It was the PAK FA. It was the T-50. Now it’s the Su-57.

It’s Russia’s answer to the United States’ cutting-edge F-22 “Raptor” stealth fighter.

Now, more than 15 years after the US fighter entered service, Russia has pitted the best its military aviation industry can offer against its rival .

F-22 versus Su-57: It’s the ultimate face-off between East and West.

And their appearance in Syria came amid an increasing number of aggressive stand-offs between Russian and US air force tactical aircraft.

At least two of the still experimental combat jets landed at the Russian Khmeimim air base in Latakia province on the Mediterranean coast earlier this year.

This was a surprise.

About 11 of the aircraft exist, each representing a different stage in the stealth fighter’s development over the past ten years.

Some Western military analysts had thought only two of them had sufficiently completed electronics, sensors and avionics to make them combat capable.

Only one is believed to have new engines capable of giving it true fifth generation capabilities, including “super cruise” — the ability to idle at high speeds over long distances.

But the Su-57 is a secret program.

Surprises should be expected.

Much of what we know is, at best, speculation. The rest may well be little more than propaganda.

So here’s what we think we know.

THE HUNTER

Moscow has placed an order for an initial production run of 12 Su-57 stealth fighters. Those that it already has may be upgraded to combat standards, or remain as technology test beds.

None were expected to achieve operational status before 2019.

But up to four are set to become active on the frontline in Syria.

From the outset, the Su-57 was designed to be an F-22 killer. To achieve that goal it has been equipped to find its agile and fast opponent in the open skies by overcoming their expensive stealth properties.

It has all the mod-cons. Data linked and integrated sensors. Electronic warfare components. Advanced composite construction materials.

Likewise, the US F-22 was built to present a minimal radar reflection.

This does not make it ‘invisible’. It just significantly reduces the distance at which any radar signal is strong enough to ’see’ it.

So Moscow gave its Su-57 as many different kinds of ‘eyes’ as possible.

It has radar antennas of various frequencies scattered about the aircraft. These can sense, and pinpoint, active radars from all directions. And its powerful Active Electronically Scanned Array N036 Byelka transmits on more sensitive frequencies than most.

It’s generally expected that if the Su-57 and F-22 wander within 50km of each other, they will detect each other.

This is yet to be seen.

But the Su-57 doesn’t just rely on using its opponent’s radar against it, or blasting the skies with its own electronic pulses. It has a wideranging infra-red search-and-track system looking for telltale traces in the skies — be it an engine’s exhaust, or even the friction of a wing cutting through the air. The F-22 as yet has no such ability.

It even has an electrically-enhanced optical sensor (essentially a glorified telescope) in its front cockpit. This enables the pilot to use the Mk1 Eyeball to locate a stealth fighter even if his sensors have provided little more than a general direction.

THE HUNTED

Exactly how stealthy the Su-57 is itself is a cause of much heated debate.

India, which had engaged in a long-term development contract with Russia for a variant of its own, appears to have its doubts. The cooperative program is in trouble. Instead of buying 108 of Moscow’s finest, New Delhi appears to want out.

Among the problems cited is the effectiveness of its stealth features, as well as integrating its avionics and weapon systems with Indian requirements.

It has since asked the US to provide it with details of the F-35 Strike Fighter, with the potential for a future purchase in mind.

Stealth profiles are closely guarded secrets. But the F-22 is believed to have a reflective area of just a fraction of a millmetre. Conversely, the Su-57’s reflection is believed to be measured in centimetres.

But like all military weapons, the Su-57 is a balance of compromises.

And its mission determines that balance.

It’s required to be fast. It’s required to have range. It’s required to be supermaneuverable. It’s required to have an extensive load of internal weapons.

This makes the aircraft bigger. Therefore, there is more surface area that stealth technology must negate.

Some military analysts argue the Su-57 has been deliberately optimised to be mostly invisible from its front profile. This is because it is a hunter, not wanting to be seen as it tracks down its prey.

But once it turns away, a weaker stealth profile is presented to those trying to find it.

SEE FIRST, SHOOT FIRST

Any potential weakness in the stealth arena may help explain the Su-57’s emphasis on long-range weaponry.

Sukhoi boasts their fighter’s internal payload bays can house anything from next-generation hypersonic weapons through to the best available long range air-to-air missiles.

That such big missiles can be carried internally — unlike with the F-35 — is a bonus for the Russian jet’s stealth ability.

The F-22 can also carry big anti-air missiles inside. And it appears to carry six of them over the Su-57’s four. In the Russian’s favour is the K-77M’s range — at more than 200km. The US weapon (AIM-120D) can travel about 160km.

While this means older aircraft such as US F-15 and Russian Su-35s can be blasted out of the sky without ever knowing there was another aircraft out there, it’s a different matter when it comes to stealth-versus-stealth.

The idea is to be invisible at long or medium ranges. And in close, the Russian pilot can unleash nimble infra-red guided missiles simply by looking at their target.

But analysts point out much of the Su-57’s arsenal appears designed to pick out and shoot-down key support aircraft, such as lumbering AWACS and mid-air-refuelling tankers.

Simply put, it means the Su-57 has the potential to supercruise (fly faster than Mach 1 without the use of fuel-guzzling afterburners) unnoticed to within launching range against such high-value targets.

Without air-to-air refuelling and the broad sweep radar picture relayed by AWACS, stealth pilots will find themselves flying blind over much shorter distances than anticipated.

It is scenarios such as this that can quickly change the course of a war.

HEART OF THE MATTER

Russian-built aircraft engines have long had a reputation for unreliability and needing extensive maintenance.

