Lifting gas

Heated atmospheric air is frequently used in recreational ballooning. According to the Ideal gas law, an amount of gas (and also a mixture of gases such as air) expands as it is heated. As a result, a certain volume of gas has a lower density as the temperature is higher. The average temperature of air in a hot air balloon is about 212 °F (100 °C).

Hydrogen, being the lightest existing gas (7% the density of air), seems to be the most appropriate gas for lifting. It can be easily produced in large quantities, for example with the water-gas shift reaction, but hydrogen has several disadvantages:

• Hydrogen is extremely flammable. Some countries have banned the use of hydrogen as a lift gas for commercial vehicles but it is allowed for recreational free ballooning in the US, UK and Germany. The Hindenburg disaster is frequently cited as an example of the safety risks posed by hydrogen. The high cost of helium (compared to hydrogen) has led researchers to re-investigate the safety issues of using hydrogen as a lift gas; with good engineering and good handling practices, the risks can be significantly reduced.
• Because the hydrogen molecule is very small, it can easily diffuse through many materials such as latex, so that the balloon will deflate quickly. This is one reason that many hydrogen or helium filled balloons are constructed out of Mylar/BoPET.

Helium is the second lightest gas. For that reason, it is an attractive gas for lifting as well.

A major advantage is that this gas is noncombustible. But the use of helium has some disadvantages, too:

• The diffusion issue shared with Hydrogen (though, as Helium's molecular radius is smaller, it diffuses through more materials than Hydrogen).
• Helium is expensive.
• Although abundant in the universe, helium is very scarce on Earth. The only commercially viable reserves are a few natural gas wells, mostly in the US, that trapped it from the slow alpha decay of radioactive materials within Earth. By human standards helium is a non-renewable resource that cannot be practically manufactured from other materials. When released into the atmosphere, e.g., when a helium-filled balloon leaks or bursts, helium eventually escapes into space and is lost.

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 so 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.[2] 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.

The gaseous state of water is lighter than air (density 0.804 g/L at STP, average molecular mass 18.015 g/mol) due to water's low molar mass when compared with typical atmospheric gases such as nitrogen gas (N₂). It is non-flammable and much cheaper than helium. The concept of using steam for lifting is therefore already 200 years old. The biggest challenge has always been to make a material that can resist it. In 2003, a university team in Berlin, Germany, has successfully made a 150 °C steam lifted balloon.[3] However, such a design is generally impractical due to high boiling point and condensation.

Ammonia is sometimes used to fill weather balloons.[4] Due to its high boiling point (compared to helium and hydrogen), ammonia could potentially be refrigerated and liquefied aboard an airship to reduce lift and add ballast (and returned to a gas to add lift and reduce ballast). Ammonia gas is relatively heavy (density 0.769 g/L at STP, average molecular mass 17.03 g/mol), poisonous, an irritant, and can damage many metals and plastics.

Methane (density 0.716 g/L at STP, average molecular mass 16.04 g/mol), the main component of natural gas, is sometimes used as a lift gas when hydrogen and helium are not available.[4] It has the advantage of not leaking through balloon walls as rapidly as the smaller molecules of 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. However, methane is highly flammable and like hydrogen is not appropriate for use in passenger-carrying airships. It is also relatively dense and a potent greenhouse gas.

Hydrogen fluoride is lighter than air and could theoretically be used as a lifting gas. However, it is extremely corrosive, highly toxic, expensive, is heavier than other lifting gases, and has a high boiling point of 19.5 °C. Its use would therefore be impractical.

In the past, coal gas, a mixture of hydrogen, carbon monoxide and other gases, was also used in balloons. It was widely available and cheap; the down side was a higher density (reducing lift) and the high toxicity of the carbon monoxide.

Acetylene is 10% lighter than air and could be used as a lifting gas. Its extreme flammability and low lifting power make it an unattractive choice.

Hydrogen cyanide, which is 7% lighter than air, is technically capable of being used as a lifting gas at temperatures above its boiling point of 25.6 °C. Its extreme toxicity, low buoyancy, and high boiling point have precluded such a use.

Neon is lighter than air (density 0.900 g/L at STP, average atomic mass 20.17 g/mol) and could lift a balloon. Like helium, it is non-flammable. However, it is rare on Earth and expensive, and is among the heavier lifting gases.

Pure nitrogen has the advantage that it is inert and abundantly available, because it is the major component of air. However, because nitrogen is only 3% lighter than air, it is not an obvious choice for a lifting gas.

The de Lana-Terzi's vacuum airship (1670)

Theoretically, an aerostatic vehicle could be made to use a vacuum or partial vacuum. As early as 1670, over a century before the first manned hot-air balloon flight,[5] the Italian monk Francesco Lana de Terzi envisioned a ship with four vacuum spheres.

In a theoretically perfect situation with weightless spheres, a 'vacuum balloon' would have 7% more net lifting force than a hydrogen-filled balloon, and 16% more net lifting force than a helium-filled one. However, because the walls of the balloon must be able to remain rigid without imploding, the balloon is impractical to construct with all known materials. Despite that, sometimes there is discussion on the topic.[6]

Another medium that in theory could be used is a plasma: Ions repelling each other could give a pressure intermediate between vacuum and hydrogen and hence that counteracts the atmospheric pressure. The energy and the containment requirements are extremely impractical, so that it may only be interesting for science fiction.

It is also possible to combine some of the above solutions. A well-known example is the Rozière balloon which combines a core of helium with an outer shell of hot air.

In 2002, aerogel held the Guinness World Record for the least dense (lightest) solid.[7] 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.[8] 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.[9]

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).[10][11] 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.