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Engineering a vacuum zepelin?

quarky

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Oct 15, 2007
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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.
 
This sounds great, but it will probably turn out that there is some proportional constant whereby the strength of the materials required to withstand the external pressure will always make it too heavy to float, or something like that. If you make it bigger so you need less vacuum, then there is too much material mass; make it smaller to reduce weight (thus requiring greater vacuum %) and it's not strong enough to resist collapse. I'd be very surprised if this hasn't been tried already.
 
yeah, but the deal gets better, as per weight of contained air/ weight of shell, as the radius increases.
 
A variant sprouted from Iain M. Banks' fertile imagination has the zeppelin filled with evacuated buckyballs. This also has the benefit of turning the "vacuum" into a pumpable medium. Net gain in lift, in his opinion (presumably not strictly calculated - we are talking rip-roarin' SF here), would be just a few percent beyond hydrogen filling, and the entire apparatus "something that tended to be done because it could be done, and not for any real pactical purpose".
 
is it possible to float an airship(or anything) with a vaccumm? i mean theoretically?

ive never thought about it before but i assume you need molecules to push the craft upward?

very confused! can you help.

lxxx
 
s_pepys said:
is it possible to float an airship(or anything) with a vaccumm? i mean theoretically?

ive never thought about it before but i assume you need molecules to push the craft upward?

very confused! can you help.

lxxx
Click to expand...

A container filled with nothing will be lighter than one filled with H2 or He.
Problem is, instead of a balloon, you need a structure that can withstand the relatively increased exterior pressure.

It would be advantageous to start out with a helium or hydrogen filled container and evacuate that gas for lift-off, rather than evacuating the much heavier air.

That would sort-of be cheating, yet, it's a start.
 
Floyt said:
A variant sprouted from Iain M. Banks' fertile imagination has the zeppelin filled with evacuated buckyballs. This also has the benefit of turning the "vacuum" into a pumpable medium. Net gain in lift, in his opinion (presumably not strictly calculated - we are talking rip-roarin' SF here), would be just a few percent beyond hydrogen filling, and the entire apparatus "something that tended to be done because it could be done, and not for any real pactical purpose".
Click to expand...

Cool.

One of my thoughts on the matter was to create a sphere of balloons, wrapped in mylar. The balloons would merely comprise the outer shell of the structure, possibly being a lot lighter than a geodesic frame work. As the interior air was evacuated, the shell of balloons would squish against each other tightly, gaining structural integrity with the increase of outside pressure.
 
quarky said:
A container filled with nothing will be lighter than one filled with H2 or He.
Problem is, instead of a balloon, you need a structure that can withstand the relatively increased exterior pressure.

It would be advantageous to start out with a helium or hydrogen filled container and evacuate that gas for lift-off, rather than evacuating the much heavier air.

That would sort-of be cheating, yet, it's a start.
Click to expand...

It iss much easier to design a vessle to contain pressure--the equations are well known--than it is to design one to keep pressure out.
In the first case, you can use load paths and pressure stiffening to make it work and be light weight.
In the second case, the only consistent solution is more material--witness the wall thickness in very deep sea exploration vessels, and submarines...
 
How about: instead of a geodesic dome, you take 4 strong straight struts, arrange them like a giant caltrop (look it up), and stretch fabric over that? Now your rigid beams are feeling straight-up compression (rather than hoop stress, as the struts of a compressed geodesic dome) and the fabric is under pure tension.

Maybe an advantage, I'd have to do the math.
 
ben m said:
How about: instead of a geodesic dome, you take 4 strong straight struts, arrange them like a giant caltrop (look it up), and stretch fabric over that? Now your rigid beams are feeling straight-up compression (rather than hoop stress, as the struts of a compressed geodesic dome) and the fabric is under pure tension.

Maybe an advantage, I'd have to do the math.
Click to expand...
Column buckling becomes a factor, especially with the crippling effect (perpendicular load to the beam/column) of the load due to P*A on the fabric.
 
s_pepys said:
is it possible to float an airship(or anything) with a vaccumm? i mean theoretically?

ive never thought about it before but i assume you need molecules to push the craft upward?

very confused! can you help.

lxxx
Click to expand...

Yes. In order for things to float under the influence of a buoyant force, only one condition must be met: the weight of the fluid displaced by the object must be more than the weight of the object itself. And, as quarky pointed out, a hollow object will weigh less when it is completely evacuated versus being filled with hydrogen or helium gas.

So, theoretically, a hollow completely evacuated "balloon" could float in the atmosphere. The challenge is to prevent the thing from collapsing under the external pressure pushing inwards on it, thereby eliminating any buoyant effect, because there is no internal pressure pushing outwards.
 
rwguinn said:
Column buckling becomes a factor, especially with the crippling effect (perpendicular load to the beam/column) of the load due to P*A on the fabric.
Click to expand...

