This is a video of the mouth of a pressurized plastic soda bottle rocket as it blasts off, as viewed at 50,000 frames/second. Some more info from somebody who actually knows what they're talking about because up until five seconds ago I still thought this was a firework and not a plastic bottle rocket:
The "Mach discs" or "Mach diamonds" emerge as the flow reaches under-expanded supersonic flow from the nozzle. These objects are the result of standing shockwaves caused by abrupt pressure changes. The direction-indicating colour Schlieren technique used visualizes the density gradients produced.
Fascinating. Granted I have no clue what that means, but you don't have to understand something to find it fascinating. Take women for example. "What about them?" My girlfriend isn't talking to me for some reason. "And that's fascinating?" No, that's because I got too drunk in the woods last night and she had to come find me with a flashlight. *rips open flannel to reveal 'I SELF-QUARANTINE WITH BEARS' t-shirt* Man my head hurts.
Keep going for the full video, as well as the Mach diamonds formed behind an F-15 taking off because goshdang that thing looks sweet.
Thanks to Salman, who agrees there better have been a fat man's belly on the receiving end of that bottle rocket, and the impact better have been filmed in ultra-slow motion as well.