China's most advanced YF-77 Lox/LH2 rocket engine for Long March 5 has highest specific impulse China's YF-77 Lox/LH2 (liquid oxygen-liquid hydrogen) rocket engine type has the highest specific impulse, which means it is the most efficient. In comparison, SpaceX uses an inferior kerosene-liquid oxygen mixture. There is another important reason that the SpaceX engine fuel technology is inferior to China's YF-77. Liquid oxygen cannot be used as a coolant, because it's an oxidizer. It will react with the hot metal and burn it. China's YF-77 uses its liquid hydrogen to cool the rocket engine. The cryogenic cooling allows for a very large rocket engine. In comparison, SpaceX's main engine is kerosene cooled. Kerosene is only at refrigeration temperature and not cryogenic. This doesn't cool the rocket engine as much. Thus, the SpaceX Falcon 9 requires nine small Merlin engines. In conclusion, the SpaceX Falcon 9 is technologically inferior to China's YF-77 for two important reasons. Since the SpaceX Falcon 9 has lower specific impulse (ie. less efficient), it requires more fuel to send the same payload. Also, due to the inefficiency of the Merlin engines and the inability to properly cool them, the SpaceX Falcon 9 must use nine engines (which increases manufacturing complexity, costs, and failure rates). China's liquid oxygen-liquid hydrogen (Lox/LH2) rocket engine type has the highest specific impulse (ie. most efficient). Notice the increased density of circular rings near the bottom. Form follows function. The rocket engine is hottest at the bottom and requires the most rings (or tubes) to circulate liquid hydrogen coolant. References. Lox/LH2 | Encyclopedia Astronautica "In Russia hydrogen fuelled upper stages were designed and developed by the mid-1970's, but the Russians never seem to have found the extra performance to be worth the extra cost. Europe and China developed liquid oxygen/liquid hydrogen engines for upper stages of the Ariane and Long March launch vehicles." Liquid Hydrogen--the Fuel of Choice for Space Exploration | NASA "Despite criticism and early technical failures, the taming of liquid hydrogen proved to be one of NASA's most significant technical accomplishments. . . . Hydrogen -- a light and extremely powerful rocket propellant -- has the lowest molecular weight of any known substance and burns with extreme intensity (5,500°F). In combination with an oxidizer such as liquid oxygen, liquid hydrogen yields the highest specific impulse, or efficiency in relation to the amount of propellant consumed, of any known rocket propellant."