The term airship, also known as dirigible, is an aerostat, an aircraft that is lighter than air. This type of aircraft can be flown in air by steering and propelling with the use of rudders as well as propellers or also by using thrust mechanisms. Aerostatic aircrafts depend on aerodynamics to stay aloft in the air by making a large-sized "envelope" which is filled with a gas for lifting purposes that is much less denser than surrounding air. The initial versions of dirigibles possesses hydrogen as the lifting gas even though hydrogen was well known for being highly flammable.
Airships can be of three types - rigid airship, nonrigid blimps and semirigid airship. Blimps is the term used to refer to pressure airships in which the internal pressure is maintained by air being forced into a ballonet placed internally. This ballonet helps in maintaining the shape and structural integrity of the airship. Semirigid airships are good at maintaining an envelope shape using internal pressure. But semirigid airships have an internal support like the fixed keel. Control gondolas and engine gandolas, steering surfaces and stabilizers are mounted on the keel. The structural framework of rigid airships helps in maintaining the shape and carrying loads like gondolas, engines, etc. This framework consists of balloons called "gas cells", also known as "gasbags", which provide the static lift thus eliminating the need for structural loading. Rigid airships are also known as zeppelin as they were created by Count Zeppelin. A majority of them were built by his firm.
Prior to the 1940s, the only widely used aircraft to have controlled and power-enabled flight were airships. But with the introduction of aeroplanes, the use of airships gradually decreased. A number of highly fatal accidents has led to the decline of airships, for example the burning of the Hindenburg, a hydrogen-filled airship, in 1937; similarly the USS Akron which got completely destroyed. However, airships have been of great use in the 21st century in the fields like tourism, geological surveys, platforms for cameras at sporting events, advertising, aerial observation, etc. These are areas of application where the airship’s ability to hover at a spot for longer periods of time outshines its incapability of better speed and easiness of transportability.