to entering the preburner. Full-flow staged combustion cycle 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 on the Starhopper test rocket, at their South Texas Launch Site. Applications Oxidizer-rich staged combustion S1.5400—First staged combustion rocket engine used on the Blok L upper stage. NK-33—Soviet engine developed for the never-flown upgraded version of the N-1 launch vehicle. Later sold to Aerojet Rocketdyne and refurbished/remarketed as the AJ-26 (used on Antares block 1 launch vehicles in 2013–2014). In use on the Soyuz-2-1v. P111 - liquid oxygen/kerosene demonstrator engine developed between 1956 and 1967