| RD-861 (РД-861) | |
| Country Of Origin: | Soviet Union |
| First Date: | October 27, 1967 |
| Designer: | Yuzhnoye Design Bureau |
| Manufacturer: | Yuzhmash |
| Purpose: | Upper stage |
| Associated: | Tsyklon-3 |
| Predecessor: | RD-854 |
| Successor: | RD-861K |
| Status: | Out of production |
| Type: | liquid |
| Fuel: | UDMH |
| Mixture Ratio: | 2.1 |
| Cycle: | Gas generator |
| Combustion Chamber: | 1 + 4 |
| Nozzle Ratio: | 112.4 (main) |
| Thrust(Vac): | 78.71kN |
| Chamber Pressure: | 8.88MPa |
| Specific Impulse Vacuum: | 317 seconds |
| Burn Time: | Up to 130s |
| Restarts: | 1 |
| Length: | 1.56m (05.12feet) |
| Diameter: | 1.53m (05.02feet) |
| Dry Weight: | 123kg (271lb) |
| Used In: | Tsyklon-2 and Tsyklon-3 third stage |
The RD-861 is a Soviet liquid propellant rocket engine burning UDMH and nitrogen tetroxide in a gas generator combustion cycle. It has a main combustion chamber, with four vernier nozzles fed by the gas generator output. It can be reignited a single time.
When the Soviet military developed the Fractional Orbital Bombardment System, Yangel's OKB-586 proposed a new version of their R-36 ICBM, called the R-36-ORB (GRAU Index: 8K69). It incorporated an orbital warhead called OGCh (GRAU Index: 8F021), for which the RD-854 engine was developed in-house. Since the Outer Space Treaty of 1967 banned nuclear weapons in Earth orbit, but did not ban the launch systems, the Soviet Union proceeded to test their FOBS albeit without placing nuclear warheads in orbit.
There are three versions of this engine: