Because Raptor is a staged combustion engine — like the main engines of NASA's now-retired space shuttle fleet — it is expected to be far more efficient than the open-cycle Merlin engines used by the Falcon 9.
While the Falcon 9’s engines use liquid oxygen (LOX) and kerosene, Raptor will use LOX and methane. Musk explained that "the energy cost of methane is the lowest, and it has a slight ISP [specific impulse] advantage over kerosene and doesn’t have any of the bad aspects of hydrogen." (Hydrogen is difficult to store at cryogenic temperatures, makes metal brittle and is very flammable.)
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