Vacuum airship | Hacker News Reader

Vacuum airship

en.wikipedia.org

56 pointsejr59 comments

The linked articles from the 1887 New York Times are wonderful:

"The means by which passengers will be landed en route is also in doubt. Some suggest that they will simply be dropped overboard and their luggage thrown after them, while others maintain that a coil of greased rope for the purposes of descent will be carried on each and every machine."

Truly the Hyperloop of its day.

http://query.nytimes.com/mem/archive-free/pdf?res=9904E7D815...

How does this compare at all to the Hyperloop?

It exists only on the paper.

LOL.

This is a problem that every zeppelin faces too. They can't easily gain or lose height, because that would mean losing the gas they need to stay airborne.

A vacuum ship that actually has a vacuum pump aboard would have the great advantage that it can easily change it's buoyancy using, say, electrical power. It could use this to change height, or compensate for being loaded with materials.

Vacuum ships would actually be a solution to the zeppelin problem.

A zeppelin could use compressor tanks to control its buoyancy in an exactly analagous way. Lots of details on how the problem was handled in practice here:

http://en.wikipedia.org/wiki/Buoyancy_compensator_(aviation)

There are multiple solutions to the "Zeppelin problem", as you call it, to the point that it's not much of a problem.

If you had a powerful enough pump to create a reasonable vacuum it's far more powerful than the pump you need to compress helium if you need to lose buoyancy.

At atmospheric pressure, a given volume of hydrogen weighs approximately 7% (there is some variation with temperature) of an equivalent volume of air. All that is needed to contain hydrogen at atmospheric pressure is a membrane it can't pass through. The inwards force exerted on the membrane by the atmosphere is perfectly balanced by the outwards force exerted by the hydrogen gas. The only structural material required is for the purpose of harnessing the lifting power of the enclosed gas, providing an aerodynamic shape, etc..

Conversely, a vessel containing a vacuum must be built to withstand the 101,325 N/m^2 net inward force exerted upon it by the atmosphere that is not balanced by the outward pressure of the contents of the vessel. Consider, for a moment, what you would need to build a 1 m^2 coffee table out of if 20 elephants were to be able to stand on top of it at once.

For the sake of argument, let's say that we had a magical material that would let us build a replica of the Hindenburg that is of equal weight to the original, only with vacuum vessels replacing the hydrogen bags. The Hindenberg contained 200,000 m^3 of hydrogen lift gas, weighing 1,798 kg and displacing enough atmosphere to provide a total lift of 25,850 kg, 10,000 kg of which were considered it's cargo capacity beyond the passengers it carried. This new version of the Hindenberg, despite being built with materials and techniques that are probably well beyond our current level of technology, would carry the same number of passengers and just 18% more cargo.

For those of you who think that vacuum vessels are a lot safer than hydrogen, I'd like to remind you that the atmosphere surrounding 200,000 m^3 of vacuum provides a rather huge amount of potential energy waiting to be released. When vacuum vessels of a large size fail the resulting implosion can be both spectacular and devastating.

https://www.youtube.com/watch?v=Zz95_VvTxZM

I have wondered if such a thing could be designed for use in the upper atmosphere and dropped in from space. That would avoid the need to keep out the full atmosphere. But then so would pumping out the air as it gained altitude...

I like this thinking! Like dropping a beach ball into a swimming pool.

But if dropped into the upper atmosphere where there is little air pressure, it would sink until it got to an altitude where it's weight equaled the mass of the atmosphere it was displacing.

That would be the point.

This is an idea that seems interesting at first glance, but it's really not. What's important for lift isn't the absolute weight of the gas; it's the ratio of the weight of the lifting gas to the surrounding air. What that means is while hydrogen gives you about 1kg/m3 lift a perfect vacuum bottle only gives you about 1.3kg/m3. It probably wouldn't make sense even if we had the necessary exotic materials.

The major weakness of a vacuum airship is that a breach of the pressure hull will compromise the vacuum and thus the buoyancy. So any truly useful solution will require a number of independent vacuum cells.

The strongest shape made of compressive members is a tetrahedron, if surfaced with an impermeable and inelastic membrane an evacuated aerostat could end up looking like http://www.theatlantic.com/video/archive/2012/01/a-gorgeous-...

