The staged combustion cycle (sometimes known as topping cycle, preburner cycle, or closed cycle) is a power cycle of a bipropellant rocket engine. In the staged combustion cycle, propellant flows through multiple combustion chambers, and is thus combusted in stages. The main advantage relative to other rocket engine power cycles is high fuel efficiency, measured through specific impulse, while its main disadvantage is engineering complexity. Typically, propellant flows through two kinds of combustion chambers; the first called preburner and the second called main combustion chamber. In the preburner, a small portion of propellant, usually fuel-rich, is partly combusted, and the increasing volume flow is used to drive the turbopumps that feed the engine with propellant. The gas is then injected into the main combustion chamber and combusted completely with the other propellant to produce thrust. Tradeoffs The main advantage is fuel efficiency due to all of the propellant flowing to the main combustion chamber, which also allows for higher thrust. The staged combustion cycle is sometimes referred to as closed cycle, as opposed to the gas generator, or open cycle where a portion of propellant never reaches the main combustion chamber. The disadvantage is engineering complexity, partly a result of the preburner exhaust of hot and highly pressurized gas which, particularly when oxidizer-rich, produces extremely harsh conditions for turbines and plumbing. History Staged combustion () was first proposed by Alexey Isaev in 1949. The first staged combustion engine was the S1.5400 (11D33) used in the Soviet planetary rocket, designed by Melnikov, a former assistant to Isaev. About the same time (1959), Nikolai Kuznetsov began work on the closed cycle engine NK-9 for Korolev's orbital ICBM, GR-1. Kuznetsov later evolved that design into the NK-15 and NK-33 engines for the unsuccessful Lunar N1 rocket. The non-cryogenic N2O4/UDMH engine RD-253 using staged combustion was developed by Valentin Glushko circa 1963 for the Proton rocket. After the abandonment of the N1, Kuznetsov was ordered to destroy the NK-33 technology, but instead he warehoused dozens of the engines. In the 1990s, Aerojet was contacted and eventually visited Kuznetsov's plant. Upon meeting initial skepticism about the high specific impulse and other specifications, Kuznetsov shipped an engine to the US for testing. Oxidizer-rich staged combustion had been considered by American engineers, but was not considered a feasible direction because of resources they assumed the design would require to make work. The Russian RD-180 engine also employs