The Su-35 could revive Iran's Air Force

The Su-35SE's projected purchase aims to revive an air force that is a shadow of its former glory; once the most powerful in the region, the IRIAF is now burdened by a fleet of tired, ageing aircraft. The service's prominence in Iran's military strategy has diminished as a result. Depending on how well the Su-35 is integrated and received by Iran, the 'Super Flanker' could reverse that trend and alter priorities for Iranian military planning going into the future.

The Platform

The Flanker platform on its own is a feat of airframe design. The design requirements for the Flanker called for the perfect air superiority fighter - one with long range, powerful sensors, heavy armament, high (Mach 2+) speed, and excellent agility. Sukhoi delivered in all aspects. The Flanker has a huge airframe to accommodate its massive internal fuel load and engines, as well as ample room for a large and varied array of weapons and sensors. Despite its size and weight, it was the first serial-produced supermanoeuvrable aircraft, owing to advanced aerodynamic features such as a blended body and LERX.

A unique quality of the Flanker design is its shunning of external fuel tanks, especially when compared to western designs. Comparable designs such as the F-15C and F-14 with 13,500 lb (6.1t) and 16,200 lb (7.3t) of internal fuel respectively carry external fuel tanks (EFTs) on practically all missions. Despite EFTs being jettisonable, in practice they are held on the aircraft even during air-to-air combat. This means that even when the weight penalty of extra fuel is gone, EFTs place a drag penalty on the aircraft, restricting it from reaching its advertised performance. This is not to mention the extra Radar Cross Section (RCS) of keeping the tanks on the aircraft.

The Su-27 meanwhile carries all 20,700 lb (9.4t) of its fuel internally, increased to 25,400 lb (11.5t) in the Su-35 - that's about equivalent to an F-15C with all three EFTs. The extra fuel that western fighters carry externally is all carried internally in the Su-35, leading to a much cleaner aircraft in real world missions that can get closer to its advertised performance - and with a lower Radar Cross Section. In fact, the Su-35 has had numerous RCS-reducing treatments including RAM on and around the air inlets and compressor face, as well as an electro-conductive canopy coating. These changes have reportedly reduced the RCS from 10-15m2 to 0.7-2m2.

The Su-35 can use EFTs but they are rarely carried due to its huge internal fuel capacity

Kinematics

Even though the legacy Flanker was already a very high performance aircraft, the Su-35 has earned its 'Super Flanker' moniker in every respect. To that end, the Su-35 has uprated 3D-thrust vectoring AL-41F-1S engines with much greater thrust than the AL-31s found on the Su-27/30s. These allow the Su-35 to supercruise in Beyond Visual Range (BVR) combat, and execute impressive post-stall manoeuvres in Within Visual Range (WVR) combat.

The Su-35's supercruise claim actually comes from Sukhoi chief test pilot Sergey Bogdan, who said that in a test flight the Su-35 accelerated to Mach 1.1 at medium altitude without the use of afterburner. Bogdan also said that the aircraft was still accelerating when they had to end the test due to running out of supersonic testing airspace. Accelerating through the transonic phase of flight (Mach 0.8-1.2) without afterburner is impressive, and suggests that the Su-35 could settle at a supersonic cruise above the transonic range if afterburner is used to push it past Mach 1.2 before being disengaged.

The Su-35's AL-41F-1S engines can deflect ±15° and generate 31-32,000 lbs of thrust

Much has been said about the Su-35's supermanoeuvrability, with detractors pointing out that WVR combat is 'dead' due to the prominence of BVR combat and the HOBS capabilities of modern 'dogfight missiles' such as the AIM-9X. One might therefore question why the US and Russian 5th generation designs - which by nature of having stealth features are more focused towards BVR - both incorporate thrust-vectoring and internal guns. The answer is two-fold; firstly there is still a reasonable probability of air-to-air combat reaching the WVR stage. Secondly, thrust vectoring aircraft are capable of staying close enough to their opponent that they can min-range contemporary dogfight missiles, requiring the use of the gun. A thrust vectoring jet would also be much more likely to get the first shot (in WVR) against a non-TVC jet due to superior nose-pointing ability, though this would come at the cost of airspeed. WVR combat is here to stay at least until unmanned or 6th generation fighters enter the scene.

