A City in the Sky

A companion piece to Touch Me in the Third Place, by Eli Dourado

“The Third Place” by Colby Green

The first manned flight was on a vehicle created by two brothers. Not the Wright brothers in 1903, but the Montgolfier brothers in 1783. The Montgolfier balloon used 1,700 cubic meters of hot air to carry the first two aeronauts, physicist Jean-François Pilâtre de Rozier and François Laurent d'Arlandes, a French marquis, from the center of Paris to the city’s suburbs in November of that year.

Hot air balloons, blimps, Zeppelins, and other lighter-than-air vehicles operate on principles that are much more basic than the aerodynamics that provide lift for airplanes.

All that matters is buoyancy. A cubic meter of air at standard pressure and 15ºC has a mass of 1.225 kilograms. A cubic meter of any less-dense gas (including hot air) will generate an upward force equivalent to the weight difference between it and the surrounding medium. At constant pressure, air gets a tiny bit lighter per cubic meter for every degree of warming. If the air in the balloon is 100 degrees Celsius hotter than the ambient air, then a 1,700-cubic-meter balloon can lift about 536 kg, counting the gross weight of the balloon itself. That 536 kg reflects a force capable of lifting 0.316 kg per cubic meter. That’s not a lot, but we can make up for it with volume. A small force per cubic meter, multiplied by a lot of cubic meters, can equal a large force.

Very few engineers have taken this observation to its logical conclusion, but one of them was the great architect and systems thinker Buckminster Fuller. Fuller was fond of geodesic structures, which have a high strength-to-weight ratio. He imagined a mile-wide geodesic sphere with thousands of people living in it. A mile-diameter sphere contains more than 2 billion cubic meters. If the activity of the people in the sphere were enough to raise the air inside it by 1ºC, the resulting buoyancy would be able to lift about 8,900 metric tons. Merely being big enough, and optimizing the strength-to-weight ratio of the structure, is sufficient to create a floating community in the sky.

Of course, we can do even better—much better—if we don’t limit ourselves to hot air.

Substituting hydrogen gas for hot air in a mile-wide sphere would produce 2.3 million metric tons of gross lift. Humans cannot breathe hydrogen, but with the added gross lift, they wouldn’t have to. With the added weight budget, we could build living structures along the outside of the sphere, being careful to avoid top-heaviness so that the sphere doesn’t flip upside-down. If 2.3 million tons isn’t enough lift, we could increase the size of the sphere. A doubling of diameter to two miles would increase the volume of lifting gas and the total gross lift eightfold.

The sphere wouldn’t just be lifting gas. It would need structural support. Designing this structure to have adequate strength while minimizing the necessary mass is a key challenge. While the structure itself would probably be geodesic, selecting a material is one of the most consequential engineering decisions in floating city design. The structure of the sphere would face forces that compress it, tear it apart, and shear it. The material should also not be brittle, deforming instead of shattering when pushed past the breaking point. It should be lightweight, cheap, and corrosion resistant.

Using today’s technology, carbon fiber composites could be a good choice if their poor compressive strength is taken into account when laying out the structure. Overbuilding in key spots could make it work—that’s how modern rigid-body airships are now being designed. Alternatively, the Zeppelins of the early 1900s used aluminum structures. While aluminum is denser than carbon fiber, it is better at handling the compressive loads to which lighter-than-air vehicles are subject. It’s even more fun to speculate what the materials of the future could do. The ideal material likely remains a composite, perhaps super-strong diamond fibers in some more-ductile polymer matrix.

Even with today’s technology, the mass of the structure would take up a relatively small fraction of the gross lift—perhaps 20–30 percent. That would leave significant payload capability—over a million tons—for humans and their living area. That’s plenty for a community space in the sky.

