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All the current and upcoming 5th generation fighter aircraft have increasingly similar physical layouts regardless of their origin. This seems to be due to similar design choices around stealth and aircraft performance. Refer to the image. All of these aircraft have a "box" fuselage, blending the engines, fuselage and wing roots, engine intakes located on the sides of the main fuselage, high-mounted trapezoid wings, slanted out twin vertical stabilizers, and the like.

The Su-57 still uses the typical Su-27-like engine layout, which has many more edges and corners, potentially making it more challenging for a stealthy design.

Why does Su-57's design prefers this layout, while all other 5th generation fighters go for a blended "box" cross section of the fuselage?

enter image description here

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  • $\begingroup$ Edges and corners don't in themselves reduce stealthiness. If anything, stealthy airplanes look more like origami than non-stealthy ones. $\endgroup$ Jan 10 at 22:49
  • $\begingroup$ This answer should help $\endgroup$
    – sophit
    Jan 11 at 9:53
  • $\begingroup$ @sophit the accepted answer on that question seems to put roughly the same point I am trying to make. Having less number of edges and bends helps in stealth. However, Su57's distinct nacelles increases the reflectivity. The sharp edges near the intakes may even be very good corner reflectors. "With better computational methods, also curved surfaces could be simulated and it became possible to reduce edge count. But ideally the surface should be horizontal near all edges, which explains why fuselages and wings look like they had ironing edges." $\endgroup$
    – paki eng
    Jan 12 at 17:56
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    $\begingroup$ Russian designs are most stealthy in the forward aspect angles. Side and especially rear angles receive much less attention. The thinking is to sneak up on the enemy, but once the bombs fall or missiles fly, stealth is moot anyway. $\endgroup$ Jan 13 at 7:47
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    $\begingroup$ @Pilothead It's not exposed, there's a shallow S-duct with radar blocker grating inside. Not as good as deep S-ducts, but consistent with limited stealth. $\endgroup$
    – Therac
    Jan 19 at 9:17

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There are different definitions of what to call "stealth". Some aircraft like the B-2 are total all-aspect stealth, others like the F-15SE proposal only make small RCS reductions.

The Su-57 is in between. It's a stealth design, but it focuses on forward aspect stealth. This is similar to the F-35, which also forgoes rear hemisphere stealth. The Su-57 applies major RCS reductions, like eliminating vertical surfaces and shielding the engines, but still builds upon the Su-27 Flanker platform, so they couldn't completely rearrange the planform.

The reasons are historical. The Soviet Union's program to develop a 5th-gen fighter started in 1979, 2 years before the US ATF program (the F-22). Both programs wanted a low-observable, supercruising, supermaneuverable, fused-sensors fighter. Here is what the US considered:

enter image description here

The USSR also had all its design bureaus compete, but I can't find open sources for their designs. In 1987, Mikoyan won the competition with their Project 1.44. Sukhoi, Yakovlev, Ilushin, Antonov, Tupolev lost. In 1991, the USSR collapsed, and the program was defunded. Russia was considering joining NATO, or disarming completely.

Meanwhile, Sukhoi's Su-27 was raking in the cash. It briefly overtook even the F-16 as the most-exported fighter, though not for long. Heavy fighters usually suffer from poor sales, but the Flanker's fly-by-wire, supermaneuverability, payload, reliability made it popular for export.

Sukhoi knew the future is stealth, and invested into developing their own design further. In 2000, when the new government revived the 5th gen program, Sukhoi had a prototype ready to go. The program took just 10 years and $1 billion from that to first flight in 2010. The only way Sukhoi could accomplish this was by reusing the Su-27 as much as possible. Early Su-57 prototypes were just modified Su-27s, testing one thing at a time. Later, they changed the airframe, but they had to keep the overall layout.

Some prototypes tested flat combiner nozzles, suited for deep stealth. Some had complete S-ducts and blended nacelles:

enter image description here

None were picked for the production version. S-ducts are heavy. IR-lined flat nozzles are even heavier. This results in a notable drop in performance. The F-22's useful load is only 0.48 of MTOW, down from 0.58 for the F-15. That's a massive drop, and part of why the F-35 lost the stealth nozzle. Sukhoi didn't put as much trust in stealth as Lockheed did, and wanted a plane that would work well even if stealth failed.

The practical fact is that the Su-57 is a transitional design, rather than a new all-stealth concept. That's not necessarily a bad thing - stealth is as of yet untested in peer-level combat, and it can be countered. To that end, the Su-57 has the most comprehensive counter-stealth sensor suite to date, with two IR systems and a dual-band radar, which might offset its higher RCS.

From the top, the Su-57's design (left) is consistent with stealth, if following the concept of low-level penetration. But Sukhoi's all-stealth design (right) is different:

enter image description here

The Su-57 makes compromises. It can take different engine models, be quickly modified for A2A or A2G, uses intake radar blockers rather than deep S-ducts. Stealth-wise, while the B-2 and F-22 are estimated at roughly -25 dBsm median RCS (-40 for head-on aspect), the F-35 at -20, the Su-57 only claims around -10 dBsm median. That's a lot better than +10 to +15 of most 4th gen fighters, but it's not state-of-the-art stealth.

