Main-stage hydrocarbon engines are drawing the most attention and money as the U.S. Air Force and Congress look for a way to shift national security launches off the Russian-built RD-180, but a separate engine competition is underway at the other end of the rockets. Already flying is a modified version of the SpaceX Merlin 1D, which powers the Falcon 9 first stage in an array of nine engines. A single Merlin, with a larger nozzle for maximum efficiency in vacuum, takes over after staging to ...


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