"Airships, or dirigible balloons, are lighter-than-air aircraft that operate from a lifting-gas that is less dense than the surrounding air, keeping them afloat. The three main types of airships are rigid, semi-rigid, and non-rigid. Non-rigid airships are what people commonly call blimps. Blimps rely on internal pressure to maintain their shape. Semi-rigid airships also rely on internal pressure to maintain their shape, but the structure is also supported by metal keel at the bottom of the structure. Rigid airships, also called zeppelins, have a structural framework that keeps the shape and the lifting gas is contained in one or more cells within the airship"
https://www.rd.com/article/why-you-dont-see-blimps-anymore/
