Big is Beautiful: The Mind-Boggling Size of Giant Airships

The most counterintuitive thing about airships is their immense size. The recently built British hybrid airship Airlander 10, which first flew in 2012, was almost 300 ft long, and was the largest aircraft in the world before it was retired after wind damage in 2016. That's big!

Or not. The Graf Zeppelin, a German airship that ran regular transatlantic commercial flights between 1928 and 1937, was 776 ft long. The Hindenburg was over 800 ft long. Considering that, it stops seeming strange that the Airlander 10 was considered only a “prototype.”

The enormous size of airships is the biggest barrier to the re-emergence as a major mode of transportation. Often, when something is successful, people ask if it is scalable.

Scalability is good because it means success can be reproduced. Sometimes success isn't scalable because it depends on a single personality or unique local conditions. If a successful project isn’t scalable, that is a sad limitation on its value. Scalable successes are far more transformational and profitable than things that only work well once or a few times or on a small scale. The great high-flying companies of recent decades, like Google, Amazon, and Facebook all have this property: that their central concepts and technologies can be, and were, implemented on a small scale at first, and then could grow to a limitless extent at modest or slight marginal cost. Venture capitalists in Silicon Valley are constantly on the lookout for this kind of idea, which can start small, and indeed may be brought almost to maturity by a handful of founders, and then can massively scale up.

Airships have a major scalability problem, too, though it's the opposite of the usual one. They only work well on a large scale. They can't scale down. Consequently, the good old-fashioned model of a visionary entrepreneur pulling himself up by his bootstraps and slowly, patiently building an empire while winning investors’ trust along the way can't work for airships. Or at least, it works very badly. There are markets for small airships, like advertising and surveillance, and aspiring airship builders sometimes try to survive on these while they chase bigger dreams. But not only are the market small, the small airships are technologically quite different, and much less useful, than the giant airships that could revolutionize global transportation, so different that they can hardly even serve as proper prototypes.

Why do airships have to be so big?

First, it takes a lot of lifting gas to lift much of anything. Lifting gas creates lift by displacing air. One cubic foot of air weighs less than one tenth of a pound. One cubic foot of hydrogen weighs about one-twentieth of that, so most of the weight of the displaced air turns into lift. A thousand cubic feet of hydrogen or helium can lift about 100 pounds. An airship can't fly until it contains enough lifting gas that its lift is more than the weight of the frame and equipment that comprise the airship and give it its functionality. The difference between the lift from the gas and the weight of the frame and equipment becomes the usable payload. For that payload to be very large, an enormous amount of lifting gas needs to be contained.

But there's more to it than that, because the lift of an airship, for a given design, is roughly proportional to the volume, while the weight of an airship is roughly proportional to the surface area. Table 1, below, shows the pure math of how lift overtakes weight and creates payload, using unitless numbers to keep it abstract rather than tying it any actual, particularly airship design:

Table 1
Length (x)
Lift (10x^3)
Weight (60x^2)
Payload (lift-weight)

In the example shown in Table 1, the airship can’t fly at all until it reaches a length of 6. At that point, it can only just fly, and has no room for any payload at all. At just above 6, say length 6.1, it can carry a small payload. By length 7, it can carry about one-sixth of the weight of the frame as payload. Increasing the length to 8, though it’s only a 14% increase in length, and a 31% increase in cost of construction materials, more than doubles the payload, and the next two increments in length nearly double it again, so that the length-10 airship, though less than half again as long as the length-7 airship, and only twice as costly in construction materials, has nine times the payload capacity!

In practice, as airship designers scale up their proposed designs, they tend not to grow just the payload, but also the features. The smallest feasible rigid airship is a lot bigger than the smallest feasible non-rigid airship. A rigid frame is costly in weight and therefore requires a bigger airship, but it enables faster speeds and non-pressurized lifting gas, a safety feature. Add thrusters to maneuver, and you have to get bigger to accommodate that. Add a fancy buoyancy control system so that you can land anywhere, and you have to get bigger again.

After their huge size, the next most counter-intuitive thing about airships is their extreme lightness. Of course, the whole point of an airship is that, as a whole, including the lifting gas, it's lighter than air. To achieve this, it's crucial not only that it contain a lot of lifting gas, but also that everything else about it be as lightweight as possible. Every pound of frame or equipment is a precious pound of payload capacity lost. And so the airship won't have a hard exterior. The envelope is made of fabric. The rigidity inheres only in a frame.

