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Copyright © Alexei Gretchikhine, 1997-98.

The Photograph

After stealing dozens air shows and breaking thousands hearts, Sukhoi had established as a world known aircraft design bureau, beating a rival Mikoyan after decades of bad luck and political opposition. Sukhoi's workhorse, basic Su-27 had been rebuild as the Su-30 interceptor, Su-33 carrier born fighter, Su-34 tactical bomber, Su-32FN ASW and coastal defence platform, Su-35 and Su-37 air-superiority fighters. However, busy with development of the Su-27, Sukhoi is working on at least two fighter-sized aircraft, one of which is intended to replace Su-27, Su-33, Su-35 and Su-37.

S32310196
Sergei Skrynnikov, Air Fleet Herald

The final configuration of the new generation fighter was revealed at enlarged meeting of the Russian air force Military Council (RusAF MC) which was held on 31 January and 1 February of 1996. The photo, showing RusAF top brass and Russia's top designers participated in the meeting, appeared in the 1996 March-April issue of "Vestnik Vozdushnogo Flota" (Air Fleet Herald formerly known as "Aviatsiya i Kosmonavtika" and recently reincarnated as "Vesnik Aviatsii i Kosmonavtiki"). It is hard to say if chief editor - Sergei Skrynnikov - was aware of the healthy curiosity of western aircraft enthusiasts to fifth-generation Russian fighters, whose focus was mainly attracted to the scale model sitting at the desk in front of Glavcom VVS (RusAF commander) Piotr Deinekin (front row, center) and Sukhoi constructor general Mikhail Simonov (front row, second from right). The type is believed to be new air-superiority fighter referred by west as S-37 (formally known as S-32 -- do not mix with Su-32!).

S32310196b
Gareth Burgess, Flight International

Filling MFI Requirement

The Sukhoi's candidate for the Russian air force requirement for a Mnogo-funktsional'ny Frontovoy Istrebitel' (MFI - multifunctional frontal fighter) is less known than its rival Mikoyan article 1.42. Vladimir Ilyushin, Sukhoi's veteran test pilot, revealed in mid 1997 that the aircraft was "close to completion", adding that it will be a "worldwide sensation" when it is unveiled. The scarce information on Simonov's new fighter indicate that it had already underwent high-speed taxi tests by the end of the summer and made its maiden flight at Zhukovsky at September 25th, 1997, in hands of Sukhoi's test pilot Igor Votintsev.

Although the S-37 was expected to be unveiled at Moscow Aerospace salon (MAKS'97) along with Mikoyan's 1.42, both remained wrapped in veils of secrecy due to decisions made by Russian military. Both MFI contenders were shown to Prime Minister Chernomyrdin at closing days of the air show.

The Sukhoi's vision of fifth-generation fighter is considered a lower cost solution (in a ballpark of $40 mln) for the MFI requirement than officially cancelled article 1.42. The savings from this slightly smaller, but equally sophisticated aircraft are believed to be due to the less radical and yet very capable design, demonstrating a mix of the fourth-and-a-half (4++) generation technologies and revolutionary new developments.

The S-37 might well be the latest and the last of Russia's aerospace industry, owning its concept to the very final iterations of fundamental aircraft research of former Soviet Union. The exact reasoning behind Sukhoi's decision to pursue heavy fifth-generation fighter is not clear. As recently stated by Russia's former Defence Minister Igor Sergeyev, currently Russia develops six types of fighter aircraft. The funding from RusAF remains scarce at best and the competition between ANTK Sukhoi and MAPO MIG for this money is fierce. Clearly, the investment of the capital gained from export sales of Su-27 to China and Su-30MKI to India in the S-37 project can serve as a final blow to the MAPO MIG fighter business. If the S-37 will indeed turn out a cheaper MFI, Simonov can count on possible orders from RusAF in very distant future. The S-37 can also have a great export potential as the Su-27 replacement and compete with F-22 exports.

six5th_s37
S-37 Berkut in flight, Kommersant Daily

What's in the Name?