There's no perpendicular load; the fabric is arranged symmetrically on all sides of the column. There's a huge compressive force, which, yes, may cause ordinary buckling. But hoop compression on a sphere can buckle it, too, so the real question is whether "buckling of a column" and "crushing of a sphere" have different r-dependencies, and maybe whether one is more favorable than the other. I kinda doubt it, but I'd like to see the math.
 
I have thought about this for years. About the best idea I can come up is for gaining high altitude. The pressures would be very low so you might get some advantage over using Helium.
 
Lift generated by a vacuum would make a nice twist on the regular perpetual motion machine designs.
 
Psi Baba said:
If you make it bigger so you need less vacuum, then there is too much material mass; make it smaller to reduce weight (thus requiring greater vacuum %) and it's not strong enough to resist collapse.
Click to expand...
DISCLAIMER: Very dirty handwaving follows.

Both surface and cross-sectional area increase as the square of the size (characteristic linear dimension, e.g. diameter); volume as the cube.
The "buckling moment"* (sucking in normal to the skin) is presumably proportional to the underpressure and size.
The compressive hoop force (compressive loading tangential to the skin) is proportional to the underpressure and the square of the size.

So... at a given internal pressure, doubling the size (diameter) of a "thick skinned" float would imply:

23 = 8 times displacement (raw buoyancy)
22 = 4 times the skin-tangential compressive strength
21 = 2 times the skin-normal stiffness

A quick guess suggests all those factors are about proportional to the amount of underpressure. That is, if external pressure is 14.65 psia then internal pressure of 12.65 psia (2 psi underpressure) will generate twice the lift and require twice the strength and stiffness of 13.65 psia internal pressure.

Quartering the underpressure while doubling the size would imply:

23/4 = 2 times displacement (raw buoyancy)
22/4 = 1 times the skin-tangential compressive strength
21/4 =0.5 times the skin-normal stiffness

That is, doubling the size increases volume enough to gain buoyancy even if internal pressure is increased so the skin doesn't have to be made stronger or stiffer. Maybe a very large, stiff-skinned container (say, a 3D truss structure covered by thin mylar skin) at slightly reduced internal pressure could manage to float.

Using ordinary air at small pressure differential would also mitigate the problem of diffusion, possibly allowing a thinner and lighter outer skin (ISTR that's a practical limitation on the thin-ness of a H2/He balloon skin?). With sufficient buoyancy reserve, a small compressor (solar power?) might eliminate that problem entirely.

Of course, the above "analysis" ignores the loading of the skin structure by its own weight. Whether a further internal pressure increase could compensate for that and still gain buoyancy has been left as an exercise; I fear that analysis may depend on construction details.

At high altitude, of course, external pressure is already low... leaving little to gain by reducing internal pressure. A light bag filled with low molecular weight helium can get bigger when it gets higher; that might be a tough act for a rigid pressure-deficit container to follow.

*I'm duly ashamed of such sloppy use of engineering terminology, but I did warn you about the handwaving.
 
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I see I neglected that skin cross-sectional area resisting hoop forces is proportional to enclosure size. So, for a given pressure deficit, doubling the size will impose four times the hoop force on twice the skin cross-section. That is, the skin will need to have twice the compressive strength per unit length.

Reducing the pressure deficit by half while doubling the size won't require stronger or stiffer skin... but it will quadruple not only the raw buoyancy, but also the skin area... and weight.

I stand self-corrected: Increasing size won't save the thick-skin concept.
 
For the same reasons a balloon filled with lighter than air gas has to be a large structure to fly then so would a no-air structure. In other words the no-air container would have to be large and I don’t believe it would ever be possible to build it both strong enough and light enough to ever fly. Tis all but a flight of fantasy. Sorry to be a party pooper.
 
quarky said:
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.
Click to expand...

This is incorrect. First off, all the scaling in the problem is polynomial (xn), none of it is exponential (nx). Second, the scaling for volume (r3) is actually the same as the scaling for the container weight. The surface area scales as r2, but the bigger r is, the less curvature you have, which means the same external pressure exerts more lateral force on a surface element. So you need to keep making the surface thicker, adding back another factor of r. Both weight and displaced volume grow as r3, and increasing the size of your structure gains you nothing.
 
ynot said:
For the same reasons a balloon filled with lighter than air gas has to be a large structure to fly
Click to expand...

A balloon with lighter-than-air gas doesn't need to be a large structure in order to fly. It only needs to be large if you want to lift something other than the structure itself, which is usually the case. But party balloons prove that you can fly small structures with boyant gasses.

Small hot air balloons have the additional problem of faster heat loss, but that's a limitation on flight time, not on flight itself.
 
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