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How are vacuum optimized engines tested without disintegrating them? is an excellent question as it's attracted quite a number of informative and diverse answers.

@Uwe's answer informs us that there exist at least one or two low-pressure test chambers where engines can be fired for tens of seconds and the exhaust cleverly managed/diverted to maintain a pressure below 50 Torr for tens of seconds which is pretty amazing!

But the news discussed in the linked question says that

SpaceX recently test fired vacuum optimized raptor engines of starship... operated in atmosphere

@PcMan's answer tells us that this was indeed done at sea level, and says:

Apparently, the Raptor just bulls its way past the problem by virtue of very high chamber pressure (Meaning the nozzle is not so very overexpanded)

Question: How unusual is it to test vacuum engines at sea level? Is this yet another one of the many it's pulled off? Or has it been done before?

This may be difficult to answer unambiguously; an engine meant to run in vacuum might be tested with a shorter nozzle at sea level, I don't think that should count. It should be a test that has the same configuration as designed for use in vacuum.

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    $\begingroup$ Do SSMEs count? If so, it was quite common once. $\endgroup$ Oct 23, 2021 at 12:22
  • $\begingroup$ @OrganicMarble dunno, do space folks call the SSME a vacuum engine? Is its nozzle called a "vacuum nozzle"? I'm not qualified to make the call. $\endgroup$
    – uhoh
    Oct 23, 2021 at 12:24
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    $\begingroup$ It's an engine that ran in vacuum. Perhaps the terminology is insufficiently precise. $\endgroup$ Oct 23, 2021 at 12:33
  • $\begingroup$ @OrganicMarble why does everything Shuttle have to be so exceptional? ;-) I think you can write an answer noting this interesting case, burning continuously from sea-side to orbit, I think that would be great! $\endgroup$
    – uhoh
    Oct 23, 2021 at 13:30
  • $\begingroup$ The theoretically optimal vacuum nozzle is infinitely long. Which means from a theoretical standpoint, no nozzle is truly a "vacuum nozzle" and no engine is truly a "vacuum engine". So, a "vacuum nozzle" always has to be sub-optimal, but then, every nozzle is a "vacuum nozzle" and every engine is a "vacuum engine" – just to different degrees. As mentioned, the SSMEs ran from sea-level to orbit. Also, the "sea-level Raptors" on Starship are going to be used for landing in vacuum on the Moon, since the RVacs can't gimbal, so the sea-level Raptors are also going to be running in vacuum. $\endgroup$ Oct 23, 2021 at 17:13

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