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I know SRBs have a quite bit of thrust oscillation, and it's pretty much part and parcel of burning solid propellant. Do liquid-propellant engines experience something similar albeit at a much smaller magnitude? If so what is the main cause of this constant small variation in thrust level? Is it because of the combustion process inside the bell? In Falcon 9 launches you often see photos like this Falcon 9 exhaust The streak patterns of the exhaust plume indicate there is at least some non-uniformity of the pressure and temperature field inside. I assume every liquid engine actually behaves like this, but it's only apparent on LOX/RP-1 engine because of soot acting like temperature indicator through the intensity of the glow for the pattern.

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    $\begingroup$ I think the vertical streaking is closely related to the physical arrangement of the fuel/oxidizer injectors and resulting flow patterns, but I'm not sure why I think that. $\endgroup$ – Russell Borogove Sep 17 at 22:15
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    $\begingroup$ Agreed, and if it was perfectly mixed, you likely wouldn't see the streaking. $\endgroup$ – Organic Marble Sep 18 at 0:01
  • $\begingroup$ If I find out that KSP streak patterns have changed in KSP2, I'm coming back to this question... $\endgroup$ – Erin B Sep 19 at 13:18
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Yes, in liquid engines this phenomenon is called "rough combustion".

Sutton (4th edition) says

Combustion that gives pressure fluctuations greater than about +/- 5% of the mean pressure at a chamber wall location and which occur at completely random intervals is called rough combustion.

enter image description here

It's different from "combustion instability" defined as activity which

displays organized oscillations ocurring at well-defined intervals with a pressure peak that may be maintained, may increase, or may die out.

pp. 257 - 259

The streak patterns you mention are more likely due to non-uniform mixing resulting in slight variations in local mixture ratio, than due to rough combustion, IMHO.

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  • $\begingroup$ F1s had problems with non-uniform mixing (combustion instability) behind the single injector plate, so not technically in the bell. Source: Apollo's Daring Mission (~14 min. in). Solved by adding baffles before the plate so that it was more like the individual nozzles found on V2s. $\endgroup$ – Mazura Sep 19 at 4:45
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In addition to the "rough combustion" that Organic Marble mentions, liquid engines also characteristically suffer from pogo oscillation. This is the phenomenon in which the thrust causes acceleration of the rocket, which changes the flow of propellant in the lines fuel and oxidizer lines, which then causes a change in the thrust, which is now a loop.

Pogo oscillation was a major problem in the development of the Saturn rockets, and I believe the Jupiters before them.

enter image description here

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    $\begingroup$ I see figures of up to 34g oscillation at 16Hz (zero-to-peak, at the engine's mount point). This would make for a VERY uncomfortable ride. $\endgroup$ – jeffB Sep 18 at 17:13
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    $\begingroup$ The Titan (ICBM) boosters used in the Gemini program had some serious (i.e., destructive) pogo oscillations in their initial flight tests. Fortunately the were largely resolved before the rockets were carrying human passengers. $\endgroup$ – FKEinternet Sep 18 at 21:57
  • $\begingroup$ What methods can be used to prevent it? $\endgroup$ – DrMcCleod Sep 19 at 11:17
  • $\begingroup$ @DrMcCleod: I know that the Saturns were fixed by making the thrust plate more rigid. I'm not sure about general solutions to the problem, though. There are some rocket scientists who frequent this site, I hope they chime in. $\endgroup$ – dotancohen Sep 19 at 13:15
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    $\begingroup$ Space Shuttle Main Engine had a "pogo accumulator" built in, a pressurized volume connected to the LOX feedline which helped to damp out oscillations. $\endgroup$ – Organic Marble Sep 29 at 23:04

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