The RD-866 engine of a space tug is installed in the rocket payload bay and intended for the following:
- fuel delivery into the steering unit used for the rocket flight control in the flight end of the first and flight of the second and third stages;
- creation of roll forces during third the stage flight;
- creation (at the leg of spacecrafts distribution) of thrust and steering forces various in level and direction, during multiple runs of the main LRE and liquid thrusters;
- fuel delivery into hydraulic actuators for gimballing the main engine's chamber at the leg of spacecrafts distribution.

- Multifunctional engine;
- Multiple restarts and thrust regulation;
- Provision of consumers with propellants within a broad range of flowrates and pressures.

Consumer's characteristics of RD-866 engine

The multifunctional non-regulated engine operates under the generator scheme with multiple activation of both main engine and thrusters.
Propellants are fed under a combined scheme (expulsion and pump delivery). Broad range of flowrates and pressures is ensured for consumers.
The engine involves:
- centralized supply source composed of two turbopumps with gas generators and two feeders;
- one-chamber main LRE;
- 16 liquid thrusters.