Lighter-than-air ships are of three classes, rigid, semi-rigid and non-rigid. The rigid airship has a complete metal skeleton, which gives the ship strength and shape. Into the metal frame of the rigid airship are built quarters, shops, communication ways, even engine rooms in the case of the Akron and Macon, with only the control car, fins, and propellers projecting outside the symmetrical hull. The lifting gas is carried in a dozen or more separate gas cells, nested within the bays of the ship.
The non-rigid airship has no such internal support. The bag keeps its taut shape only from the gas and air pressure maintained within. Release the gas and the bag becomes merely a flabby mass of fabric on the hangar floor. Ship crews do not live in the balloon section, but in the control car below.
The British, apt at nicknames, differentiated between the two types of airships by calling them "rigid" and "limp" types, and since an early "Type B" was widely used in the first World War, quickly contracted "B, limp" into the handier word "Blimp."
The third type, semi-rigid, has a metal keel extending the length of the ship, to which control surfaces and the car are attached, and with a metal cone to stiffen the bow section.
The rigid ship is of German origin. Developed by Count Zeppelin, retired army officer, and largely used by that nation during the war of 1914-18, it was taken up after the war started, by the British and Americans, and to a small extent later by France and Italy.
Non-rigid ships were widely used by the British and French, to a less extent by Italy and United States.
The intermediate semi-rigid was largely Italian and French in war use, though United States bought one ship after the war from the Italians, built one itself. The Germans also built smaller Parseval semi-rigids.
The rigid airships are the largest, the non-rigids smallest. The rigid has to be large to hold enough gas to lift its metal frame along with the load of fuel, oil, crew, supplies, passengers and cargo. The blimps can be much smaller.
The Army's first airship, built by Major Tom Baldwin, old time balloonist, had 19,500 cubic feet capacity. Goodyear's pioneer helium ship "Pilgrim" had 51,000 cubic feet. These contrast with the seven million feet capacity of the Hindenburg, and the ten million cubic feet of ships projected for the future.