Comment by credit_guy | Hacker News Reader

Comment by credit_guy | Hacker News Reader

Here's how to think about airships using numbers.

You need about 1000 liters of helium or hydrogen to lift 1kg of cargo (the difference is only about 10%, despite the fact that H2 is twice as light as He they are both so much lighter than air, that you can almost think they are weightless, so all that matters is the mass of air displaced).

Let's say you want to lift 1 ton. You need a cube of balloon that's 10m x 10m x 10m. Let's put 4 of those next to each other, so you can lift 4 tons with a box-shaped balloon that's 40m x 10m x 10m. Let's make it a cylinder instead, and make the Soviet assumption that pi=3, we end up with a cylindrical balloon that's 40m long and 10m tall and can lift 3 tons. Since the ends of an actual dirigible are not quite flat, let's say you need a 50m long and 10m tall dirigible to lift about 3 tons. The ratio of lengt/height of Hindenburg was 7:1 and of Graff Zeppelin was 6:1. Current Goodyear blimps have a ratio of about 4:1.

So our intuition building block is a dirigible that's 50m long and 10m tall and can lift 3 tons.

Multiply that by 10 in all dimensions and you get a 500m long by 100m tall giant that can lift a whopping 3000 tons of cargo.

Multiply by only 5 in all dimensions and you get a 250m long by 50m tall dirigible that can lift 375 tons of cargo. That is a large dirigible, but not impossibly large. That was roughly the size of Hindenburg: 41 m tall and 245 m long.

Hindenburg was build 100 years ago. We can probably build one like that nowadays for much less. How about one that's double in size in all dimensions? I find that conceivable. And such a dirigible would have 3000 tons of lift. Can you imagine that? For comparison, the cargo capacity of the famous Liberty ships was about 4300 tons. You could get airlift capacity that's only available nowadays as sealift.

The math isnt that simple. As they get bigger the structure needed for very heavy lifts also multiplies. The old airships only carried light/small objects like people and mail. Lifting large individual objects is a totally differnt game. Ok, the new beast has a 3000kg capacity, but the crane/truss necessary to suspend a 3000kg object will itself weigh perhaps 1000kg. You then need a ballast system, separate from the crane, that also can hold 3000kg (big water bags or equivalent) to hold the airship down when it isn't loaded. That also requires more structure and equipment. The cargo-carrying behemoth that can quietly lift giant refinery parts into place is not a simple equation.

It's not that bad. Without reaching for any modern supermaterials, ordinary Victorian-age 2GPa music wire needs to be 4.4 millimeters thick to support 3000kg; 1000 kg of 4.4-millimeter music wire would be 8.3 kilometers of wire. 100 meters of it only weighs 12 kg. (Of course this doesn't include the safety factor you'd want.)

Tension members like wire scale linearly with load; a cable of 2 GPa music wire that can support 3000 tonnes would be only 140 mm in diameter. 100 meters of it would weigh 12 tonnes. This almost doesn't change if you split it into separate cables going to separate parts of the airship envelope.

It's not the Victorian age anymore, and we actually do have modern supermaterials like flash bainite, S-glass, Tyranno fiber, basalt fiber, carbon fiber, kevlar, and gelspun UHWMPE. Basalt fiber, declassified in 01995, is 2½ times as strong as music wire and one third the weight, so roughly you need 1.6 tonnes of it (1600 kg) to support 3'000'000 kg at a 100-meter span. It's not all that expensive either; it's mostly used to reinforce concrete.

Of course you need more than just a single cable, and you need a safety factor. But the available margin is much greater than you're implying. Airships don't need big trusses to carry compressive loads because the compressive load is carried by the gas, and they have to carry the load below the envelope anyway to be stable, so the load is necessarily connected to the envelope with tensile members.

I don't mean to say that it's a simple project, in particular because the failure mode is so exciting, but it definitely seems feasible.

I don't think England, Sweden, or the US is going to do it, though. They can't even fly to the moon any more, they're embroiled in partisan fighting among their ethnicities that threatens to boil over into genocide, and California's been trying to build the US's first high-speed railway for 26 years without any success. They aren't the kind of place anymore where Enrico Fermi built the humans' first nuclear reactor under the football bleachers; when Richard Handl tried to build one in Sweden in 02011 the police detained him and confiscated his supplies, due to the irrational paranoia of the public. They aren't even the kind of place my friend Aaron built RECAP anymore.

Meanwhile, cargo ships are out there carrying hundreds of thousands of tons. I don't want to be a naysayer, but ships will always have the luxury of the fact that water is denser than air. You can float a hell of a lot more on it. Plus, we have hundreds of years of infrastructure built to support it.

I'm having a hard time thinking of what the place of these air ships will be. I realize that large cargo ships can't go everywhere, but again, we already have things like rail that far outclass what these airships are claiming.

The only real potential commercial use case for cargo airships is delivery to remote locations which lack any other transportation infrastructure. Think of building a new wind farm and electrical lines out in the middle of nowhere.