Sensors, Avionics, and Cockpit

The greatest weakness of Russian fighters has usually been the avionics and radars, and while the systems in the Su-35 are slightly behind their Western counterparts such as the F-15EX (which to be fair is over a decade newer), the gap in capability is far narrower than in the Cold War, especially in export versions. Russia has realised that export customers won't accept severely downgraded products anymore. To that end, the Su-35 has done away with legacy Soviet fighter avionics such as the infamous SPO-15 RWR and replaced them with modern systems.

The Su-35's upgraded systems translate to much more situational awareness provided to the pilot, who sits in a cockpit dominated by two large high definition MFDs. These display all the main tactical and flight information. Their large size makes them well suited to displaying the complex and highly detailed information provided to a pilot in modern air combat, as well as providing growth potential for future missions.

Su-35 cockpit showing prominent 15in MFI-35 displays. Right display shows aircraft and flight information

Claimed Su-35 TSD (Tactical Situation Display)

The main source of tactical information to these MFDs is the Tikhomirov NIIP N035 'Irbis-E', a high-powered PESA with great improvements over the N011 'Bars' radar series in earlier Flankers. The Irbis-E, as with most Russian radars, places great emphasis on emitting power, with average and peak power ratings of 5 kW and 20 kW respectively. This is an attempt to match contemporary AESA radars in range through raw power, overcoming the noise deficiencies of PESAs compared to AESAs. Consequently, the Irbis-E can detect a 3m2 RCS target (roughly equivalent to an F-16, with the F-15 regularly being quoted at 10-25m2) in a narrow-FOV cued search out to a claimed 400km. In a more conventional volume search this drops to 200km. This is still more than enough range to employ R-77s against fighters, and sufficient for the Irbis-E to engage larger tactical aircraft as well as ISR and refuelling aircraft with the R-37M (if purchased).

The Irbis-E has a 900mm phased array that can be mechanically steered by 60° in azimuth and 120° in roll

While not possessing the extreme frequency agility and beam flexibility of AESAs (which are beneficial from an Electronic Warfare perspective, and for simultaneous multimode operation), the Irbis-E is still a fine air defence radar. Being an Electronically Scanned Array (ESA), it shares many advantages AESAs have over Mechanically Scanned Arrays (MSAs), especially electronic beam steering and improved LPI capabilities. The former is the most significant benefit; electronic beam steering allows near-instant scanning (0.4 ms in the Irbis-E's case) of a desired sector, eliminating the need for the radar antenna to be mechanically steered to gather a picture of a given azimuth and elevation. This allows for targets to be acquired quickly and tracks to be maintained with high quality and reliability. Another benefit of the Irbis-E's electronic beam steering (±60 degrees) coupled with the mechanical steering of the array (±60 degrees) is that it allows for very high gimbal limits of the radar, up to a maximum of ±120 degrees. This would allow the Flanker to execute an extreme F-pole manoeuvre (or 'crank') while guiding its missiles. Despite not being an AESA, the Su-35 can track 30 targets and engage 8 with R-77s simultaneously (engaging 4 simultaneously when using R-37Ms at long-range). And with 12 hardpoints (including multiple ejector racks for carrying 2x R-77-1s per compatible hardpoint), it can make full use of that capability.

Supporting the Irbis-E is the OLS-35 IRST, which can detect non-afterburning fighters in a head-on aspect from up to 50km away. This fully passive system (giving no RWR indication) can be used to launch infrared BVR missiles such as the R-27T/ET, and can be of particular use against stealth aircraft or in a highly jammed environment. Rounding out the Su-35SE's situational awareness toolkit is the S-108 datalink, which is encrypted and reported to operate in a similar manner to Link-16. Chinese journalists say the PLAAF has made it compatible with their indigenous AWACS aircraft, which is promising for the potential of integrating it with Iran's IADS.

OLS-35 is the latest evolution of Flanker IRSTs, which Russia has traditionally assigned more importance to than Western designs

The Su-35's HUD is the IKSh-1M, and from limited combat footage - mainly A2A launches in the War in Ukraine - we can see that it displays all the information expected of a modern HUD in those modes. However there has not been much more of the HUD shown to determine if it has more advanced features such as indicating multiple targets or RWR indications. What we do know is that the Su-35 does not have a Helmet Mounted Display (HMD), only the monocle-type NSC-T-04/NSTs-T-04 Helmet Mounted Sight (HMS) designed for acquiring an R-73/74 lock at short ranges. This is adequate for a dogfight - and was revolutionary when the technology was first introduced in the 1980s-90s - but falls short of providing a full HMD which can effectively act as a HUD, showing speed and altitude, a pitch ladder, selected weapons, targeting symbology, and even locked or tracked targets and RWR contacts. In fact, the F-35 is noted for not having a HUD at all, which has been fully replaced by the HMD. Lacking a HMD isn't a massive disadvantage, as all the information relevant for air combat is presented on the HUD and MFDs. But a HMD would provide a great deal of ergonomics, and help the pilot maintain visual situational awareness by not needing to look at their HUD for flight or targeting information, especially during a dogfight. Russia is reportedly developing a HMD for the Su-57, but there is no information on if this will be integrated to the Su-35.

