# Would a blimp "filled" with vacuum float?April 2, 2009 7:00 AM   Subscribe

If you were to take a blimp like structure and instead of filling it with hydrogen, or helium, or hot air, you instead "filled it" with vacuum, that is you removed everything from the inside (think mercury vacuum), would that structure float in earths atmosphere...This is driving me nuts, help me physics!
posted by stilgar to Science & Nature (34 answers total) 2 users marked this as a favorite

Assuming the structure were rigid (so it didn't collapse due to the external pressure), yet no heavier than the material ordinarily used to make a blimp, then yes, it would.

Whether both of those conditions could be achieved in practice is doubtful.
posted by DevilsAdvocate at 7:03 AM on April 2, 2009 [1 favorite]

If the container were rigid and it had less mass than that of the volume of air it displaced, yes.
posted by Optimus Chyme at 7:03 AM on April 2, 2009

what they said. As long as the blimp can keep its volume, it will float.
posted by JauntyFedora at 7:05 AM on April 2, 2009

No, it wouldn't. The reason that helium et al floats is that it's lighter than air and the upwards pull of the helium is enough to counteract the weight of the container.

If Hx + ObjKg > 'Weight' of the air pushing down (where Hx is the lifting force of helium) then an object will rise.

If the Hx is zero (in the case of a vacuum), the equation would be just 0 + ObjKg or just an empty object.
posted by unixrat at 7:05 AM on April 2, 2009

What? Apparently I'm totally wrong.
posted by unixrat at 7:06 AM on April 2, 2009

Also, I was totally wrong about the airplane/treadmill too, so the opposite of what I think is probably correct.
posted by unixrat at 7:09 AM on April 2, 2009 [12 favorites]

HowStuffWorks says yup.
posted by steef at 7:09 AM on April 2, 2009 [1 favorite]

You want to compare the density of the blimp with the fluid it sits in. If you can make a light envelope that will maintain a vacuum such that the average density of your blimp is less than the average density of your fluid, the blimp should float.

Actually building such a blimp could be difficult. Maybe a really big buckyball would work.
posted by b1tr0t at 7:11 AM on April 2, 2009

The reason helium is lighter than air the fact that it's less massy. And what's less massy than a vacuum? There is no "upward pull." There is just massier stuff, pulled by gravity, trying to fill the space where the vacuum is.
posted by bricoleur at 7:11 AM on April 2, 2009 [4 favorites]

No, it wouldn't. The reason that helium et al floats is that it's lighter than air and the upwards pull of the helium is enough to counteract the weight of the container.

No more answering physics questions for you, mister!
posted by Optimus Chyme at 7:17 AM on April 2, 2009 [13 favorites]

The problems with unixrat's explanation is that helium has mass, and just like any mass in the general vicinity of the earth, it's attracted towards the earth. It's just that it has less mass than an equal volume of air, and is attracted to the earth less strongly than air is. Helium balloons rise only because that rising allows an equal volume of more massive air to sink. A rigid helium balloon on the moon, with no atmosphere, would still fall to the ground. A rigid, "vacuum-filled" balloon (with sufficiently low mass) would rise in the earth's atmosphere, because that also allows the more massive air to sink.
posted by DevilsAdvocate at 7:17 AM on April 2, 2009 [3 favorites]

The lifting force of a vacuum-filled container in air is not zero, if you ignore the mass of the container itself.

If you have a truly "empty" container, it will be lighter than one filled with helium. Helium is not massless, after all.

When I was in high school we did an experiment to demonstrate this; we took a bell jar (do they even have those around anymore?) and pumped it out to a fairly good vacuum, then weighed it, then broke the vacuum and weighed it again. The difference is very slight, but it's there — and it's almost exactly what you'd predict using the ideal gas law. In fact you can measure the strength of the vacuum this way, if your scale is good enough.

The reason you can't float a balloon or blimp full of vacuum is an engineering problem: there's no container available that would withstand a vacuum while simultaneously being lighter than the lifting force the vacuum would create in air. Instead, it's easier to take a bladder-like structure and fill it with a light gas like hydrogen or helium, pumping it up to positive pressure so it will retain its shape. The latter approach is in some ways more counter-intuitive than using vacuum, but we're familiar with it so it seems logical.

It's a fairly common feature in science fiction to have vacuum-filled blimps; the caveat is generally some form of advanced materials that makes them possible. (Carbon nanotubes, synthetic diamond, magic, etc.)
posted by Kadin2048 at 7:18 AM on April 2, 2009 [3 favorites]

Another way to think about this is a hot air balloon. You have air in the balloon and air outside the balloon. You heat the air in the balloon so that its density decreases. The density difference creates a buoyant force.

This example is slightly more complicated because air in the hot air balloon can leave the balloon, but the fact remains, you change/control its density to create lift. Take this principle to the extreme, of zero density. A vacuum contains no mass in a given volume (density = 0 kg/m3) and you will get more lift than usual.

Keep in mind the difficulty of making a container that can support a vacuum, maintain its shape/volume and still be lighter than a comparable structure filled with helium.

