page_title: Staged combustion cycle
section_title: Full-flow staged combustion cycle
hierarchical_section_title: Staged combustion cycle / Variants / Full-flow staged combustion cycle
context_page_description: The staged combustion 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 is combusted, and the increasing volume flow is used to drive the turbopumps that feed the engine with propellant. In the main combustion chamber, the propellants are combusted completely to produce thrust.
  The fuel efficiency of the staged combustion cycle is in part a result of all propellant ultimately flowing to the main combustion chamber; contributing to 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.
context_section_description: Full-flow staged combustion (FFSC) is a twin-shaft staged combustion cycle that uses both oxidizer-rich and fuel-rich preburners. The cycle allows full flow of both propellants through the turbines; hence the name. The fuel turbopump is driven by the fuel-rich preburner, and the oxidizer turbopump is driven by the oxidizer-rich preburner.
  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 the DLR (German Aerospace Center) in the frame of the SpaceLiner project, up to 1000 flights are expected for Raptor from SpaceX. 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, 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 called gas-gas engines. 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 increased engineering complexity of two preburners, relative to a single-shaft staged combustion cycle, as well as an increased parts count.
  As of 2019, only three full-flow staged combustion rocket engines had ever progressed sufficiently to be tested on test stands; the Soviet Energomash RD-270 project in the 1960s, the US government-funded Aerojet Rocketdyne Integrated powerhead demonstration project in the mid-2000s, and SpaceX's flight capable Raptor engine first test-fired in February 2019.
  The first flight test of a full-flow staged-combustion engine occurred on 25 July 2019 when SpaceX flew their Raptor methalox FFSC engine at their South Texas Launch Site.
caption: Full-flow staged combustion rocket cycle