In order to provide buoyancy, any lifting gas must be lighter, i.e. less dense, than the surrounding air. A hot air balloon is open at the bottom to allow hot air to enter, while the gas balloon is closed to stop the (cold) lifting gas from escaping. Common lifting gases have included hydrogen, coal gas and helium.
When heated, air expands. This lowers its density and creates lift. Small hot air balloons or lanterns have been flown in China since ancient times. The first modern man-lifting aerostat, made by the Montgolfier brothers, was a hot air balloon. Most early balloons however were gas balloons. Interest in the sport of hot air ballooning reawoke in the second half of the twentieth century and even some hot-air airships have been flown.
Hydrogen is the lightest of all gases and a manned hydrogen balloon was flown soon after the Montgolfier brothers. There is no need to burn fuel, so a gas balloon can stay aloft far longer than a hot-air balloon. It is also safer if there is no flame on board, since the materials used to make aerostats are flammable. Hydrogen soon became the most common lifting gas for both balloons and, later, airships. But hydrogen itself is flammable and, following several major disasters in the 1930s, it fell out of use.
Coal gas comprises a mix of methane and other gases, and typically has about half the lifting power of hydrogen. In the late nineteenth and early twentieth centuries municipal gas works became common and provided a cheap source of lifting gas. Some works were able to produce a special mix for ballooning events, incorporating a higher proportion of hydrogen and less carbon monoxide, to improve its lifting power.
Helium is the only lifting gas which is both non-flammable and non-toxic, and it has almost as much (about 92%) lifting power as hydrogen. But it was not discovered in quantity until early in the twentieth century, and for many years only the USA had enough to use in airships. Nowadays, almost all gas balloons and airships use helium.
- Ege, Lennart; Balloons and Airships, Blandford, 1973, Pages 110 ff.
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