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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?

Ashvin
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    Where did you get that idea? – GdD Oct 22 '21 at 08:38
  • I read that somewhere. – Ashvin Oct 22 '21 at 08:45
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    @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? – David Hammen Oct 22 '21 at 10:15
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    @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. – David Hammen Oct 22 '21 at 12:35
  • @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. – Christopher James Huff Oct 22 '21 at 12:49
  • @ChristopherJamesHuff No, they are not. Flow separation occurs precisely because an engine designed for use in vacuum is instead used in a region of the atmosphere with sufficiently high pressure. The Shuttle's OMS engine was not allowed to be used below 70000 feet above sea level because of this. The OMS engine was designed for use in vacuum. There was a risk of the nozzle collapsing in on itself below 70000 feet. – David Hammen Oct 22 '21 at 12:55
  • A risk, not a certainty, and in a nozzle not designed for sea level operation. There would be a mass penalty, but there's no reason not to think a vacuum-optimized nozzle could be designed to operate at sea level. – Christopher James Huff Oct 22 '21 at 13:01
  • Unrelated to pad tests, but here are two posts about the old school way of testing upper stage engines: https://space.stackexchange.com/q/41901 https://space.stackexchange.com/q/37749 – Organic Marble Oct 22 '21 at 18:39
  • Don't you think at some stage, all engines need to be tested to destruction? – Robbie Goodwin Oct 24 '21 at 21:02

4 Answers4

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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

CuteKItty_pleaseStopBArking
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    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. – David Hammen Oct 22 '21 at 10:29
  • 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? – leftaroundabout Oct 23 '21 at 00:23
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    @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. – Level River St Oct 23 '21 at 01:10
  • @leftaroundabout the perfect vacuum nozzle is of infinite size. Reality causes rocket builders to accept a smaller , less efficient nozzle size due to mass and vehicle size contraints. If the Raptor starts at 270Bar, and expands that over a 80:1 ratio, the end result is still < 3 bar. Of course the pressure in the choked flow region in the nozzle is not the same as inside the combustion chamber, but it is somewhat related. – CuteKItty_pleaseStopBArking Oct 23 '21 at 05:17
  • @LevelRiverSt “It would appear that the vacuum raptor isn't that overexpanded at all” – but even the sea level Raptor is overexpanded, as visible from the shock diamonds. — Sure, of two engines with the same expansion ratio (and same fuel), the one with the higher chamber pressure will also have a higher exhaust pressure, but that comparison doesn't make any sense. The whole point of higher chamber pressure is to get a higher expansion in the end, because that's what (after the choke point) makes the exhaust fast and thus Isp high. High-pressure chamber and not use? Thrown away potential. – leftaroundabout Oct 23 '21 at 10:50
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    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. – John Doty Oct 24 '21 at 19:50
  • It occurs to me that another mitigation would be a test stand that partially plugs the nozzle, limiting the expansion. – John Doty Oct 24 '21 at 19:52
  • @leftaroundabout the main advantage of higher chamber pressure is the ability to get more thrust from the same equipment and footprint, thus higher TWR and more allowed rocket mass per rocket base area. The added ISP possibility is nice, The greater expansion ration tolerance is nice, but both of these is really a side-effect of the true goal. – CuteKItty_pleaseStopBArking Oct 25 '21 at 00:38
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    @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. – CuteKItty_pleaseStopBArking Oct 25 '21 at 00:41
  • @PcMan that's certainly not the “main” advantage. Thrust is best optimised by increasing the mass flow rate, which can be done without pressures that put so insane stress on the engine. Increasing the chamber pressure without then also suitably expanding it hardly helps with thrust, it just produces a wider plume behind the rocket. – leftaroundabout Oct 25 '21 at 08:34
  • @PcMan It doesn't seem hard to design a plug that would constrain the expansion in such a way that the gauge pressure inside the bell at sea level should match the vacuum expectation, thus stressing the bell as if it was in vacuum. That's about as realistic as you can get without a huge vacuum chamber. – John Doty Oct 25 '21 at 14:42
  • @JohnDoty if you can do that, you will will a Nobel prize.. That's like saying you can easily add weight to a scale until it reads zero. – CuteKItty_pleaseStopBArking Oct 25 '21 at 14:44
  • @PcMan If you have a scale with an extra applied force pushing upward, you most certainly can zero it by adding weight. The cause of the problem with overexpansion is that the gauge pressure within an overexpanded flow is negative (the absolute pressure is, of course, always positive). The negative gauge pressure allows atmosphere to enter the bell and produce the destructive flow separation. So, raise use the plug to raise the absolute pressure by one atmosphere, so the gauge pressure is guaranteed to be positive. – John Doty Oct 25 '21 at 15:19
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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

Uwe
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    another non-SI unit I didn't know about. 1/760 standard atmosphere, because why not – njzk2 Oct 23 '21 at 22:10
  • @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. – Uwe Oct 25 '21 at 20:39
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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.

caInstrument
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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.

tckosvic
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