Airships are pretty dang cool. Airplanes need a continuous expenditure of energy to stay in the air, but if you just fill a bag with a light gas, you can stay up in the air with no energy expenditure at all. The two lightest gases are hydrogen and helium. Though the hydrogen atom is 4 times lighter than helium, hydrogen gas is made of two hydrogen atoms bonded together, so it’s only twice as light as helium gas. The difference in lifting power is even more minor: What matters for lifting power of a gas is the difference between its density and that of ordinary air. Hydrogen and helium are both much lighter than air, so the lifting power of either gas is nearly equal to the density of air.
Hydrogen and helium both have problems as lifting gases. Helium’s problem is that it’s very expensive. Helium comes out of the ground, usually as a byproduct of fossil fuel extraction. There’s a finite amount of it in the Earth’s crust, and effort must be expended to obtain more of it. On the demand side of the equation, helium is a very useful gas, with many applications in cooling, spaceflight, balloons, etc. It’s quite ironic the second most common element in the universe is so hard to come by on Earth. Hydrogen is much cheaper. It can be created from methane, or by electrolysis of water. The main problem with using hydrogen in an airship is safety. Hydrogen is very flammable, and great care must be taken to avoid sparks, and to avoid allowing any hydrogen and oxygen to mix.
Some people have proposed vacuum airships. Vacuum would not be a much better lifting gas than hydrogen or helium. Its lifting power is exactly equal to the density of air, and hydrogen and helium are already pretty close to that. But maybe we’d like to use vacuum because its cost is equal to the energy cost of pumping out the air in a given volume of space, and it’s not flammable. There are some problems with this idea, though.
The obvious problem is that an air sac filled with vacuum would simply collapse. The working principle of airships is that the gas inside the balloon can supply enough outwards pressure to keep the atmosphere from collapsing the balloon, while still weighing less than an equivalent amount of air would. Vacuum does not push back, and this is a fairly huge problem. The first thing one might think of is to make a rigid spherical shell instead of a balloon. The problem here is weight. To be light enough to still fly, the shell must be very thin, which in turn makes it susceptible to buckling. See this and the linked patent application for details, but this engineering is pretty hard to pull off. A simple uniform shell would not work for any known material, though there are ways to get around this.
Alternately, what if we kept the floppy structure, but made the electromagnetic field provide the counter-pressure for us? If we had a perfectly reflective material, a photon gas could be used. We don’t have that, though, so an electrostatic solution is necessary instead.
The proposal is simple: Make a spherical balloon with a conductive layer and charge it up. The charges repel and this repulsion holds the balloon open, even with a vacuum inside. How much charge is needed to pull this off? We can calculate it without too much difficulty. Electric field from a uniformly charged spherical shell is
If we write in terms of surface charge density it’s
Energy density in the electric field is
Energy density has the same units as pressure. Think about squeezing the balloon a bit. This would increase the amount of space occupied by the electric field (remember the shell theorem: electric field is 0 inside the balloon), and so increases the energy. So the energy density tells us the outwards pressure produced by the charge on the balloon.
If we plug in atmospheric pressure for we get the following charge density:
This is a disturbingly large charge density. What will the electric field be at the surface?
Yes, also disturbingly large, nearly 100 times the breakdown voltage of air. The electrostatic airship would shoot out lightning long before it actually worked.
Okay, so what if we didn’t demand a vacuum and filled the ballon with mere slightly depressurized air? Say we think we can tolerate an electric field of 1MV/m. How much pressure can that produce? Unfortunately, the quadratic scaling of energy density works against us! The supported pressure would only be about 5Pa. The air inside the balloon would need to have 99.995% of the density of the air outside. Making a hot air balloon sounds way easier!
So how do we make airships instead? Something that I think we should consider is: Just use hydrogen lifting gas! Avoiding sparks is just a matter of being careful, I’d like to think that materials science and engineering has improved since the Hindenburg days. We can also add a layer of safety by making one air sac inside another. The inside one is full of hydrogen, while the outer is full of nitrogen. A leak in either sac will not cause the mixing of hydrogen and oxygen, and sensors could be placed inside the the nitrogen region to detect if too much hydrogen or oxygen is going across the boundary.
Yes, helium costs would be a problem for large-scale use of airships. Yes, it’s possible to use hydrogen in airships safely. This has been noted by many people.
Hydrogen has some properties that make it relatively safe:
it’s light so it rises instead of accumulating on the ground or around a leak
it has a relatively high ignition temperature
and some properties that make it less safe:
it has a wide range of concentrations where it will burn in air
fast diffusion, that is, it mixes with air quickly
it leaks through many materials
it embrittles steel
it causes some global warming if released
Regardless, the FAA does not allow using hydrogen in airships, and I don’t expect that to change soon. Especially since accidents still happen despite the small number of airships.
In any case, the only uses of airships that are plausibly economical today are: advertising and luxury yachts for the wealthy. Are those things that you care about working towards?
Your ‘accidents still happen’ link shows:
One airship accident worldwide in the past 5 years, in Brazil.
The last airship accident in the US was in 2017.
The last airship accident fatality anywhere in the world was in 2011 in Germany.
