Rostec reports that the excellent AL-51F1 engine has passed all tests successfully, exceeding all expectations. The new engine provides a robust thrust of 11,000 kg and an afterburner capable of reaching 18,000 kg. Thanks to this improvement, the Su-57 can now reach a cruising speed of Mach 2 while significantly reducing its visibility in the infrared range, a decisive advance over its predecessor.
Interestingly, the AL-51F1 not only improves the Su-57 but also revitalizes another promising fighter, the Su-75. According to Russian Deputy Prime Minister Yuri Borisov at the Petersburg International Economic Forum [SPIEF], the Su-75 is scheduled for its first test flight in mid-2025. The initial model of this revolutionary aircraft is currently being built at the Gagarin aircraft factory in Komsomolsk-na-Amure.
AL-51F1 changes classification
Currently, Russian Su-57s are powered by the AL-41F1 engine or its modified version, the AL-41F1S. This important factor led world aviation authorities not to recognize the Su-57 as a fifth-generation fighter. The AL-51F1 arrives, a revolutionary engine that places the Su-57 among the fifth-generation fighters.
The AL-41F1 engine was initially developed for the Mig-1.44 project. Over time, it was upgraded and evolved into the AL-41F1S we see today, used on the Su-35S and later the Su-57. The “experts” maintain that these engines do not allow the Su-57 to reach supersonic cruising speed without afterburning, a characteristic expected of a 5th-generation fighter. [It should be noted that the F-35 also does not reach supersonic cruising speed, but this does not seem to worry anyone.]
However, this is not all. The Su-35C, with 75% fuel on board, can reach Mach 1.15 speed without afterburning. This achievement is on par with the powerful French Dassault Rafale. The Su-57, being a ton lighter than the Su-35S and equipped with a pair of AL-41F1 engines upgraded to increase thrust by 1,400 kg [700 kg each], logically suggests that the Su-57 does not should have trouble reaching a cruising speed of Mach 1.25 – 1.31 without afterburning.
S-shaped air intakes
Over time, the Su-57 fighter went from a basic fighter to a sophisticated piece of machinery. It underwent multiple upgrades to its avionics and various systems and received improvements to its landing gear and fuselage design.
Despite its advances, critics began to express concerns about the lack of S-shaped air intakes, which significantly increase the equivalent radar [EPR] cross-section. However, the Su-57 deploys a different technical strategy to compensate for the absence of these air intakes.
In 2020, an intriguing practice was observed in photos and videos of the fighter: the air intakes were meticulously retouched or completely hidden because the turbine blades were not visible. But that does not mean that they do not exist but are not visible. The experts of the Sukhoi Design Bureau did not neglect the S channel but opted for a different approach due to the specific characteristics of the engines.
Some of the engine’s output is vertical, and the exposed turbine blades are protected by what is known as a coaxial radar jammer [a specific type of grille], similar to that found on the F-117.
Nozzles and turbines
The updated Russian Su-57s are expected to hit the market in 2024 with several improvements. It is not only a new engine but also innovative flat nozzles and ceramic turbines. Last year, Rostec and AUC revealed that tests were underway for this new configuration. Recently, these companies have reported that the test results conform to the initially planned technical specifications.
The first time we heard about the Su-57’s flat nozzles was on a presentation slide. Russian media quickly began referencing this same slide during a United Engine Corporation [UEC] talk, hinting that progress was underway.
The work was not exclusively about flat nozzles; there were more. They discussed the development of high-performance bearings, components called “ceramic turbines,” and cutting-edge electrical systems.
But the conversation doesn’t end there. The speech also focuses on the construction of a three-circuit power plant for the engine. This advance could lead to a notable improvement in effective subsonic fuel efficiency, perhaps an increase of up to 12.5%. Now, that is something to keep in mind.