According to Indian reports, the Su-57 is no different.

One of their complaints was the design of the fighter made it difficult and time-consuming to service

Engine troubles have been the Su-57’s weakest link in claiming true fifth generation status. But Moscow believes this is about to change.

The next model, the Saturn Izdelie-30, is said to be 30 per cent lighter, have greater thrust and fuel efficiency — and fewer moving parts for improved reliability.

It’s already been fitted to at least one Su-57 prototype. It began testing late last year.

Unlike the troubled existing engine, it is said to be true fifth generation fighter technology. It can propel the stealth fighter at up to 2000km/h without the need for afterburner.

This conserves fuel. It also reduces the enormously visible heat flare an afterburner produces.

The Su-57’s future may depend on the success or failure of this new engine.

But in one aspect, the Russian stealth fighter’s engines are already well ahead of its US rivals.

It has what is called three-dimensional vector-thrust. It’s a wordy way of saying the engine’s nozzles can twist to point in any direction.

The US Raptor has nozzles that operate in only two dimensions — up and down. This makes it the US air force’s most manoeuvrable fighter.

The Su-57 is better.

But it comes at a cost — greater radar and infra-red visibility.

Doubts surround the value of being a ‘supermaneuverable’ fighter in the modern age. They’re not expected to get into a turn-and-burn gunfight.

In a stealth versus stealth scenario, however, just such a close encounter does appear much more likely. And the extra ability to jink and twist could give the Russian jet an edge when it comes to dodging missiles.

AERIAL ASSASSINS

Moscow itself seems to have doubt about the Su-57’s potency. It initially planned to buy 150 of the aircraft before 2020.

That’s since been cut back to just a dozen examples.

Technical problems have beset the program from the outset.

Engines — and aircraft — have caught fire. Manufacturing suitable quantities of advanced materials, such as composite carbons, at suitable quality is no easy task.

And Russia has lost a lot of the cash necessary to fix them due to sanctions over its invasion of Crimea along with low world oil prices.

Only the final three of the 11 Su-57 airframes so far delivered to Russia are believed to have all the stealth features — composite materials and integrated sensors — needed to make them fully operational. Only the ninth is thought to have been retrofitted with the new engine.

The United States has more stealthy fighters. These could be able to sneak to within the effective reach of their shorter-ranged weapons — if they have the AWACS and tanker support they need.

But Russia’s less stealthy Su-57 doesn’t need to get as close. It can stand off, unobserved, before unleashing a deadly volley of very fast, long-range weapons. Moscow hopes new low-frequency band radars being used by new anti-air systems such as the S-400 will detect roughly where the F-22’s are before the Su-57 makes its move.

Which scenario works best depends on one thing: Is the fighter’s radar cross-section small enough to allow it to do its job?

This could be what Moscow aims to test in the skies above Syria.

It has already been pitting its most advanced anti-aircraft missile and sensor system, the S-400, for the past year against the F-22.

Now the Su-57 will be able to measure its mettle against the very aircraft it was designed to destroy.

WICKED GAME

The Israeli Eros B satellite at the weekend captured at least two Su-57 stealth fighters on the ground at Russia’ Khmeimim air base in Syria’s northern Latakia province. A separate unconfirmed aerial image appears to show a third, and some witness accounts say they saw four.

This is possible. Russia’s combat aircraft are usually deployed in groups of four.

However, the second photograph may just be of one of the first two aircraft after being repositioned in a more secure spot.

Chairman of Moscow’s Military Industry Committee Vladimir Gutenov told media on Friday that he would not confirm the deployment of the Su-57, but that he “wholeheartedly welcomed” the idea as they “need to be tested in combat conditions, in conditions of resistance.”

This follows Russian Deputy Defence Minister Yuriy Borisov saying earlier this month the fighter had completed its first phase of testing, and that it was now ready for combat trials.

This could be them.

The potential for an encounter between the Su-57 and F-22 is now very real.

And Israel is currently operating nine F-35 Lightning II stealth strike fighters on home soil.

And US F-22s in Syria have already hit the headlines in the past year.

It’s been involved in several ‘close encounters’ with Russian combat jets straying across the Euphrated demarcation line between Coalition and pro-Syrian air operations.

Moscow has boasted its older Su-35 ‘scared off’ F-22s during these stand-offs last year. The US air force tells a different story.

But tensions were ratcheted up a notch just last week when two F-22’s repeatedly intercepted Russian aircraft crossing the Euphrates ‘deconfliction line’. The aggressive probes came after a clash between US forces and Moscow-backed mercenaries. The mercenaries were reportedly ‘wiped out’.

Any high-tension encounter between stealth fighters will have an added layer of confusion given their ‘low visibility’.

But that’s not the only risk Russia’s stealth fighter faces.

It is unlikely to suffer the fate of the Su-25 ‘Frogfoot’ that was recently shot down by a shoulder-launched missile. Nor is it likely to be detected and shot-down as the Su-24 ‘Fencer’ was by a Turkish F-16 in 2015.

Instead, the threat comes from a much smaller, much cheaper, and much less complex direction.

Drones.

Syrian rebels have already applied this ‘asymetric warfare’ approach on Russia’s Khmeimim air base. Flock of drones carrying little more than hand grenades and mortar shells struck the base in December and January.

Moscow claims they did no damage as they were all successfully jammed or shot down.

But should even one Su-57 be damaged by such a strike, the cost to Moscow’s development program — and prestige — will be enormous.

Video from Russia’s Ministry of Defence / United Aircraft Corporation / US Air Force promotional material.