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The semi-rigid airship consists of a rigid keel, sometimes running the whole length of the ship, suspended below an envelope containing the lifting gas. The keel provides the prime attachment of the Gondola(s), engines etc. The semi-rigid airship maintains its shape mainly by the pressure of the lifting gas in the envelope. In the 1920s an Italian semi-rigid airship flew from Norway to Alaska and is now recognised as the first aircraft to fly over the North Pole.
The recent Zeppelin NT airship (see left), designed and constructed in the late 1990s, incorporates an internal load-carrying frame constructed of carbon fibre. This frame is used to carry the engines and gondola, but it does not give the shape of the envelope. This construction, while it has been successful, was chosen to fulfill very specific criteria, and may not have been selected without those customer requirements. The CargoLifter design from the turn of the new century also contained a rigid keel, and while this was never completed it would have been the largest semi-rigid constructed.
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