RD-855

N₂O₄ / UDMHMixture ratio1.97CycleGas generatorConfigurationChamber4PerformanceThrust, vacuum328 kN (74,000 lbf)Thrust, sea-level285.4 kN (64,200 lbf)Chamber pressure6.570 MPa (952.9 psi)Specific impulse, vacuum292 s (2.86 km/s)Specific impulse, sea-level254 s (2.49 km/s)Burn timeUp to 127 sRestarts1Gimbal range±41°DimensionsLength1.74 m (5 ft 9 in)Diameter3.81 m (12.5 ft)Dry weight320 kg (710 lb)Used inR-36, Tsyklon-2 and Tsyklon-3 first stage vernierReferencesReferences^[1][2][3]

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 combination of N₂O₄, an oxidizer, and UDMH, a propellant.^[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

• R-36 – The Soviet ICBM for which the RD-855 was created.
• Tsyklon-2 – A Soviet small rocket that uses the RD-855.
• Tsyklon-3 – A Soviet small rocket that uses the RD-855.
• Tsyklon-4 – A Ukrainian small rocket project that would have used the RD-855.
• Yuzhnoe Design Bureau – The RD-855 designer bureau.

References

External links

• Yuzhnoye Design Bureau English-language home page Archived 2021-06-29 at the Wayback Machine
• Yuzhmash Home Page