F-16 and F/A-18s for example.

That's not for the IR signature, that's to increase pressure in the exhaust for more thrust. Action = Reaction and all.

[Lord Squishy]

Problem with minimizing exhaust heat is that hot exhaust = more energetic exhaust, which means more thrust.

[primer]

Originally Posted by Lord Squishy

Problem with minimizing exhaust heat is that hot exhaust = more energetic exhaust, which means more thrust.

Shielding and cold airflow can help against that but you won't see that on a fighter.

[Lord Squishy]

Originally Posted by primer

Shielding and cold airflow can help against that but you won't see that on a fighter.

Tell me, what about a system whereby you have forward swept wings with exhausts under the wings and then a very forward-mounted engine with a long exhaust tailpipe that runs through the aircraft frame (with the frame itself either being cooled by cryogenic fuel or external airflow?

Might actually disguise the worst part of the plume from everything by directly behind.

[primer]

Originally Posted by Lord Squishy

Tell me, what about a system whereby you have forward swept wings with exhausts under the wings and then a very forward-mounted engine with a long exhaust tailpipe that runs through the aircraft frame (with the frame itself either being cooled by cryogenic fuel or external airflow?

Might actually disguise the worst part of the plume from everything by directly behind.

So, split exhaust and a lot of cooling? Split exhausts are inefficient. And cold would make the pressure in the exhaust go down so you'll lose a lot of thrust.

[Clerik]

F-22 supposedly has TONNES of stealth design features, many aimed at reducing IR signature.

They have spread out as much of the heat generating components and systems all over the airframe as possible, so that heat is not concentrated in one part of the aircraft.

On top of that, they do have cold air (or liquid, not sure) running all over to further smooth out temperatures, especially away from the engines. The result is that heat is distributed very uniformly, and the hottest point is significantly cooler than other aircraft.

And then of course as someone mentioned, you don't need to use afterburners that often due to super-cruising engines.

[primer]

Originally Posted by Clerik

On top of that, they do have cold air (or liquid, not sure) running all over to further smooth out temperatures.

The F-22 and F-35 use liquid equipment cooling, older gen fighters like the F-16 just use ram air and cold air from the environmental control system (fancy aircon).

[Lord Squishy]

Originally Posted by primer

So, split exhaust and a lot of cooling? Split exhausts are inefficient. And cold would make the pressure in the exhaust go down so you'll lose a lot of thrust.

No, split intakes through the leading wing edge and a single, heavily recessed exhaust into the airframe. You're not cooling the exhaust itself, you're just cooling the airframe around the exhaust to ensure it doesn't, I don't know, melt, but the exhaust is regular temperature.

[primer]

Originally Posted by Lord Squishy

No, split intakes through the leading wing edge and a single, heavily recessed exhaust into the airframe. You're not cooling the exhaust itself, you're just cooling the airframe around the exhaust to ensure it doesn't, I don't know, melt, but the exhaust is regular temperature.

Sounds needlessly complicated.

[Lord Squishy]

Originally Posted by primer

Sounds needlessly complicated.

It actually doesn't sound that much more complicated to me than what we have now, which is often basically the same design, just in different places, but what do I know, maybe it is.

[Major Diarrhia]

Years ago, I remember the F-22 was going to have some sort of system which could produce hot spots beside the wings, with an electric system, in quickly alternating patterns in order to confuse IR missiles. But, I haven't heard anything about it since then.

[Admiral Vice]

Originally Posted by Major Diarrhia

Years ago, I remember the F-22 was going to have some sort of system which could produce hot spots beside the wings, with an electric system, in quickly alternating patterns in order to confuse IR missiles. But, I haven't heard anything about it since then.

So what does that tell you?

*winks knowingly*

[Major Diarrhia]

The Senate deemed it too expensive so they pulled it just before it went into its final phase, only to be replaced by a seemingly cheaper system that ended up more expensive?

[Desert Fox]

One item of curiosity is that for years it sounded like the F-22 was going to be only partly stealthy but it turns out to be extremely stealthy (as far as radar)

[Apocal]

Depends on what kind of radar. I could pick them up on a SPS-48E from a further distance than they'd probably like. And this was radar video, not IFF video I was seeing, our UPX was busted at the time. A SPY-1B 4kyds behind us was holding the track as well, although I'm not sure who picked it up first, I'd just taken the watch and it was a part of the passdown that USAF spent a lot of money on some worthless shit.

[Desert Fox]

I have heard that the Navy wanted to test the SPY-1 radar system against the F-117 but the airforce did not want too?

[Apocal]

Originally Posted by Desert Fox

I have heard that the Navy wanted to test the SPY-1 radar system against the F-117 but the airforce did not want too?

Never heard about that, except for vague statements that could have been taken out of context. If that actually did happen I can imagine the USAF said no because it proves absolutely nothing about the F-117's capability. F-117 was designed with Soviet radars and by now we've learned our lesson about designing our shit to beat our own shit.

The only reason we could get F-22s on was because they were based at Langley AFB and floating off the coast of Virginia during pre-deployment workups, we're able to track everything coming and going from there. Listening in on their comms chatter told us that they were doing a stealth run to make sure everything was good to go. And since we're all AAW types, "If it flies, it dies" kinda guys, that's like challenging Mike Tyson in his prime.

[Desert Fox]

You can be certain that the Navy will test their F-35 against their radar platforms

[Apocal]

Originally Posted by Desert Fox

You can be certain that the Navy will test their F-35 against their radar platforms

Oh yeah. If nothing else they'll get the data when our guys go to play bad guys for a ship's deployment training.

[iBorg]

That's not surprising. VLO means only that : VLO. Pour out enough radar energy and there will be enough return to track even an F-22. OTOH, the F22's ESM systems will sniff out the big powerful radar long before it enters detection range and take measures to avoid it.
Obviously the Raptor drivers didn't expect the US Navy to launch Standards at them :-)