A rigid airship is a type of airship (or dirigible) in which the envelope is supported by an internal framework rather than by being kept in shape by the pressure of the lifting gas within the envelope, as in blimps (also called pressure airships) and semi-rigid airships. Rigid airships are often commonly called Zeppelins, though this technically refers only to airships built by the Luftschiffbau Zeppelin company. In 1900, Count Ferdinand von Zeppelin successfully performed the maiden flight of his first airship; further models quickly followed. Prior to the First World War, Germany was a world leader in the field, largely attributable to the work of von Zeppelin and his Luftschiffbau Zeppelin company. During the conflict, rigid airships were tasked with various military duties, which included their participation in Germany's strategic bombing campaign. Numerous rigid airships were produced and employed with relative commercial success between the 1900s and the late 1930s. The heyday of the rigid airship was abruptly ended by the destruction of the Hindenburg by fire on 6 May 1937. The disaster not only destroyed the biggest zeppelin in the world but caused considerable reputation damage to rigid airships in general. Amid widespread public safety concerns, several nations opted to permanently ground their existing rigid airships and scrap them in subsequent years. Construction and operation Rigid airships consist of a structural framework usually covered in doped fabric containing a number of gasbags or cells containing a lifting gas. In the majority of airships constructed before the Second World War, highly flammable hydrogen was used for this purpose, resulting in many airships such as the British R101 and the German Hindenburg being lost in catastrophic fires. The inert gas helium was used by American airships in the 1920s and 1930s; it is also used in all modern airships. Although airships rely on the difference in density between the lifting gas and the surrounding air to stay aloft they can also generate a certain amount of aerodynamic lift by using their elevators to fly in a nose-up attitude. Similarly, by flying nose-down, down-force can be generated: this may be done to prevent the airship rising above its pressure height. Typically airships start a flight with their gasbags inflated to about 95% capacity: as the airship gains height the lifting gas expands as the surrounding atmospheric pressure reduces. The height at which the internal pressure of the gasbags equals external atmospheric