a staged-combustion rocket engine cycle. Lockheed Martin began purchasing the RD-180 in circa 2000 for the Atlas III and later, the V, rockets. 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 2022. The first laboratory staged-combustion test engine in the West was built in Germany in 1963, by Ludwig Boelkow. Hydrogen peroxide/kerosene powered engines such as the British Gamma of the 1950s may use a closed-cycle process by catalytically decomposing the peroxide to drive turbines before combustion with the kerosene in the combustion chamber proper. This gives the efficiency advantages of staged combustion, while avoiding major engineering problems. The RS-25 Space Shuttle main engine is another example of a staged combustion engine, and the first to use liquid oxygen and liquid hydrogen. Its counterpart in the Soviet shuttle was the RD-0120, similar in specific impulse, thrust, and chamber pressure specification to the RS-25, but with some differences that reduced complexity and cost at the expense of increased engine weight. Variants Several variants of the staged combustion cycle exist. 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 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 called full-flow staged combustion. Staged combustion designs can be either single-shaft or twin-shaft. In the single-shaft design, one set of preburner and turbine drives both propellant turbopumps. Examples include the Energomash RD-180 and the Blue Origin BE-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 the Rocketdyne RS-25, the JAXA LE-7, and the Raptor. 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. In addition to the propellant turbopumps, staged combustion engines often require smaller boost pumps to prevent both preburner backflow and turbopump cavitation. For example, the RD-180 and RS-25 use boost pumps driven by tap-off and expander cycles, as well as pressurized tanks, to incrementally increase propellant pressure prior