[faitswulff]

A few frames of the video caught my eye: https://screenshots.firefox.com/eOm06NUSg0CSkb1a/arstechnica...

It looks like there's some sort of harmonic wave in the blast. Does anyone have any insights on why the design would result in that?

[trothamel]

I believe those are Mach/Shock diamonds, which are common in rocket engine exhaust. It wouldn't surprise me if the propellant and the time of the test made them more visible.

https://en.wikipedia.org/wiki/Shock_diamond

[tim333]

As also seen on page 2 of "Ignition" with page 3 showing some other interesting rocket effects. http://library.sciencemadness.org/library/books/ignition.pdf

[fkfk]

They are called shock diamonds and are caused by standing shock waves in the exhaust plume: https://en.wikipedia.org/wiki/Shock_diamond

[paulyg]

Its supersonic shock waves. https://en.m.wikipedia.org/wiki/Shock_diamond