Lifting Gas in Airships

Most people know that zeppelins like the Hindenburg were filled with a flammable gas known as hydrogen, and that the Hindenburg disaster was caused by this gas igniting. Some also know that American airships were filled with the inert gas helium, but that the Zeppelin company was not allowed to purchase helium from the Americans. But what are the other characteristics of these gases, and are there any other alternatives for providing lift to airships?

Hydrogen is the lightest element in the universe (and the most abundant). It occupies the first position on the periodic table, consisting of a single proton orbited by a single electron. Most of the hydrogen found on Earth is bound up with oxygen to create water (H20), but it is relatively easy to split the water molecules using electrolysis into their constituent elemental hydrogen and oxygen.

Because hydrogen has only a single proton in its nucleus it is extremely reactive, and liable to form bonds with other atoms to create compounds like water; its volatility is the reason why it is so flammable. In contrast, helium, which occupies the next position on the periodic table, has two protons in its nucleus and is therefore a stable element. It doesn’t try to connect with other atoms, and is thus stable and inert, and not flammable.

One of the interesting things about helium is that it was discovered in space before any was found on Earth. Spectrometry is the process by which the presence of elements can be detected using their spectral signatures in light; so a particular element will produce a particular frequency of light in the visible spectrum. Helium’s spectral signature was first detected in the light coming from our Sun, and thus the element was named for the Greek god of the Sun, Helios. This was in 1868, but helium was not discovered on Earth itself until 1903, in natural gas fields in the US.

Helium is quite rare, being found in natural gas at concentrations of up to 7%, from which it is separated using fractional distillation. Helium possesses escape velocity; any helium released into the atmosphere escapes the earth’s gravitational pull and is lost into space.

Both hydrogen and helium produce lift in airships by their property of being lighter than air. The relative buoyancy of both gases means that any enclosed envelope which is filled with the gas will exert an upward force if placed within the atmosphere. Hydrogen is slightly more buoyant as it has one less proton and is therefore lighter than helium. The density of air at standard temperature and pressure is 1.28 grams per litre, so 1 litre of displaced air has sufficient buoyant force to lift 1.28 grams. But since the airbag of an airship uses hydrogen or helium to displace the air, the weight of the gas must be subtracted from that. The mass of one litre of helium is 0.18 grams, so the lifting force is thus reduced to 1.1 grams when helium is used.

What if, instead of using a gas to displace the air, one used only vacuum? Then there would be no weight from the gas, and the maximum lifting force would be obtained. This would seem to be the most efficient option — except for the problem posed by the fact that the pressure of the air would tend to collapse the vacuum container, unless it was very strong. A strong container would not be very light, so any weight advantage would be lost, at least with the current state of material science. (In Gene Wolfe’s Book of the New Sun series, the flyers do seem to make use of vacuum chambers to stay aloft, presumably by using some novel super–strong, super–light material.)

But what is truly amazing to me is how little advantage the vacuum setup would achieve over using even helium, the less efficient of the two commonly used lifting gases. Helium is only 14% less efficient than vacuum. The molecules of helium gas are so negligible that they add very little to the weight of the airship, yet they exert sufficient force on the bag to counteract the pressure of the air that they are displacing. Remove that tiny amount of matter, and now you need a very strong vessel to fight back against the crushing pressure of the air on the enclosed vacuum.
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