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RD-859

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RD-859 Credit: via Dietrich Haeseler

Status: Hardware. Date: 1964-72. Thrust: 20.05 kN (4,507 lbf). Unfuelled mass: 57 kg (125 lb). Specific impulse: 312 s. Burn time: 400 s. Height: 1.09 m (3.57 ft). Diameter: 1.10 m (3.62 ft).

The RD-859 engine was used as the backup engine for the Soviet LK manned lunar lander in the event the primary RD-858 engine failed during landing on the lunar surface or ascent and return to lunar orbit. The engine was capable of a restart within three seconds after shut down. Testing of the engine on the ground and in earth orbital tests confirmed the high reliability of the design.

The autonomous twin-chamber single-mode double-start pump-fed engine featured deep thrust throttling using hypergolic propellants and used an open generator scheme (using the main engine propellants). During engine startup the turbine's rotor was spun up by a pyrotechnic device. The inlet valves were opened using gas tapped from the manifold of the pyrotechnic device. The valves were operated by fuel pressure delivered from the pump's discharge branch. Engine shutdown was performed by pyrovalves under command of the control system.

The engine was designed for a total burn time of 400 seconds. This included as little as 3 seconds in full-thrust mode during the first burn; followed by up to 400 seconds of full thrust during the second firing. The full engine installation on the LK consisted of the RD-858 main engine and the RD-859 back-up engine:

Engine                      RD-858                   RD-859
No of chambers:                1                      2
Operating regime    nominal         throttled            
Thrust in vacuum    2050 kg         858 kg           2045 kg
                    20104 N         8414 N           20055 N
spec. vac. impulse  315 s           285 s            312 s
                    3089 Ns/kg      2795 Ns/kg       3060 Ns/kg
Mixture ratio O/F   2.03            1.6              2.0
Thrust variation    +/- 9.8 %       +/- 35 %         +/- 9.8 %
Mixture ratio deviation        3 %                   3 %
Chamber pressure    80 kg/cm^2      33.8 kg/cm^2     80 kg/cm^2   
                    78.5 bar        33.15 bar        78.5 bar
Nozzle exit pressure 0.07 kg/cm^2    0.021 kg/cm^2    0.07 kg/cm^2
Operation time      50 + 350 s      100 s            400 s
Propellants               N2O4 / UDMH                N2O4 / UDMH
Engine mass                  53 kg                   57 kg
Propulsion system height                  1090 mm
Propulsion system width                   1102 mm

Chambers: 2. Engine: 57 kg (125 lb). Chamber Pressure: 78.50 bar. Thrust to Weight Ratio: 35.87. Oxidizer to Fuel Ratio: 2.