I wonder if a modern materials would let us build lift cells in this vein - maybe thin-skinned vessels filled with aerogel for structural support.

> "This cannot even be achieved using diamond"

The whole concept totally absurd. Helium is ~15% the density of air, so even if you created a material 100x stronger than diamond, it would only outperform helium by a tiny amount.

The helium in a balloon/dirigible is under pressure, so its density would be somewhat greater than 15% that of air. But point taken.

No. It's always the same pressure as air. That's the point. It'll always be ~15%.

immediate thought was aero gel too. I wonder if graphene would be useful as well.

Misses the point hydrogen is safer, easier, could be done today with existing tech for commercial travel (Already used a lot for scientific balloons) and pretty much has the same lifting power.

(Also Helium is obviously currently is used in airships commercially with no real problems on top of a theoretical vacuum ship other than perhaps cost of balloon contents, which I'd guess is not the limiting factor anyway)

A problem with Hydrogen is that it is very reactive -- that doesn't necessarily mean fire, but it does mean that any surface it comes into contact with will be affected. For example, you can't fill a latex balloon with hydrogen and expect it to last very long.

Hydrogen, at normal pressures, isn't that reactive at all if you ignore it's combination with oxygen.

This rail-bound tank was probably not designed with weight savings as a prime considerations. https://www.youtube.com/watch?v=Zz95_VvTxZM

Still, this is a fascinating idea and I would love to see someone make a serious attempt at it.

Yeah, but as the Wikipedia article states: not even diamond is strong enough. So don't hold your breath.

Its a cool idea but as the post suggests air pressure on the vacuum spheres is a major problem. The crush force on the sphere will scale 40x the lift factor. So 1 gram of lift, 40 grams of crush.

Once you reach scales of 10,000 kilograms you'll be dealing with keeping 400,000 kilograms of weight out of your vacuum.

Its worth noting that this force is not due to air preasure per se, but the difference in prrssure between the inner and outer volumes; if you can distribute the 'crush force' over a series of concentric partially pressured shells for example, the force is significantly reduced.

...not sure how it would apply to balloons, but its used in a number of things to contain high pressure on the inner ring.

Technically true, but then that weights more. So you change the mass to mass to lift ratio.

The advantage over hydrogen at atmospheric pressure is really minor, too.

That's true for buoyancy but vacuum definitely has a major advantage in area of flammability.

What happens if a vacuum sphere that can lift 10t implodes?

You lose 10t of lift.

If a hydrogen envelope of 10t ruptures, you lose 10t of lift and have a major fire hazard.

You lose 10t of lift in a horrifyingly destructive implosion: https://www.youtube.com/watch?v=Zz95_VvTxZM

And here's what hydrogen does.

Note the comparison is of a pure H₂ vs. 2 χ H₂ + O₂ mix.

https://www.youtube.com/watch?v=qOTgeeTB_kA

Hydrogen is not as major of a hazard as it looks.

It's lighter than air, and will rapidly leave if it escapes. The real trick is making sure no oxygen gets in. In contrast a leak is not super dangerous.

It will still burn of course, but if the skin is flameproof it won't do much harm to the ship as a whole. So what you do is make sure it's enclosed in cells of a flameresistant material. That way a fire won't spread and will rapidly rise away and put itself out.

The range of explosive concentrations of hydrogen gas means that either oxygen in low concentration or hydrogen in low concentration are highly explosive.

If you've a leak, pretty much by definition, oxygen is getting in.

Other than vacuum not burning.

Yeah, your balloon just implodes at the speed of sound.

In terms of disastrous outcomes, it's pretty much the same thing. Hydrogen explosions are scary, but the ball of flame also rises very quickly. Most of the people on the Hindenburg survived.

I wonder what would have happened if a Vacuum airship would have had a similar accident. Meaning a hull breach at a 200m (650ft) altitude. No fire, just a bit of a drop. Maybe shrapnel? Well, I guess it would drop much faster.

Given that there are no vacuum containment systems large enough to support the Hindenberg, it would never have taken off in the first place.

Given that what precipitated the failure of the Hindenberg appears to have been either an initial slow leak which caught fire, or a fire which consumed the gas envelopes, possible as little as a gradual loss of lift.

Even in the case of sudden failure, it would depend on how many lift compartments existed -- you'd lose 1/n of your total lift, so presumably a pronounced but possibly controlled descent.

