Skypirates: lessons in zeppelin aircraft carrier design

When I worldbuild, I put great importance on doing the background work, even background work which doesn’t feature in the foreground very often. So it is with zeppelins in the Skypirates world. Not only do we design them in the same modes and manners as real rigid airships, we’ve discovered that, to our surprise, they’re not quite as implausible as we thought.

Which isn’t to say that they aren’t implausible. We’re making a couple of assumptions rooted in our alternate history that are, well, unlikely. First: we assume that the limit on the size of airships was not one of 1920s and 1930s materials science, but rather one of insufficient ambition. Second: we suppose that engine technology in about 1925, owing to a few more years of the First World War to incubate, has reached levels not seen until ten or fifteen years later1. Third: we assume that large airships are much cheaper than they were in actuality, and that they’re much more common. Fourth: we assume a couple of highly-specific technological advances with respect to gas handling.

If you’re unfamiliar with airships, that last one might seem oddly specific, but it turns out to be critical. There are exactly two practical lifting gases2: hydrogen and helium. You may remember from high school science that hydrogen makes a tremendous *thump* as it blows up in the presence of flame. This doesn’t necessarily rule it out for zeppelins-of-war, but it does push it firmly into the realm of the sub-optimal. That leaves us with helium.

Helium is a pain. Its most notable characteristic3 is its mad delight in escaping every container you try to put it in, including Earth’s atmosphere. Its second most notable characteristic is its relative rarity. Because of its eagerness in leaving the planet once it’s free, there’s very little of it in the air, and it’s difficult to get out of the air. In the timeframe in which the Skypirates stories takes place, the United States historically controlled most of the world’s helium supply4, and, for political reasons, the Germans were unable to buy any for the Hindenburg. (And we all know how that turned out.)

Now, we also had another problem, not all that closely related, but ultimately in the same vein: aircraft-carrying zeppelins are heavy, and anything you put inside a zeppelin’s hull doesn’t just count against your total lift, it reduces it5. I’m going to introduce a term here, and I’m sure it’ll make airship engineers tear their hair out, but here we are: reserve displacement.

If you have a fixed amount of gas inside of a stretchable container, then reduce the pressure outside the container, the container will expand, and the pressure inside will drop. Although less air mass is displaced per unit volume, the container’s volume grows, and the container still makes lift. This effect explains why high-altitude weather balloons look tiny when they take off, and then get huge when they reach high altitudes. High-altitude weather balloons have high amounts of reserve displacement.

Rigid airships aren’t designed with lot of reserve displacement. Their gas cells start out almost fully inflated, for a very simple reason: over the course of a flight, airships get a lot lighter. Your choices are either to vent your lifting gas when you’re nearly to your destination, or rely on complicated ballast recovery systems to capture water vapor from your engine exhaust. One puts you at the mercy of the ground facilities at your destination6, and the other only works in the absence of heavy flight operations. Launching and recovering lots of planes is the same thing as dropping and taking on tens or hundreds of tons of ballast over the course of a few hours, and reality has no way to mitigate that.

So, we’re presented with three little plausibility concerns that get in the way of storytelling: zeppelins don’t have much room for gas cell expansion, limiting them to a narrow band of altitudes; realistic methods for landing require a loss of lifting gas unlikely to be available at your friendly neighborhood jungle ruin filling station; and air operations break all normal procedures for trimming and ballasting. We invented two pieces of retro-scientific fictional technology to gloss over those plausibility issues, both products of fictional Imperial German zeppelin pioneer Karl von Rubenstein.

The von Rubenstein cell is a specially-treated fabric gas cell with a fantastically7 useful characteristic: it is helium-impermeable (don’t ask me to explain it; I just said it’s fiction). von Rubenstein cells can be used as trim tanks, in a sense—if you limit a cell’s expansion, you can pump helium into it, and plain air into the others. The air-helium mix generates less lift per unit displacement, and the trim cell is holding more helium, but isn’t displacing any more air, and the lift goes down.

The von Rubenstein pump is what lets us move helium around so easily. Passing a mix of gases through its machinery yields helium as one output, and everything else as another. The air-helium mix inside a gas cell can easily be separated back into nearly-pure helium, thus raising the trim altitude again, and even atmospheric helium can be extracted, albeit slowly.

Combining the two, we have a model for airship operations. The secret is that modern zeppelins in the Skypirates world aren’t built with a lot of ballast. They’re built for a ceiling, with helium capacity and reserve displacement for that ceiling. In their untouched configuration, that’s their ‘trim altitude’—where they’ll end up, absent other concerns. To descend, or to land, or to launch aircraft, the crew pumps extra helium into the trim cells, and the zep’s lifting capacity goes down.

Practically speaking, there are a few inescapable limitations. For one, a zeppelin’s initial trim altitude—its pressure height, as the technical term goes—is inversely related to its lifting capacity. To leave room in the gas cells for high-altitude expansion, the gas cells must not be filled at sea level, and doing so leaves buoyancy on the table. To some degree, trim cells mitigate this—the excess can be pumped in and held at pressure, and eventually the reduced amount of helium in the lift cells will balance out the zeppelin’s weight, or either trim or lift cells will explode8. Between that and limited ballast, though, we have the ability to let our zeppelins cruise at a broad range of altitudes, which was our aim in the first place.

Another of those inescapable limitations comes from structural strength and weight. It strikes me as unlikely that zeppelins of the bulk we’re talking about could be built as lightly as we say the are. Assuming I’m wrong, we’re within about 10-15% of actual, possible zeppelin designs, in an ideal world, with our fantasy technologies. If I’m right, the figure is more like 25% or 30% off, I’d wager.

Even so. When I first went over the design rules parvusimperator found for zeppelins, I thought to myself, “This is awesome, but utterly impossible.” From ‘utterly impossible’ to ‘we just need to be about one-third lighter than aluminum actually is’? That’s progress.

Expect a few more posts on zeppelin design in the near future. For one, I’ve completed a first draft of a vaguely technical cutaway drawing of Inconstant, which may eventually show up on a mug, and I want to go over it some. For another, I mentioned the zeppelin construction rules, and I feel like I should provide those, too, pending parvusimperator’s approval.

1. This is not zeppelin-specific, but it does explain our extreme fuel efficiency.
2. Impractical lifting gases include water vapor, ammonia, methane, simple hot air, and (yes, I know it isn’t a gas) vacuum, all of which don’t work in large airships for various reasons.
3. Besides being lighter than air and making your voice funny when you inhale it, that is.
4. It comes from natural gas wells, mostly.
5. Airships work by displacing air with a lighter lifting gas. When you’re putting aluminum hangar plating or a library in the place of gas cells, you’re cutting into your displacement.
6. “Top me up! I need about five hundred thousand cubic meters!”
7. In the sense of fantasy, too.
8. This is bad.

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