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Engineering a vacuum zepelin?

quarky said:
Idle curiosity, but have any of you made soap bubbles with helium?
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No, but I've made them with hydrogen. More fun to ignite. Aluminum scraps in muriatic acid makes a fair source of hydrogen, then bubble through soapy water. Chase the bubbles around with a blowtorch and make them go boom. You can fill rubbers that way too. They hold more hydrogen and make bigger booms.
 
I believe the idea is as old as the realization that air has a mass. It will be cool to see material science catch up to it. :)
 
To me, the definition of vacuum matters. If vacuum is a measure of the impact force by gas molecules on one side of a membrane trying to move to the other side of the membrane, then a vacuum balloon is a daunting engineering problem. The forces involved scale in abusive ways since the volume increases much slower than the surface area, with the lift proportional to volume but the crushing force proportional to surface. There's probably a little bubble of 'this might work' somewhere in the middle where the surface area is small enough to make the forces manageable and the volume is large enough to make enough lift to offset the structure that manages the crushing force, but it's not a big bubble.

If vacuum is defined as a quantity of matter in a certain volume though, then it's not a daunting problem, just a matter of making a smaller amount of matter exert the same amount of force on one side of a membrane as a larger amount of matter does on the other. There's actually an easy and cheap way to do this: make your smaller amount of matter move faster, so that each individual molecule from the smaller amount hits the membrane proportionately harder. At the extreme case, you can have a balloon inflated by a pretty decent vacuum, consisting of a few molecules of gas moving very fast. Basically, a hot air balloon IS a vacuum balloon and the hotter you can make it, the more lift. It might be more productive to develop better heat resistant membranes and methods of heating than to try to devise an internal structure to withstand pressure. Picture a balloon made of spun refractory enclosing a wisp of plasma heated by magnetic pinch or microwaves.

There are other applications of this: vacuum tubes rely on quantity of matter in a volume rather than pressure measurement, because the actual critical factor is the mean free path of the electrons. Electrons have to be able to pass from one electrode to the other without encountering matter that would absorb them. You can get such a vacuum at atmospheric pressure inside a hot flame. Put the emitter, grid, and collector of a vacuum tube within a flame and it will function just like it's inside an evacuated glass envelope. Some flame detectors for furnaces operate in this manner. They act as a diode if and only if the flame is present. Otherwise it's just two unconnected wires.
 
Andrew Wiggin said:
The forces involved scale in abusive ways since the volume increases much slower than the surface area, with the lift proportional to volume but the crushing force proportional to surface. There's probably a little bubble of 'this might work' somewhere in the middle where the surface area is small enough to make the forces manageable and the volume is large enough to make enough lift to offset the structure that manages the crushing force, but it's not a big bubble.
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This is wrong. We've been through the scaling arguments already, and the size doesn't matter until you get to absurdly large volumes.
 
Andrew Wiggin said:
No, but I've made them with hydrogen. More fun to ignite. Aluminum scraps in muriatic acid makes a fair source of hydrogen, then bubble through soapy water. Chase the bubbles around with a blowtorch and make them go boom. You can fill rubbers that way too. They hold more hydrogen and make bigger booms.
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Yes, i used to make hydrogen bombs with zinc in coke bottles with nitric acid; with a 'rubber' on the bottle. We floated these crafts over the skies of Cambridge, until the neighbors got upset. But I never tried it with soap bubbles.
This is one of the big regrets in my life.
 
rjh01 said:
What was the density of the concrete? The weight would mostly be stone. However some stones (volcanic) can contain a LOT of air and actually float in water. Do your calculations again using concrete that has the same density as water or less.
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If you're thinking about cementicious concrete, the conventional desity is 2400kg/m3. With Cement RD at 3.14, water at 1.0, you can use low-density aggregates and sand, but you'd probably still get to a 1500kg/m3 blended density.

Depending on mix proportions for your required strength, your concrete density could get as low as 1600kg/m3, but I doubt much lighter.

The reinforcing should be added at 7850kg/m3 stell at a nominal 150kg/m3 concrete. Glass Pultruded Re-bar could be considered for this purpose with a reduced weight per volume for the same strength.

The resulting product would not be airtight by a long shot and would still need a membrane of some sort.
 
Non-fluffy concrete generally weighs about as much as sandstone or limestone; in the area of 150 lbs/cu. ft. Re-bar raises that number slightly.
 
When the Romans built the Pantheon 2000 years ago its 43m diameter dome was the largest un-reinforced concrete dome in the world. It still is.

One of the techniques they employed in building this remarkably sophisticated structure was to grade the aggregate they used for the making the concrete. The higher up the dome they went, the lighter and finer the aggregate, culminating in pumice being used at the top around the occular (opening).

From Wiki:

The stresses in the dome were found to be substantially reduced by the use of successively less dense aggregate stones, such as small pots or pieces of pumice, in higher layers of the dome. Mark and Hutchison estimated that, if normal weight concrete had been used throughout, the stresses in the arch would have been some 80% greater. Hidden chambers engineered within the rotunda form a sophisticated honeycomb structure.[35] This reduced the weight of the roof, as did the elimination of the apex by means of the oculus.

Just thought you'd like to know. :)

Mike
 
Toke said:
I believe the idea is as old as the realization that air has a mass. It will be cool to see material science catch up to it. :)
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Well it took a little less than twenty years to go from aerogels to a microlattice structure that's somewhat lighter than air, so give it a couple of decades. Plus time to figure out large scale production.
 
quarky said:
Greetings.

