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 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.
The main (only!) difference between them is the length of the nozzle (the vacuum one is a bit longer).
Because of flow separation, as pcman correctly pointed out, some engines would see buffeting and might be damaged/destroyed by running at sea level pressure.
There are several ways to work around this:
- Test in vaacum. This is done for the RL 10 engine
- Tweak the nozzle to work in both condition. This was done for the space shuttle SSME which had to work in both conditions.
- For the raptor, it seems that an increased chamber pressure did the trick
A nice article that goes deeper. With videos of flow separation: Ars technica