Maybe what you’re thinking of, from Futurama:
Bolding mine
There’s the crunch, with the same pressure inside and out, we’ll need no reinforcing, thus no extra weight.
@ LSLGuy - If we built our airship out of anti-gravity material, what would be the point of filling it with Helium?
Or may be was:
“Nothing is impossible for the person who does not have to do themselves.”
I think that was either Winston Churchill or Dilbert.
I talked about this back in post #181. In a blimp the size of the Goodyear Lightfoot One, you have only 1500 kg of lifting gas (about 3300 lbs). If you could reduce lifting gas use by your 1% target, you would increase your lifting power by about 33 pounds (not “a ton or two”). To do that, you’d need to design a shell that encapsulates the entire Goodyear blimp’s lifting envelope, that could resist the 1% negative pressure, and weighs less than 33 pounds. Could you cover the whole blimp with a sufficiently strong, rigid material that weighs less than 33 pounds? Everyone says it’s impossible with any known material and I believe them.
Let’t try this another way. The Goodyear Lightfoot One has a volume of 8425 cubic meters. I don’t know its surface area but its envelope has a maximum width of 46.45 feet. If we assume that it’s a tube with a diameter of 46.45 feet it would be 175.57 feet long to have our given volume. That tube would have a surface area of 25,621 square feet. Atmospheric pressure is about 14.7 lbs per square inch. That means that you want to put an average negative pressure of about 0.147 lbs per square inch on your shell (1% of atmospheric pressure), or about 21.168 lbs per square foot on your shell. That means the whole shell would need to withstand 614,093 lbs of pressure (21.168*25,621). To benefit from the 1% reduction in lifting gas you would use at your target, you would need to design your 33 pound shell to support 614,093 pounds of pressure. Again, everyone says this is impossible with any known material. Richard Vaughn has apparently tried to find a buoyant negative pressure vessel and the best material he could come up with is only 2% as strong as it needs to be.
Oh no … there’s a recent patent application for a design to make vacuum balloon envelope - {PDF} - a layered shell with honeycomb structures … awful thick reading but enough to say Scylla will be spending big bucks to license this design.
Yeah, I wasn’t trying to start a tangent on electric cars, just make a point about quantitative vs. qualitative thinking. I would say that VBSU is not quite as mysterious as you make it sound, though. The properties are well within what normal chemical bonds can accomplish.
To slightly repeat what’s been done in other threads, let’s go through the equations again:
We want a material that allows the buoyancy force to equal the gravitational force on the hull.
r = radius of hull
t = thickness of hull
p[sub]a[/sub] = density of air = 1.225 kg/m[sup]3[/sup]
p[sub]h[/sub] = density of hull material
F[sub]b[/sub] = gp[sub]a[/sub]4/3pir[sup]3[/sup]
F[sub]h[/sub] = gp[sub]h[/sub]4pir[sup]2[/sup]*t
Set equal and simplify:
gp[sub]a[/sub]4/3pir[sup]3[/sup] = gp[sub]h[/sub]4pir[sup]2[/sup]t
p[sub]a[/sub]/3r = p[sub]h[/sub]t
r/t = 3p[sub]h[/sub]/p[sub]a[/sub]
Now use the hoop stress equation for a spherical pressure vessel (q is pressure):
S = qr/2t = (q/2)*(r/t)
And substitute:
S = 3/2qp[sub]h[/sub]/p[sub]a[/sub]
Rearrange so that the variables are on the left, constants on the right, and fill in the known stuff:
S/p[sub]h[/sub] = 3/2*q/p[sub]a[/sub] = 124,163 J/kg
So energy/mass is the parameter we care about, and equal to compressive strength divided by density. You can also see that radius doesn’t enter into the final result (the scale-independence that’s been mentioned).
Diamond for instance is 17000 MPa and 3520 kg/m[sup]3[/sup], giving a parameter of 4,829,545 J/kg. That’s plenty! Even the much more common silicon nitride has a parameter of 937,500 J/kg.
But all of this ignores buckling forces, which are difficult to deal with. The walls are thin and will crumple at the smallest imperfection. So instead you need to lower the density of the material while keeping S/p constant. You can do that with sophisticated microstructuring; instead of solid diamond, you instead have a kind of fractal truss structure at the atomic level. You can thus make the walls, say, a million times less dense and thus much thicker. The overall strength doesn’t change but the buckling resistance increases dramatically.
All of this is of course sci-fi for now, though I think you could do it at a small scale via laser ablation or similar technique.
The thing is, although silicon nitride and other materials could make a balloon that just floats, they are still not as good as an ordinary helium balloon. Only microstructured diamond has a chance at beating hydrogen, and even then it’s fairly uncertain that there’s no other problem you’d run into.
That’s it exactly. Thank you.
Funny but I associated the name “Farnsworth” with the quote, but got confused when wiki reminded me that the prof in BttF was named “Brown.” I shoulda pulled “BttF” out of my search & added “Farnsworth”; might’ve found it that way.
Bad (or at least insufficient) explanation on my part. Sorry.
