system
Given the following post, respond as if you're commenting on Reddit:
human
Why do russian RD-180 engines run oxygen rich?

As far as I know, every other rocket engine in existence favours fuel-rich operation. I know efficiency is inversely proportional to molar mass, and oxygen is lighter than RP-1, so is that the main reason why ther RD-180 is such a good engine? If so, what made it possible to run it oxygen rich and what prevented the US from developing the same engine?
gpt
The fuel mix for the preburner needs to be oxygen-rich, because a fuel-rich mix with RP-1 could generate soot and clog stuff up.

In a hydrogen-oxygen engine that's not a concern, so for example the SSME uses a fuel-rich mix in its preburner. That's also the easier thing to build, because an oxygen-rich preburner means you're piping a hot mix of oxygen and combustion products around, and that's really, really aggressively corrosive. The Americans didn't actually believe the Russians had achieved that until the Soviet Union collapsed and they got to see it for themselves.