What is Flexyshell? Flexyshell is a new kind of pressure vessel, designed for storing hydrogen and other gases safely and efficiently. Unlike traditional tanks, Flexyshell uses a flexible cylindrical wall that carries only the round (hoop) stress created by internal pressure, while a separate system of strong external tendons carries the lengthwise (axial) stress. This separation of forces allows the vessel to be lighter, safer, scalable, and easier to manufacture.