Around the dinner table, as I was explaining it to my daughter, my wife came up with a good analogy: the airship is a little like a TENT. It derives its form from one substance, like aluminum or carbon fiber rods fitted together, and its substance (mostly) from another, the fabric. A good tent can shelter a family for a weekend, keep them dry, resist high winds, and create a little island of homey comfort amidst the wilderness. Yet the tent can be packed into a small bag and carried by hand with ease. It's actually rather small in substance, but it has a structure that spreads that substance out to contain a relatively large space, and a shape and non-transparency that resemble the built houses people live in. Like tends, airships would get their structure from one material, a frame made of something like aluminum or carbon fiber, and their look and ability to contain gases from another material, some kind of light yet strong and impermeable fabric.

That means the cost of an airship is not proportional to its size. When Mike Voorhees of Skylite Aeronautics claims that his GeoShip would cost about the same to build as a jetliner, even though, at 500 meters long, it would about six times the length of a large jetliner and hundreds of times larger in overall volume, the claim sounds implausible. Yet although a GeoShip could fit (I think) hundreds of jetliners inside it, it would weigh only a few times more, because it’s a like a tent.

Now that you know why gigantic size is so valuable for airship technology, you’re ready to be surprised by the fact that all the designs most actively pursued by important players are smaller than the great airships of the past, and to recognize in that fact a measure of how much the airship industry is being held back by the timidity of capital.

Thus, the Aeroscraft, which the LA company Aeros proposes to build, would be 647 feet long, compared to the Hindenburg’s 804 feet. Flying Whales’ LCA60T would be 505 feet long. Hybrid Air Vehicles of the UK has plans to build the non-rigid Airlander 50, 390 feet long. Lockheed Martin has a hybrid airship program and plans to build the LMH-1, 280 feet long. The Atlant 100 airship that RosAeroSystems of Russia wants to build would be 427 feet long.

Exceptions (that prove the rule?): CargoLifter, the German airship company that ran out of money in 2002, was planning to build the semi-rigid CL160, which would have been 853 feet long. And Mike Voorhees’ GeoShip, mentioned above, would be over 1,500 feet long, but I’m pretty sure he's nowhere close to building.

If big is beautiful, why do all the big players in the emergent giant airship industry seem to want to build ships smaller than the Hindenburg? They must know their business too well to be ignorant of the extreme economies of scale inherent in airship technology, and the huge sacrifices of both payload and features that are involved in downsizing. Surely, if mankind could build 800-foot-long airships in the 1930s, we could do better now. We have fancy structural materials like carbon fiber and high-performance fabrics like Vectran. We have strain gauges and wireless communications to monitor structural failings throughout a giant airship hull. We can monitor and communicate weather patterns far more effectively, and there have been improvements in weather forecasting. Computers can help optimize the use of thrusters and weight shifting so as to stabilize an airship. And so on. If we could build an 800-foot airship in the 1930s, surely we could build a much bigger one today. So why are airship companies setting their sights lower? It’s as if, instead of seeking the pot of gold at the end of the rainbow, I were to propose to go three-quarters of the distance to the end of the rainbow, and hope to pick up a few gold pieces that spilled along the way.

Actually, this probably makes sense as a commercial strategy: capital’s hard to raise, there are all sorts of risks inherent in innovation, and there are niche markets where airships’ unique capabilities, such as (relative) infrastructure independence, should enable them to succeed even if they aren’t cost-competitive with other modes. Probably the airship industry players know, or at least dimly guess, that much bigger payoffs await when a little bit more bigness can be achieved. They’re like settlers with backpacks full of tools, groping their way up a cliff of innovation, trying to find little handholds and footholds of profits here and there to ascend by, on their way to the verdant, bountiful uplands of the Roadless Revolution.

But they may mislead the public about the potential of airships. When Lockheed Martin claims their hybrid airship is “Revolutionizing Remote Transport,” they might give people the impression that remote transport is all airships are good for. But grow them some more, and they can start revolutionizing routine transport, and that’s when airships really start to matter.


Popular posts from this blog

Why Giant Airships Could Be a Trillion Dollar Industry

Ryan Cooper on "The Return of Airships"

The Roadless Revolution