The S-37 is an internal Sukhoi OKB designation which is rationalized in terms of commonly used yet controversial indexing originated with Sukhoi Su-7 and Su-9 prototypes. These were designated S-1 and T-1 respectively, with "S" being a first letter of swept wing in Russian "Strelovidnoe krylo" and "T" from the Russian for delta wing "Treugol'noe krylo". Clearly, the "S" in S-37 implies that new Sukhoi has a swept wing but the index conflicts with another S-37 formerly allocated to a single-engined lightweight multirole combat aircraft broadly similar to French Rafale which was cancelled in 19941. In any event, S-37 is an internal bureau designation, and could become Su- anything. Reported name of S-37 is Berkut (âÅÒËÕÔ/Ber-koot) which means golden eagle in Russian.

Tandem Triplane with Forward Swept Wing

The fifth-generation Sukhoi fighter features forward-swept wings (FSW), canards and widely spread twin outward-canted vertical stabilisers and incorporates low-observable and thrust-vectoring technologies. Comparing blown up photograph of the scale model and early speculative sketches and artist's renderings, which appeared in western aviation press, it is clear that S-37's forward-swept wing is closely coupled to canards. First photographs revealed that aircraft retains the horizontal stabilisers, evolving from pure canard to a tandem triplane layout. In addition, photographs show that the stabilizer are highly swept (ca 70 deg) and their leading edges appear to extend aft from the wing roots of FSW. This is very different from X-29A strakes which were extensions of the wing roots themselfs ending with a trailing edge flaps.

The tandem triplane configuration was test flown in 1985 on T-10-24 which served as one of the naval Flanker prototypes. The addition of the canards, referred PGO in Russian (Perednee Gorizontal'noe Operenie - Forward Horizontal Stabilizers), solved the control problems encountered at high angle-of-attack (AOA) flight regimes when the tailerons lost their efficiency in the wake of the wing. The PGO's seemed to be a favourite choice for forth-and-a-half generation of Sukhoi's Su-34, -32FN, -35 and Su-37 and fifth-generation S-37 and S-54.

six5th_f16.six5th_x29
General Dynamic's entry in the superagility field was a FSW F-16 (left). As opposite to X-29 (right) it was a true tandem triplane configuration. None work out for USAF. Courtesy NASA and Grumman.

The FSW had an edge in the trans- and subsonic performance but because the aluminum alloys commonly used in aircraft design fail to withstand the higher structural loading of the wing, the supersonic FSW concept was kept on the ground until introduction of the composites. A titanium-composite winged flight-by-wire Grumman X-29A had proved the feasibility of supersonic FSW concept. However, due to its poor performance at supersonic speeds, FSW was nowhere to be found in United States Air Force next-generation air superiority fighter.

Genesis of S-37

The early Soviet designs to feature moderately forward swept wing were Belyaev's DB-LK and Babochka aircraft and Mikoyan Gurevitch PBSh-2 (MiG-6) biplane. Captured at the end of WWII, German FSW Junkers Ju-287 was test flown by German and Russian crews. A six engined EF-131 was build and underwent extensive structural and flight testing until 1947, when theme was closed. At about the same time Pavel Tsybin build several testbeds LL (Letauchaya Laboratoriya) -1, -2 and -3 with stright, swept back and forward swept wings respectively (40 degrees). The LL-1 and LL-3 rocket powered gliders performed number of powered flights and provided Central Aerohydrodynamics Institute (TsAGI) with much needed FSW data. In one of the flights LL-3 reached Mach 0.97 in dive. The modern history of the TsAGI's FSW research started in 1977. Numerous scale models with FSW were "flown" in wind tunnels. The FSW MiG-23 models were extensively tested in TsAGI while SibNIA concentrated on FSW subscales of Su-27. These were complemented by Sukhoi's own experimental SYB-A.