The Ukrainian 'Sura-M', which Russia has replaced with the NSC-T-04/NSTs-T-04. There are no images of the Russian replacement, but it is said to have a similar mode of operation

The Su-35's defensive suite is also improved, with the Soviet-vintage SPO-15 replaced by the SPO-32 (L-150-35 Pastel) RWR. It provides warning from the L-band to Ka-band frequencies, can be used to automatically release countermeasures, and provide targeting information for anti-radiation missiles. Its user interface and display is not shown in cockpit images, but is likely to be much more informative than legacy Soviet systems. Countermeasures can also be controlled by the I-222 SOER electro-optical reconnaissance suite comprised of the SOAR Missile Approach Warning System (MAWS) and SOLO Laser Warning System, the former of which uses six cameras mounted all over the aircraft to detect even passive (e.g. heat-seeking) missiles from up to 50km away.

SPO-32 (L-150-35 Pastel) specifications

SOAR MAWS claimed detection ranges. Note that it is not specified if these ranges are for when the threat rocket motor is still on, which would greatly enhance detection range

Not much is known about the Su-35's EW suite. Not being an AESA, the radar itself can't act as a jammer. The main form of ECM for the Su-35 are the wingtip-mounted Khibiny-M pods. The capabilities of these pods are kept under wraps, but the loss of a similar (but not identical) pod nearly intact in Ukraine may pose a vulnerability in the Flanker's ECM/ECCM capabilities once it has been studied by Western intelligence services.

Weaponry

The primary BVR weapon of the Su-35 is the Active Radar Homing (ARH) R-77-1 (export version RVV-SD), with an improved seeker and 110km range. This is roughly equivalent to the AIM-120C, but there is no R-77 variant in service with similar range to the AIM-120D, which is quoted as having ~160km range. Though the AIM-120C is the most widely used version, the AIM-120D is proliferating, including with exports to countries in the Middle East. Iran should monitor development (and export possibilities) of the R-77M, which meets the AIM-120D in range. In the meantime, Su-35s have been seen carrying the ~300km range "AWACS-killer" R-37M (RVV-BD). The R-77-1 and R-37M have been effective in Ukraine, while the R-27 series (including IR guided T/ET variants) have not been seen in use.

Typical combat load of Su-35s in Ukraine. Here shown with 6 AAMs (2x R-37M, 2x R-77-1, 2x R-73L), and one ARM (1x Kh-31PM). Su-35s have been seen carrying as many as 3x Kh-31 ARMs in the earlier stages of the war. Not visible in this picture are the 'Khibiny' jamming pods on the wingtips.

In WVR combat the Su-35's main weapon is the R-74M (RVV-MD) infrared-guided missile, supplemented by the GSh-30-1 30mm cannon. Though the RVV-MD is a fine dogfight missile with 60-75 degree off-boresight capability and 40km range, it uses an (improved) IR rather than IIR seeker, making it more susceptible to countermeasures compared to its latest competitors. It also lacks LOAL capabilities being added to the latest western dogfight missiles such as the AIM-132 ASRAAM and AIM-9X Blk II. The K-74M2 will have LOAL, but whether it has a IIR seeker is unknown and subject to disagreement between western sources.

The Su-35 can carry a variety of anti-surface munitions including Anti-Radiation Missiles (ARMs), guided and unguided bombs, glide bombs, and cruise missiles. These will not be covered in this post given the Su-35's role once it joins the IRIAF.