The last airship accident fatality in the US was in 1986.
I think that this compares favorably with very nearly everything.
Have to divide by number of airships, which probably makes them less safe than planes, if not cars. I think the difficulty is mostly with having a large surface-area exposed to the wind making the ships difficult to control. (Edit: looking at the list on Wikipedia, this is maybe not totally true. A lot of the crashes seem to be caused by equipment failures too.)
No, and I don’t work on airships and have no plans to do so. I mainly just think it’s an interesting demonstration of how weak electrostatic forces can be.
I think this is mostly about how weak air is against dielectric breakdown.
On the other hand, the hydrogen pushing against the airship membrane is also an electrostatic force.
We can create a vacuum airship if we make its hull based on rigid ribs with fabric stretched between them. I heard that so-called black triangle UFOs could be military blimps based on such technology.
Another interesting idea on these lines is a steam airship. Water molecules have less molecular weight than air, so a steam airship gets more lift from steam than from air at the same temperature.
Theoretically it’s possible to make a wet air balloon. Something that floats just because it’s full of very humid air. This is how clouds stay up despite the weight of the water drops. But even in hot dry conditions, the lift is tiny.
What about a hot air blimp with the membrane being quilted, and filled with aerogel. The super light, super insulating aerogel combined with the large volume to surface ratio would make it pretty efficient to keep hot.
https://en.m.wikipedia.org/wiki/Thermal_airship
With aerogel insulation, the hot air plus steam idea seems quite plausible. Claude s3.6 says:
With modern aerogel insulation (U-value ~0.015 W/m²K):
For 10m radius: Heat loss = 0.015 × 1,257 × (100-15) = 1,602 W ≈ 5,500 BTU/hr
For 20m radius: Heat loss = 0.015 × 5,027 × (100-15) = 6,409 W ≈ 22,000 BTU/hr
Converting to fuel consumption (using propane as example):
Propane contains ~91,500 BTU/gallon
Assuming 80% heating efficiency:
10m radius: ~0.08 gallons/hour 20m radius: ~0.30 gallons/hour
The efficiency improves dramatically with size due to the cubic/square relationship. Each doubling of radius:
Increases volume (and lift) by 8×
Increases surface area (and heat loss) by 4×
Improves fuel efficiency per kg of lift by ~2×
Ok. I just had another couple of insane airship ideas.
Idea 1) Active support, orbital ring style. Basically have a loop of matter (wire?) electromagnetically held in place and accelerated to great speed. Actually, several loops like this. https://en.wikipedia.org/wiki/Orbital_ring
Idea 2) Control theory. A material subject to buckling is in an unstable equilibrium. If the material was in a state of perfect uniform symmetry, it would remain in that uniform state. But small deviations are exponentially amplified. Symmetry breaking. This means that the metal vacuum ship trying to buckle is like a pencil balanced on it’s point. In theory the application of arbitrarily small forces could keep it balanced.
Thus, a vacuum ship full of measurement lasers, electronics and actuators. Every tiny deviation from spherical being detected and countered by the electronics.
Now is this safe? Probably not. If anything goes wrong with those actuators then the whole thing will buckle and come crashing down.
You can use magnetic instead of electrostatic forces as the force holding the surface out against air pressure. One disadvantage is that you need superconducting cables fairly spread out* over the airship’s surface, which imposes some cooling requirements. An advantage is square-cube law means it scales well to large size. Another disadvantage is that if the cooling fails it collapses and falls down.
*technically you just need two opposing rings, but I am not so enthusiastic about draping the exterior surface over long distances as it scales up, and it probably does need a significant scale
To hold the surface out, you need to have a magnetic field tangent to the surface. But you can’t have a continuous magnetic field tangent to every point on the surface of a sphere. That’s a theorem of topology, called the Hairy Ball Theorem. So there has to be some area of the ball that’s unsupported. I guess if the area is small enough, you just let it dimple inwards in tension. The balloon would be covered in dimples, like a golf ball.
Yes, for that reason I had never been considering a sphere for my main idea with relatively close wires. (though the 2-ring alternative without close wires would support a surface that would be topologically a sphere). What I actually was imagining was this:
A torus, with superconducting wires wound diagonally. The interior field goes around the ring and supports against collapse of the cross section of the ring, the exterior field is polar and supports against collapse of the ring. Like a conventional superconducting energy storage system:
I suppose this does raise the question of where you attach the payload, maybe it’s attached to various points on the ring via cables or something, but as you scale it up, that might get unwieldy.
I suppose there’s also a potential issue about the torque applied by the Earth’s magnetic field. I don’t imagine it’s unmanageable, but haven’t done the math.
My actual reason for thinking about this sort of thing was actually because I was thinking about whether (because of the square-cube law), superconducting magnetic energy storage might actually be viable for more than just the current short-term timescales if physically scaled up to a large size. The airship idea was a kind of side effect.
The best way I was able to think of actually using something like this for energy storage would be to embed it in ice and anchor/ballast it to drop it to the bottom of the ocean, where the water pressure would counterbalance the expansion from the magnetic fields enabling higher fields to be supported.