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Could we make a city or even a building float in the air using hydrogen gas?

This is specifically about hydrogen—helium is too expensive.

This would be on a planet identical to Earth, with the same atmosphere, gravity, etc.
The city would be like a large disc or plate, with the gas bladder beneath it. Picture an upside-down dome with a chunk of land on top of it.
This city would not move; instead it would be anchored to the surface by one or more large cables, that also stop it from flipping upside down due to how physics work.

My main question: would this model be efficient?

Joachim
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Science and fantasy entwined
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    Mythbusters or other science enthusiasts could give you a quite interesting "blimp" at how big the issue you'll have. But sometimes reality's boring, therefore all you have to do is to push forward and up your ambitions to make a truly wonderful world ! – Tortliena - inactive Nov 14 '23 at 22:12
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    (Without any semi-camouflaged meaning, on Stack-Exchange you're expected to do some research before coming here. I fear you'll have quite a few downvotes because you don't show much of that preliminary research . To reduce the risks of this happening, can you explain what you looked for, and why it didn't help you find the answer you wanted? This will also help us find exactly the answer you need.) – Tortliena - inactive Nov 14 '23 at 22:19
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    One cubic meter of hydrogen can lift about one kilogram. One common-or-garden variety brick weighs about 2 kilograms, so you need 2 cubic meters of hydrogen to lift it. A regular sized room, say 6 by 4 meters with a height of 3 meters has a volume of about 72 cubic meters, so that an entire room filled with hydrogen could lift about 36 bricks, which seems dramatically less than the number of bricks needed to build the walls of the room. – AlexP Nov 14 '23 at 23:26
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    One question. Where is this city supposed to be located? On Earth or a planet with dense (non breathable) atmosphere? – Mon Nov 15 '23 at 01:37
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    Good point @Mon. there are actually a lot of undefined details we need to answer : What is the allowed technology level? You seem to be worried about money, do you have an upper limit? What is the size you consider it's a city? Shall the city be relatively autonomous (they can produce some of their own goods?), or is it just a giant cruise ship in the sky? – Tortliena - inactive Nov 15 '23 at 01:55
  • @Tortliena Yep. I was going to ask about tech levels as well but then I thought 'what's the point' until we know where the city is supposed to be located? In reality even using high tech materials like aluminum or titanium alloys and carbon fiber etc a 'floating city' is never going to get off the ground in an Earth like atmosphere, not if you define 'city' by any normal measure and we don't even know the population size required. – Mon Nov 15 '23 at 05:40
  • I was the final close vote. This question was close by five votes all for the same reason: needs more details. We really don't know what you're trying to do. If you research blimps, dirigibles, and air ships you'll discover that the volume of gas needed to keep the gondola afloat is enormous compared to the gondola itself. On the surface, that suggests an obvious no, this can't be done. But we know nothing about the context of your question. Are we on Earth? What city and year should we use to judge technology? (*Continued*) – JBH Nov 15 '23 at 15:19
  • Can we heat the hydrogen? How safe must this be? Should we use the city/year for technology as an example of the city you're trying to float? If not, what year/city for that reference? Are we trying to float existing structures (stone, wood, metal) or are we building strictly for light-weight? As you can imagine, floating a medieval England castle is a much bigger deal than floating a building built from today's expanding foam or tomorrow's carbon nanotube structures. Should we deal with infrastructure (power gen, water access, sewer dump/processing, etc.)? Details... Wherein the devil dwells. – JBH Nov 15 '23 at 15:23
  • The question in the heading is asking whether a "city" can be made to float, without enough detail about the city. The final question is about whether it is "efficient" with insufficient detail to know whether the efficiency being measured is economics (no background to determine) or some other measure. Until there is only one question with enough definition there is nothing to work with here. – KerrAvon2055 Dec 22 '23 at 00:21

2 Answers2

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A quick calculation - giving it a bash anyway!