Tomcat Successor

Being an air superiority fighter, the Su-35 is more suited to Iran's needs than the multirole Su-30SM. In the face of large regional air forces (to say nothing of the USAF/USN), Iran cannot risk tactical aircraft on offensive missions. Therefore the Su-35 - likely purchased in small numbers - will serve a defensive role for the IRIAF. Important areas of interest will be Iran's nuclear facilities in the centre of the country, as well as other hotspots such as the Persian Gulf. Consequently, Su-35s will likely be collocated with IRIAF F-14s in TAB-8 Khatami (Esfahan), with the possibility of deployments in TAB-1 Mehrabad (Tehran) and TAB-3 Shahid Nojeh (Hamadan). From Esfahan, the Su-35s will effectively take over the F-14's role (though Iran's Tomcats will not be retired anytime soon) by using their large combat radius and powerful radars to provide effective coverage of the whole country. This will be especially important in monitoring lower altitudes, where threats use Iran's mountainous terrain to mask their approach from air defences. If these low-flying threats increase altitude to engage Iranian Su-35s, they would find themselves in the engagement envelope of Iranian SAMs. Su-35s can also protect Iranian SAMs by engaging threats trying to launch standoff weapons from the edge of SAM range. In this manner Iran will hope to use Su-35s in close collaboration with its existing IADS to create a defence that is greater than the sum of its parts.

Possible Su-35 basing, which due to reported small purchase will likely focus on nuclear facilites

The Su-35 may also allow Iran to stage a limited defence against an attack by 5th generation fighters such as the F-35. The most likely source of this threat would be from Israel, whose F-35s would be under significant fuel pressure and unable to sustain air-to-air combat for long. Ground-based VHF radars such as the Matla-ul Fajr 4 (thought to be Iran's version of the Nebo-M, detection range of an F-35 greater than 300km) could provide early warning and cue the Su-35 to use its long-range (but narrow FOV) radar mode, or the OLS-35 IRST. Assuming a 0.005m2 RCS, the Su-35 could detect the F-35 at approximately 80km, though a firing solution would be acquired at a slightly reduced range. The OLS-35 would be of use if the F-35 is forced to engage afterburner.

Keep in mind the above is a very simplistic scenario; in reality Iranian defences would comprise of a wide variety of radars positioned in different locations, dense SAM and AAA protection, and ground-based jammers and passive sensors. Israeli F-35s would make heavy use of jamming, follow optimised routes and altitudes, and would likely be escorted by fighters with an air-to-air load. Su-35s would complicate Israeli plans further by providing an airborne interception platform, whereas previously attacking stealth aircraft would only have to contend with ground-based X-band fire-control radars which may not be able to obtain firing-quality tracks until it's too late.

If purchased in larger numbers, Iran may be able to stage a more robust defence against large scale attacks, or deploy Su-35s in a more proactive role nearer the Persian Gulf. These could help provide cover for Iranian naval forces and island bases, or even vessels conducting operations in the Gulf of Oman and Arabian Sea. With their heavy payload capacity and long range, Su-35s could operate as launch platforms for Iranian anti-ship missiles, though with their air superiority role and probable low numbers they will more likely escort F-4Es operating in the anti-shipping role. Su-35s could also conduct limited Offensive Counter Air (OCA) missions against high value targets such as ISR aircraft, though these would ideally be destroyed on the ground by ballistic missiles. Unfortunately due to financial constraints and Iran's military-political environment, Iran will likely opt for a small purchase, and Su-35s would most likely only be seen in the Persian Gulf during military exercises or peacetime air policing missions, supplementing the usual radio warnings IRIADF controllers give to snooping western spy planes.

Intangibles

Aside from operations of the aircraft itself, there are a number of indirect effects the Su-35 purchase can have on Iran, especially the IRIAF.

The IRIAF's long-running budgetary issues cause greater problems than just dated equipment. The existing fleet has only seen modest upgrades, and most AAM stocks date back to the 1970s-80s. While there's no public data on the average Iranian fighter pilot's annual flight hours, they have likely been kept to a low(er) level to preserve a tired fleet which saw intensive service in the 1980s. This has a negative effect on the readiness and proficiency of Iranian pilots. IRIAF physical infrastructure (with the notable exception of the new Oghab-44 airbase) is at best the same as in the 1980s, at worst crumbling and decrepit, with certain bases such as TAB-3 Shahid Nojeh (Hamadan) showing signs of physical decay on their runways, taxiways and aircraft shelters.

Barely able to keep its existing fleet going, the IRIAF hasn't had the funds for many R&D projects. Development on the F-5 platform and Yasin trainer is relatively low-end when compared to progress made by other services. These projects include modern features like a glass cockpit but are only light combat/training aircraft. As a result the IRIAF is well behind on key technologies used in modern air combat. Isolation from these technologies will certainly have held back IRIAF doctrine and training, which only has access to at best 1980s platforms. Through exposure to the Su-35, the IRIAF would see first-hand the calibre of aircraft it is up against in regional and transregional air forces. After a long period of decline, IRIAF commanders may realise the organisational revamp needed to modernise the force. Higher tier military and political leaders may then be convinced to provide greater technical, financial, and developmental resources to the IRIAF. Thus the Su-35 could serve as a catalyst for wider IRIAF modernisation and its evolution into force fit for the 21st century.

