Lifting gas
Carrying gas is a gas that is used as a filling for airships and gas balloons . It has a lower density than the aircraft surrounding air . This results in a static buoyancy according to the Archimedean principle .
The most important carrier gases are hydrogen , helium and hot air .
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For this purpose, gases can be used that have a lower density than air (approx. 1.293 kg / m 3 under normal conditions ). From a physical point of view, hydrogen is ideal for this as the lightest of all elements with an atomic mass of only ~ 1.0u, molecular mass H 2 of 2u and a density of only 0.0899 kg / m 3 . Since this gas is also relatively easy and cheap to produce, it was predominant as a lifting gas well into the 20th century. However, hydrogen is highly flammable and even explosive when mixed with oxygen , for example from the air ( oxyhydrogen ). Because of these disadvantages, it was gradually displaced by the inert noble gas helium as soon as it could be produced in sufficient quantities. Before the Second World War, however, only the United States could do this . In balloon sport , however, only hydrogen is still used for gas balloons, since a helium filling would be far too expensive.
Helium, the second lightest element, has four times the atomic mass with ~ 4.0 u, but since it exists as a noble gas in the form of atoms and not diatomic molecules, it has only one double, according to Avogadro's law , 0.1785 kg / m 3 as great a density as hydrogen. Since it is not the absolute density but the difference to the air density that is decisive for buoyancy, a helium filling only generates about 8% less buoyancy than a hydrogen filling.
Both helium and hydrogen have the property of diffusing through many substances (balloon envelope) . It can also happen that gas has to be released to compensate for buoyancy . In the case of constantly filled balloons or airships, a small part of the gas must therefore be replaced at regular intervals.
With one cubic meter of hydrogen, a static lift of 1.203 kg can be generated, with one cubic meter of helium a lift of 1.1145 kg; however, these values only apply under normal conditions . At some altitude there is a lower air pressure (see barometric height formula ), which requires a larger lifting gas volume (but with the same lifting gas mass) for the same buoyancy.
The stated values result in a generous rule of thumb: To lift a mass of one kilogram with balloon buoyancy, around one cubic meter of lifting gas is required, corresponding to one gram of one liter .
In the past, the easily available and cheap illuminating gas was also used as a lifting gas. In addition to the disadvantage of a significantly lower buoyancy, the fire hazard and toxicity had to be accepted. Since the public gas supply was switched to natural gas , there is no longer any coal gas available.
Over 100 ° C, superheated steam is also available as a lifting gas, which (molecular mass = 18) generates about twice as much lift per gas volume as air of the same temperature, and thus almost three quarters of that of helium or hydrogen. In 2006, the scaled model of a hot steam balloon, the hot steam aerostat HeiDAS UH, was presented in Germany. At a steam temperature of 150 ° C, a lift of 735 grams / cubic meter was achieved. The price for this service is paid for by unusual materials, because the shell material is steam-resistant like stainless steel, but light like plastic. Water vapor is the most effective non-flammable lifting gas, which is not threatened by scarcity of resources.
Since the helium deposits are limited and the gas is becoming increasingly scarce and expensive due to the increasing consumption, balloon and airship technology has switched to mixing hydrogen and helium in a certain volume ratio. This gas mixture then has a higher load-bearing capacity than helium alone, but is cheaper and, on the other hand, unreactive and non-flammable (inert) in contrast to hydrogen.