AIRSHIP
An airship is a buoyant aircraft that can be steered and propelled through the air. Unlike aerodynamic craft (e.g. airplanes and helicopters) which stay aloft by moving an airfoil through the air in order to produce lift, aerostatic craft such as airships (and balloons) stay aloft primarily by means of a cavity (usually quite large) filled with a gas of lesser density than the surrounding atmosphere.
In the early days of airships, the primary lifting gas was hydrogen. Until the 1950s, all airships, except for those in the United States, continued to use hydrogen because it offered greater lift and was cheaper than helium. The United States (until then the sole producer) was also unwilling to export helium because of its rarity and the fact it was considered a strategic material. However, hydrogen is flammable when mixed with air, a quality that some think contributed to the Hindenburg disaster, as well as other rigid airship disasters. The buoyancy provided by hydrogen is actually only about 8% greater than that of helium[1] . The issue therefore became one of safety versus cost. American airships have been filled with helium since the 1920s and modern passenger-carrying airships are often, by law, prohibited from being filled with hydrogen.
Nonetheless, some small experimental ships still use hydrogen today. Other small experimental ships (i.e. thermal airships) are filled with hot air in a fashion similar to hot air balloons.