Quote:
Originally Posted by AlexanderHowl
I can guarantee you that the each stage is not getting an ISP of 480.

You have a figure of 13 fuel tanks giving 3.12 mps or 5.02 km/s. DeltaV is equal to ln( full/empty ) * ISp, ln( 1/0.35 ) = 1.05, so ISp = 4780m/s or 487s.
The RS25 (SSME), which is still a quite respectable engine operating near theoretical limits for its fuel, achieves 452s vacuum, 366s at sea level. That corresponds to 0.138 mps per tank, and you should probably set aside one module per stage (call it armor) to account for structure, tank mass, and incidental weight associated with staging, which will wind up reducing your deltaV from 3.12 mps to 2.53mps.