The concept for metal-clad airships has been around since the mid-19th century, but the materials sciences of the time was unable to support such a concept. Only one truly metal-clad airship has actually been built, the ZMC-2 built for the US Navy in 1929.
ZMC02 used thin aluminium panels (2 mm thick) on internal supporting rings, stitched together with 3.5 million alloy rivets, giving a durable gas-tight construction. The structure, however, needed to be pressurised to sustain flight loads. Additionally, to increase stability of the shell the design gave a double curvature to virtually all points of the skin so the overall design resulted in a short, fat profile. Despite eight tail fins, the chaotic airflow pattern at the rear of the vehicle resulting from the low length/diameter ratio gave an unsteady flight path, and despite ten years successful service the design has not been repeated.
Designs for other metal-clads using similar techniques to the ZMC-2, have frequently been proposed. These include the Wren Skyships RS1, and the current proposal by Varialift of the UK. Note: the initial Varialift proposal is planned to test a buoyancy control mechanism.
An evolution of the metal-clad, is the composite-clad hull, of the type proposed by Worldwide Aeros. This proposal comprises a framework of composite construction, covered with a gas-tight composite shell. Work on the prototype is underway, funded by the US Military under the Pelican Programme. This project is intended for the Heavy Lift Transport segment.
Source: 'An introduction to the Airship' - Edwin Mowforth