The RD-855 (, GRAU index: 8D68M), also known as the RD-68M, is a liquid-fueled rocket engine with four nozzles. As a vernier engine, it provides fine steering adjustments for rockets. It is powered by a hypergolic mixture of unsymmetrical dimethylhydrazine (UDMH) fuel with dinitrogen tetroxide oxidizer. This combination is hypergolic, meaning the two substances ignite on contact, eliminating the need for an external ignition source.
The RD-855 can orient its chambers within a range of ±42° using hydraulic cylinders. Each chamber on the Tsyklon rockets is protected by an aerodynamic cover, and these covers are equipped with small retro-rockets allowing for separation from the second stage.
The RD-855 utilizes the gas generator cycle, in which a portion of the propellant is burned in a pre-burner to produce hot gases that drive the engine's turbopumps, and then the gases are discarded without passing through the main combustion chamber.
It played a key role in rockets such as the R-36, Tsyklon-2, and Tsyklon-3, steering these rockets by adjusting its nozzles. The engine is housed in a cylindrical structure that surrounds the primary RD-251 engine. This arrangement provides protection for the nozzles and ensures the successful detachment of the rocket's first stage. The RD-855 is activated two seconds before the main RD-251 engine.
Manufactured from 1965 to 1992, the RD-855's inaugural flight was on December 16, 1965, aboard the R-36 missile. Its most recent known flight was on January 30, 2009, with the Tsyklon-3.