As I understand it the Hindenburg used hydrogen exactly because of the (politically) unavailability of helium, the extra lift was just a bonus. And yes it’s a substantial bonus.
This site seems to agree https://css.history.com/news/the-hindenburg-disaster-9-surprising-facts
If we had materials strong enough to make a vacuum filled Hindenberg, you’d be better off using those materials to make the structure lighter and fill it with helium.
This is exceedingly unlikely. We don’t currently have any materials with a strength-to-weight ratio high enough to float in earth’s atmosphere simply by enclosing a vacuum. What’s more, shape memory alloys are kind of a terrible choice for this: they’re not all that stiff or strong. The most common one, Nitinol (made from nickel and titanium) is only about 15% less dense than steel, while its strength is less than half that of the strongest steels. If you could make a vacuum balloon out of a shape-memory alloy, you could make it out of steel. (You definitely can’t make one out of steel).
I don’t know what you saw, but either you’re misremembering or you were actively being deceived. Any material with the requisite strength-to-weight ratio for a vacuum balloon would be an enormous breakthrough in material science. It would be all over the aerospace industry (in which I work) for starters. It certainly wouldnt be put aside as a curiosity with no practical applications.
As an aside, if vacuum balloons were possible in thebearth’s atmosphere, they’d be spherical. A sphere is the most efficient shape for such a thing on every level.
The problem is not strength to weight ratio–current materials already have sufficient strength for the application.
The problem is buckling. The balloon walls have to be very thin; so thin that they easily fold in and collapse the structure.
The formulas that govern the buckling threshold have material density as a parameter. If you can make a material with the same strength-to-weight ratio as something we have now (diamond, say), but extremely low density, then a vacuum balloon is possible.
You can lower density via microstructure. A fractal truss structure would do the trick–imagine a radio tower type structure, itself made with rods that resemble the whole structure, and then those made from rods that also resemble the whole. You can achieve arbitrarily low density structures this way.
Yes, the space between the rods must also be vacuum. This is fine; you build a low-density shell and then wrap the whole thing in an impermeable sheet. The sheet can be arbitrarily thin and won’t add much to the mass.
Of course, the whole idea is still pointless. Finding a way to make hydrogen safer gets you almost all the benefit.
What buoyancy? Or rather, buoyancy in which medium? Buoyancy in water has very different requirements from buoyancy in air.
Can you cite a design criteria that gives material density as a parameter? I have not seen that before, only geometry and stiffness.
In one of the Pellucidar novels by Edgar Rice Burroughs (the Tarzan guy), they make the trip to the Earth’s center in a zeppelin made of the usual magic substance (called, I believe, jasonite), which is strong enough to hold vacuum but light enough to float. IIRC the lifting capacity of the blimp is 225 tons, but it only weighs 75 tons.
It is the usual unobtainium, but blimps were high-tech at the time it was written.
Even as a child I recognized that it wasn’t practical, but I also recognized that the Earth isn’t hollow and filled with dinosaurs and beautiful cave women, worse luck. Straining at a gnat and swallowing a camel is fatal to the willing suspension of disbelief.
Regards,
Shodan
Cylinders are a close second. There are countless video examples of large, sturdy cylindrical vessels imploding (due to buckling failure) when subjected to internal vacuum. Railroad tanker cars are made of steel that is 1/2"-5/8" thick, but apparently that’s not enough. That video list also shows backyard demonstrations of the implosion of 55-gallon drums under vacuum; these are thinner metal, but they are also smaller and include reinforcing ribs, so should be more resistant to buckling than very large vessels like tanker trucks and railroad cars. But they’re not.
standard railroad tanker cars has a volume of 114 cubic meters. Sea-level air has a density of 1.225 kg/m^3. So if you completely evacuated the tank, you’d develop a lifting capacity of about 140 kilograms. That’s enough buoyant force to lift about one square meter of the material that’s used to make the tank (which we already know needs to be much thicker to resist buckling failure). A tanker car is made from about 90 square meters of such material. So to build an envelope of this scale with any actual usable lifting capacity, you’d need to come up with a material that’s about 1% as dense as, and substantially more resistant to buckling failure (on this scale) than, 1/2" steel.
In other words, it’s not even close.
Clearly. We already have submarines, which are vessels that are positively-buoyant while maintaining in internal pressure substantially less than their immediate ambient environment.
It requires combining and munging some formulas, but here’s a handwaving argument:
Imagine that we have a spherical pressure vessel. Also imagine that we have two materials available: solid diamond, and a kind of diamond foam that has had half the material removed. It’s half the density and half the strength. Specific strength (strength divided by density) remains the same.
The stress on a spherical pressure vessel is pR/2t (p=pressure, R=radius, t=thickness). You can see that both materials behave the same here: our low-density foam is only half as strong, but we can meet that requierment by doubling the thickness. We end up with the same mass since we have twice the volume at half the density.
Buckling is a different story. The classical stress limit is 2Et[sup]2[/sup]/BR[sup]2[/sup]. E is Young’s modulus, which will also be halved for our low-density diamond. B is a unitless constant. The important thing is that it’s proportional to thickness squared. So our diamond foam has twice the buckling resistance here: E got halved, but t[sup]2[/sup] went up by 4x.
If we have some means of making arbitrarily low-density materials (say, with a fractal microstructure), then we can achieve the same overall compressive strength (which depends on specific strength) but arbitrarily high buckling resistance for a given mass target.
In fact you don’t even really need fancy microstructure. Give me a few truckloads of 1 cm x 25 cm diamond rods (and some Tinkertoy connectors) and I can build a truss that achieves the same effect.
That’s changing the geometry and the density, not changing the density. The entirety of the buckling resistance gain is from the change in geometry. It’s like saying an I-beam with slightly smaller linear weight but much higher I has better buckling resistance because of density change.
If the density is what mattered you could have two materials A and B with the same modulus and make shells out of them with the same thickness and have the less dense one have a higher buckling resistance.
All I said is that density is a parameter, not that it was the only parameter. It’s impossible to build a vacuum balloon as a thin-wall spherical vessel using a dense, homogeneous material. Lower the density, however, and it becomes possible.
There’s no real distinction between materials and geometry, anyway. The microscopic geometry of a material has an enormous effect on its properties.
As for your I-beam, I *would *say it has better buckling resistance–per unit mass, that is.