I remember that one. An excellent thread on the SD and science coupled with one of the infrequent occasions when GQ posters grind another’s hopes and dreams mercilessly to dust (to mix metaphors), which always gives enjoyment by mixing in the human element for the popcorn crowd.
We could never imagine Hannah Montana swingingnaked on a wrecking ball, but that happened, so you know, I’m still hopeful for my vacuum blimp.
Still gonna go with no on that.
Imagine a bicycle spoke, long and thin. If you stretch it in a materials testing machine you can probably develop about 250 pounds of force before it breaks. Now try compressing one instead: I’ll wager you’ll peak at a few pounds of resistance before it buckles and bends. The problem is that as soon is the material deforms and moves away from the load axis, you create a large bending moment that assures larger stresses and larger deformations, unto complete structural failure.
For very short very fat columns (think “hockey puck”), buckling isn’t an issue, and so it just comes down to compressive strength. But at some sufficiently high aspect ratio (somewhere between “hockey puck” and “bicycle spoke”), buckling becomes a concern, and the only solution is rigidity, which comes from both cross-section geometry and the material’s modulus of elasticity. Example, Imagine a fat tubular aluminum column, and a skinny solid steel column. The aluminum column, despite having a lower modulus of elasticity and lower total compressive strength (compared to the skinny solid steel column), may have greater resistance to buckling failure because of the greater width of the column.
If we’re trying to make an atmospherically buoyant vacuum vessel, then the skin will need to be so thin as to be a 2-dimensional analog of “bicycle spoke” in terms of the thickness compared to span. In fact, even thinner.
According to that Wikipedia link, graphene has a stiffness about 5X that of steel - which means the section modulus of your vacuum vessel’s skin can be decreased by a factor of five, which in turn means you can only lower your skin thickness by a factor of 5[sup]1/3[/sup] = 1.7 compared to the thickness of steel that you would need to avoid buckling. Bottom line, anything larger than a microballoon will still have to be built far too heavy to be buoyant, even if it’s made of graphene.
Maybe you couldn’t…
I’ve actually done a lot more thinking about the concept of a vacuum dirigible since my last thread and have a brans new idea for a hot air/vacuum hybrid airship for everybody to shit on and crush my dreams.
Picture something similar to aerogel or a honeycomb. Because it is hard to make something big that holds any amount of negative pressure, we simply stack lots of smaller ones together into a honeycomb type of structure. These micro capsules are not airtight. They are somewhat permeable to air, so that they slowly acquire whatever the current surrounding pressure is. Your pressure envelope is made out of a 10 foot thick layer of this honeycomb structure with a hollow center in which you have your heater.
When you turn the heat on it heats the the air inside the envelope which expands, is outgassed and provides lift. It also heats your microbead/honeycomb structure, the air inside the microbeads heats up, expands, and is also outgassed throught the permeable membrane of the beads themselves.
Such a setup retains heat better than a standard hot air balloon as that honeycomb will be a great insulator. When you turn the heat off, the air will take some time to flow into the microbeads while that’s happening, you are generating some (it might be tiny) amount of lift from the low pressure inside the microbeads and the pressure envelope itself.
Picture a giant styrofoam shell that leaks really slowly.
Ta-da!
So what you’re positing is a well-insulated hot air balloon. The “slow leak” thing doesn’t help you: If it’s slower than the cooling, then the pressure difference will crush your balloon, and if it’s faster than the cooling, then it’s just hot air as a lifting gas, which is nothing new.
We don’t insulate hot air balloons because it isn’t necessary; the loss of heat is low because the air inside the balloon and the surrounding atmosphere don’t directly interact, so there is essentially no convection, only conduction, and because the density of the gas is very low conduction is very slow. (There is also radiation but because the temperature difference between in the inside and outside of the balloon is so small radiation is nearly insignificant. However, if we filled a vessel with very hot gas we would want a reflective layer on the inside to prevent radiation losses.) Hot air balloons, once aloft, can float for many hours with minimal use of the burner provided that there aren’t shifts in air density.
