What are 1st stage and 2nd Stage engines of an airplane?

[Jooyandeh]

TL;DR Summary

In this text, will they work both together or the 2nd stage must replace the 1st stage engine?

Salaam
I was reading an article about a new engine for Sukhoi Su-57 called AL-51F1 with flat nozzle. The text says it is a second stage engine. The engine currently in use is AL-41F1 with circular nozzle. is a first stage engine.
I had two questions:

1. What's the meaning of 1st and 2nd stage engines in airplanes? (I know what they are in rockets)
2. Based on the texts below, will the new 2nd stage engine replace the old engine in the future or will they both work togther?! (the red-colored part of the text confused me a bit. What does it mean by the second engine?!).

The text says:

In terms of propulsion systems, the Su-57 is adapted to use both first and second stage engines. Even with the first stage engine, the fighter meets the basic requirements for a fifth-generation aircraft. Aircraft with the second stage engine are now undergoing flight tests.

If you need to have more information about the subject, here is some other data:

The second stage engine with a flat nozzle and controlled thrust vector for the Su-57 fighter, known as ‘product 30’ or AL-51F1, is undergoing flight tests on a prototype aircraft with the flight number 052 (PAK FA T-50-2). At the same time, the second engine with axisymmetric circular nozzle remains standard – AL-41F1. This is told and shown in the documentary film of Channel One Masters of the Sky, dedicated to the 85th anniversary of the Experimental Design Bureau named after Pavel Sukhoi.The engineering approach to the Su-57 implies modularity of its design, which makes it possible to quickly replace various components, including the propulsion system. As Mikhail Strelets, director of the Sukhoi Design Bureau and chief designer of the Su-57, noted, in the future combat aircraft fitted with the first-stage engines may be equipped with a new flat-nozzle engine, which will significantly improve their combat performance and also reduce the fighter’s visibility in the rear hemisphere.

Thanks in advance for the help.

 

[jack action]

It is "stage" as in "development stage" and there are two versions of the engines, namely the AL-41F1 (first engine) and the AL-51F-1 (second engine):

While the current Su-57 production tranches are powered by the AL-41F1, the aircraft is to be the basis for a family of stealth combat aircraft; future improved variants are planned to be powered by the Saturn izdeliye 30, later designated AL-51F-1, a new design that fits into the same footprint as the AL-41F1.

The Su-57 is powered by a pair of NPO Lyulka-Saturn izdeliye 117, or AL-41F1, augmented turbofans. [...]

The aircraft employs thrust vector control (TVC) where the vectoring nozzles' rotational axes are each canted at an angle, similar to the nozzle arrangement first employed on the Su-30MKI and also used on Su-35S. The nozzles themselves vector in only one plane; the canting allows roll and yaw moments by vectoring each nozzle differentially, thus enabling the aircraft to produce thrust vectoring moments about all three aircraft axes, pitch, yaw and roll. The engine inlet incorporates variable intake ramps for supersonic efficiency and retractable mesh screens to prevent engine damage from foreign object debris ingestion especially when operating in short, austere runways. In 2014, the Indian Air Force openly expressed concerns over the reliability and performance of the AL-41F1; during the 2011 Moscow Air Show (MAKS-2011), a Su-57 suffered a compressor stall that forced the aircraft to abort takeoff.

The planned Su-57M will be equipped with a new engine from NPO Saturn in the mid-2020s under the development designation izdeliye 30 and eventually designated AL-51F-1. The powerplant is designed with an estimated thrust of 11 tonnes (107.9 kN, 24,300 lbf) dry and 17 tonnes (167 kN, 37,500 lbf) in afterburner. In addition to improved performance, reliability, and costs compared to the AL-41F1, the AL-51F-1 will also reduce the aircraft's radar and infrared signature with glass-fibre plastic IGVs and a new nozzle with serrated flaps. In 2023, it was reported that UEC Saturn is developing an alternative non-axisymmetric "flat" nozzle for the Su-57; flight testing began in late 2024. Because the flat nozzle was requested well after Sukhoi had finalized the Su-57 design, Saturn designed the nozzle to fit with minimal changes to the airframe.

From the latter quote, there are even 2 stages of the AL-51F-1, with different nozzles. The second stage engine will have a 'non-axisymmetric "flat" nozzle'.

With this info compared with your last quote, the 'axisymmetric circular nozzle' of the first stage engine AL-51F-1 (being carried over from the first engine, the AL-41F1) will remain "standard" with the second engine, i.e. the AL-51F-1; meaning the "second stage" nozzle design should be available upon request.

Anyway, that is how I read it.