Some sources suggested that original layout was much closer to X-29A tailless scheme and that aerodynamic of early S-37 directly benefited from TsAGI wind-tunnel tests with X-29A scale models. This, however, totally neglects the roots of the S-37 and the history of the soviet FSW research which started out in late 70s. Clearly, S-37 did not start its life as a technology demonstrator, built to examine FSW aerodynamics and composite wing structures.

S-37 is by no means the first FSW designed by Sukhoi. At one point Su-9 was fitted with FSW to explore the aerodynamics of the layout. It is not clear which Su-9 was used for the FSW tests, likely a single engine fighter, rather than postwar twin engine Me-262 look alike. Although German FSW influence is tempting, there were by far more time and funds for experimental work in 60s, when few Su-9 aerodynamic testbeds were flown.

LL3.six5th_mig23
Early FSW studies of Tsybin's LL-3 (left) were followed in late 70s when interest to FSW was revived. This TsAGI photo shows MiG-23 scale model tested hydro tunnel (right).

Sukhoi's SYB-A first flew in 1982, more than a year earlier than Grumman X-29 FSW demonstrator2. Initially discovered by US intelligence at Saki airfield, close to Syberski in the Black Sea region Sukhoi FSW was codenamed SYB-A, in tradition of indexing Soviet experimental aircraft according to the places these were first seen.

FSW Su-9 and SYB-A are likely to be closely related if not the same aircraft. Moreover, few sources suggested that S-37 in early stages of its development was to be much smaller, single engined attack aircraft, very much like Su-9 and SYB-A. What is interesting, SYB-A was spotted on what appears one of the naval airbases which give additional clues about the genesis of the S-37.

This naval (VMF) connection of the S-37 is coherent with efficiencies of the FSW in reduced takeoff run, a superb forward view from the S-37 cockpit, strengthened landing gear and finally and folding wing. All these features can be found extremely handy in carrier operations. By late 90s the soviet Navy planed to acquire two 1143.5 and two nuclear powered Ul'yanovsk class arcraft carriers (55,000 and 75,000 tons displacement respectively) which should have been at one point backed up with development of the new generation carrier based aircraft replacing aging MiG-29K and Su-27K. These plans, however, were shattered by demise of the Soviet Union. Today the naval prospects of S-37 (if there was one in first place) are limited to land based AVMF (Aviatsiya Voenno Morskogo Flota).

Officially stated starting point of S-37 is 1983, five years before the larger Ul'yanovsk was laid down at Nikolaev. S-37 is certainly more than Russian super Hornet, at least what it became due to realities Sukhoi had to face.

Sukhoi Fifth-generation Fighter Philosophy

The FSW is a better performer at high angles of attack in post-stall manoeuvring much needed in close-in dogfight. The fact that Simonov had chosen FSW for his fifth-generation fighter once again confirms Sukhoi's commitment to the superagility as a crucial requirement for the next generation air-superiority fighter. This approach, so much different from western concepts of stealth, supercruise and BVR engagements, was taken to the limits in Su-37. The FSW S-37 fitted with TVC expected to outperform its stalemate in close-in dogfight involving post-stall flight regimes. Having the edge in manoeuvring, the S-37 is clearly catching up in stealth with US and European new-generation fighters. However even with its internal weapon bay and RAM coating, the new Sukhoi is a very different concept than F-22. The heavy accent on RAM rather than radar absorbing structures (RAS) is obvious.

The reason for such attitude is not clear, although a combination of the technology limitations and operational doctrine is most likely candidate. The major components of radar stealth -- RAM coatings and surface quality -- are subject to the production and maintenance tolerance as it was shown by USAF F-117 and B-2 operational experience. Untightened screws, scratches or unfastened access panels were known to greatly deteriorate the RCS of the aircraft, reducing the engineering efforts put into aircraft design. It remains to be seen how Sukhoi will overcome the looser production standards of the Russian aircraft plans.

