| RD-856 (РД-856) | |
| Country Of Origin: | Soviet Union |
| First Date: | December 16, 1965 |
| Last Date: | January 30, 2009 |
| Designer: | Yuzhnoye Design Bureau |
| Manufacturer: | Yuzhmash |
| Purpose: | Vernier |
| Associated: | R-36, Tsyklon-2 and Tsyklon-3 |
| Status: | Out of production |
| Type: | liquid |
| Fuel: | UDMH |
| Mixture Ratio: | 1.98 |
| Cycle: | Gas generator |
| Combustion Chamber: | 4 |
| Thrust(Vac): | 54.23kN |
| Chamber Pressure: | 7.16MPa |
| Specific Impulse Vacuum: | 280.5isp |
| Burn Time: | Up to 163s |
| Restarts: | 1 |
| Gimbal: | ±30° |
| Length: | 0.9m (03feet) |
| Diameter: | 3.35m (10.99feet) |
| Dry Weight: | 112.5kg (248lb) |
| Used In: | R-36, Tsyklon-2 and Tsyklon-3 second stage vernier |
The RD-856 (GRAU Index 8D69M), also known as the RD-69M, is a four-nozzle liquid-fuel rocket vernier engine, burning N2O4 and UDMH in a gas generator cycle. It was used on the R-36, Tsyklon-2 and Tsyklon-3 second stage as thrust vector control by gimbaling of its nozzle. The engine is distributed through a cylindrical structure that is integrated around the main engine RD-252 module. The structure includes aerodynamic protection for the nozzles. The engine was started by a pyrotechnic ignitor.
The engine was serially produced between 1965 and 1992. It was first launched on December 16, 1965 on an R-36 and its last launch was on January 30, 2009 with the last launch of the Tsyklon-3. The production capability was restarted for the Tsyklon-4 but with the apparent cancellation of the program the engine would still be out of production.