Examples of Su-35 technologies completely new to the IRIAF:

Phased Array Radars
Modern IRST
Medium-range ARH missiles
Helmet Mounted Sight
Thrust Vectoring
Tactical Datalink
MAWS
...and numerous improved avionics and aviation subsystems e.g. modern RWR, ECM/ECCM

The technologies embedded in the Su-35 and its related systems can also have a wider effect on the Iranian defence industry. The engines would be a subject of great interest in the long term, but in the short term the Su-35's avionics, ECM/ECCM equipment, and air-to-air missiles will be studied for any gaps that the Iranian industry has. Iranian development of air-to-air missiles especially has lagged behind modern standards for a long time, suffering from most developments being kept within the underfunded and inexperienced IRIAF, instead of seeking assistance from organisations with superior missile expertise such as the DIO and IRGC.

S-107/108 'Polet' datalink capabilities. The manufacturer claims it operates similarly to JTIDS, MIDS, and Link16. S-108 would be the IRIAF's first known datalink system and could feed air defence radar data directly to the Su-35's MFDs

Sustainment

Iran has experience with high-turnover components such as hydraulics, landing gear, and other general mechanical and airframe components, which will help Iran keep the Su-35 operational in the event of a cutoff of support. The Irbis-E will also be more reliable and easier to maintain than the analogue MSAs found on Iran's current maintenance-intensive aircraft like the F-4 and F-14, purely by virtue of being a modern, digital system.

However, Iran will have trouble sourcing domestic replacements for the greater share of titanium parts used in the Su-35. It would be prudent of Iran to seek technology transfers and cooperation with the aim of internalising Su-35 maintenance and even future upgrades. While the AL-41F-1S engines are relatively reliable - with 4000 flight hours lifetime and MTBO of 1000 hours - their advanced materials and technologies (particularly the thrust-vectoring system) will be a new to Iranian maintenance personnel. Finally, the Su-35's use of RAM will be a challenge for Iran, particularly as it will need replacing over time. Iran would probably receive a supply of RAM for maintenance purposes, and should also look to reverse engineering it.

If the Su-35's purchase is well received, Iran could be encouraged to seek greater technical cooperation with Russia, and even look into making further purchases in a wider gamut of defence products. There is a precedent for this; Iran sought more Russian aircraft after being exposed to some types such as MiG-29s and Su-24s when Iraqi aircraft fled to Iran in the 1990s. Even today there are reports that Iran is interested in the S-400 after buying the S-300, despite Bavar-373 being at the end of its development. Indeed, with western pressure reducing customers for Russian arms, this may be a good opportunity for Iran to tick some important items off its shopping list, such as by fulfilling its reported interest in attack helicopters.

Issues

Though the Su-35 will bring a massive qualitative uplift to the IRIAF, the most prevailing speculation is that Iran will only take up Egypt's cancelled order of just 24-30 aircraft - a reasonable assumption given the IRIAF's usual budgetary issues. This is simply not enough when the size of Iran and the air forces of her enemies are taken into account. Western-style forces send as many as 12-20 aircraft on OCA missions; such a force is half or more of Iran's entire speculated Su-35 order.

This problem is compounded by the proliferation of 5th generation aircraft in the region. Iran knows the value of a generational advantage in fighter aircraft first-hand from its experience in the Sacred Defence where the F-14 Tomcat reigned supreme. Even in that case Iran had more F-14s than the number of Su-35s it is predicted to receive. So adversaries building numerical and generational superiority is a significant concern for Iran that needs to be remedied.

Iran will hope to avoid the product support woes that have plagued its past purchases. Russia last supplied engines for Iran's MiG-29s in 2007, but since UN and Western nuclear sanctions were imposed in 2010 there were no more significant transfers of parts. Military-technical cooperation only recovered after 2015, the S-300 sale being the most public example of such. With the Su-35 being one of its premier export products, Russia will also not want to give the impression that it is an unreliable partner, especially considering the pressure put on their sales by Western sanctions such as CAATSA (ironically this is the same sanction that changed the destination of Su-35s from Egypt to Iran!).