Having a bunch of smaller nearly evacuated vessels is not an improvement on the concept; you now have the parasitic mass of each of these vessels each of which are orders of magnitude more dense than air. This is really a basic scaling problem and there is no size or shape of a rigid vessel made of a real material which can be evacuated to the extent that it will be buoyant in air. This is why we use pressure supported flexible skins (sometimes with an internal structure to help control inflation or provide aerodynamic properties) for lighter-than-air-flight.
Stranger
Saying that it’s “just” a well-insulated hot air balloon reveals a knee-jerk depecratory stance. A well-insulated hot air balloon is a very neat thing as it saves dramatically on fuel necessary to maintain lift.
The second part is also untrue. Sure, there would be a buckle rate for the microspheres but the greater the pressure differential, the faster they would equalize, and while they are equalizing we are gaining something.
It’s ok, you can admit it. It’s a cool concept
Ok. These are good objections, but if the temperature loss is so low why does my yeti cooler need such thick walls? Why isn’t it just made of balloon skin? Why does my oven have thick insulation? Why isn’t that made of balloon skin?
We are talking a lot of surface area to radiate heat from in a hot air balloon. I find it difficult to believe that loss of heat over that huge are is not an issue when it is such a big issue in coolers and ovens.
If you were well insulated you could easily maintain a greater temperature differential. The hotter the air inside your envelope the less the volume that envelope needs to be.
I think I have a solution that’s on the edge of working, though some might consider it a cheat.
The problem isn’t raw material strength. Extant materials have the compressive strength requirements; the problem is buckling resistance, which is a problem with all compressive structures. The way to improve buckling resistance is to make the walls thicker, lowering the average density, but the walls themselves are made from stuff and have internal structures which may themselves buckle.
So it would be nice to convert a compressive structure into a tensile one. Luckily, there’s a way–using inflatables! A column built from a high tensile strength fabric and filled with compressed gas can be made as fat as necessary and thus has arbitrary buckling resistance.
I propose making an icosahedron out of fat columns filled with compressed hydrogen. An envelope surrounds the whole thing and the interior is pumped to a vacuum.
The solid material (fabric) is always in tension and so there are no buckling problems in that regard. Can we ensure there’s enough pressure that the columns as a whole don’t buckle?
I posit that the columns can always be made large enough that it works–if you imagine the limiting case with really fat columns, the structure becomes not all that different from a superpressure balloon with a tiny bit of vacuum at the center.
Of course that indicates why it’s pretty much a cheat, since obviously the dominant component of the lift is coming from the air displaced by the gas-filled structure and not the vacuum. So take that as you will, though my intuition is that you can get at least half the lift from the vacuum if you designed it right.
To have enough strength, your hydrogen tubes would have to be at high pressure, high enough that the mass of the extra hydrogen inside would more than offset the vacuum.
But she didn’t do it from a skyhook.
Hydrogen is so much less dense than air that you can get away with a lot of compression. Suppose a cross-section of your structure is 50% gas/50% vacuum. The gas needs to be at least 2 atmospheres to resist the overall forces along that plane. But hydrogen can be at 14 atmospheres before it reaches the same density as air, so there’s plenty of headroom.
Of course the whole thing is in every way worse than a simple hydrogen balloon, but no one asked about that…
First of all, heat loss from a hot air balloon is primarily by conduction, not radiation, as addressed previously. Because the air, both inside the balloon and in the atmosphere is of such low density, the rate of conduction is low under normal conditions. The air inside the balloon does flow (cold air from the outside falls and forces hot air to rise through the center) but because of the large mass of air and small temperature differentials the flow is slow, and the difference in average temperature is not large; hot air balloons have an average internal temperature of around 200 °F, whereas your stove probably doesn’t even have a measurement below 300 °F, and the primary reason for insulating your stove is to prevent your kitchen from getting hot (unless you have a convection stove) as it cooks largely by radiation. Your Yeti cooler, on the other hand, has to protect a small volume from heat coming from the ambient air into a cold temperature reservoir with essentially infinite heat capacity, and unless you are using dry ice you have a working temperature of not much less than 32 °F which is maintained as long as the total energy flux doesn’t exceed the latent heat of fusion of your ice or other coolant.
