One fun counter-argument to my thesis: the FastTrac engine which was developed in the late 1990s: https://en.wikipedia.org/wiki/Fastrac_(rocket_engine) https://

One fun counter-argument to my thesis: the FastTrac engine which was developed in the late 1990s: https://en.wikipedia.org/wiki/Fastrac_(rocket_engine) https://en.wikipedia.org/wiki/Fastrac_(rocket_engine) At the time, however, I recall hearing endless griping from various corners that NASA had the gall to fund a non-LH2 engine. Internally, this was justified as being nothing more than a cheap and cheerful way to get the X-34 to its flight regime -- definitely not a serious rocket engine, no sir! That was LH2's job! Everyone knew that! The support of the RP1 rebels within NASA wasn't enough to keep it alive, and the program was cancelled after a few years. Later, it was resurrected as the template for SpaceX's first generation engines. So I personally consider this to be the exception that proves the rule.