I understand that there are sea level-optimised and vacuum-optimised nozzles, but why optimise for sea level when the rocket doesn't spend much time there? I would expect the first stage nozzles to be optimised for maximum specific impulse over the mission profile. Is "sea level nozzle" a misnomer, or is there only a slight difference between my expectation and a true "sea level nozzle", or is there a real need to get a bit better specific impulse for the first minute or so?
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$\begingroup$ One can see the exhaust from a rocket engine suddenly bloom as the vehicle reaches a certain altitude. That is when reduced atmospheric pressure makes the engines change from being overexpanded to underexpanded. Launch vehicle designers choose different altitudes at which the exhaust is perfectly expanded, but that design level is always well above sea level. The one choice that every launch vehicle designer avoids is having the exhaust be grossly overexpanded. A thruster optimized for vacuum conditions would almost certainly be grossly overexpanded at sea level atmospheric pressure. $\endgroup$– David HammenCommented Apr 9, 2021 at 8:04
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It's not quite a misnomer. Sea level nozzles aren't optimized for sea level, they are designed to be operable at sea level. You can see the "sea level" Raptor for instance produce Mach diamonds due to the overexpanded exhaust being compressed by the surrounding atmosphere. These go away as the vehicle approaches the altitude the nozzle was optimized for.
A vacuum-optimized nozzle with no altitude-compensating features wouldn't just produce slightly overexpanded exhaust, it would experience flow separation. This could cause control issues, vibrations, or directly damage the nozzle, so engines with such nozzles generally can not be operated at sea level.
answered Apr 8, 2021 at 17:44
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$\begingroup$ SpaceX posted a video of testing a vacuum Raptor at McGregor, and you can see that they had to install additional bracing to stabilize the nozzle. IIRC, they also weren't able to throttle it up to 100%. The SSMEs are an example of engines that have to really thread the optimization needle because they are lit before liftoff and operate up to orbit. $\endgroup$ Commented Apr 8, 2021 at 18:12
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$\begingroup$ The SSMEs also have some unusual nozzle features designed to prevent flow separation at sea level. The Raptor testing: the usual flow separation problems are worse at low throttle, though they may have had some other issue limiting max throttle. $\endgroup$ Commented Apr 8, 2021 at 18:28
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$\begingroup$ Indeed, that makes perfect sense. I remember there were throttling limitations for the test, likely I misremembered the direction of the limit. $\endgroup$ Commented Apr 8, 2021 at 18:29