Wouldn't you want some sort of access for electrical lines anyways? Maybe for components air lifts would make sense. Building those with just airlift capacity seems rather messy and complicated. Getting people parts tools, possibly machinery at each pylon...

Still, I could see parts of wind turbines moved. As they are large and unwieldly, but not that heavy.

I can envision intermodal commercial use cases. Airships could lift a group of containers from a ship, while at sea, and deliver them to inland marshaling yards. That would skip port bottlenecks as well as truck/train transport from, for example, Los Angeles to Chicago. The improved economics of unloading containers as a group, instead of one by one alone might make it worthwhile.

Doubt it. A huge airship would be needed to lift multiple shipping containers. It would be cheaper to expand the ports and lay more rail lines.

OP article makes a compelling argument that there's an untapped market for faster-than-shipping and cheaper-than-air freight:

> The new airships can carry heavier loads farther and cheaper than helicopters can, with lower emissions than fixed-wing aircraft—potentially zero emissions, if the ships are powered by hydrogen fuel cells.

> To Buerge the aerospace engineer, helicopters seem like a reasonable analogy to the new airships. “I don’t ride on a helicopter with any regularity, and you probably don’t either,” he says. But even though the civilian helicopter market isn’t near the size of the commercial aircraft industry, “I wouldn’t call helicopters an unsuccessful technology, or a technology that doesn’t exist in a serious way.”

Airships make the entire Earth a shipping port on the air ocean. Rail shipping doesn't come close to ships in cost, even in the few places to which it has been so expensively extended. Airships might.

How would you mitigate the safety concerns of hydrogen gas?

Maybe surround the balloon with another balloon and fill the thin space between with an inert gas or helium? I guess you put some kind of gas sensor inside the baloon that would alert you to any oxygen getting in, but that would necessitate having electricity near the hydrogen, which is probably a no-no.

Sensor could be optical. Gas sensor using fiber to carry the signal. But probably this is almost paranoia, electrical sensors would trip much earlier than a critical concentration would form. Also possible to design a very well encapsulated electrical sensor.

Isn't helium extremely expensive and even getting more so every year?

Yes that's true. On the other hand, much of the helium we currently extract along with natural gas is currently being wasted and vented into the atmosphere. It could be captured but that would require some investment in additional equipment.

Yeah, but hydrogen is cheap.

Not really.

Helium is extracted from natural gas fields. It is continuously produced inside Earth by the decay of Radon; the alpha particles from the decay become helium once they acquire 2 electrons. Since helium is light, it keeps seeping upwards. Geological formations that trap natural gas will also trap helium.

Some fields have a lot of helium. There's one in the US that has 7% of it, but that's an extreme case. There are plenty in Kansas, Oklahoma and Texas with around 1-2% helium.

According to the US Geological Survey [1] the average price for private consumers for crude helium was less than $5 per cubic meter. The total reserves were a bit more than 20 billion m3, split into various levels of confidence (measured, probable, possible, speculative). You can read that to mean that essentially the reserves of helium are infinite.

Why? Oil and natural gas companies are all about reserves. Their reserves are the assets against which they raise debt, there's an entire finance area about oil and gas exploration and production. The best reserves are those that are producing right now. Below that are those other levels of reserves. The more certain a reserve is, the more money you can raise against it to finance your activities. But elevating a reserve from "speculative" to "possible" or "probable" is costly, and risky. Companies do that based on a long term plan.

But they do it. Journalists like to take the current level of reserves, divide by the current usage, and voila, the world has only x years of resource y left. Where generally x is a fairly small number, like 10. But years pass, and that x does not dwindle to zero. People think that somehow there were some new and unexpected discoveries, but otherwise, we'll still run out of the resource in a few years.

But it's not like that. The reserves get renewed as part of the normal business. Not "renewed" as in "renewable", but just as in elevating some reserves from possible to probable, some from speculative to possible, and finally expanding the speculative reserves too.

Anyway, even at the level of a few billion cubic meters of helium, we'd still have enough helium for a few thousand of these ginormous airships.

[1] https://pubs.usgs.gov/periodicals/mcs2020/mcs2020-helium.pdf

This is really interesting, I hadn't thought of it like that. It looks like airships are one of those things that work much better as you scale them up.

Thanks for this. This is the first reference I've seen to the soviet pi=3 thing. What's the origin of that?

Perhaps it comes from the Soviets' notorious ineptitude for mathematics and aeronautics.

Which is surprising to me. Coming from a physics and mathematics background, a significant portion of my lecturers (and generally the better ones) at university came from ex-USSR countries.

...although maybe all that tells us is that the best ones left.

Presumably the throwaway troll is trolling, sarcastically, not attempting to describe reality literally.

Are you making the assumption that the structure, engines, fuel, and cargo hold are a negligible proportion of the cargo mass? Payload actually looks like around 20% of gross weight.

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