Low Observable

One of the early Sukhoi exposures to the low visibility technologies were tests with Sukhoi Su-25 Frogfoot prototypes T8-11 and T8-12. First trial, code named "Astra", involved T8-11 which is now displayed in Monino Air Force museum. A graphite based filler was applied to the basic T8-11 airframe for reduction of the aircraft radar cross section (RCS). Additional tests involved the first series production T8-12, which became a testbed for radar absorbing material (RAM) coatings and special camouflage schemes, intended to lower radar and visible signatures of the aircraft. This low visibility paint tests were similar to those carried out for USAF A-10 in early 1990s. The cannon port was faired-over and forward fuselage of the stealthy Frogfoot was painted with rubber like material. For a brief period this RAM coated T8-12 joined T8-16 at Khodynka, but its prompt removal suggested that the hightech and state secrets were at stake and it was put on display by a mistake or ignorance in a first place.

701
Sukhoi Su-35

The Afghanistan experience where Sukhoi's encountered a thread of the shoulder launched infrared homing surface-to-air missiles such as Redeye, Stinger and SA-7, forced Sukhoi team to work on the reduction of the infrared signature of the Su-25. The results materialized in the Su-25T development -- Su-25TM (Su-39 in Sukhoi's nomenclature). The installation of the intake cones hiding the turbine blades and efficient mixing of the exhaust with cold air reduced the IR signature of the Frogfoot from front and rear aspects. This fourfold reduction at expense of 2% lower SFC is indeed an impressive achievement.

Further experiments with low visibility involved the advanced Flanker development prototypes, aircraft of 700 (Su-35,-37) and 600 (Su-30) series. These fighters wear eye catchy new camouflage schemes designed to reduce the visual signature of the aircraft on the ground and in the sky. One of the most interesting examples of Sukhoi experiments was a scheme applied to 701, designed to deceive space based optical systems.

Some effort was directed in reduction of the radar cross sections of advanced Flankers as well. The Su-34,-32FN have optimized forward fuselage shape, lack variable geometry intakes and were reported to have partial RAM coating. Recently Sukhoi stated that basic export models of Su-30MK can be treated with RAM to fulfill customer requirement for a lower RCS aircraft.

Clearly benefitting from previous research, the S-37 prototype relies heavily on the Sukhoi's state of the art low observable technology. The forward swept wing, a conformal underfuselage weapon station(s), use of RAM, fixed intakes with S-ducted air ducts, suggest a further reduction of the aircraft radar signature down from similarly sized Flanker's 3-5 sq m. The extend of the reduction of the IR signature of the S-37 exhausts will depend on the choice of the trust vectoring nozzle. The F-22 type flat 2D nozzle can give a better results while 2D nozzle might contradict to Simonov's superagility ideas favouring 3D exhaust. The Saturn-Lulka was reported to work on reduction of the IR signature of the axi-symmetric trust vector controlled (TVC) Al-37FU power plant on non-afterburning regimes.

Powerplant

The scarce availability of trust vectoring Saturn-Lulka Al-41F engineered for the Mikoyan's article 1.42 forced Sukhoi to seek a replacement for the originally planned powerplant. According to MAPO MIG sources, the limited number of Al-41F are involved in Mikoyan's Article 1.42 tests and not available to Sukhoi's competitor. Reluctance of MAPO MIG made a trust-vectoring control (TVC) Al-37FU (sometime referred as Al-31FU where FU stands for Forsazh, Upravlaemoye soplo - afterburning, articulated nozzle) powerplant used in Sukhoi's Su-37 a natural choice for fifth-generation fighter, but would have been premature for the first S-37 airframe. Additionally, the availability of the Al-37FU could be a problem since all prototypes are involved in flight tests on the Su-37 and in the bench endurance tests. At the time of the Su-37 first flight only three Al-37FU were built.