The Su-75 (on the right), slated to fly in 2024, doesn't compromise. It's designed from scratch, has no predecessor, follows every rule of stealth design there is. It was started in 2000s Russia, enriched by high oil, global trade, and Sukhoi's own export success. Yet, it doesn't yet have a single customer signed up.

enter image description here

The reasons are trust, reliability, familiarity. It will take a long time to get acquainted with the Su-75 and iron out all the kinks. Even longer to learn how to keep it stealthy - a challenge in itself, which took impractical restrictions for the B-2 and time for the F-22. India and China realize it will take a long time to learn, and China's more eager to invest that time in their homegrown platform.

On the domestic side, there's the cat-and-mouse game between stealth and counter-stealth. Playing is definitely better than not. But Russia has previously claimed the S-300 to have strong counter-stealth. If true, it means the Su-57 and even Su-75 won't be able to rely on stealth in former Soviet countries, which are stock-full of S-300's, originally meant to guard against US/NATO. This might be why the compromise option was picked for the PAK-FA over full-stealth prototypes.

Also, it could be that the shape works better than it seems to. But Sukhoi's own patent only claims -10 dBsm. Here is an independent evaluation of the Su-57's RCS, compared to F-35 and J-20. The conclusion of this modeling is that the Su-57 has about 10 dB worse RCS than the F-35, and its engine placement plays a role in that. It is, however, a major improvement over 4th gen fighters.

enter image description here

Summary: Sukhoi hedged their bets with the Su-57, building a plane that can sell as either a stealth fighter or as a better Flanker. It doesn't look as stealth-oriented as the F-35, J-20, or Su-75, because it isn't. But from key aspects, its RCS reduction is significant, about 100-fold.

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  • $\begingroup$ Thanks. Can you please give the source of the RCS numbers? Also the source of "Sukhoi itself says it's a compromise". $\endgroup$
    – paki eng
    Jan 12 at 17:49
  • $\begingroup$ So you mean to say they didn't want it to be stealthy as bad as to try out a new new layout, they just went with the old flanker design? $\endgroup$
    – paki eng
    Jan 12 at 17:57
  • $\begingroup$ @pakieng Stealth on a tight design budget isn't easy. By starting with the Flanker, they got a plane that works right away, stealth or not. Plus, they were able to reuse Su-37 parts and rigging, until new models were developed. Early Su-57 prototypes were just modified Su-27s, testing one thing at a time. The old source went down, I'll look for one. $\endgroup$
    – Therac
    Jan 13 at 1:06
  • $\begingroup$ @pakieng Linked the source, at the end. $\endgroup$
    – Therac
    Jan 17 at 4:41
  • $\begingroup$ Thanks for the source Therac $\endgroup$
    – paki eng
    Jan 18 at 20:22
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The F22 uses blended airframe that was designed for stealth very onset. Russia knew stealth was a terrible design alone so sacrificed capacity in exchange for improvements of engines. More likely nacelles are easier to open to access engine mechanics and part replacement befitting Russian simplicity philosophy. And being slightly larger, the engines are straight and ducts largely straight as possible. enter image description here

enter image description here

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  • $\begingroup$ Interesting!! Looking from the front ±10 degrees, you could look straight on to the engine blades. Terrible terrible for stealth. $\endgroup$
    – paki eng
    Jan 22 at 15:35
  • $\begingroup$ @pakieng There's an angled duct and a radar blocker in front of them. $\endgroup$
    – Therac
    Jan 22 at 16:06
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The simple answer is the Su57 is not a fifth generation fighter. Stealth is arguably the most important characteristic of fifth gen fighters, and the Su57 is not stealthy. It has the RCS of an F18, or about 1000 times more detectable than an F35. Global security lists the F18 at 1sqm, within the range claimed for the Su57, and the F35 at .005 or -30dbm. From the front at certain angles it even has a 10x higher RCS spike due to exposed engine face in the air intakes. Marketing hype does not change reality.

This doesn't mean it is a poor aircraft that isn't useful. It was just a low budget attempt at stealth that failed. Militaries are still buying upgraded fourth gen aircraft, but anyone trying to conduct missions that require stealth using an Su57 is in for a rude awakening.

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    $\begingroup$ The first source is very poor. First, it distorts the facts: Sukhoi's patent refers to average RCS, which can't be compared to frontal RCS. Second, from no angle is the F-18 a 0.1 m2 RCS target. Third, it contains flights of fancy like -50 dB RCS for the F-35. Fourth, it cites Quora answers as its sources. $\endgroup$
    – Therac
    Jan 19 at 2:34
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    $\begingroup$ @Therac So instead of complaining about my source, offer your own comparison of a Su57 RCS to an F18 and F35. This is not the only place where this is stated. Global security lists the F18 at 1sqm, within the range claimed for the Su57, and the F35 at .005 or -30dbm. $\endgroup$
    – Pilothead
    Jan 19 at 16:43
  • $\begingroup$ Here is the F/A-18's RCS modeled. The Hornet has a rounded all-metal fuselage, its sides light up to +15 dBsm and more. $\endgroup$
    – Therac
    Jan 20 at 4:56

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