- Mueller, Joseph B.; Michael A. Paluszek; Yiyuan Zhao. Development of an aerodynamic model and control law design for a high altitude airship (PDF). American Institute of Aeronautics and Astronautics: 2. 2004.
In general, airships fall into one of two categories: pressurized or rigid. With pressurized models, the shape of the hull is maintained by a small pressure differential between the internal lifting gas and the external atmosphere. With rigid airships, the hull is formed by a lightweight skeletal structure with a fabric skin, while the lifting gas is stored in internal “gasbags”. The Graf Zeppelin II and the Hindenburg were both rigid airships. Pressure airships are sometimes made “semi-rigid” by adding a structural keel to share the bending loads generated from aerodynamic forces.
- Konstantinov, Lev. The Basics of Gas and Heat Airship Theory. Montgolfier (Kyiv, Ukraine: AEROPLAST Inc). 2003, 1: 4–6, 8.