Staged combustion cycle
The staged combustion cycle, also called topping cycle or preburner cycle, is a thermodynamic cycle used in some bipropellant rocket engines. In staged combustion, one propellant is sent through a preburner and partially burned using a small portion of the second propellant. The resulting hot gas is used to power the engine's turbines and pumps, then injected into the main combustion chamber along with the remainder of the second propellant to complete combustion. There are two main variants of the cycle depending on which propellant is sent through the preburner: oxidizer-rich staged combustion (ORSC) and fuel-rich staged combustion (FRSC).
The advantage of staged, or "closed", combustion is that all of the cycle's gases and heat go through the combustion chamber. (An alternative design, called the gas-generator cycle, is an "open" cycle where the gas used to drive the turbopumps is exhausted overboard without passing through the main combustion chamber, reducing efficiency.) Since no propellant is dumped overboard, staged combustion allows for high power turbopumps which permit very high chamber pressures, allowing the use of high expansion ratio nozzles at low altitude for better performance. Disadvantages of staged combustion include harsh turbine conditions, exotic plumbing to carry the hot gases, and complicated feedback and control. In particular, ORSC requires advanced metallurgy due to extremely corrosive oxidizer-rich gas, while FRSC requires use of a fuel that will not coke (form solid carbon byproducts from incomplete combustion), such as liquid hydrogen.
Staged combustion (Замкнутая схема) 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 around 1963 for the Proton rocket.
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