In a solid rocket motor, what structure serves as both the combustion chamber and the primary propellant containment vessel?

The rocket casing containing the pre-mixed grain

Solid rocket motors are structurally simple because the fuel and oxidizer are pre-mixed and cast into a single, solid, rubbery or granular material known as the grain. This grain is packed directly inside the rocket casing. This casing serves the dual critical roles of containing the propellant during storage and transport, and simultaneously acting as the primary combustion chamber once ignition is initiated. This inherent simplicity avoids the need for complex plumbing, injectors, and turbopumps required by liquid systems, contributing to their high thrust-to-weight ratio, although this simplicity comes at the cost of zero control once burning commences.