> Aren't we using Russian RD-180 https://en.wikipedia.org/wiki/RD-180 engines which also seemed to punch above their weight class. Allegedly it was hard it believe it was possible to run engine at that efficiency level.
I don't think professionals were that much surprized. George Sutton (author of an encyclopedia on the subject, https://www.amazon.com/History-Liquid-Propellant-Engines-Lib...) writes: "The history of liquid propellant rocket engines (LPREs) in the former Soviet Union is remarkable because they have developed a larger variety and a larger number of LPREs than any other nation and the number of their LPREs that have flown is considerably higher than that of any other country." http://arc.aiaa.org/doi/abs/10.2514/2.6943 So having RD-180 performing as they do could be rather expected.
I don't think professionals were that much surprized. George Sutton (author of an encyclopedia on the subject, https://www.amazon.com/History-Liquid-Propellant-Engines-Lib...) writes: "The history of liquid propellant rocket engines (LPREs) in the former Soviet Union is remarkable because they have developed a larger variety and a larger number of LPREs than any other nation and the number of their LPREs that have flown is considerably higher than that of any other country." http://arc.aiaa.org/doi/abs/10.2514/2.6943 So having RD-180 performing as they do could be rather expected.