When I was a kid, back in the long ago, I would scour the elementary school library for any and every book on aircraft. One of my third-grade art projects featured a construction-paper airplane with a proper airfoil. In other words, I was into flying machines of every variety in a very serious way. However, nothing quite filled my nerdy heart like airships.
Let’s face it, while airplanes are remarkable machines, the experience of traveling in those fixed-wing speedsters of the skies isn’t that much different than traveling by bus. There’s something about the idea of traveling in a machine more akin to a floating hotel that sounds simply spectacular. It’s no wonder they are a staple of the steampunk universe.
There was little doubt that I would include an airship in my first steampunk story, but I had to think about how a steampunk airship would actually work. The story was aimed toward middle-grade readers and the airship is merely a backdrop for a small part of it, so I had no intention to include lots of details, but I wanted what did show up to make at least some sense.
The first item to figure out was what type of airship it would be. Airships basically fall into three categories: blimps which hold their shape purely with air pressure, semi-rigid airships which also hold their shape with air pressure and include some metal structure to distribute forces, and rigid airships whose shape is defined by its metal structure (sometimes called Zeppelins even though the Zeppelin company wasn’t the only manufacturer of such machines). Weight is everything with airships so the less of that heavy structure the better. However, as size increases more structure is needed to deal with the loads the airship has to carry. This means that when we look toward history, all the truly large airships have been of the rigid variety. If large airships were produced today, advances in engineering would likely result in the use of a semi-rigid design to reduce weight. My airship needed to be big, so a rigid design was the only possible choice.
That led me to the tricky part. How do you propel a huge airship? In the real world, airships were powered with internal combustion engines rather than the external combustion engines that powered the age of steam. External combustion means burning fuel to heat water in a boiler to produce the steam pressure that powers the engine. That means a heavy engine and plenty of hot flame. Heavy is always bad in an airship and flames… well, everyone knows about the Hindenburg.
As far as weight goes, I think it means that steampunk airships need to be big. A huge airship can carry enough lifting gas to support the structure needed for a powerful steam engine and boilers. The airship would need to carry water as well, but that is historically the case anyhow. In order to control altitude an airship needs to be able to change either the amount of lifting gas or its weight by adding or removing ballast and that ballast was generally water. Removing water for an airship is easy enough, you just release it. Adding water is less obvious, but it can be added by capturing engine exhaust and condensing the steam in the exhaust back into water. A lot of engine exhaust is water vapor. The United States Navy had some large airships for a short period of time (like the USS Macon that served as a flying aircraft carrier) and they relied on capturing water from exhaust to manage altitude. So a steam-powered cousin of those big U.S. Navy airships would probably be larger and slower to carry the same load, but it seems plausible.
Now about the whole flame thing. The best lifting gas for an airship is hydrogen–it’s the lightest element after all. Unfortunately, hydrogen really likes to burn which makes the airship a fireball waiting to happen. There’s a reason that the big airships that used hydrogen for lift mounted their engines outside of the hull.
Well, the obvious solution is to use helium–the second lightest element–as the lifting gas. Unlike hydrogen, helium doesn’t burn and is quite effective at smothering fires. Unfortunately, there’s a problem with helium. Now, helium is about twice as heavy as hydrogen for a given volume, but that’s less of an issue then you might think. Lift in an airship is produced by displacing air and air is so much heavier than either helium or hydrogen that the difference between the two doesn’t have a huge impact. The real problem with helium is the supply. Hydrogen is easy to produce from water so there’s no problem filling the vast volume of an airship. In fact, hydrogen airships would just vent off hydrogen as it consumed fuel and became lighter during a journey. Nobody had to worry about replacing that hydrogen before the next trip. Helium, on the other hand, can’t be produced unless you happen to have a fusion reactor handy. The only significant source of helium is as a by-product of drilling for natural gas. In current times, there’s actually a fair amount of concern about the helium supply. There might not be a lot of airships around, but helium is an important coolant for superconductors.
The infamous Hindenburg was originally intended to use helium rather than hydrogen, but the Germans didn’t have access to enough of it. They also considered surrounding a central section of hydrogen gasbags with helium, but the supply was too short to make that work as well. When the U.S. Navy deployed its large airships, helium was used since the United States had a much greater helium supply than Germany. Maybe the Germans couldn’t make the idea of a hybrid airship work because of their helium shortage, but the idea could work for my imagined steampunk vessel.
With a hybrid design, my airship started to make sense. It would be bulky and slow compared to its real-world counterparts, but using helium to provide a protective layer around its dangerous load of hydrogen meant that the flame and boilers could be used with a plausible degree of safety. Even if hydrogen leaks, it goes up keeping it away from the engine mounted below.
Carrying all that hydrogen meant that it doesn’t even need to carry a separate supply of fuel. It can burn its hydrogen to heat the boilers and power the ship. As I mentioned earlier, Airships normally carried water for ballast and the steam from powering the engines can be captured or released to help control altitude. Since most of the lifting gas would be hydrogen, I can better hand-wave around where all the helium comes from to support fleets of such machines.
With a bit of thought and research, I felt I could justify my steampunk airship. That’s a very good thing since airships are awesome.