Staged combustion cycle

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.

The first staged combustion engine was the S1.5400 (11D33) used in the Soviet Molniya rocket, designed by Melnikov, a former assistant to Isaev.

[1] About the same time (1959), Nikolai Kuznetsov began work on the closed cycle engine NK-9 for Korolev's orbital ICBM, GR-1.

The non-cryogenic N2O4/UDMH engine RD-253 using staged combustion was developed by Valentin Glushko circa 1963 for the Proton rocket.

The purchase contract was subsequently taken over by United Launch Alliance (ULA—the Boeing/Lockheed-Martin joint venture) after 2006, and ULA continues to fly the Atlas V with RD-180 engines as of 2025.

[3] Its counterpart in the Soviet shuttle was the RD-0120, which had similar specific impulse, thrust, and chamber pressure, but with some differences that reduced complexity and cost at the expense of increased engine weight.

Preburners that burn a small portion of oxidizer with a full flow of fuel are called fuel-rich, while preburners that burn a small portion of fuel with a full flow of oxidizer are called oxidizer-rich.

The SpaceX Raptor has both oxidizer-rich and fuel-rich preburners, a design called full-flow staged combustion.

Hydrolox engines are typically twin-shaft designs due to greatly differing propellant densities.

[5][4] Benefits of the full-flow staged combustion cycle include turbines that run cooler and at lower pressure, due to increased mass flow, leading to a longer engine life and higher reliability.

Fuel-rich staged combustion cycle. Here, all of the fuel and a portion of the oxidizer are fed through the preburner, generating fuel-rich gas. After being run through a turbine to power the pumps, the gas is injected into the combustion chamber and burned with the remaining oxidizer.
Oxidizer-rich turbine exhaust from a SpaceX Raptor preburner shown during a 2015 sub-system test on a test stand at Stennis Space Center . In the full-flow rocket engine, the preburner exhaust is fed into a turbine and then into the main combustion chamber.
Full-flow staged combustion rocket cycle
SpaceX Raptor FFSC rocket engine, sample propellant flow schematic, 2019