It isn’t that a hot air balloon doens’t lose heat energy, but it does so slowly enough that it is not worth adding the extra mass of insulation to offset any savings of fuel for the propane burner. Complex schemes that add more mass in order to achieve a marginal increase in theoretical buoyancy are highly unlikely to be a net improvement.
This isn’t a cheap per se, but realize that once you start increasing the pressure of the hydrogen inside the columns you are increasing density and thus reducing buoyancy. Compared to the external loads (14.7 psia AMSL) which add up quickly you’re going to need a considerable amount of internal structure to support that. You’ve proposed an icosohedral structure (presumably because it is the simplest “sphere-like” classic solid with triagonal faces) but it would be instructive to look at an even more simple structure for scaling without dealing with only axial loading, i.e. take an annular structure, like a backpacker’s alcohol stove, seal and fill the annulus with hydrogen or helium, and cap the open ends (we’ll assume for the moment that the end caps are perfectly rigid) to form an evacuated chamber in the center. In this case buckling is not a concern as long as you keep the pressure in the annulus sufficient to assure positive tension. Now calculate the buoyancy of the structure (both the evacuated center and the high pressure annulus) versus the mass of some real world textile material for the skin. You’ll find that the necessary ratio of pressure in the annulus to the ambient to support the end load is larger than the inner diameter of the annulus squared over the difference of the squares of the outer and inner walls. In short, you’ll end up with a very large annulus to support an evacuated center, even assuming rigid caps. The scaling on a icosohedran will be different but it is actually worse because of the lower mechanical advantage of that configuration. Getting more than a small fraction of lift from the evacuated volume just isn’t plausible.
Stranger
Also hydrogen is going to leak and the higher the pressure, the faster the leak. And where will it leak to? All directions to some extent, but if there’s a vacuum on one side and air on the other, it’ll leak more to the vacuum side, since there’s less resistance that way. Anyway, what you end up with is a hydrogen balloon.
Indeed, since you’ll have all that internal tubing, which will be useless mass.
It’s interesting to me how this idea of vacuum balloons seems to get a hold on people. I wonder what’s special about it? Probably conceptual simplicity when looked at through the lens of Warner Brothers physics.
I asked some similar questions to the OP in a recent thread.
Because hydrogen is dangerous and helium is rare and running out (until we are able to mine the Sun)
For me at least, it’s because it falls in the domain of extreme engineering problems. It doesn’t violate any laws of physics, and existing materials seem almost good enough if you could arrange them right, but it’s never been done. So it’s like nanomachines and nuclear rockets and electromagnetic launchers and maglev vacuum tunnels and so on.
Pretty much. Can you describe your proposed structure a bit better? I know what an alcohol stove looks like but if you tried to inflate one with a gas, the inner part would collapse as it’s under compression, not tension. So I must have an incorrect picture of the proposal.
To get a slightly better feel for my intuition, consider just the material required by an infinite cylinder at 2 atm (1 atm above ambient), 1 m radius. Zylon fiber has a tensile strength of 5800 MPa and density of 1540 kg/m^3. Hoop stress is Pr/t, which if you solve for t (wall thickness) gives 1.75e-5 m. Since Zylon is a fiber with anisotropic strength, you have to handle the axial stress separately. That’s half of hoop stress so the wall thickness ends up as 2.62e-5 m. The final density of the cylinder is thus 0.253 kg/m. The density of the hydrogen is 0.564 kg/m, so the fabric is actually the smaller part. And if you work the numbers a bit further, you find that you can get a neutrally buoyant cylinder compressed at 7.65 atm. That’s a heck of a structural member that weighs the same as air. Well, I’m ignoring the end caps but they’re relatively small if the cylinder has a decent aspect ratio (10:1, say).
Anyway, this isn’t proof that it’s possible to get a decent ratio of vacuum but hints that there is reasonable margin available with this approach.