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 The above statement is wrong and so is the author calling it a "misnomer". According to Program Designations documents here at Rocketdyne from *1971* (~40 years ago!) the RS-24 was reserved for the Space Shuttle Auxilary Propulsion System (SSAPS) and that RS-25 was reserved for the Space Shuttle Main Engine (SSME).  Please update the article as the SSME is the RS-25.