Surprisingly, as a temporal solution, instead of similar and widely available Su-27 Flanker's Al-31F powerplants, the S-37 prototype received a pair of Perm Aviadvigatel D-30F6 engines used on MiG-31 Foxhound interceptors. Designed by the 1980, this full authority digital engine control (FADEC) engine comprises six interchangeable modules and a core module. Although powerplant accumulated several thousand flight hours and experienced no operational drawbacks, it has estimated 300 hrs life between overhauls (Russian engine maintenance is very different from western philosophy and term "overhauls" has a different meaning). There were no reports on TVC versions of D-30F6.

  Aviadvigatel D-30F6 Saturn-Lulka Al-37FU
Trust: 15,500 kgf (34,170 lbs) 14,500 kg (31,966.5 lbs)
Dry weight: 2,416 kg (5,315 lbs) 1,600 kg (3,527 lbs)
Bypass ratio: 0.4 0.6
Inlet diameter: 1,020 m (40.2 in) 910 mm (36 in)
Mass flow rate: 150 kg/s (330 lb/s) 120 kg/s (264 lb/s)
Fuel consumption    
    non-afterburning: 0.72 kg/kgf hr 0.67 kg/kgf hr
    afterburner: 1.90 kg/kgf hr 1.92 kg/kgf hr
Turbine inlet temperature: 1,660K (2,528.5F) 1,665K (2,537F)

The first photographs of S-37 revealed a pair of auxiliary intakes which could be used during take offs for increased air flow to the engines. These could have been repositioned from the underside of the aircraft due to the reduces radar cross section considerations or/and lack of the space taken by internal missile bay(s).

Avionics

In early September, defence-ministry acquisition chief Col Gen Anatoly Sitnov noted: "What is the use of developing the Sukhoi fifth-generation fighter, if the aircraft's cockpit dates back to a second- or third-generation design?" While Sitnov statement clearly implying the state of the art of the S-37, one can hardly expect that a first test airframe will incorporate all innovations planned for the series production. First cockpit images of the aircraft during the second round of the flight testing show that instrumental panel of the S-37 is broadly similar to that of the Su-35 and Su-37 family. Noteworthy are mix of the russian and english labeled buttons and gauges3, conventional centre flight stick and two large color MFDs. It is not clear whether the inadequate cockpit was promptly updated (during enlargement of the fins) to counter Sitnov's critics or this was the original layout.

Undoubtly, many parts of the S-37's avionics are missing on the first airframe. Similarly, the sole Su-37 demonstrator flies with a counter weight instead of the advanced radar hence the aircraft is intended to explore among other things the trust vectoring modes of the new powerplant. However, the Su-37 fighter will have the top notch avionics suit which is tested on other 700 series airframes -- Sukhoi Su-35s.

It is expected that the sophistication of S-37 cockpit and avionics suit should at least match that of forth-and-a-half generation Su-35 and Su-37 aircraft. The cockpit of the S-37 does most certainly feature the color liquid crystal MFDs and wide angle HUD. The test proven in Su-37 demonstrator inclined pilot seat, a fixed pressure sensitive throttle and side-stick controller will also find its way to the cockpit of further development airframes expected to impose even greater G-loads on pilot than superagile Su-37.

Su-37cockpit
The throttle, dash and side-stick of Sukhoi Su-37

The type of the radar intended for S-37 is not known. The size of the radom seems to be somewhat smaller than that of Su-27 family, possibly implying the smaller diameter antenna. The photographs of S-37 Berkut, show that the starboard tail sting is slightly longer than the port one. This asymmetry is explained by the fact that longer sting contains a new rear ward fasing radar developed by Phazatron while shorter houses RWR gear. Sukhoi shortened the length of the port sting to eliminate the interference between these sensors.

Armament

The armament of the S-37 will most likely never get close to the air-to-air arsenal of Mikoyan's article 1.42, enjoying super long range K-37. However the ram jet version of AA-12 Adder, R-77PD (RVV-AE-PD), seems to be the most appropriate long stick for the new fighter. The missile's collapsible lattice stabilizers give R-77 family the compactness well suited for the internal weapon bay(s) of the stealth S-37. However, the aerodynamically superior lattice stabilizers have reportedly a much greater RCS than conventional surfaces, thus potentially revealing the position of the aircraft at the moment of the missile launch. Owning the rear ward facing radar, the orientation of the ejected missile is less predictable than that on the external pilon.

