RD-861

The RD-861 () is a Soviet liquid propellant rocket engine burning a hypergolic mixture of unsymmetrical dimethylhydrazine (UDMH) fuel with dinitrogen tetroxide () oxidizer 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.

==History== 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.