I've brought this up in the past, but it felt like a good time to try again...probably because of the fun discussing the human powered helicopter challenge.

The challenge is to build a craft that is lighter than air, without the use of hot air, He or H2. Specifically, it would need to float via evacuating air from a container...one that would need the structural integrity to not be crushed from pressure increase on its exterior surface.

In this math, to the small extent that I've pondered it, a very large geodesic framework of carbon fiber struts, covered with a lightweight, air tight, strong membrane is the likely approach.

I think it is possible, given the existing materials. The available weight that can be evacuated; i.e., the air inside the sphere, grows exponentially with the radius, with respect to the weight of the container itself.

Hence, at some huge size, a very small % of the inner air need be evacuated...decreasing the demands of the structural components and membrane.

If this is even hypothetically possible, I suspect it would be as impractical as the human powered helicopter. Yet, I find it intriguing in its own right, and worthwhile.

So, I'm asking curious engineers to offer some thoughts on this.

Imagine going aloft by pulling air out of a container, and coming back down by letting air back in. No expensive leaky helium; no potentially explosive hydrogen; no costly gas fired hot air.

In futuristic space exploration, this type of structure could grab a chunk of vacuum, and then drop into a planet's atmosphere, slowly allowing gasses to enter, and making a very gradual descent...or choosing to remain aloft at a specific altitude.

Pre-thanks for the willing.
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Just wanted to say I beat you to this idea

http://forum.nasaspaceflight.com/in...d894457bf206f746964d0bff43&topic=14620.0;wap2
 
I couldn't sleep last night, so I took to trying to do some mental maths. I tried to work out how many 100mm diameter balls would fit in a sphere 700mm in diameter. I failed.

I have had another go at it this morning, because it was annoying me, and I came up with a figure of 286. Please feel free to tell me I'm wrong! :)

If each of these balls was made of 1mm thick carbon fiber, they would weigh about 55.4 grammes. Anyone remember those "holy holey" plastic balls you used to get, which meant cricket in the garden didn't threaten your window panes? Anyway, imagine removing half the shell of the carbon fibre balls by moulding some sizeable holes. Your ball now weighs 27.7 grammes, and 286 of them weigh 7.92kg.

These balls, in my mind, represent the most efficient way of re-inforcing our sphere such that it would take the pressure of being totally evacuated (OK, I know that isn't poss.)

Stuff this lot of holy balls into your 700mm diameter sphere, again made of 1mm thick carbon fibre (which itself weighs 2.76kg), and the total weight of carbon fibre is now 10.69kg. Extract all the air.

The weight of the air that the sphere is displacing is, I reckon, about 0.2155kg.

Therefore, the evacuated spheroid, presuming it survived the evacuation, is some 49 times heavier than it needs to be before it would float off into the sky.

Even if you made the material 0.2mm thick, it would still be nearly 10 times heavier than it needs to be to fly.

Even just the outer shell alone, 1mm thick carbon fiber, is some 12.8 times heavier than the air it displaces. To fly, it would have to be .078mm thick max, and there is no way on this planet that this would be strong enough to cope with a perfect vacuum inside.

I'd be happy for anyone to check my figures, which are bound to be wrong......but they aren't going to be 50 times too high, and that illustrates the huge issues required to be overcome before any vacuum zeppelin would fly.

Mike (ducking for cover, for fear of having my maths shredded!)
 
A sphere filled with small spheres, non-compressible, of the same diameters, in their tightest configuration, leaves aprox. 27% voids.

I think the approach I mentioned earlier, creating a shell of tight fitting units, is the way to go.
 
What might approach a vacuum balloon in performance is a plasma balloon.

A very hot plasma could create enough pressure to keep a balloon inflated while still being nearly as tenuous as a vacuum.

Left as an exercise for the reader;

1. How to maintain such a plasma
2. What material will withstand that heating
 
quarky said:
I don't usually say this, but I think I'm right.
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Well, help me out then! How is that figure calculated? I made one huge assumption in calculating mine, and if I have gone wrong, it is in that assumption. But I'm curious........

Mike
 
I wonder if the idea of vacuum-filled internal spaces could be used in static construction? For example, strucutural members of a skyscraper using a support beam with a vacuum-filled, honeycomb/foam type interior? Wouldn't make a large difference, but would it make a measureable difference at all in the amount of weigh thte lower portions of the building would have to support? Is there any material that could fit the bill for this?
 
Hellbound said:
I wonder if the idea of vacuum-filled internal spaces could be used in static construction? For example, strucutural members of a skyscraper using a support beam with a vacuum-filled, honeycomb/foam type interior? Wouldn't make a large difference, but would it make a measureable difference at all in the amount of weigh thte lower portions of the building would have to support? Is there any material that could fit the bill for this?
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People are looking at just that already. Some years away however, the largest block of their LTA material would fit in the palm of a hand. It'd be especially interesting for thing bridges. Or space elevators. :)
 
Structural engineers build in a safety factor of x3 for domestic-scale structures. I am sure they are even higher for commercial buildings and civil engineering projects. Thus, any benefit from having small volumes of vacuum, even if remotely practical, would be infinitesimal in comparison. Probably about the equivalent of having one less photocopier per floor of the completed building.

Mike
 
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