For my rocket motors, I’ll be using convolute paper tubes as the casing. Paper casings are weaker than aluminum or fiberglass, but have several major benefits. First and most importantly, they are much safer. They can’t withstand quite as much pressure as an aluminum tube but when they fail they don’t generate metal shrapnel. Also, they are much cheaper. All it takes to make one is a bit of kraft paper and some white glue. Lastly, paper tubes have a little bit of flexibility, which actually aids in the retention of clay nozzles. When the clay nozzle is rammed into place, it bulges the tube outward ever so slightly, creating an interference fit that keeps the nozzle from sliding out (at least in theory). In total I made about 100 tubes, split evenly between 1/2″ ID, 3/4″ ID, and 1″ ID (inner diameter). I marked them based on which batch they were just in case there are variations between the strength of each batch. I can already tell that some of them didn’t come out as well, since you can sort of tell how strong they are when you squeeze the walls with your fingers. I may later try to make an apparatus for measuring this strength in order to do some rudimentary quality control. By the way, convolute tubes are the opposite of spiral-wound tubes, which would not be suitable for the task of being a rocket motor casing, as the spiral creates a small channel on the inside that would allow the flame to shoot down, prematurely igniting fuel and causing the engine to explode. Convolute tubes are rarely used for anything else other than rockets/fireworks, and are therefore really hard to purchase, especially if you have specific size and thickness requirements.
