Raptor’s claimed vacuum ISP is 380 [...] I also don’t know where I’d go if I wanted to prove to myself that the number is legit (wikipedia just cites an Elon tweet...).
The Isp of a closed cycle rocket engine with a given propellant mix is largely a function of its chamber pressure and expansion ratio, so one can use a program like RPA to plug in known numbers and see what other claims are consistent with an Isp of 380. Example (for SL variant) in this tweet.
My guess is that 380 is achievable if they close the throat and use a large enough nozzle, but they’ll opt for slightly lower in order to cram 9 engines into the upper stage. With Starship staging at record low velocities, reducing gravity losses through higher thrust might matter more than a 1% efficiency gain.
The Isp of a closed cycle rocket engine with a given propellant mix is largely a function of its chamber pressure and expansion ratio, so one can use a program like RPA to plug in known numbers and see what other claims are consistent with an Isp of 380. Example (for SL variant) in this tweet.
My guess is that 380 is achievable if they close the throat and use a large enough nozzle, but they’ll opt for slightly lower in order to cram 9 engines into the upper stage. With Starship staging at record low velocities, reducing gravity losses through higher thrust might matter more than a 1% efficiency gain.