There are three types of airships: the nonrigid airship, often called a blimp; the semirigid airship, and the rigid airship, sometimes called a Zeppelin.
The first effort at building an airship involved stretching the round balloon into an egg shape that was kept inflated by internal air pressure. These non-rigid airships, commonly called blimps, used ballonets, air bags located inside the outer envelope that expanded or contracted to compensate for changes in the gas.[]p Because these blimps often collapsed under stress, designers added a fixed keel under the envelope to give it strength or enclosed the gas bag inside a frame. These semirigid airships were often used for reconnaissance flights.
