Lighter than air
Lighter than air refers to materials (usually gases) that are buoyant in air because they have average densities lower than that of air. Dry air has a density of about 1.29 g/L (gram per liter) at standard conditions for temperature and pressure (STP) and an average molecular mass of 28.97 g/mol.
Some of these gases are used as lifting gases in lighter-than-air aircraft, which include free balloons, moored balloons, and airships. Heavier-than-air aircraft include airplanes, gliders and helicopters.
Usage as lifting gas
Neon (density 0.900 g/L at STP, average atomic mass 20.17g/mol) is lighter than air and will lift a balloon. However, it is relatively rare on Earth, expensive, and is among the heavier of the lifting gases.
Water vapor (density 0.804 g/L at STP, average molecular mass 18.015 g/mol) is lighter than air, and has successfully been used as a lifting gas. It is generally impractical due to high boiling point and condensation.
Ammonia (density 0.769 g/L at STP, average molecular mass 17.03 g/mol) has sometimes been used to fill weather balloons. Due to its relatively high boiling point (compared to helium and hydrogen), ammonia could potentially be refrigerated and liquified aboard an airship to reduce lift and add ballast (and returned to a gas to add lift and reduce ballast).
Methane (density 0.716 g/L at STP, average molecular mass 16.04 g/mol) is the chief component of natural gas and is sometimes used as a lift gas when hydrogen and helium are not available. It has the advantage of not leaking through balloon walls as rapidly as the small-moleculed hydrogen and helium. Many lighter-than-air balloons are made of aluminized plastic that limits such leakage; hydrogen and helium leak rapidly through latex balloons.
Hydrogen and helium
Hydrogen (density 0.090 g/L at STP, average molecular mass 2.016 g/mol) and helium (density 0.179 g/L at STP, average molecular mass 4.003 g/mol) are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both much lighter than air that this difference only results in hydrogen having 8% more buoyancy than helium.
In a practical dirigible design, the difference is significant, making a 50% difference in the fuel-carrying capacity of the dirigible and hence increasing its range significantly. However, hydrogen is extremely flammable and its use as a lifting gas in dirigibles has decreased since the Hindenburg disaster. Helium is safer as a lifting gas because it is inert and does not undergo combustion.
In 2002, aerogel held the Guinness World Record for the least dense (lightest) solid. Aerogel is mostly air because its structure is like that of a highly vacuous sponge. The lightness and low density is due primarily to the large proportion of air within the solid and not the silicon construction materials. Taking advantage of this, SEAgel, in the same family as aerogel but made from agar, can be filled with helium gas to create a solid which floats when placed in an open top container filled with a dense gas.
In 2012, discovery of aerographite was announced, breaking the record for the least dense material at only 0.2 mg/cm3 (0.2 kg/m3). These solids do not float in air because the hollow spaces in them become filled with air. No lighter-than-air matrix or shell containing a hard vacuum has ever been constructed.
- Balloon (aircraft)
- Heavier than air
- Hot air balloon
- Vacuum balloon
- Vacuum airship
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