The heat generated by artificial carving is small and will not cause combustion. For example, machining or 3d printing will either produce high heat or rely on high heat, which is easy to cause solid fuel combustion and is too dangerous. The burning speed of the outer end of A4 paper is obviously much faster than that of the middle part. Similarly, the same solid rocket can't burn at the same time because of its different burning speeds, which will affect its stability and thrust. Therefore, in order to make the solid rocket burn at the same cross section at the same time, the solid fuel in the solid rocket is not cylindrical as everyone imagined. As for what shape it is, this is a doctoral thesis. Similarly, in order to accurately process these shapes, the processing personnel need to constantly trim them by hand.