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?

  • 2
    $\begingroup$ Where did you get that idea? $\endgroup$
    – GdD
    Commented Oct 22, 2021 at 8:38
  • $\begingroup$ I read that somewhere. $\endgroup$
    – Ashvin
    Commented Oct 22, 2021 at 8:45
  • 1
    $\begingroup$ @Ashvin You might have read it on this site in the answer to a question you asked linast January, Can the space shuttle use OMS engines during landing? $\endgroup$ Commented Oct 22, 2021 at 10:15
  • 4
    $\begingroup$ @GdD There are many questions and answers on this site about grossly overexpanded nozzles. In an overexpanded nozzle the exhaust pressure is less than ambient pressure (atmospheric pressure at the rocket's altitude). In a grossly overexpanded nozzle, the exhaust pressure is so much less than ambient that the exhaust flow separates from the nozzle before exiting the nozzle. Engines have blown up because of this. $\endgroup$ Commented Oct 22, 2021 at 12:35
  • $\begingroup$ @DavidHammen "flow separation will occur, which can potentially damage or destroy the engine" and "vacuum optimized engines disintegrate when operated in atmosphere" are rather different statements. $\endgroup$ Commented Oct 22, 2021 at 12:49

4 Answers 4


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):

screenshot of Twitter of Elon Musk regarding Raptor vac testing at sealevel

  • 8
    $\begingroup$ The problem isn't with an overexpanded nozzle per se. It's with a grossly overexpanded nozzle. Almost all engines are overexpanded at launch. In many launches one can see the first stage exhaust suddenly bloom when the rocket reaches a high enough altitude. That's the altitude at which the exhaust transitions from overexpanded to underexpanded due to the reduced pressure at that altitude. $\endgroup$ Commented Oct 22, 2021 at 10:29
  • $\begingroup$ Hm... Elon Musk generally knows his stuff, even when it sounds inconsistent, but... how does this explain anything? Of course higher chamber pressure means you're going to have more expansey nozzles. It means even the sea-level nozzles will have more expansion than those of other booster engines. But surely it also means that an efficient vacuum nozzle is going to have even more expansion, at which point flow separation should again be just as much of an issue? $\endgroup$ Commented Oct 23, 2021 at 0:23
  • 2
    $\begingroup$ @leftaroundabout take two engines with the same expansion ratio and different chamber pressures. The one with the higher chamber pressure will have the higher nozzle exit pressure. It would appear that the vacuum raptor isn't that overexpanded at all - presumably making it so would make the nozzle so vast it would either be uneconomic or wouldn't fit in the ship. $\endgroup$ Commented Oct 23, 2021 at 1:10
  • 1
    $\begingroup$ I wonder whether the "higher chamber pressure" was a special condition for a sea-level test. It makes sense to test beyond the normal operating envelope to verify margins, and that would also help mitigate the trouble with over-expansion. $\endgroup$
    – John Doty
    Commented Oct 24, 2021 at 19:50
  • 1
    $\begingroup$ @JohnDoty partially plugging the nozzle would do more to invalidate the engine test than sawing off the whole nozzle, that's like testing a racing car's stability under speed by having the car tow a train, so it doesn't go as fast. $\endgroup$ Commented Oct 25, 2021 at 0:41

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.

See https://ntrs.nasa.gov/api/citations/20180005322/downloads/20180005322.pdf and https://www.nasa.gov/sites/default/files/files/WSTFTestStands.pdf

There is also an ESA testfacility in UK: https://www.esa.int/ESA_Multimedia/Images/2021/06/Maintaining_vacuum

  • 1
    $\begingroup$ another non-SI unit I didn't know about. 1/760 standard atmosphere, because why not $\endgroup$
    – njzk2
    Commented Oct 23, 2021 at 22:10
  • $\begingroup$ @njzk2 Torr or mm Hg is not a SI-unit but it is still used for blood pressure. Torr was used for vacuum technology during many decades, even some centuries. So if your blood pressure was ever measured you missed the opportunity to learn about Torr. $\endgroup$
    – Uwe
    Commented Oct 25, 2021 at 20:39

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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.