The RD861K engine is intended for thrust creation and flight control of LV third stage along the active leg of trajectory in pitch and yaw channels. The thrust vector control is effected by gimballing the combustion chamber in two orthogonal planes.

The engine is designed on the basis of the high-reliable serial RD-861 engine used on the Cyclone LV third stage.
Utilization of basic components of the previous engine without their principal alterations ensure high performance and reliability.
The RD861K engine has the following advantages in comparison with its forerunner:
- the engine's specific impulse is increased by 15.6 s;
- number of restarts in flight is increased;
- the burn time is increased by 3 times;
- the thrust vector control system is fed by high-pressure fuel;
- the mixture ratio is ensured with accuracy of ±1,0%.

Consumer's characteristics of RD-861K engine

The one-chamber pump-fed with hyperholic propellants engine of tripe run is designed under the generator scheme.
The turbine's working medium is a gas produced by the gas generator through combustion of main propellants. During the startup the TPU's rotor is spinned-up by helium. The latter is stored in a high presse vessel of the propellant system.
The control units function with helium delivery into their pilot cavities after action of respective electropneumatic valves. Helium is delivered from a pressure vessel installed on the engine and charged in the course of LV flight from a stage vessels.