Rockets operate on a pretty simple principle: if you throw something out the back really fast, the rocket goes forward. Practically speaking, we accomplish this with a combination of chemistry and physics, by burning fuel and oxidizer together and accelerating the exhaust out a nozzle. Solid rocket propellant, like that found in the model rockets shown here, is a combination of fuel and oxidizer that don’t react until they’re ignited. You don’t want your rocket to just explode as soon as it’s lit, though, so solid rocket motors are carefully designed to burn in a particular way. By packing the propellant into different shapes – and even including patterns of propellants with different burn rates – engineers can create a rocket that burns with the thrust pattern they want.
In the case of this model rocket motor, what we observe is not really how it is intended to burn; you can see how some of the combustion products are working their way out of cracks that wouldn’t normally exist. But the video and animation do show how the burn front moves gradually through the engine, allowing it to produce a relatively steady amount of thrust for a longer period before reaching the darker burning propellant on the left, which would normally launch the model rocket’s parachute. (Image and video credit: Warped Perception; via Gizmodo)