|This article needs additional citations for verification. (October 2012)|
Because of Archimedes' principle, a lifting gas is required for aerostats to create buoyancy. Its density is lower than that of air (about 1.29 kg/m3, 1.29 g/L). Only certain lighter than air gases are suitable as lifting gases.
- 1 Gases theoretically suitable for lifting
- 2 Hydrogen versus helium
- 3 High-altitude ballooning
- 4 Submerged balloons
- 5 Balloons on other celestial bodies
- 6 See also
- 7 References
Gases theoretically suitable for lifting
Heated 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 mass 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 (14 times less dense than air), seems to be the most appropriate gas for lifting. Moreover, it is very cheap and easy to produce, e.g., by electrolysis of water or by displacement from an acid or water by a reactive metal. 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 and Germany. The Hindenburg disaster is frequently cited as an example of the hydrogen 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, so that the balloon will deflate quickly. (That's the reason why most hydrogen- or helium-filled balloons are coated by a thin layer of aluminium.)
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 same diffusion problem as above described with hydrogen;
- Helium is very 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 the 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, it eventually escapes into space and is permanently lost.
Steam / water vapor
The gaseous state of water is lighter than air, incombustible 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. However, such a design is generally impractical due to high boiling point and condensation.
Ammonia is sometimes used to fill weather balloons. 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 is relatively heavy (0.6 kg/m3), poisonous, and an irritant.
Methane, the main component of natural gas, 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 smaller molecules of hydrogen and helium. 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.
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.
Neon is lighter than air and could lift a balloon. Like helium, it is incombustible. 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. Nevertheless, an aerogel named SEAgel (Safe Emulsion agar gel) has been produced that floats in air if it is filled with pure nitrogen.
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, the Italian monk Francesco Lana de Terzi postulated a ship with four vacuum spheres.
In a theoretically perfect situation with weightless spheres, a 'vacuum balloon' would be 7% lighter than a hydrogen-filled balloon, and 16% lighter 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.
Another medium that in theory could be used is a plasma: Ions repelling each other could give a pressure that counteracts the atmospheric pressure. But this seems to be 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.
Hydrogen versus helium
Hydrogen and helium 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 is minute. Both provide about 9.8 N of lift (1 newton is the force required to accelerate 1 kg at 1 m/s2) per cubic meter of gas at STP.
The lifting power in air of hydrogen and helium can be calculated using the theory of buoyancy as follows:
The density at sea-level and 0 °C for air and each of the gases is:
Thus helium is almost twice as dense as hydrogen. However, buoyancy depends upon the difference of the densities (ρgas) − (ρair) rather than upon their ratios. Thus the difference in buoyancies is about 8%, as seen from the buoyancy equation:
- Buoyant mass (or effective mass) = mass × (1 − ρair/ρgas)
- Therefore the buoyant mass for one m3 of hydrogen in air is:
- 0.090 kg * (1 − (1.292 / 0.090) ) = −1.202 kg
- And the buoyant mass for one m3 of helium in air is:
- 0.178 kg * (1 − (1.292 / 0.178) ) = −1.114 kg
The negative signs indicate that these gases tend to rise in air.
Thus hydrogen's additional buoyancy compared to helium is:
- 1.202 / 1.114 ≈ 1.080, or approximately 8.0%
At higher altitude, the air pressure is lower and therefore the pressure inside the balloon is lower. That means that while the mass of lifting gas and mass of displaced air for a given lift are the same as at lower altitude, the volume of the balloon is much greater.
A balloon that is designed to lift to extreme heights (stratosphere), must be able to expand enormously in order to displace the required amount of air. That is why such balloons seem almost empty at launch, as can be seen in the photo.
A different approach for high altitude ballooning, especially used for long duration flights is the superpressure balloon.
Because of the enormous density difference between water and gases (water is about 1,000 times more dense than most gases), the lifting power of underwater gases is very strong. The type of gas used is largely inconsequential because the relative differences between gases is negligible in relation to the density of water. However, some gases can liquefy under high pressure, leading to an abrupt loss of buoyancy.
A submerged balloon that rises will expand or even explode because of the strong pressure reduction, unless gas is able to escape continuously during the ascent or the balloon is strong enough to withstand the change in pressure.
Balloons on other celestial bodies
A balloon can only have buoyancy if there is a medium that has a higher average density than the balloon itself.
- Balloons cannot work on the Moon because it has almost no atmosphere.
- Mars has a very thin atmosphere – the pressure is only 1/160th of earth atmospheric pressure – so a huge balloon would be needed even for a tiny lifting effect. Overcoming the weight of such a balloon would be difficult, but several proposals to explore Mars with balloons have been made.
- On Venus, the density of the CO2 atmosphere at the surface is 65 kg/m3, fifty times that of Earth, and gravity is comparable to that on the Earth's surface. A small balloon could lift substantial weights. In 1985, the Soviet Vega program sent two balloons to float in Venus' atmosphere at 54 km altitude. Because CO2 is so dense, ordinary (Earth) air would be a lifting gas on Venus. This has led to proposals for a human habitat that would float in the atmosphere of Venus at an altitude where both the pressure and the temperature are earthlike.
- Titan, Saturn's largest moon, has a dense atmosphere of mostly nitrogen that is appropriate for ballooning. There are currently plans for exploring Titan using balloons.
- "HeiDAS UH – Ein Heissdampfaerostat mit ultra-heiss-performance". Aeroix.de. Retrieved 2012-10-21.
- Tom D. Crouch (2009). Lighter Than Air
- "[physics/0610222] Stability Analysis of an Inflatable Vacuum Chamber". Arxiv.org. Retrieved 2012-10-21.
- "Exploring Mars With Balloons". Spacedaily.com. Retrieved 2012-10-21.
- [dead link]