Lighter than air

Lighter than air gases are buoyant in air because they have a density that is less than the density of air. Lighter than air gases are used to fill balloons, airships, and aerostats. (Heavier than air aircraft include aeroplanes and helicopters.)

Hot air

The density of a gas can be reduced by raising its temperature. Heated air is widely used as a lifting gas in hot-air balloons. (The gas in a hot-air balloon is not only heated air, but also includes the products of combustion from the balloon's burner.)

Hot air balloons have the advantage of straightforward lift control. To increase lift, more heat is applied. To decrease lift slowly, the hot air is allowed to cool. To decrease lift quickly, hot air is released (vented). However, unlike balloons using low molecular mass gases (see below), hot air balloons require nearly continuous burning of fuel in order to remain aloft. (See Hot air balloon.)

Low molecular mass gases

Since the average molecular mass of air is 28.97, any gas with a lower molecular mass will be lighter than air (at the same temperature and pressure). A balloon containing lighter than air gas will expand as it rises. Weather balloons can be made with strong "envelopes" (buoyant containers) so that they maintain maximum height at maximum volume (instead of exploding).

Determination of lighter than air gases is straightforward. These gases must have an atomic or molecular mass less than 28.97 and exist as a gas at room temperature. (This temperature requirement is based on our definition of a lighter than air gas; other definitions are possible.)

The following elements with atomic masses below 28.97 have high boiling points: silicon, aluminum, magnesium, sodium, carbon, boron, beryllium and lithium. The hydrides, fluorides and oxides of aluminum, magnesium, sodium, beryllium and lithium are solids at room temperature and either decompose before boiling or have high boiling points. (See Ionic compound.)

Water is a special case. Although water is not a gas at room temperature, steam has been used as a lighter than air gas. See discussion below.

**Lighter than air gases**
Compound	Formula	Mass	Comments
Nitrogen	N₂	28	Majority component of air (~78%)
Carbon monoxide	CO	28	Toxic, flammable
Ethylene	C₂H₄	28	Flammable, reactive
Diborane	B₂H₆	27.6	Spontaneously flammable in air
Hydrogen cyanide	HCN	27	Very toxic, flammable, water soluble
Acetylene	C₂H₂	26	Very flammable, reactive
Methyllithium	LiCH₃	21.9	Very flammable, very reactive, explodes on contact with moisture
Neon	Ne	20.2	Noble gas, similar price to helium with much less lift
Hydrogen fluoride	HF	20	Very toxic, very corrosive, water soluble
Ammonia	NH₃	17	Toxic (at higher concentrations), slightly flammable, water soluble, easy to liquify
Methane	CH₄	16	Flammable, inexpensive, widely available
Helium	He	4	Noble gas, expensive, very safe, small atomic size makes it prone to leakage
Hydrogen	H₂	2	Very flammable, inexpensive, prone to leakage

Many of these gases are not practical for use in balloons. The following combine poor lift with objectionable properties: carbon monoxide, hydrogen cyanide, hydrogen fluoride, methyllithium, diborane, ethylene and acetylene. Nitrogen has negligible lift. Neon is harmless and offers a modest degree of lift; however it costs roughly the same as helium, another noble gas with far superior lift. The four remaining gases (ammonia, methane, helium, and hydrogen) have been used as balloon gases.

Ammonia 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 easily reduce lift and add ballast (and returned to a gas to add lift and reduce ballast).

Methane (the chief 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 hydrogen and helium. (Most lighter than air balloons are made of aluminized plastic that limits such leakage; hydrogen and helium leak rapidly through latex balloons.)

Hydrogen and helium are the most commonly used lifting gases. Helium and hydrogen both provide about 1 kg of lift per cubic meter of gas at room temperature and sea level pressure. Although hydrogen is slightly more buoyant, helium is usually preferred because it is not flammable.

Many countries have banned the use of hydrogen as a lifting gas for manned vehicles. The Hindenburg disaster is frequently cited as an example of the risks posed by hydrogen. The high cost of helium (compared to hydrogen) has led researchers to reinvestigate the safety issues of using hydrogen as a lifting gas: with good engineering and good handling practices, the risks can be significantly reduced. A sensible policy might allow hydrogen for cargo airships (both those unmanned and those manned only by pilots) and require helium for passenger airships.

Steam

Although water is not a gas at room temperature and sea level pressure, water combines readily with dry air (until the partial pressure of water reaches its saturation vapor pressure). Moist air is lighter than dry air. Most hot air balloons burn propane to provide heat; the combustion products have an average molecular mass of 29.1; the "light" water vapor compensates for the "heavy" carbon dioxide. Pure water vapor (steam) could be used to lift balloons; however, the balloons would need to be double-walled to provide insulation, and condensation would be a serious problem. Nonetheless, two research efforts are currently underway to build steam-based aircraft. (See external links below.)

Low pressure buoyancy

The average density of an aircraft can be reduced, at least in principle, by creating a partial vacuum. The concept of an airship supported by the buoyancy of a vacuum has been explored in science fiction. The envelope must be strong enough to resist crushing by external atmospheric pressure and light enough to be lighter than air. No such device has ever been constructed.

Derivation

At low densities, the behaviour of gases is well approximated by the ideal gas law

pV = nRT

where $p$ is pressure, $V$ is volume, $n$ is the number of moles of gas, $T$ is absolute temperature, and $R$ is the universal gas constant.

Dividing both sides by $V$ , $R$ and $T$ gives

\frac{p}{RT} = \frac{n}{V}.

Now multiply each side by $M$ , the molecular mass of the gas in question:

\frac{pM}{RT} = \frac{nM}{V}

Notice that $nM$ , the number of moles multiplied by the mass per mole, is simply the total mass of the gas. And mass divided by volume is density. So,

\rho = \frac{pM}{RT}

where $\rho$ is the density of the gas. This equation shows that a gas with low density can be achieved by:

Lowering $p$ , the pressure;
Lowering $M$ , the molecular mass;
Raising $T$ , the absolute temperature; or
Some combination of the above.

R

is a physical constant and so cannot be changed.

External links

Fluid mechanics | Atmosphere