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Here is a photo of a test firing of the Space Shuttle main RS-25 engine from 1981 from here:

enter image description here

The top of the blue luminous area (a static shock wave?) looks like it has a corner.

Here is a screen shot from a YouTube Video of a July 17, 2015 test of the SLS RS-25 engine:

enter image description here

Below are animated GIFs to outline what looks to me like a rectangular cross-sectional shape. This is not a scientific analysis - it just appears rectangular to me.

Since the expansion and nozzle are (presumably) circular in cross-section, I wouldn't expect the profile to appear to have that corner.

Is it just me, or is there a significantly non-circular shape to the cross-section at this boundary, and if there is, why?

note: The question What (actually) is the cause of the blue light from LH2/LOX rocket engines? also addresses the abrupt luminescence at the shock wave boundary, as does the question Why do the exhaust flames from cryogenic stage engines appear to be separated from the nozzle?

1981:

enter image description here

2015:

enter image description here

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    $\begingroup$ I don't see a rectangular shape (only the ones you have drawn). To me this looks like a upside-down cone with an uneven base. $\endgroup$
    – asdfex
    Commented Jul 1, 2016 at 12:16
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    $\begingroup$ @asdfex You are probably right. I'm hoping to find more technically useful imaging, and/or other metrology performed on this luminous volume - I don't even know exactly what it's called - this sudden onset of extremely bright light. $\endgroup$
    – uhoh
    Commented Jul 1, 2016 at 12:28
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    $\begingroup$ If you look at some launch videos the luminous volume appears much more cone shaped. I think you're seeing static captures of a dynamic system and reading too much into them. I'm pretty sure it is really cone shaped but am away from my references right now. Example video youtube.com/watch?v=JPdk9M5BGMw $\endgroup$
    – Organic Marble
    Commented Jul 2, 2016 at 2:37
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    $\begingroup$ @OrganicMarble Once I got started, I think I spent half the day just watching (and downloading) Shuttle videos for frame-grabbing. Wow! Some amazing footage - stuff we never saw on TV during the live broadcasts. Yep all nice and circular, will add a variety of images tomorrow. $\endgroup$
    – uhoh
    Commented Jul 3, 2016 at 14:14
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    $\begingroup$ Is the engine exhausting into a rectangular flame channel? Back pressure could be affecting the flow. $\endgroup$
    – Bob Jacobsen
    Commented Mar 27, 2019 at 5:17

4 Answers 4

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The shock cone is not rectangular in any way when the engine is being tested, just as it is not in flight.

I finally ran across a different camera angle view of the same test stand (from slightly above the nozzle exit plane looking down, not below and looking across the plume as most of the available shots were taken).

It's clear that the base of the shock cone is actually circular. Your "corners" are irregularities in the Mach disk, seen from the side.

enter image description here

Source - still frame from engineering video (hence the poor quality) - see below for source.

Edit:

I found a version of this engineering video online. Sadly whoever posted it stuck a watermark right over the Mach disk. sigh. But the circular disk is visible at these times:

  • 6:06 (briefly, the SSME blows up a few seconds later)
  • 24:13 (I think this is one I screen-shotted above, again the SSME blows up a few seconds later)

I'm happy this video, surely one of the most expensive ever made, is online, even with the watermarking.

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    $\begingroup$ Wow!! I don't know if there is a perseverance badge but you certainly deserve one. This view from a new angle puts the issue to rest once and for all. Thank you for continuing to pursue through "exhaustive research" through to conclusion. (pun intended) $\endgroup$
    – uhoh
    Commented Jul 26, 2019 at 21:42
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    $\begingroup$ It's awfully hard to see the shape of a planar object when your eyepoint is nearly at the level of it. $\endgroup$
    – Organic Marble
    Commented Jul 26, 2019 at 21:54
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    $\begingroup$ I think this nabs you a second perseverance badge, thank you for the update and "newly discovered footage"; the PeriscopeFilm YouTube channel looks downright addictive! $\endgroup$
    – uhoh
    Commented Jun 1, 2020 at 2:35
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This should be considered a supplementary answer.

The Rectangular shape looks quite convincing, but remember we can only see the back part of the flame clearly. There are other mechanisms by which the 'rectangle shape' may form by way of circular harmonics and the like. If viewed from just the right angle, the rectangular corner appearance can be replicated.

Imagine the crown of a perfect mach diamond looking like a flat circle viewed from the side: enter image description here

Add a small vertical sinusoidal oscillation whose period is twice the circumference: enter image description here

Add a smaller sinusoidal oscillation whose period is four times the circumference: enter image description here

At this point, you're getting a shape that isn't too far away from what you observed above. In particular, look at the long end of the 'rectangle'. The line isn't perfectly straight and has a tiny kink to it.

If we superimpose it onto one of the original images: enter image description here

We can see that while they don't match up perfectly, they still match up pretty good. (And no, before you ask, I am not going to spend a huge amount of time trying to get them to match up perfectly.)

This is what the ring looks like from a different angle:

enter image description here

If you look carefully at the non-rectangular-looking mach diamonds in the pictures above, their ellipses seem slightly out of plane compared to the ellipse of the engine nozzle.

You'd probably have to ask a rocket combustion simulation expert why these slight instabilities occur.

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    $\begingroup$ @uhoh Yay! Thankyou. I know my impressionistic answering style of "I don't know what it is, but this is what it kind of looks like" sometimes clashes with what you're after in an answer, but I'm glad you liked this particular answer. $\endgroup$
    – Ingolifs
    Commented Jul 29, 2019 at 5:22
  • $\begingroup$ I'm still getting used to Stack Exchange ;-) $\endgroup$
    – uhoh
    Commented Jul 29, 2019 at 6:00
  • $\begingroup$ Extremely underrated answer. +1 $\endgroup$
    – Magic Octopus Urn
    Commented Jul 30, 2019 at 15:35
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tl;dr/update: The corner that makes the shock wave appear to have aspects of a rectangular top is real and it is reproducible! But @OrganicMarble's new answer now puts the issue to rest.

Together with the images from 1981 and 2015 here is a video of another test from 2017.

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.

GIF of "rectangular" shape in shock wave

The Feb 22, 2017 NASA video A Rainbow View of NASA's RS-25 Engine Test cued at 00:50 just before the inside view:

However, the shock wave appears to be "circular" in cross-section during flight. Here are some still photos taken from below, during launch, from Wikipedia.

enter image description here

above: STS-79 (NASA) from here - CLICK to ENLARGE

enter image description here

above: STS-51f, Spacelab, (NASA) from here - CLICK to ENLARGE

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i found a clear video of the shock diamond its at around 1:25

it only looks rectangular because they keep cutting of the image

this is a good video of the shock diamond forming

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