| Country of origin | Soviet Union |
|---|---|
| First flight | December 16, 1965 |
| Last flight | January 30, 2009 |
| Designer | Yuzhnoye Design Bureau |
| Manufacturer | Yuzhmash |
| Application | Vernier |
| Associated LV | R-36, Tsyklon-2 and Tsyklon-3 |
| Status | Out of production |
| Liquid-fuel engine | |
| Propellant | N2O4 / UDMH |
| Mixture ratio | 1.97 |
| Cycle | Gas generator |
| Configuration | |
| Chamber | 4 |
| Performance | |
| Thrust, vacuum | 328 kN (74,000 lbf) |
| Thrust, sea-level | 285.4 kN (64,200 lbf) |
| Chamber pressure | 6.570 MPa (952.9 psi) |
| Specific impulse, vacuum | 292 s (2.86 km/s) |
| Specific impulse, sea-level | 254 s (2.49 km/s) |
| Burn time | Up to 127 s |
| Restarts | 1 |
| Gimbal range | ±41° |
| Dimensions | |
| Length | 1.74 m (5 ft 9 in) |
| Diameter | 3.81 m (12.5 ft) |
| Dry mass | 320 kg (710 lb) |
| Used in | |
| R-36, Tsyklon-2 and Tsyklon-3 first stage vernier | |
| References | |
| References | [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]
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