First Look at the Stealth Optimised AL-51F-1 Engine Powering Russia’s New Su-57M Fighter

<p >The first images of the new flat nozzle AL-51F-1 engine developed for an upcoming enhanced variant of Russia’s Su-57 fifth generation fighter have been released, with images on December 10 showing one of the engines fitted onto the fighter program’s second flight prototype. Although photographs of prototypes of the AL-51F-1 have been released in the past, the release of the first image of the engine with its final intended nozzle design, which significantly contributes to stealth capabilities, have never before been seen. Confirmation that the engine’s development has reached this advanced testing stage indicates that Russian sources’ projections that the engine will enter service before 2027 may be credible. Although the Su-57 first entered service in 2020, with the scale of production having since <a href=" >grown rapidly</a>, the aircraft currently rely on the AL-41F-1 engine, a derivative of the AL-41F-1S that currently powers the Su-35 ‘4++ generation’ fighter. Fighters equipped with the new AL-51F-1, and benefitting from a number of other enhancements, were according to Russian state media reports in 2021 <a href=" >scheduled to replace</a> the baseline variant in production in 2025, although it remains uncertain whether disruptions to Russian industry have affected this schedule. The enhanced new variant of the fighter will be designated the Su-57M.</p><p ><img src=" title="Su-57 with AL-41F-1 Engines Upon Delivery to the Russian Air Force in November 2024"></p><p >Integration of the AL-51F-1  is expected to improve stealth, facilitate high speed supercruise, reduce operational costs and maintenance needs, increase power available for onboard subsystems such as radars, and significantly improve range and all aspects of flight performance. The engine is speculated to provide the Su-57 with the highest thrust levels of any fighter in service worldwide, if not second to the Chinese J-20 which is expected to soon integrate <a href=" target="_blank">WS-15 next generation engines</a>. The integration of a new variant of the Al-51F-1 with flat nozzles, which helps minimise the rear radar cross section, is one of multiple incremental upgrades that have helped to <a href=" the Su-57’s stealth capabilities, with new air intakes and the use of new radar absorbent materials in the airframe having been reported. The Russian Air Force has demonstrated high confidence in the Su-57’s stealth capabilities, and has <a href=" target="_blank">deployed the fighters</a> for high intensity combat operations to parts of the Ukrainian theatre with particularly high concentrations of Ukrainian air defences.  </p><p ><img src=" title="AL-51F-1 on Su-57 Second Prototype" ></p><p >Despite significant enhancements integrated over time, the Su-57 notably places a far lower emphasis on stealth capabilities than Chinese or American fifth generation fighters, and was not designed for penetration flights deep into enemy territory. It was instead intended to retain a high degree of survivability while operating behind and in tandem with a screen of ground based air defence systems. The Su-57 program has prioritised reducing maintenance needs and operational costs to avoid the very low availability rates that have plagued America’s F-117, F-22 and F-35 stealth fighter fleets. One notable means by which this had been achieved is through reduced reliance on radar absorbent coatings, which was achieved by using innovative solutions such as <a href=" >radar absorbent fibreglass</a>. In contrast to American stealth fighters which consistently cost far more to operate than their fourth generation predecessors, this approach allows the Su-57 to potentially achieve lower operational costs than its <a href=" target="_blank">direct predecessor the Soviet Su-27</a>, thus allowing Russia to move its fleet into the fifth generation without either significantly raising sustainment funding or contracting the number of fighters in service. </p>