SpaceX recently test fired vacuum optimized raptor engines of starship. Doesn't vacuum optimized engines disintegrate when operated in atmosphere? If yes, what additional modifications are made to the engines to test them in sea level?
Flow separation can occur in a rocket nozzle that is overexpanded.
This can cause quite severe turbulence and thus buffeting of the rocket nozzle.
The SSME used a special rocket nozzle shape to partially compensate for this.
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):
There are several test stands with altitude capability of up to 100 K ft (30.5 km or 8 Torr) for engine firings using the steam ejector system and up to 250 K ft (76 km) non-firing capability with vacuum pumps. 8 Torr is 1 kPa or about one hundredth of the sea level air pressure.
The NASA White Sands Test Facility Propulsion Test Stands and the Glenn Research Center, Plum Brook Station.
At the White Sands Test Facility there are 6 altitude test stands and 3 ambient pressure units.
There is also an ESA testfacility in UK: https://www.esa.int/ESA_Multimedia/Images/2021/06/Maintaining_vacuum
The “simplest” (mechanically) way is to chop the nozzle down for ground tests, but of course then you’re not testing as you fly- you’re testing a modified engine, not the literal flight design. For component testing, or just a first crack at engine operation, management may approve this as an initial step.
The next option is to place a liquid-cooled “donut” in the nozzle, at the rim. This occupies the space taken by separated flow, which would ordinarily take in ambient gas. The engine is now in a flight-like build, but not truly flight-like operation. Again, this is progress, not a definitive solution.
And of course, it’s the 2020s, not the 1950s. The question of nozzle flow is hardly new, and computers have had 50+ years of Moore’s law on their side. In some cases, particularly small engines, management may simply allow nozzle verification by analysis. Of course, small engines have more options for test facilities anyway, but such is the industry. That’s (partly) why plenty of firms will sell you the small thrusters.
The Arnold Air Force (AEDC) base at Tullahoma Tn has major altitude rocket firing test cells. Witnessed Apollo Service Module firing tests there in the mid 60s. That engine was around 25k lbf. It had an expansion ratio of 62.5. I believe the capabilities of that facility have expanded greatly to larger engines since then.
Seem to recall an altitude firing cell being constructed at NASA Stennis. Do not know it's current status.
Engines designed for use at altitude must be test fired at simulated altitude or the nozzle will collapse inward during a sea level firing for a flight weight nozzle.