The airship evolved from the spherical balloon first successfully flown by the Montgolfier brothers in 1783. Airships are basically large, controllable balloons that have an engine for propulsion, use rudders and elevator flaps for steering, and carry passengers in a gondola suspended under the balloon.
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.
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.
The rigid airship was the most useful type of airship. A rigid airship has an internal framework of steel or aluminum girders that supports the outside material and gives it shape. Only this type of airship could reach sizes that made it useful for carrying passengers and cargo.