We steampunks love our airships. Not because they’re particularly efficient or fearsome flying machines, but because they provide the most amusement per pound than any other vehicle.
There is a body of science that describes the performance of airships—much of which is blithely ignored or at least subverted in steampunk stories and artwork. My stories—the Airship Flamel Adventures—feature an airship whose characteristics have at best a tenuous relationship with actual Airship Science. So I know whereof I speak. However, I recently discovered a novel airship technology that seems completely impractical (and being more impractical than a standard airship is quite an accomplishment) yet which contains just enough real science to keep things interesting.
Airships, including hot air balloons, work because they have a large volume filled with a gas that is less dense than air. The gas weighs less than air, so it wants to float. If you add in the weight of the rest of the airship (such as the gondola and the cells containing the lifting gas) and the ship still floats, then you’re in business! You’ve got an airship that will fly. (If not, however, your airship sits obstinately on the ground.)
In the history of airships, hydrogen was found to have the most lifting power. Air, at STP (chemist-speak for standard temperature and pressure—0 degree Celsius and 1 atmosphere pressure) weighs1.28 g/L (grams per liter. Hydrogen weighs only 0.09 g/L, so every liter of hydrogen gas that replaces a liter of air in the gas cell can lift 1.28 minus 0.09 or 1.19 grams of payload. However, Hydrogen has the unfortunate tendency to burn of explode under certain conditions, so a replacement would be helpful. Helium, the next lightest gas, provides only 1.1 grams of lift per liter, but is inert and doesn’t burn. So, after the Hindenburg disaster, there was a great push to use Helium in airships.
If only there were a way to increase the lift power of Helium without losing its inertness. Well, remember STP? If we operate at a lower pressure than 1 atmosphere, then the density of the Helium in the lifting cells would be lower than 1.1 g/L, and the lifting power greater. Of course, if we reduce the pressure of the Helium further, eventually the cells will be filled with nothing—just a vacuum. Of course, a vacuum would be the ideal lifting “gas” as its density is 0 g/L.
But there’s a problem. As we reduce the pressure in the gas cells, they will tend to collapse unless we build a stronger (and heavier) structure to keep the cells expanded. But the idea would work theoretically, if we had structural materials strong enough and light enough to prevent the gas cells from collapsing. Theoretically.
Surprisingly, this idea is not new. It was first proposed by Francesco Lana de Terzi, an Italian Jesuit priest in 1670 in the very early days of balloons. He conceived large copper spheres from which the air was evacuated. Theoretically, his ship could life six people. Theoretically. In reality, thin and light copper spheres could not be manufactured, and they would be so thin that they would collapse in any case.
However, the concept was rediscovered in the 1800s by Arthur de Bausset who attempted to patent his designs as well as raise operating funds in his “Transcontinental Aerial Navigation Company of Chicago”. He was unsuccessful in both undertakings.
Buckminster Fuller conceived of gas cells supported by geodesic spheres which would provide higher compressive strength to prevent the structure from collapsing.
Even NASA is looking into vacuum airships. Saturn’s moon, Titan, has a cold and dense atmosphere, so a rather small-sized vacuum cell might be sufficient to provide lift. Other groups are looking into airships for the thin atmosphere of Mars.
Vacuum airships are an interesting concept that might be feasible with sufficiently advanced materials. The big question is whether they’ll be perfected first by aeronautical engineers or steampunk air captains.