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.
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.
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.
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.
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)
. . . 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).
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.
The throttle, dash and side-stick of Sukhoi S-37
The type of the radar intended for S-37 is not known. The size of the random 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.
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.