In a recent video, Warped Perception filmed a model rocket engine firing underwater. Firstly, it’s no surprise that the engine would still operate underwater (after its wax waterproofing). The solid propellant inside the engine is a mixture of fuel and oxidizer, so it has all the oxygen it needs. Fluid dynamically speaking, though, this high-speed footage is just gorgeous.
Ignition starts at about 3:22 with some cavitation as the exhaust gases start flowing. Notice how that initial bubble dimples the surface when it rises (3:48). At the same time, the expanding exhaust on the right side of the tank is forcing the water level higher on that side, triggering an overflow starting at about 3:55. At this point, the splashes start to obscure the engine somewhat, but that’s okay. Watch that sheet of liquid; it develops a thicker rim edge and starts forming ligaments around 4:10. Thanks to surface tension and the Plateau-Rayleigh instability, those ligaments start breaking into droplets (4:20). A couple seconds later, holes form in the liquid sheet, triggering a larger breakdown. By 4:45, you can see smoke-filled bubbles getting swept along by the splash, and larger holes are nucleating in that sheet.
The second set of fireworks comes around 5:42, when the parachute ejection charge triggers. That second explosive triggers a big cavitation bubble and shock wave that utterly destroys the tank. If you look closely, you can see the cavitation bubble collapse and rebound as the pressure tries to adjust, but by that point, the tank is already falling. Really spectacular stuff! (Video and image credit: Warped Perception)