ULA always had a lower-stage engine problem. They started with a combination of baffling RS-68 and imported Russian RD-180. Then they switched to an engine manufactured by their biggest and TBH only competition in the anything-but-SpaceX launch market (for some reason this is a surprisingly big market).
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ULA always had a lower-stage engine problem. They started with a combination of baffling RS-68 and imported Russian RD-180. Then they switched to an engine manufactured by their biggest and TBH only competition in the anything-but-SpaceX launch market (for some reason this is a surprisingly big market).