Is hydrogen the right lifting gas? It is flammable, after all. I believe the risks from using hydrogen as a lifting gas have been overstated, but as the Hindenburg showed, failure can be catastrophic. However, helium, the main alternative, in addition to supplying 8 percent less gross lift than hydrogen, is much more scarce. The price of helium is currently $14 per cubic meter. At more than 2 billion cubic meters of volume, to lift our mile-diameter sphere with helium would cost $30 billion. Hydrogen is more than 100 times cheaper.

What if in the future we could manufacture helium? Helium is a byproduct of hydrogen fusion. Some current fusion designs use helium to cool superconducting magnets, and the associated leakage makes them net helium users, not producers. But future fusion technology could be much better. Just as in the 20th century we mastered the control of electrons (i.e., electronics), in the 21st century we could get much better at controlling atomic nuclei—protons and neutrons. Fine control over these subatomic particles would mean not only abundant energy, but also low-energy transmutation of matter and control over isotope ratios.

Helium, in other words, could someday be cheap. Helium-3, the lightest isotope of helium, so rare that people have seriously proposed mining the lunar surface to acquire it, could even be cheap. Since pure helium-3 would have only a 4% gross lift penalty relative to hydrogen, while remaining inert and noncombustible, it’s probably the ideal lifting gas if cost is no object or if economical alchemy is invented.

The other lifting gas worth mentioning is no lifting gas at all.

350 years ago, a Jesuit priest proposed a vacuum airship. Why use hydrogen or helium when you could use something even lighter—nothing. To this day, the concept has proved impossible. The challenge is that the container must both be strong enough to withstand atmospheric pressure and light enough that the system is able to achieve net lift. Scientists have shown that no homogenous shell of any known material, not even one made of diamond, would be able to meet these requirements. Research has lately turned to non-homogenous honeycomb structures as a possible solution, but nobody has yet produced a design that would work. Even if we could crack this problem and achieve net lift, it’s probable that the structure would be heavier than hydrogen and helium gas.

Finally, we should think a little about propulsion. A floating city would need a way to actively manage its location in space. If required to maintain position against 30-mph winds, a mile-diameter sphere weighing 2 million tons would require about 1.4 GW of propulsion. This is about an order of magnitude more than a 747 uses on takeoff. With adequate energy storage on board, it would be possible to maintain position against such winds for some period of time using only solar panels to charge the batteries. If we want to do it indefinitely, we need some other power source as well as the associated fuel delivery service. The biggest single nuclear reactor that exists today is 1.6 GW. Fusion is more power dense (at the fuel level) than fission, and aneutronic varieties of fusion require less shielding than fission, so it may be the best choice to power station-keeping.

The amount of power required for station-keeping scales with wind speed to the third power, so there is a benefit to allowing the sphere to drift. It might make sense to have enough propulsion for active stabilization and for the city to hold position against a gentle breeze. This would also enable the city to move to a desired location when winds are sufficiently calm. The city might actively ride the winds to stay out of the way of storms where winds exceed 30 mph.

Maintaining altitude amid changes in payload—as human beings get on and off, for instance—could be done primarily using an active buoyancy control system. Essentially, pumps could compress lifting gas into a tank when the payload got too light, preventing the city from rising too high. The pumps could release the compressed gas from the tank into the gas cells when the payload got too heavy. Because the pumps do not work infinitely quickly, vectored thrust from the propulsion system could provide a redundant altitude stabilization system, keeping the city where it should be while the pumps adjust.

Although some of the technologies discussed above are speculative, some version of a floating city—or floating recreational center, which would have fewer resupply requirements—should be possible, even with today’s technology, with enough commitment and fortitude.

By the end of the century, with sufficient technological advances, it could move from theoretically possible to maybe even probable. It’s a good goal to work toward.

Concept art by Colby Green

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This companion piece was written by Eli Dourado, who is also working to create and promote optimism about the future at the Abundance Institute! He can be found promoting progress on his substack and on Twitter/X.

This essay is a nonfiction companion piece to last week’s story, Touch Me in the Third Place, by Taylor Stuckey which can be read here:

Touch Me in the Third Place

The illustrations for Touch Me in the Third Place were created by Colby Green.