I'm surprised nobody else is talking about the "trombone" engine design of this thing.

Initially, the aircraft was supposed to be equipped with one engine with a variable bypass ratio. At that time, the idea of ​​creating a multi-mode aircraft received great attention. Distribution. Variable-bypass turbofan engines ("twin-pipe") made this possible. The turbofan engine with flat nozzles and a variable bypass ratio was developed at the Rybinsk Engine Design Bureau, chief designer - P.A. Kolesov. The engine passed bench tests, and various sources indicate a thrust of 18,500 to 23,500 kg. Perhaps these are different modifications. Strictly speaking, for the T-12 project, the engine thrust was assumed to be 18,500-19,000 kg at maximum thrust and about 11,000 kg at nominal thrust. At the Sukhoi Design Bureau, this engine was called "Trombone." A little more detail about the twin-pipe engine, all information taken from the internet.

The operating principle of such an engine is as follows. The main module forces atmospheric air through a common air intake. At maximum bypass ratio, all the air enters the main module, where it is divided into two streams: one goes into the combustion chamber, the other bypasses it. To reduce the bypass ratio, special dampers behind the first stages of the main compressor open and some air is bypassed into the second module. Simultaneously, the exhaust device of the second circuit of the first module closes. Thus, both modules of the turbofan begin to operate in single-circuit mode, providing the necessary thrust."

In more detail, the upper engine is a dual-circuit engine with a bypass ratio of more than 1, i.e., a turbofan. In the figure, number 1 is not a compressor, but rather a fan. It is not located immediately in front of the compressor, as in a turbofan, but slightly shifted forward. When necessary, the upper engine operates as a dual-circuit engine (turbofan), the lower engine does not operate at all, and the air duct to it is blocked. The fan forces air only into the upper engine. In this mode, fuel consumption is minimal, producing 9-11 tons of thrust.

To increase thrust, the air duct for the lower engine opens, and it begins to operate. The fan forces air into both the upper and lower engines. The lower engine, unlike the upper one, is a classic turbojet, without a secondary circuit. At the same time, the secondary circuit of the upper engine is shut off, and it also becomes a turbojet, not a turbofan. In total, we have two operating turbojets with a common air intake and a fan within it. Thrust in this mode can reach 19 tons. It is believed that the twin-tube engine was called the RD-179 or RD-179-300, but it never entered production, and its name was transferred to the production engines, the T-12-1 03.