> American engineers went in the opposite direction. They believed that a fuel-rich mixture in the preburner was possible and could be done with existing metal alloys, so long as hydrogen was used as the fuel instead of kerosene. This ultimately lead to the development of the Space Shuttle Main Engine, which to date remains the most efficient liquid fuel rocket engine ever flown.

SSME performance was due to H2 vs kerosene, it was not a full flow engine.

Edit; also no mention of Blue Origins BE-4 which is also a full flow engine.