Could you get around this by using many small vacuum-filled spheres instead of one large sphere?

The scaling of sphere SA to Volume isn't exactly linear, its more logarithmic Source: http://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/Com...

sure, but many small spheres are heavier than a large one.

More specifically: weight/lift ratio is a cube to square relationship. Lift increases with the cube of the radius, weight (of the envelope) with the square. So a collection of smaller spheres would have a higher mass.

Would another shape reduce the amount of surface area/external pressure?

I've long dreamed of having one of these ships, but it doesn't seem to be in the cards.

No, a sphere has the minimal surface area volume ratio. Wait, that does not really answer your question.

Yes it does. It would be a major mathematical revolution if you discovered a new shape that exists in our world that has a better SA:V ratio.

But SA/V isn't the important ratio here. The important ratio is weight to volume.

Surface area only matters in terms of how much pressure the system needs to resist, and using compression members means you have some room to play with. But if you have a a structure from which you've removed the air that is too heavy to float in air you get no prize.

> But SA/V isn't the important ratio here. The important ratio is weight to volume.

Don't forget crush resistance with a minimum of construction material. But as it turns out, a spherical shell is the optimal shape for all the stated requirements -- it evenly distributes the compressive force across its surface, it represents the optimal ratio of surface area to volume, and it therefore follows that a sphere represents the smallest mass per enclosed volume.

> But if you have a a structure from which you've removed the air that is too heavy to float in air you get no prize.

That's certainly true. But for a vessel with a fixed wall thickness, making it larger greatly increases the chance that it will rise, because the enclosed volume increases so much faster than the surface area.

This would all be easier on Venus, where the atmospheric pressure is vastly greater than here, but the gravitational force is less. If only it weren't so damn hot.

I think it is mathematically proven that that's a sphere. Once something is mathematically proven there won't be anything ever that invalidates this.

Also: Imagine any shape. Now imagine to inflate it. What shape will it more and more approximate?

Reminds me of an idea I had for lighter than air packing material. It was going to be something like over-sized bubble wrap filled with helium, or balloons you'd insert into the spaces and inflate with a helium canister. Seems a great idea at first, but then you do the maths, and think about the materials. Unfortunately the weight savings are likely to be so minimal they're unlikely to save on postage costs, and even if they did reduce postage they're likely to be outweighed by the cost of materials. Plus you later find out that lots of other people have had the same idea and posted on the internet about it.

This reminds me of Buckminster Fuller's cloud 9 concept. His idea was that since volume scales so much more strongly than surface area for a sphere, if you make a larger and larger sphere you will enclose cubed more air for only squared more structure. Build a large enough sphere and heat the enclosed air a couple degrees above ambient and the entire structure would be buoyant, like a soap bubble with human breath in it.

http://en.wikipedia.org/wiki/Cloud_Nine_%28tensegrity_sphere...

"Geodesic spheres (structures of triangular components arranged to make a sphere) become stronger as they become bigger, due to how they distribute stress over their surfaces."

I happened upon some Bucky Fuller stuff recently, and wondered just how large geodesic spheres have been made... and it's not particularly large. Which makes me wonder: is this really the whole story? What is the flaw in geodesic spheres that keeps them from being the basis of huge enclosed structures?

The difficulties of construction for one, and such large structures are vulnerable to buckling I would imagine.

These are fun (also in Diamond Age I believe) and I like how they are both impossible to create and there is a patent application on them :-)

It is interesting to run the structure density vs the vacuum numbers. I always wondered if we would be able to make vacuum filled bucky balls out of carbon at some point.

Does the vacuum have to be perfect? Maybe you can reduce the pressure on the surfaces, and therefore simplify the engineering, by leaving some air in there. I think it is an interesting idea that varying the internal pressure would change your lift. You only ever need "just enough" lift anyhow.

14% advantage over helium makes it seem like it's not worthwhile. Am I missing something?

Helium is a finite resource- escaped Helium eventually leaves our planet forever; nuclear fusion will probably never produce sufficient helium for current industrial use.

Helium is quite expensive, and a limited resource:

"His patent application was eventually denied on the basis that it was 'wholly theoretical, everything being based upon calculation and nothing upon trial or demonstration.'"

He'd get that patent today. Little as it might avail him.

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