**1. Introduction**

The history of aviation is inextricably linked to the history of engine development. Throughout its existence, progress in aviation has been based primarily on progress in the development of aircraft engines. In turn, the ever-increasing demands for improving the efficiency of air transportation have been a powerful driver for the development of aircraft engines.

There have been many attempts to create a heavier-than-air, powered aircraft. One of the first aviation pioneers to create such an aircraft was the Russian researcher A.F. Mozhaisky [1]. Between 1881 and 1885, he created an airplane powered by a steam engine. The energy efficiency of the engine was not high, and this, along with other structural specifics, influenced the failure of this project. About 20 years later, American aviators, the Wright brothers, created the first successful airplane [1]. But this aircraft was already equipped with a piston engine. The piston engine provided sufficient energy efficiency for airplane flights at an early stage of aviation development.

In the 30s of the twentieth century, engine engineers developed engines with selfigniting fuel, also known as diesel engines. Thanks to it, an airplane achieved a speed

record of 756 km per hour [2]. But by that time, internal combustion engines had exhausted their energy efficiency reserves.

Problems with the power-to-weight ratio of aircraft required new approaches from engine designers. Since the beginning of the twentieth century, researchers had been developing the theory of gas turbine engines. Scientists from Russia, Great Britain, and Germany were active in this field and were awarded several patents on the principle of operation and design of such engines. The first aircraft to successfully fly with a turbojet engine was the He-178, developed in Germany in 1939. However, the first generation of turbojet engines provided high power but had a very high fuel consumption. This did not allow the use of these engines for commercial aviation. With the development of turboprop and bypass turbojet engines, it became possible to achieve sufficient energy efficiency.

The first aircraft with a turboprop engine was the British Trent-Meteor, which made its first flight in 1945 [3]. The first bypass turbojet engine was the Rolls-Royce Conway, developed in the late 40s of the twentieth century. It was installed on the first commercial airliners such as Boeing 707-420, Douglas DC-8-40, etc. [4]. From that moment on, gas turbine engines became the main engines for civil aviation.

Currently, commercial aviation has several developers of high-performance engines. However, as in the past, existing engines have reached their limits in the new operating conditions. Without the development of new approaches to propulsion, the new conditions proposed by ICAO in the field of environmental protection cannot be satisfied.

ICAO has stated that the amount of harmful emissions must be reduced by a factor of 2 by 2050 [5], and ICAO is also tightening the requirements for noise [6]. Stringent emission reduction requirements can be achieved in several ways. But the main direction that can ensure the reduction of emissions is, of course, the development of new engines with new operating principles. Such engines are: electric, hydrogen and, as an intermediate stage, hybrid. But a separate engine cannot provide the required own efficiency without related subsystems.
