I think I’m right in saying that someone smarter than me has done the math to demonstrate that vacuum buoyancy in the Earth’s atmosphere is theoretically impossible (i.e. that there is no material theoretically strong and light enough to be able to enclose a vacuum in any concievable configuration such that the whole apparatus displaces more than its own weight of air).
Well,this guy thinks it can be done with graphene. But he may not be smarter than you, I doubt he’s succeeded in doing it. The concepts I’ve seen are based on a rigid structure that holds up a very light membrane. So given a large enough structure you might get significant lift. But it’s not just the strength and weight of the materials. The membrane has to be sufficiently impermeable to keep from filling with air through billions of tiny holes.
And strong enough that it will not puncture in real world conditions, ranging from bird landings to lightning strikes to collisions.
Grapheme. It’s a floor wax, it’s a dessert topping. There isn’t any engineering problem that doesn’t have someone claiming grapheme is the solution for.
Actually, I think the scaling is such that it can be done with any material, provided the “balloon” is large enough. The weight of the envelope scales as the surface area, or radius squared, but the buoyancy scales as the volume, or radius cubed.
Doesn’t the atmosphere being large enough become the limiting factor with a lot of materials?
But the required thickness to maintain strength increases against external pressure increases linearly to radius, and the buckling strength increases by a smaller but still increasing fraction. So, for a real material you have to scale up the thickness by a linear factor (py = 2syt/r), which overwhelms any advantage to surface area to volume.
Stranger
OSHA considers it to be a fire and explosion hazard with flammability in air at concentrations of 12.5-74%, which is a rather large range and similar to hydrogen at 4-75% (the lower limit is probably more relevant, so it is 3x safer than hydrogen). Of course, you’ll also need to have CO detectors and oxygen for the event of a leak since it is poisonous at concentrations well below those considered a fire risk.
Methane would actually be a better choice since it has about half the density of air and is only explosive at concentrations of 5-15% (refer to second link on hydrogen) and isn’t poisonous, but is still flammable, so not what the OP wants.
Inflammable means flammable. I think you mean nonflammable.
Someone already mentioned aerogels, it should be possible to make an extremely safe and low maintenance lighter than air craft if you could mass produce it.
There are already aerogels with less than the density of air and strong enough to withstand atmospheric pressure. With a little more improvement, you should be able to make basketball (or any convenient) size balls of aerogel, evacuate them and coat them with a thin sealant layer, and have a perpetually floating ball. Make thousands, contain them in a big envelope, and you’d have an aerostat or dirigible that was pretty much leakproof for minor punctures. And when individual balls developed leaks, they’d sink down to the bottom, where you could spot them, repair or replace. The envelope could just be netting even, to save weight, if you’re going for the aerostat version.
Do you have a cite for that? Aerogel density is sometimes measured without including the air it contains for the purpose of comparison between types.
That’s the point. You evacuate the aerogel and if it’s strong enough, it holds up to ambient pressure and works as a lifting agent. Or you use hydrogen and let the aerogel structure reduce the fire risk. Obviously an aerogel with the pores filled with air at STP is always going to be more dense than air…
Ok, I’ll move to the second part then. The strength to hold up to ambient pressure if the internal pressure is sufficiently reduced to make the gel buoyant. Do you have a cite for that? If that can be done, then the problem is solved, though perhaps not yet in a practical manner.
That I don’t know (maybe Sasquatch does). Though like I said, you could instead use it to safely contain hydrogen instead.
There’s an engineered metallic structure similar to aerogels and also lighter than air. I doubt this is strong enough to resist atmospheric pressure, but I’ll bet the metal would act as a great heat conductor, and prevent any spread of flame.
It might slow down the spread of flame, or prevent a flame from igniting in the first place. But it doesn’t provide a physical barrier for the gas. It would still need a surrounding membrane to keep the hydrogen in, adding to the weight. But if it was a way to make hydrogen safer it will get more consideration if the predicted rise in helium prices occurs.
Per Wikipedia and original reference there, silica aerogels have gotten down to around 1 mg/cc (yes, not counting the air). I was just going off memory of it being at least a couple years ago I saw an article with the first lower-than-air density aerogel. I was using weasel wording on the present ability to float a coated, evacuated, version of it though, because I don’t actually know what the compressive strength of that super-light material is.
They are pretty strong in general, one very light (not record-setting though!) aerogel I played with before, meant for insulation, supposedly held 200 psi. I found an article about a modified silica aerogel on the web http://samitroy.eng.ua.edu/aerogel/leventis-paper.pdf that had a failure stress of 186MPa (27,000 psi). The versions for setting records are not really intended for engineering yet, so I haven’t seen full characteristics posted. I was actually going with the safe assumption the lighter-than-air aerogels probably were not strong enough yet, hence the “with a little more improvement” fudge.
Atmospheric pressure at sea level is only 14.7 psi, so it seems that it would be possible to contain a vacuum in an aerogel, even one that isn’t particularly strong, like the one you had. However, I imagine that the stress of containing a vacuum (or the reverse) inside an aerogel is different from simply putting external pressure on the outside, plus the internal structure would have to consist of individual sealed bubbles (like Styrofoam, as opposed to a sponge, which is porous).
Well, not talking about a hollow structure of aerogel, containing a vacuum, but a continuous mass of it. Evacuated to near vacuum, then with an external coating to seal out the atmosphere. So the whole thing would be an individual sealed bubble. The larger the better, within reason, to reduce the relative mass of the sealant vs the aerogel bulk volume.
You could make a floating chunk right now though, as some have said, by sealing a volume of the present 1 mg/cc aerogel in a hydrogen atmosphere at atmospheric pressure, thereby eliminating the strength requirement.