The most important feature of Zeppelin's design was a rigid light-alloy skeleton, made of rings and longitudinal girders. The advantage of this design was that the aircraft could be much larger than non-rigid airships, which relied on a slight overpressure within the single pressure envelope to maintain their shape. The framework of most Zeppelins were made of duralumin.
The basic form of the first Zeppelins was a long cylinder with tapered ends and complex multi-plane fins. During World War I, as a result of improvements by the rival firm Schütte-Lanz Luftschiffbau, the design was changed to the more familiar streamlined shape and empennage of cruciform fins used by almost all later airships. Within this outer envelope, several separate balloons, also known as "cells" or "gasbags", contained the lighter-than-air gas (usually hydrogen, but helium in airships operated by America). For most rigid airships the gasbags were made of many sheets of goldbeater's skin from the intestines of cows. About 200,000 were needed for a typical World War I Zeppelin. The sheets were joined together and folded into impermeable layers. Non-rigid airships often do not have multiple gas cells (though some Italian-built semi-rigid airships did).
Forward thrust was provided by several internal combustion engines, mounted in nacelles, or engine cars, attached to the structural skeleton. Some of these could provide reverse thrust for manoevering while mooring. A comparatively small compartment for passengers and crew was built into the bottom of the frame, but in large Zeppelins this was not the entire habitable space; they often carried crew or cargo internally for aerodynamic reasons.
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