The way to figure out if something will float in a fluid (like an iron cannonball in a pool of liquid mercury, a balloon in air, a blimp, etc.) is to find the density of the object and see if it's greater than the fluid. If it is less, than the object will displace less mass than an equal size of fluid would and there is buoyancy. The iron cannonball would indeed float in liquid mercury. A lead cannonball would too! (Note there are some technical limitations on this method relating to the continuity of the fluid medium and whether the density is constant. For shallow pools of mercury, and for reasonable heights in the atmosphere these constraints are satisfied.)
posted by KevCed at 7:24 AM on April 2, 2009

yay for magic vacuum blimps/. just wait till i get home.
posted by mary8nne at 7:24 AM on April 2, 2009

...more lift than usual, i.e. compared to the same magical container filled with He instead of a vacuum...
posted by KevCed at 7:25 AM on April 2, 2009

Note: Here's what happens when you create a fairly strong vacuum in a railroad tanker car. Not saying you can't do this, but the structure has to be carefully designed and use the right materials, to avoid being crushed by the air.
posted by knave at 7:35 AM on April 2, 2009 [8 favorites]

The other thing is, the air pressure is proportional to surface area (radius^2), and the density is inversely proportional to volume (radius^3), so the bigger you make this thing, the better.
posted by knave at 7:40 AM on April 2, 2009

THANK YOU EVERYONE! I was pretty sure it would float, but that the problem would be building the damn thing in the first place. Hurray for physics!
posted by stilgar at 7:56 AM on April 2, 2009

Knave, that video kicks ass.
posted by originalname37 at 8:10 AM on April 2, 2009

Larry Niven did stories with vacuum filled zeppelins, using Slaver stasis fields to provide the necessary strength.
posted by nomisxid at 8:23 AM on April 2, 2009

If I remember correctly Neil Stephenson writes about this idea in The Diamon Age. Diamond shelled 'blimps' that are strong enough to hold vacuums. Pretty sure it is this work, has been a while.
posted by Gainesvillain at 10:25 AM on April 2, 2009

Vacuum-emptied dirigibles.
posted by steef at 10:46 AM on April 2, 2009

Not only would it float, it would even float in helium (but with less buoyancy).
posted by TedW at 10:49 AM on April 2, 2009

Vaccuum will float on air, for example you may notice that the earth has an atmosphere, and there is no vaccumm near the ground.
posted by blue_beetle at 11:37 AM on April 2, 2009

OKAY PEOPLE let's get to work.

This shouldn't be too difficult to do on smaller scale. We need a rigid, light structure, we can wrap it with mylar.

mylar holds air pretty well. Who's got a good rigid, light material to built the structure out of? anything better than mylar for the skin?
posted by Baby_Balrog at 12:15 PM on April 2, 2009

Mylar would need a lot of reinforcement to support a pressure difference of one atmosphere.
posted by fantabulous timewaster at 12:35 PM on April 2, 2009 [1 favorite]

I've long been fascinated by this idea also. Buckminster Fuller suggested that, as geodesic dome/sphere structures scale up well, one could make vacuum-buoyed floating cities.

Interestingly, if you do the math, and if you assume that the limitation is the compressive strength of the material you're building from, the whole business is scale-invariant: a substance whose strength-to-weight ratio is high enough to hold out 1 atm of pressure and still be lighter than the air it displaces can do so for a sphere of any size. It's straightforward to calculate the required strength-to-weight ratio. One of the materials that's strong enough is fused quartz. So, I have a secret plan to build enormous evacuated glass spheres and drift Robur-like across the countryside. My glittering aerial empire will last until a sparrow flies into one of them, there is a deafening implosion, and shards of glass rain down on the hapless people below. Muahaha, etc.
posted by hattifattener at 1:33 PM on April 2, 2009 [1 favorite]

Here's my own crazy idea: If you take the ideal gas law and solve for density you essentially get:

D~PW/T

Where D is density, P is Pressure, W is atomic weight and T is temperature.

Ok, so you want low D to make this thing float but you actually want high P so that you don't need some superstrong container for it. How do we solve this? High T.

So, we need a thin light gas that we can "super" heat. Of course then we need a perfect insulator too...ah well, back to the drawing board...
posted by vacapinta at 2:22 PM on April 2, 2009

Actually this was the basis for an early attempt a flight (unsuccessful of course) by a jesuit Francesco Lana de Terzi, of Brescia in 1670.

He tried to build copper spheres containing a vacuum to lift a boat.
posted by compound eye at 6:46 PM on April 2, 2009 [1 favorite]

heres a model
posted by compound eye at 6:47 PM on April 2, 2009 [1 favorite]

Ok, let's super-heat a complete vacuum and see if it floats. (in our dreams perhaps?)
posted by blue_beetle at 12:30 PM on April 3, 2009

Can you give step-by-step instructions on heating a vacuum?
posted by b1tr0t at 10:21 PM on April 3, 2009

Where's Adam Savage? Sounds like this needs to be tested.
posted by kookywon at 12:34 PM on April 7, 2009

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