[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