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RD-855
The RD-855 (Russian: Ракетный Двигатель-855, romanized: Raketnyy Dvigatel-855, lit. 'Rocket Engine 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 (N2O4) oxidizer.[2] 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.[2] 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.[2] 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.[3] 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.[2] The RD-855 is activated two seconds before the main RD-251 engine.[2] 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. Although there were plans to restart its production for the Tsyklon-4, the engine remains out of production following the potential discontinuation of the program.[4] See also
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