>>6815211Merlin is not quite efficient in terms of ISP which is basically the most important characteristic of the engine. Only 311 s in vacuum because they are open-cycle (and thus simple and cheap). You can push RP1+LOX up to 338 s (RD-180, closed cycle)
LH2+LOX starts at 400 s. But hydrogen is a lot less dense and thus needs bigger tanks (also with thermal insulation), requires complex ignition system in the engine, evaporates in tanks and then leaks straight through metal, so it is rarely used. The only orbital rocket which uses it today is Delta IV.
The modern tendency is to move to CH4+LOX (Raptor, BE-4).