The exact number of weapon bays is not known, although the total number of the hardpoints will be twelve. The use of the internal/external weapon loads will depend on the mission. The underside shots of S-37 are extremely rare since they provide additional clues about conformal armament of the fighter and tabooed by extincting breed of russian censors. The news video clip featured late 1997 unexpectedly revealed additional details of S-37.

Status

As of late April 1998, Igor Votintsev took S-37 in about dozen times, including two flights after S-37 vertical fins were enlarged to cope with control problems experienced at large angles of attack. Sukhoi acknowledged that second airframe is built which most certainly will be engined with "native" fifth-generation Al-41. It is possible that first airframe will be reengined with Al-41 as well.

S-37 vital statistics

DIMENSIONS:  
  Wingspan 16.7 m
  Length overall 22.6 m
  Height overall similar to Su-37
WEIGHTS AND LOADINGS:  
  Weight empty, equipped 24,000 kg (52,910 lb)
  Max T-O weight 34,000 kg (74,960 lb)
  Max landing weight similar to Su-37
  Max wing loading NA
  Max power loading NA
PERFORMANCE (estimated):  
  Max level speed:  
    at height 2,500km/h (1,350 knots)
    at S/L 1,400km/h (756 knots)
  T-O speed NA
  Landing speed NA
  Service ceiling 18,000 m (59,050 ft)
  T-O run NA
  Landing run NA
  Range with max fuel  
    at S/L NA
    at height 1,782 nm (3,300 km/2,050 miles)
  G-limits NA
ARMAMENT:  
  Number of hardpoints: 12: 6-8 underwing, 6-4 conformal underfuselage
  Air-to-air: R-77, R-77PD, R-73, K-74
  Air-to-surface: X-29T, X-29L, X-59M, X-31P, X-31A, KAB-500, KAB-1500


Notes:
  1. First public photographs of the S-37 suggested that the front part of the fuselage including the "hooded cobra" LERX could have come from the original S-37 canard-delta. If true, this could possibly clear up the origins of the S-37 index.
  2. ðÏÄ×ÅÒÖÄÁÑ × ËÏÔÏÒÙÊ ÒÁÚ ÞÔÏ òÏÓÓÉÑ ÒÏÄÉÎÁ - ÓÌÏÎÏ×!
  3. Interestingly, the altimeter is western. Does this mean that S-37 from the start is designed as an airshow aircraft?
Sources:

  1. Full story Sukhoi's S-37: years away from production International Defense Review Thu, Jan 01 1998.
  2. AeroWorldNet's S-37: A New Fighter From Sukhoi by Vladimir Karnosov.
  3. The evolution of the S-37 configuratons. Defend.
  4. Russian Air Force Faces Cuts To Save Projects. Alexander Velovich. Flight International, 17/09/97, Page 18
  5. Coming Down To Earth - Air Force. Douglas Barrie. Flight International, 06/08/97, Page 35
  6. Mig Mapo Reveals Light Fighter Project. Alexander Velovich.
  7. Russia's Fighter Projects Put West In The Shade. Piotr Butowski. Jane's Defence Weekly, 10/09/97, Page 30
  8. Venik's S-37 estimates
  9. The First pictures of the S-37
  10. Russia Reshapes Things to Come with Radical S-37 Prototype
  11. Roy's Russian Aircraft Resource
  12. S-37 Express-vypusk. Seriya POLYGON
  13. Polet Berkuta. Vladimir Il'in. Aviatsia i Kosmonavtika 1/98
  14. Berkut vyshel na vtoroj etap. Aerocosmicheskie Novosti 16/64
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Last modified on: Tuesday, April 21, 1998.