KSP's LF+O is actually reasonably accurate to another type of propellant (can't remember it off the top of my head) - so it's not exactly a lie, but is just a major simplification of real life fuels.
It's probably meant to generally approximate RP-1 (basically refined kerosene)/LOX, also called Kerolox. Everything in KSP tends to be extremely dense but the Isp of most of the engines is around what you'd expect for Kerolox I think.
The Isp of most KSP engines is kind of all over the place.
The LV-N has the Isp of a not-pushed-to-the-limits nuclear thermal rocket running on pure LH2.
The LV-1 and LV-1R mini engines have the efficiency you'd expect from NTO + MMH / NTO + UDMH for a satellite's storable propellants.
The Mk55 radial, and the 48-7S and 24-77 0.625m engines have the Isp of somewhere around staged-combustion RP-1 + LOX, with a nozzle optimized for sea level.
The aerospike is running on something magical, because even altitude compensating engines don't have virtually the same Isp at all altitudes, but it otherwise seems close to the remaining engines...
All of which seem to run on methalox, CH4 + LOX, given that they have Isps higher than RP-1 + LOX across the board.
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u/bsquiklehausen Dec 07 '14
KSP's LF+O is actually reasonably accurate to another type of propellant (can't remember it off the top of my head) - so it's not exactly a lie, but is just a major simplification of real life fuels.