A semi-rigid airship is an airship which has a stiff keel or truss supporting the main envelope along its length. The keel may be partially flexible or articulated and may be located inside or outside the main envelope. The outer shape of the airship is maintained by gas pressure, as with the non-rigid "blimp". Semi-rigid dirigibles were built in significant quantity from the late 19th century but in the late 1930s they fell out of favour along with rigid airships. No more were constructed until the semi-rigid design was revived by the Zeppelin NT in 1997. Semi-rigid construction is lighter-weight than the outer framework of a rigid airship, while it allows greater loading than a non-rigid type. Principle More or less integrally attached to the hull are the gondola, engines and sometimes the empennage (tail). The framework has the task of distributing the suspension loads of these attachments and the lifting gas loads evenly throughout the whole hull's surface and may also partially relieve stresses on the hull during manoeuvres. In early airships which relied on nets, fabric bands, or complicated systems of rope rigging to unite the lifting envelope with the other parts of the ship, semi-rigid construction was able to achieve improvements in weight, aerodynamic, and structural performance. The boundary between semi-rigid and non-rigid airships is vague. Especially with small types, it is unclear whether the structure is merely an extended gondola or a proper structural keel. As in non-rigid airships, the hull's aerodynamic shape is maintained by an overpressure of the gas inside and light framework at the nose and tail. Changes in volume of the lifting gas are balanced using ballonets (air-filled bags). Ballonets also may serve to provide pitch control. For small types the lifting gas is sometimes held in the hull itself, while larger types tend to use separate gas cells, mitigating the consequences of a single gas cell failure and helping to reduce the amount of overpressure needed. History In the first decade of the twentieth century, semi-rigid airships were considered more suitable for military use because, unlike rigid airships, they could be deflated, stored and transported by land or by sea. Non-rigid airships had a limited lifting capacity due to the strength limitations of the envelope and rigging materials then in use. An early successful example is the Groß-Basenach design made by Major Hans Groß from the Luftschiffer-Bataillon Nr. 1 in Berlin,