So, I've been doing some digging, and I found that the chamber pressure for the raptor engine is more than 4 times that of Merlin (~4,000 psi to ~980 psi). How would SpaceX manage to pressurize the fuel/oxidizer enough to be efficient? And if they're planning for their liquid methane to be self pressurizing wouldn't that take too much of the fuel supply?


From the wording of the question I suspect you think the entire fuel/ox tanks are maintained at higher than the chamber pressure. They're not: tank pressure is a few bar. The fuel and oxidizer are pressurized in the turbopump.

Self-pressurizing means they have to boil some of the fuel (which is easy, when you have a rocket engine providing all the heat you could possibly want) and run the gases back into the tank. This means some of the fuel is not available for combustion, but it's more efficient than the alternative: a heavy tank (strong enough to withstand ~200 bar) filled with enough helium to fill the entire fuel tank at a few bar.

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  • $\begingroup$ Yeah sorry. It's really really late (or early) here and it's freezing so I can't either sleep or think xD. I get that the Raptor is a closed loop engine which gives the turbopump more of a kick, but the turbopump still needs to overcome 300 bars. I know there's tech for that, but they're heavy and dangerous. I guess what I'm trying to say is, if they're going to use like 27 raptor engines, why not expand the rocket a little and go with their Merlin C. I don't really see how the Raptor would be efficient given its current stats $\endgroup$ – user22624 Jan 21 '18 at 13:31
  • $\begingroup$ The higher chamber pressure makes the engine more efficient - i.e. the gain from increasing the pressure outweighs the extra weight necessary to make the turbopump strong enough. $\endgroup$ – Hobbes Jan 21 '18 at 13:44
  • $\begingroup$ @user22624 I think you should expand that into a whole new question. Get some sleep and warm up, think it over, then ask a new one. There's no limit to the number of good quality questions you can ask here. Comments are considered temporary, and really should be used just to clarify some details related to the question. $\endgroup$ – uhoh Jan 21 '18 at 13:44
  • $\begingroup$ @Hobbes do you know how stronger the outgoing turbopump pressure needs to be compared to the chamber pressure? $\endgroup$ – user22624 Jan 21 '18 at 13:49

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