Thestaged combustion cycle (sometimes known astopping cycle,preburner cycle, orclosed cycle) is apower cycle of abipropellant rocketengine. In the staged combustion cycle, propellant flows through multiplecombustion chambers, and is thuscombusted in stages. The main advantage relative to other rocket engine power cycles is highfuel efficiency, measured throughspecific impulse, while its main disadvantage isengineering complexity.
Typically, propellant flows through two kinds of combustion chambers; the first calledpreburner and the second calledmain combustion chamber. In the preburner, a small portion of propellant, usually fuel-rich, is partly combusted under non-stoichiometric conditions, increasing the volume of flow driving theturbopumps that feed the engine with propellant. The gas is then injected into the main combustion chamber and combusted completely with the other propellant to producethrust.
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 asclosed cycle, as opposed to thegas 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.
Staged combustion (Замкнутая схема) was first proposed byAlexey Isaev in 1949. The first staged combustion engine was theS1.5400 (11D33) used in the SovietMolniya rocket, designed by Melnikov, a former assistant to Isaev.[1] About the same time (1959),Nikolai Kuznetsov began work on the closed cycle engineNK-9 for Korolev's orbital ICBM,GR-1. Kuznetsov later evolved that design into theNK-15 andNK-33 engines for the unsuccessful LunarN1 rocket.The non-cryogenicN2O4/UDMH engineRD-253 using staged combustion was developed byValentin Glushko circa 1963 for theProton 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.[2]The RussianRD-180 engine also employs a staged-combustion rocket engine cycle.Lockheed Martin began purchasing the RD-180 in circa 2000 for theAtlas III and later, theV, rockets. The purchase contract was subsequently taken over byUnited 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.
The first laboratory staged-combustion test engine in the West was built in Germany in 1963, byLudwig Boelkow.[citation needed]
Hydrogen peroxide/kerosene powered engines may use a closed-cycle process by catalytically decomposing the peroxide to drive turbinesbefore combustion with the kerosene in the combustion chamber proper. This gives the efficiency advantages of staged combustion, while avoiding major engineering problems.
TheRS-25 Space Shuttle main engine is another example of a staged combustion engine, and the first to use liquid oxygen and liquid hydrogen.[3] Its counterpart in theSoviet shuttle was theRD-0120, which had similarspecific impulse, thrust, and chamber pressure, but with some differences that reduced complexity and cost at the expense of increased engine weight.
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Several variants of the staged combustion cycle exist. Preburners that burn a small portion of oxidizer with a full flow of fuel are calledfuel-rich, while preburners that burn a small portion of fuel with a full flow of oxidizer are calledoxidizer-rich. The RD-180 has an oxidizer-rich preburner, while the RS-25 has two fuel-rich preburners. The SpaceX Raptor has both oxidizer-rich and fuel-rich preburners, a design calledfull-flow staged combustion.
Staged combustion designs can be eithersingle-shaft ortwin-shaft. In the single-shaft design, one set of preburner and turbine drives both propellant turbopumps. Examples include theEnergomashRD-180 and theBlue OriginBE-4. In the twin-shaft design, the two propellant turbopumps are driven by separate turbines, which are in turn driven by the outflow of either one or separate preburners. Examples of twin-shaft designs include theRocketdyneRS-25, theJAXALE-7, andRaptor. Relative to a single-shaft design, the twin-shaft design requires an additional turbine (and possibly another preburner), but allows for individual control of the two turbopumps. Hydrolox engines are typically twin-shaft designs due to greatly differing propellant densities.
In addition to the propellant turbopumps, staged combustion engines often require smaller boost pumps to prevent both preburnerbackflow and turbopumpcavitation. For example, the RD-180 and RS-25 use boost pumps driven bytap-off andexpander cycles, as well aspressurized tanks, to incrementally increase propellant pressure prior to entering the preburner.
Full-flow staged combustion (FFSC) is a twin-shaft staged combustion fuel cycle design that uses both oxidizer-rich and fuel-rich preburners where the entire supply of both propellants passes through the turbines.[4] The fuelturbopump is driven by the fuel-rich preburner, and the oxidizer turbopump is driven by the oxidizer-rich preburner.[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. As an example, up to 25 flights were anticipated for an engine design studied by theDLR (German Aerospace Center) in the frame of theSpaceLiner project,[4] and up to 1000 flights are expected for Raptor from SpaceX.[6] Further, the full-flow cycle eliminates the need for an interpropellant turbine seal normally required to separate oxidizer-rich gas from the fuel turbopump or fuel-rich gas from the oxidizer turbopump,[7] thus improving reliability.
Since the use of both fuel and oxidizer preburners results in full gasification of each propellant before entering the combustion chamber, FFSC engines belong to a broader class of rocket engines calledgas-gas engines.[7] Full gasification of components leads to faster chemical reactions in the combustion chamber, allowing a smaller combustion chamber. This in turn makes it feasible to increase the chamber pressure, which increases efficiency.
Potential disadvantages of the full-flow staged combustion cycle include more stringentmaterials requirements, and the increased engineering complexity and parts count of the two preburners, relative to a single-shaft staged combustion cycle.
As of 2024, four full-flow staged combustion rocket engines have been tested on test stands; theSoviet storable propellantRD-270 project atEnergomash in the 1960s, the US government-funded hydroloxIntegrated Powerhead Demonstrator project atAerojet Rocketdyne in the mid-2000s,[7] SpaceX's flight capable methalox Raptor engine first test-fired in February 2019,[8] and the methalox engine developed for the first stage of theStoke Space Nova vehicle in 2024.[9]
The first flight test of a full-flow staged-combustion engineoccurred on 25 July 2019 when SpaceX flew their Raptormethalox FFSC engine on theStarhopper test rocket, at theirSouth Texas Launch Site.[10] As of January 2025, the Raptor is the only FFSC engine that has flown on a launch vehicle.
Musk said Lox and methane would be SpaceX's propellants of choice on a mission to Mars, which has long been his stated goal. SpaceX's initial work will be to build a Lox/methane rocket for a future upper stage, codenamed Raptor. The design of this engine would be a departure from the "open cycle" gas generator system that the current Merlin 1 engine series uses. Instead, the new rocket engine would use a much more efficient "staged combustion" cycle that many Russian rocket engines use.
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