On Earth the Hindenburg (lifting gas hydrogen) had a lifting volume of aprox 200,000 cubic meters and could lift approx 230 tonnes including 90 passengers and crew. Divide those two figures and you get about 870 cubic meters of lifting gas for every one tonne of cargo including people. Lets be generous and say they lift 100 people in a pinch by cutting back elsewhere just to make calculations easier.

If we accept the definition of a city as being a 'large town' i.e. lets say we're talking 50,000 people. This means you'd need 500 Hindenburgs tied together to get 50,000 people aloft. That's a lifting volume of aprox 100,000,000 cubic meters. IF I've got the maths right (someone else please feel free double check) That's a lifting body 1000 meters a side and 100 meters tall! And please note that's the lifting body for your city not area available for the city itself. That has to be added on.

But even then that's just people being crammed together for transport with no real living space, not really a 'city'. If we allow for proper living quarters, work spaces, storage and workshops, public spaces and farms etc !!! Well the numbers blow out even more. And everything would have to be made of the same light materials the Hindenburg used else your lift and volume troubles become even worse. (We could do better now in terms of modern light weight construction materials of course but that would just ease your problems not solve them.

At a minimum your probably going to need a structure about what? At least 500 to 1000 times more living space and that's just for the basics, no sports fields or Olympic sized swimming pools in 'Aero City'. Adding those kind of items of course blows out the lifting volume needed even more!

In reality I suggest there would be a requirement for a living area of at least 30-40 square kilometers of 'living' space for your citizens with the proportionate area of lifting volume. And that probably probably wont include all the space needed for farms, large sporting facilitates and other space critical items.

I would also suggest making your city an aerostat (tied to the ground) as well. This is because the engines needed to maintain control and move such a huge structure will add much more weight even if they are electrically powered and you've lined the top of your city with solar cells (more weight). It also makes cargo and passenger deliveries easier.

Someone else please feel free to correct all my estimates BTW. The point being is that on Earth at least, even using modern lightweight materials what your asking for would be very, very difficult to construct, let alone maintain (wear and tear will be an big issue just as they are with any complex machine).

But all that said, bon voyage and good luck with your new home.

Mon
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  • Two things to add: Such size tells that it will be insanely expensive (on top of being expansive), which undermines the choice of hydrogen for being cheap. Also it is extremely vulnerable to fire, as violently shown by the hindenburg's accident. I wouldn't want to live under what is to be a gigantic, uncontrollable fireball. – Tortliena - inactive Nov 15 '23 at 12:10
  • The hindenbergs shell, bladder, ect was flammable, contributing to the problem. If done with modern materials, the local area around the puncture would probably be warped, and the balloon would slowly sink but it wouldn't explode – MegatheriumMegafauna Nov 15 '23 at 15:05
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If you build your city inside a large geodesic sphere, you can even let it float with the warm air inside it.

See the answers to this question for calculations and practical problems with this, though.

ths
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  • Can you, though? That sounds highly unlikely. I don't see what the shape or structure of the sphere has to do with lifting capabilities. – Joachim Nov 15 '23 at 13:02
  • @Joachim if you think about a hot air balloon there is no (physics) reason that you couldn't put what is normally in the basket inside a sealed geodesic balloon of the same volume - it's just that the occupants would die very quickly in the 70-80 C temperatures inside the balloon! Once you lower the balloon/geodesic sphere temperature to something that is human-habitable it will only have significant lift if the oustide air is something like a polar winter, which requires lots of energy... you're right, not feasible and city mass/volume must be tiny relative to sphere volume. – KerrAvon2055 Nov 15 '23 at 13:34
  • @Joachim the trick of the geodesic sphere is, that it holds it's shape without needing a higher inside pressure, u like a balloon. – ths Nov 15 '23 at 14:18
  • I get that, but how does that automatically imply filling it with hydrogen would make it be able to lift a significant amount of people and their dwellings? I think the ability of a structure to hold its shape is certainly not the first concern here. – Joachim Nov 15 '23 at 14:31
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    This answer would be substantially improved if relevant portions of the answers to the linked question were brought forward. Stack Exchange dislikes link-only answers, which this is. – JBH Nov 15 '23 at 15:14