Some airship designs that use a lighter-than-air lifting gas heat a portion of the gas, which is usually maintained in enclosed cells to gain additional lift. Heating the lifting gas causes expansion of the gas in order to further lower the density of the lifting gas, which results in greater lift.
Thermal airships have the advantage of being much less expensive than helium-based airships. They are also routinely deflated after each flight and can be readily packed for storage and/or transport, making them blimps rather than rigid airships.
Hot air craft produce much less uplift per unit volume than helium- or hydrogen-filled craft (about 30% depending on air conditions). This necessitates lighter construction, with fewer controls and hence more difficulty in maneuvering. This leads to:
In recent years, the steering of these ships has improved somewhat. The most successful approach has been to use higher pressure in the tail fin structures than in the rest of the envelope, or to use an internal structure (see below).