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There are two forms of lighter-than-air-craft the balloon and the airship/dirigible/blimp; the generic term for both is aerostat. Both are able to ascend into the sky and stay there
because they contain a substance that is lighter than the air that surrounds them. Hot air was the balloon's first lifting substance, but flight time is limited because of the simple
fact that heated air will eventually cool. Sustained hot-air ballooning did not become feasible until after World War II with the innovation of the propane burner, which permits
reheating while the craft is aloft. The principal and preferred lifting substance for both balloons and airships was hydrogen, the lightest of the elements, despite it being highly
dangerous because of its extreme flammability. It was not succeeded by helium (which although somewhat inferior to hydrogen in lifting strength will not burn or explode) until
a crash American research program beginning in 1917 developed a means of extracting it cheaply in large quantities from the natural gas in which it is found.
The fundamental difference between the airship and the balloon is that the former is powered and horizontally controllable and the latter is neither. By the start of World
War One the airship had been developed into two main types - rigid and non-rigid; both types being cylindrical in cross-section, gas bouyant and propeller engine-driven. Their
military uses were apparent (having first been used in the American Civil War) and they entered service in large numbers. Contrary to popular belief the germans were not the
largest users of them - it was actually the british.
During the 1930s they passed from military use in most countries with the exception of the USA where they were used for anti-submarine work well into the Second World War.
In recent times interest in aerostats has been rekindles and any number of projects are being studied.
Most of the aerostats constructed have, at some time or another been used for military purposes so I have included all of them in this section.
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