KVD-1 (left) with its precursor 11D56

KVD-1’s prototype known as 11D56 was developed between 1965-1972 by Khimmash Design Bureau under a work statement issued by S.P.Korolyov’s OKB-1 Bureau. It was initially designed for the fourth stage of a future version of heavy Lunar N-1 launch vehicle. Bench trials of the engine commenced in 1966. KVD-1 engine is a single-chambered unit with a turbopump system designed to feed propellants; KVD-1’s design includes afterburning: a feature characteristic of any powerful Russian liquid-propellant rocket engine design. Yet, while a gas generator producing oxidizing turbine gas (the so-called sour gas, i.e., containing excess oxygen) is used to drive turbopump assemblies of Russian non-hydrogen close-loop engines, the case is different with KVD-1’s gas generator, where the propellant is burned with excess fuel to produce reducing (the so-called sweet) gas.