J-20 5th Gen Fighter Thread VI

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No other plane needs remotely as much cooling.

The only way the J20 would need so much air volume on both sides is if it was used for exhaust IR reduction.

That might tie in with the noticeable gap between the engine cowling and nozzles, which had previously been assumed to be caused by the use of interim engines rather than the WS15s the J20 is ultimately intending to use.

If we instead consider the possibility that there is additional internal ducting channeling cold air from the side intakes to blast out around the engines; not only would that help to greatly increase exhaust IR suppression, it should also create a cold air lay around the engines themselves, thereby massively reducing the plane’s own biggest IR source.

Such a theory would further not rule out my own earlier theory that they side intakes are there to allow active flow control within the intake to help work around the inherent speed limitations of a fixed DSI inlet design.

So the side intakes may actually serve the double purpose of intake flow management and IR mitigation, and would be one hell of a smart engineering solution is that is indeed what they did.


VIP Professional
I never really understood why would a side looking radar have that sort shape and dimension. Its height doesn't allow for any serious resolution, especially when scanning up and down. Yet the width is considerably different. Not only allowing for resolution but for some serious off axis scanning.

Now what would be the purpose of a side scanning radar? I can think of these:
A) addition to missile approach sensor system. Little to no resolution would be needed there, a simple radar would suffice, just to detect if a fast moving object is approaching. Scanning zones would need to big, though. Covering most of the side angles, which the long strip does provide for. But also high angles and to an extent low angles should be covered. Which the strip may not be able to cover properly.

B) contact detection/tracking system for short ranges. By short ranges I mean longer ranges than the optical systems around the plane can track contacts, but considerably shorter ranges than the main radar could track stuff at. So... perhaps a few dozen km to 50-ish km for non-stealthy fighters? Resolution may not be as important here, as the system would be there either for early warning against new contacts incoming from non-forward sector, or for continued tracking of already established contacts, once the main radar shares tracking data and the plane's nose turns away so the main radar can't track the contacts anymore. Still, I'd say the same issues of coverage apply. With the possible exception of the need for low angle and to some extent high angle threats. (low angle threat would be hard to catch a high flying J-20, though again not impossible. and high angle threat would likely not be applicable unless J-20 is flying low, which it shouldn't really do often. Threats here are planes, not missiles) I guess the narrow strip MAY be useful enough for this role, if the designers deemed that low and high angle threats are going to be so rare that it doesn't make sense to design the system to deal with them)

C) Actual radar with various identification modes. Needs resolution. There's no way that narrow strip can provide enough resolution for that. It would probably have serious issues giving basic altitude data on the contact, making the whole radar more of a 2D radar than a 3D one. The same issues applies to both A) and B) though, so all that approach warning data may not be enough to pinpoint where the threat is coming from. So J-20 might need to turn around and scan around to actually determine the altitude of the contact/threat. For A) that makes little sense as missile approaching wouldn't give enough time for such a maneuver. Nor would such a maneuver be prudent, shaving energy off J-20 at the worst possible time.

Both Su-57 side arrays and never installed SLAR arrays for F-22 were visibly less narrow than these alleged radar strips on J-20. So, unless this really is a placeholder space on J-20 with some test unit, what gives?


Junior Member
Registered Member
so WS-15 confirmed to 2025 ?
It’s just a speculation. Other speculations put it much closer. Given that it entered a mature project state in 2006, it doesn’t seem very likely that it would take 19 years to finish. And there were reports yesteryear of the cooling problems being solved.

There are significant difficulties associated with a 195+ kN engine, especially a TVC one. F135 was not TVC but suffered(suffers?) from a lot of fires, and that’s with USAF pumping floods of money into it, while the WS-15 is on China’s austere budget.

Presumably, even if the cooling is solved, there would need to be significant certification first for China to dare put a pilot in a plane flying it.


Junior Member
Registered Member
There are significant difficulties associated with a 195+ kN engine, especially a TVC one. F135 was not TVC but suffered(suffers?) from a lot of fires, and that’s with USAF pumping floods of money into it, while the WS-15 is on China’s austere budget.
Isn't the WS-15 a 180+Kn engine ? How does it compare with other aircraft engines ?
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