MOTOR CASINGS (Rocket Motor)

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The motor casings for commercial and military rocket motors are made of rigid materials like metal and fiberglass, but the casings for these homemade motors cannot be rigid. The materials that you ram into the casings expand outward, and the casings have to respond by squeezing back. The two opposing forces create a tight seal between the nozzle clay, the propellant. and the casing wall. To make the system work, the casings must must be made of something resilient, and metal and fiberglass will not work. Cardboard is strong, inexpensive, and easy to work, and it has the necessary resilience. So all the motors in this topic are made with cardboard casings.
A cardboard rocket casing is an ordinary cardboard tube that has been made for a special purpose. Cardboard tubes come in two types: spiral and convolute (aka parallel wound). The spiral tubes are the most common, but you cannot use a spiral tube to make a rocket motor. Spiral tubes are plentiful. You see them everywhere, and it would be convenient if you could make them work. But each layer of paper in a spiral tube contains a tiny spiral air channel. When a rocket motor ignites, its combustion chamber pressurizes. If the casing is made from a spiral tube, and at any time during the burn, the inside wall is exposed to the flame, the flame burns through the first layer of paper, and exposes the first air channel. The elevated pressure then forces the flame up the channel, where it prematurely lights the rest of the propellant. causing the motor to explode.
To avoid confusion. I’ve provided a photo of a spiral tube in Figure 8-1. Spiral tubes are made from multiple layers of narrow paper strips wound up like a barber pole. If the tubes you have look like the one in Figure 8-1. regardless of how-thick they are. they will not work.
To properly contain the burning gases generated during a rocket motor’s operation, you need a convolute tube. Figure 8-2 is a photo of a convolute tube, and you can see that it lacks the spiral structure of the tube in Figure 8-1. A convolute tube is made by rolling a single sheet of glue-coated paper around a rod or a mandrel. The result is a tube that is made like a jelly roll with a solid wall of paper and glue, and no voids or air pockets.