A
semi-rigid airship or dirigible 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 design was revived by the
Zeppelin NT in 1997.