Non-rigid airships, like Goodyear's current blimp fleet, is the only type in general use today. The internal pressure of the lifting gas (non - flammable helium) maintains the shape of the envelope, or the airship's polyester fabric skin. The only solid parts are the passenger car and the tail fins. Internal air compartments, called ballonets, are inflated or deflated with air to compensate for ambient pressure differences. These airships have no internal framework.
Semi-rigid airships were more popular earlier this century. They usually comprise a rigid lower keel construction and a pressurized envelope above that. The rigid keel can be attached directly to the envelope or hung underneath it. The most famous of this type was NORGE, the airship which General Umberto Nobile used on his attempt to reach the North Pole.
Rigid airships have an internal frame. The Zeppelins and the U.S.S. Akron and Macon were famous rigid airships. The rigid structure, traditionally an aluminum alloy, holds up the form of the airship. In general, rigid airships are only efficient when longer than 120 Meters (360ft.) because a good weight to volume ratio is (or was) only achievable for large airships. For a small airship the solid frame would have been too heavy.