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In last year's question Why does the luminous blue shockwave from the RS-25 engine appear to be rectangular? I showed images of an RS-25 engine test from 2015, and what I believe to be a test of the space shuttle engine which was much earlier (Photo ID: GPN-2000-000543 AND Alternate ID: 81-201-1.).

Today I went to look at the new video of the RS-25 exhaust rainbow which was seen during the "latest hot-fire test of development engine 0528 at the Stennis Space Center in Mississippi on Wednesday" (22-Feb-2017) and I still believe that I see the same darn oblique rectangular shape that I saw when asking the last question.

So here it is as a GIF, and then as the video. The GIF is 30 fps from about 00:55 to 01:02 (frames 1640 to 1860). I still see a "bump", and from this angle it is about a quarter of the way from the left edge of the shock wave, almost in line with the left vertical railing post.

enter image description here

What is producing this bump??

The geometry of a cylindrically symmetric engine and exhaust plus the cameras in these three shots should not produce an artifact or optical illusion of an offset bump, clearly to one side of the axis in all three tests. There is something persistent and reproducible in these tests that creates a bump on one side. What is it?

1981?: (annotation added to GIF)

enter image description here

2015: (annotation added to GIF)

enter image description here

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First, note that shock diamond formation is dependent on atmospheric pressure. According to wikipedia:

The distance from the nozzle to the first shock diamond can be approximated by

${\displaystyle x=0.67D_{0}{\sqrt {\frac {P_{0}}{P_{1}}}}}$

where x is the distance, D0 is the nozzle diameter, P0 is flow pressure, and P1 is atmospheric pressure.

Note that at least one side of the test chamber is open to sky, while another is a wall; a vapor plume of some kind is being drawn down in a twisted path -- I assume it's water vapor condensed out of the atmosphere by cold propellant lines above.

This suggests that while the engine and its interior situation may be cylindrically symmetrical, the atmospheric environment it's exhausting into may be significantly asymmetrical, so it's not surprising that the shock diamond might form slightly earlier on one side of the plume than the other.

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  • $\begingroup$ +1 for the speculative comment/answer! I like to call the boundary at the onset of the blue luminescence a "static shock wave" because it's static, and it's a shock wave, and the term doesn't carry the "diamond shape" bias. OK I'll let you finish typing and read later. $\endgroup$ – uhoh Feb 23 '17 at 10:24

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