Ever since Washington pressured Egypt to cancel its Su-35 deal, the jets have been sitting idle at two airfields in Russia

Ambiguities

As of now there is still uncertainty surrounding the Su-35 purchase, which has only received limited confirmation. There is no solid information regarding how many aircraft have been ordered, nor if they will be accompanied by support aircraft and infrastructure. Will Russia help integrate the S-108 datalink into Iran's IADS, to support the Su-35s with extra situational awareness? Will force multipliers such as aerial refuelling tankers and crucially, AWACS aircraft be included? And will Russia transfer enough technology (if any) to Iran to facilitate internal maintenance and even integration of Iranian upgrades/munitions?

Supposedly China has managed to integrate the Su-35 into its IADS and make the Su-35 compatible with its AEW aircraft. However due to what the Chinese describe as an 'integrated systems' approach of the Su-35 (a description running counter to the Su-35 sales brochure), this has required considerable Russo-Chinese cooperation. Iran should seek similar cooperation to make the most of the Su-35's capabilities. Sadly, IRIAF budgetary constraints and the slow development of the Beriev A-100 AWACS mean that the IRIAF is unlikely to order this asset despite Iran's challenging terrain posing a critical need for AWACS. Iran may even need to turn to China to supply AWACS designs such as the KJ-500.

NATO ISR aircraft have played a crucial role in the war in Ukraine. AWACS/AEW aircraft here are represented by E-3, E-7, S-100D, and G-550 AEW. AWACS/AEW aircraft would be a critical purchase for Iran, considering blind spots of ground-based radars created by Iran's mountainous terrains

Shortlisted questions from followers

What weapons could Iran integrate into the Su-35?

The easiest weapons to integrate into the Su-35 would be GPS-guided bombs and glide bombs, and LACMs which could be launched with preprogrammed target coordinates. Iran might also want to integrate the Fakour long-range missile, though this is thought to be a SARH missile and inferior to the long range, ARH R-37M which is of similar size and weight. The proposed Maghsoud might have similar performance characteristics to the R-37M, but there has been no news on this missile for a long time and in any case it is unclear how accepting the Irbis-E's software is to non-Russian AAMs. Ideally Iran would receive assistance from Tikhomirov NIIP for integrating weapons if required.

What domestic aircraft projects would be affected?

The Su-35 itself wouldn't directly replace any Iranian aircraft in active development. There has been sporadic talk of a "heavy/semi-heavy fighter" but there is no evidence that this project has gone anywhere past the concepting stage. The Yasin program might be indirectly affected if Iran purchases Yak-130 advanced trainers to accompany the Su-35s. However Iranian officials have only talked about the Su-35 with some mentions of SAMs and attack helicopters (which may affect the Shahed-216 program - but this again has not made much progress). On the whole the Iranian aviation industry will benefit from what it can learn from the Su-35.

Why did Iran choose the Su-35 over Chinese fighters? Will Iran buy more Russian planes in future or Chinese jets as well?

Due to Iran's expansive airspace and experience with the F-14, IRIAF planners favoured a long-range, heavy air superiority platform. With Chinese Flankers unavailable due to IP issues and 5th gens still not on the market, the only Chinese aircraft options would be the J-10 and JF-17. These are both good aircraft for their roles but they do not have the endurance of the Flanker series. Eventually Iran may look to buying 5th generation fighters and at that point there would be no non-political reason why Iran wouldn't buy Chinese fighters (which may be preferable to Russian designs). With the slow but steadily escalating cold war between China and the US, that may become politically viable.

What is the utility of Su-35s in an age of stealth?

Iran is purchasing the Su-35 in a transitional period between 4th and 5th generation fighters, and the Su-35 being a 4.5th gen aircraft instead of the latest and greatest is a symptom of that. With opportunities to buy fighters being few and far between, Iran does not have the luxury of simply waiting for 5th gen fighters to become available and buy nothing in the meantime. That's not to say the Su-35 will be of no use for Iran even in the age of stealth. Once 5th gen fighters are brought in to form the frontline force, Su-35s with their ample payload, range, and (according to Sukhoi) open avionics architecture could perform a variety of ancillary - but critical - tasks, such as carrying oversized or hypersonic weapons, air-launched drones, and even controlling or cooperating with "loyal wingman" type UCAVs. These tasks are far in the future and will require software upgrades, but as the Su-35 is slated to stay in Russian service for the foreseeable future, those upgrades will come.