>>12020892Well, it develops significantly more than twice the thrust of the Merlin 1D, and has 380s ISP compared to the Merlin's 348s. If it's ISP were to increase comensurate to chamber pressure, the increase of 10% would give it somewhere in the ballpark of 418s, I'd be surprised though if it actually managed to crack 400s though, but even that puts it closer to the efficiency of the Hydromeme rockets. Of course, it might crack that without needing the uprating, because so far we're getting the ISP of the sea-level optimized Raptor. For example, look at the SL Merlin 1D versus the VAC Merlin 1D, the expansion bell gives the vacuum Merlin 38s more ISP compared to it's SL counterpart.
For the sake of fun, let's assume Raptor 1B reaches 400s of ISP with it's increased chamber pressures, if a vacuum expansion bell is added and it confers the same 12.2% increase in ISP as the VAC Merlin 1D's bell, then the actual ISP of the uprated VAC Raptor should be 448.8s or only about 4s off the ISP of the RS-25. Even if we were to half the gains of that expansion bell, it would make VAC Raptor's ISP around 424, solidly in the same playing field as the LOX/LH2 engines, but with much smaller tankage, and much higher TWR.
