The configuration of the Sukhoi Su-57 Felon is an evolution of the Su-27’s shape adapted for the requirements of low visibility and supersonic speed and agility.
As explained by Piotr Butowski in his book Russia’s Warplanes, Volume 1, the main solution to reduce radar visibility is internal carriage of weapons. Radar blockers reduce reflections from the engine inlet guide vanes and are installed in the engine air intake ducts. The shape of the airframe has been selected to reduce the number of directions in which electro-magnetic waves are reflected, and to ensure these directions are the safest.
Another group of stealth measures concern the fighter’s equipment. The surfaces of the NO36 radar arrays are deflected from the vertical plane, thus deflecting the enemy radar’s radiation aside. The domes of the arrays are selective — they let their own signal pass through and block other frequencies. In addition, the compartments for these arrays feature radar-absorbent coatings on their edges to absorb ‘freak waves’ (which occur when a wave is amplified after multiple reflections within an enclosed space). In order to reduce their total number, the available arrays are used by multiple systems simultaneously, for example, the radar, ECM and IFF systems. Within the antenna-feeder system, use is made of antennas that do not protrude outside the airframe outlines, and the vertical empennage serves as an antenna for the communications suite. The turret of the 101KS-V IRST is rotated backwards in cruise configuration, and its rear hemisphere is covered with a radar-absorbing coating.
However, even though the Su-57 is marketed as fifth generation fighter, the Felon is not a true stealth fighter and it can’t be compared to the US-made F-22 Raptor and F-35 Lightning II. Why is that?
‘Sukhoi claims Su-57 having RCS goal between 0.1 to 1 m^2 (-10 to 1 dBsm). For comparison the F-117 had a RCS around -25 dBsm while the F-22 and F-35 has RCS better than -40 dBsm – which is at least 1,000 to 10,000 times smaller than Su-57. All of these are based on unclassified program goals and YF-117’s RCS testing data from Skunk Works,’ says Abhirup Sengupta, an aviation expert, on Quora.
‘To understand the real world difference lets take S-400’s 91N6E Search radar as reference. According to the manufacturer 91N6E has a detection range of 390 km against a 4 m^2 target. So it should be detecting a F-15 (13 m^2) from 523 km, a F/A-18E Super Hornet (~1 m^2) from 275 km, the Su-57 (assuming 0.1 m^2) from 155 km, and a F-22/F-35 (-40 dBsm) from 27 km. In short, a radar will be having between 6 to 10 times greater detection range against the Su-57 compared to F-22 & F-35.
‘It’s worth remembering that failure of Mikoyan Project 1.44 (Russia’s first attempt at Stealth aircraft) is what brought Su-57 to light. One of the lessons from Mig 1.44’s failure was Russian economy being unable to afford designing a new LO airframe from scratch. You can see that all Russian aircraft designs post Cold War that saw daylight are mere modification of an existing airframe (Su-30, Su-35 and Mig-29K, Mig-35 being derivate of Su-27 and Mig-29).
‘The Su-57 program is not too different. The goal was to modify an existing airframe (Su-27) to maximise RCS-reduction without having to design a new airframe from scratch. This is evident from Sukhoi Design Bureau not adding basic things like Serpentine intakes which are critical for achieving serious LO signature. In essence Su-57 in many ways is similar to US Navy’s F-18 Super Hornet program. It’s what you do when you’re on a really tight budget.
‘The second problem with Su-57 is that Russian Aerospace industry is simply incapable of producing a VLO aircraft. It’s not just about things directly related to Stealth (Tolerance, Material sciences for RAM/surface coatings with desired broadband, multispectral and durable characteristics) but also things indirectly related to Stealth like Engines – the Felon’s izdeliye 30 engine with circular nozzle is supposed to produce 38,600 lbf thrust, which is comparable to Raptor’s 20 year old F119 with rectangular nozzles (37–39,000 lbf). You can’t have a Fighter design with S-ducts or rectangular exhaust nozzles if you can’t produce engines with sufficient thrust to compensate the thrust-loss from those designs. Likewise you can’t have faceted glass panels to house the IRST sensor if your industry can’t economically produce one with sufficient integrity and tolerance to handle Mach 2 flight envelope.
‘Then there’s the whole RF domain dictated by the state of your semiconductor industry. When you’ve the Su-35 having comparable SAR resolution to that of F-15E’s 20 year old Mechanical radar, it highlights the inherent small bandwidth of Russian radars. You can’t have serious LPI characteristics with such a small bandwidth. The same limitations also prevent you from having highly directional (Line-of-sight) LPI datalinks – forcing you to continue relying on omnidirectional Low-band datalinks that not only ruin your RF signature but are also quite limited in terms of throughput. Stealth isn’t just about your airframe’s shape, today it’s also about your avionics suite. Without the necessary EMCON you’ll be forced to either fly blind or ruin whatever element of surprise you may theoretically have.’
‘For most part real world design objectives aren’t dictated by your group of engineers but your economic and industrial capability. Last 30 years in addition to last 6 months unequivocally demonstrate that Russia neither has the industrial capacity nor economy to develop a VLO fleet.’
Photo credit: U.S. Air Force and Sukhoi
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