My unstated assumption was that the negative gravity stuff was a gas. So you’d use that to fill the lifting envelope *instead *of the helium. My thought process went like this: according to our dear misguided OP vacuum is lighter and therefore better than helium as a lifting medium. So given that, obviously something even lighter than vacuum would be even better. Now what would that stuff be?? Eureka! 
Agree the helium would be superfluous in my scenario. And as you say, if the material was a solid better to just build the aero structure out of it and skip gas envelopes altogether.
An assumption in the buckling calculations that is making a floating vacuum sphere seem impossible is that the shell be a simple thin wall instead of structure. Even a relatively simple foam sandwich with quasi-isotropic carbon fiber epoxy composite looks like it would work. Lightweight foam between two thin composite shells will greatly increase bending stiffness.
Another assumption is isotropic materials, and designs taking advantage of composite directional properties instead of assuming a sphere might also work.
So, vacuum is not lighter than helium? All other things being equal it would not be a better lifting medium?
You condescend and say I’m misguided. Guide me correctly. Which of the above statements is untrue?
It’s misguided to think a vacuum is better than Helium … since all other thing are profoundly not equal … find your magical material and we can then figure if it will work … using diamonds may not be cost-effective …
Herzl: If you will it, it is no dream.
Cite: Walter Sobchack.
Whoever gets this thing into development, be sure to videotape the progress: according to this, there’s money even in that: Vacuum Balloon: Videorecord of Development Process: IdeasBank Memon
Further suggestions on vacuum blimps, including methods of maintaining the vacuum, are available at the same source: Today's list of IdeasBank Memons
Thanks to Q.E.D. in this thread, one of many on OP: Vacuum balloon ? - Factual Questions - Straight Dope Message Board
Yes, and that’s been apparent and agreed upon since the early part of page one. I’m showing LSLguy the courtesy of quoting him directly, so that I don’t mistake his arguments, a courtesy I wish had been returned.
I’ve been suggesting that gains could be made in lift with a pressure reduction, not vacuum, and that that is probably within the scope of what could be achievable now, even if it’s not practical or cost effective at this time.
Dammit, this happened to me once before. I made sealed aquariums filled with hair algae, bacteria and brine shrimp in empty wine bottles. Sealed them and sold them, all in high school. Later, somebody else made a business out of ecospheres
I got a different # in post 79. I may have made a mistake but I based my numbers directly off the light foot one’s specs. My calculations suggested each 1% reduction in gas saved you around 330 pounds, iirc.
You still haven’t made the case even if we had this magical material. You seem to think you’ll get tons of lift but you’re only looking at pounds … so the challenge is to build an airship with an under-pressure that only adds a single pound of weight.
If it makes you feel any better, this was first proposed in the 17th Century by Father Francesco Lana de Terzi.
If I miscalculated in post 79 and it’s only 33 pounds per 1% reduction, than it’s clearly a nonstarter. I need to see where Tired and Cranky and I diverge, to see which of us is right. I took the volume of the blimp’s envelope directly from the Goodyear website.
I’m going to reiterate this. This is where intuition fails. Unless you actually do the mathematics you don’t know the answer, and the mathematics won’t change with new materials.
You don’t gain with a composite/foam/scaffolded structure. The reason we talk about a perfect shell is that that has already optimised all the stress into a single thing - compression. There is no gain from things like a double walls separated by a lightweight matrix - this is because those structures are intended to resist loads from the sides. There are no such loads with a shell. The matrix/scaffolding is however under compressive loads, and the ability of those to resist those loads is the same problem as the walls of the shell. It turns out the best place to put the mass to resist those compressive forces is in the shell wall, and not into scaffolding to support the shell.
The mass of the shell scales linearly with the lift you gained. You drop the pressure 1%, you get lift of 1% of the mass of the air your balloon holds, and you need (minimally) 1% of the material you would need to resist 100% evacuation of the balloon. There is no sweet spot. If you can’t make a 100% vacuum balloon with lift, you can’t make a 1% balloon that is better than a 0%, only one that is worse.
The strongest materials we have are so way short of even being able to create a neutrally buoyant balloon, let alone one with actual lift that it is ridiculous. Guy got a Nobel prize for finding that, so we might assume that it isn’t just a matter of technological advance in getting there. We simply have no idea how such a material might even exist theoretically, let alone be manufacturable.
Not quite true. See my post above. Your material needs a compressive strength/density of >~124 kJ/kg. Plenty of common materials achieve this–a decent aluminum alloy might reach 600 MPa/2700 kg/m[sup]3[/sup] = 222 kJ/kg. Obviously, something like diamond is much better.
The reason for the advanced structures is to avoid buckling loads. It unavoidably reduces the compression strength but there’s no way around that.
Among other things, what makes this impractical is that hydrogen is already ~7% the density of air. So even if you just manage to create a neutrally buoyant balloon, you still have to do 13x beyond that to have something that competes with current balloons. And somewhat beyond that to have the gains needed to justify the advanced structure.
Indeed.
This thread seems to get so many posts I had missed you previous one. Useful.