PDA

View Full Version : Would this collapse under its own weight?



Desert_SW
2006-Oct-19, 02:33 AM
If you built a "hollow" pyramid-like structure out of some kind of futuristic carbon nanotube tubing that was like 60 miles x 60 miles x 60 miles? You know how they do that triangular construction for strength minus the weight? I'm not talking about just a shell but reinforced all throughout the inside too.

Not that it'd be practical to build..but would it collapse? I'm assuming it would but I'm no engineer.

antoniseb
2006-Oct-19, 12:11 PM
No, this could be built in a way that would make it strong enough to stand. Please note that the building material you describe doesn't exist yet.

I'm guessing that you'd want this not so much to be a pyramid as a 300 mile long ramp. If the tubes were made of Carbon sucked out of the air, this could have a big impact on global warming. That would be a lot of Carbon.

Desert_SW
2006-Oct-19, 12:17 PM
Maybe the pyramid would be overkill but if a simpler ramp could be built maybe that'd have benefits over a space elevator?

I think we're within a decade or two of mastering the use of CNT's. We already have the ability to make long thin sheets of it.

antoniseb
2006-Oct-19, 12:26 PM
Maybe the pyramid would be overkill but if a simpler ramp could be built maybe that'd have benefits over a space elevator?

A ramp could be used for the launch by rail idea, which might be easier to make than the space elevator, though it could also (if built on the equator) be used as the Earth-side base for the space elevator, and thereby reduce some of the concerns about the impact of weather on the ribbon.

Concerning the ramp vs. Pyramid, I think it would still need a 30 mile wide base for the part that is 60 miles tall.

I think we're within a decade or two of mastering the use of CNT's. We already have the ability to make long thin sheets of it.
I don't think we're that close to being able to manufacture it with enough control and economy of scale to do what you propose. I'm guessing sixty years minimum to get to that point. Still, I like the idea very much.

Desert_SW
2006-Oct-19, 12:50 PM
Even a 30 mile wide base would be an extreme project. I can't say we'd be able to build that in a decade or two. There seems to be so much potential though even in what we can already do with the sheets to make a composite material.

Could those sheets be "weaved" like we do with our fabrics today?

Saluki
2006-Oct-19, 08:50 PM
Interesting idea, but I was under the impression that CNTs were sought for their tensile strength, and not their compressive strenth. I havn't done any research into it, but I thought they were fairly flexible, and would buckle easily under compressive loads.

Am I wrong on this?

NEOWatcher
2006-Oct-20, 12:19 PM
Even a 30 mile wide base would be an extreme project. I can't say we'd be able to build that in a decade or two. There seems to be so much potential though even in what we can already do with the sheets to make a composite material.

Can somebody explain how much this can be saving? While reading this thread I keep thinking:
- You still need to accelerate.
- You still have atmospheric drag.
- You have drag with whatever is keeping it in line
- You have added some incredible maintenance issues
I do see benefits, mainly, you don't have to launch the launch fuel. I just don't see them outweighing the issues.

antoniseb
2006-Oct-20, 01:04 PM
Can somebody explain how much this can be saving?
There are a few advantages, but as you note they come with some huge expenses.

Imagine that we will get to a point where we will be putting a million of tons per year into orbit (good for light industry). How much chemical fuel neds to be annually combusted in the atmosphere to get that much material up there? Hint, it is a lot.

The ramp has the potential to be scaled up to launching volumes that chemical rockets can never achieve, and depending on how our energy infrastructure develops, could possibly be done without polluting the atmosphere substantially.

As you say, it is a big maintenance issue, and the development cost is very high. The required annual mass to and from orbit would be quite large to make it commercially viable.

BTW, I imagine that the structure supporting the rail would be a great place for a very large vertical integrated metropolitan zone. Imagine the view from an office sixty miles above the Equadoran Andies.

Saluki
2006-Oct-20, 03:59 PM
http://en.wikipedia.org/wiki/Carbon_nanotube


Strength
Carbon nanotubes are one of the strongest materials known, both in terms of tensile strength and elastic modulus. This strength results from the covalent sp2 bonds formed between the individual carbon atoms. In 2000, an MWNT was tested to have a tensile strength of 63 GPa.[15] In comparison, high-carbon steel has a tensile strength of approximately 1.2 GPa. CNTs also have very high elastic modulus, on the order of 1 TPa.[16] Since carbon nanotubes have a low density for a solid of 1.3-1.4 g/cm³ [17], its specific strength is the best of known materials.

Under excessive tensile strain, the tubes will undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5% [Qian et al, 2002] and can increase the maximum strain the tube undergoes before fracture by releasing strain energy.

CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional or bending stress.

[Emphasis added.]

If this is correct, CNTs would not be a good material for building a structure like this.

NEOWatcher
2006-Oct-20, 05:11 PM
There are a few advantages, but as you note they come with some huge expenses...
Ok; so the pollution issue is not quantifiable, so that changes the benefit pictures some. (Makes the picture fuzzier)
And, the scaling up of launch mass and the need for the ramp seem to be a chicken and the egg proposition. (I hate those...You can never make any definite justifications)

BTW, I imagine that the structure supporting the rail would be a great place for a very large vertical integrated metropolitan zone. Imagine the view from an office sixty miles above the Equadoran Andies.
Maybe that's it. Let's build the city as needed, and when it's done we have the launch ramp as a natural by-product.

Van Rijn
2006-Oct-21, 07:05 AM
http://en.wikipedia.org/wiki/Carbon_nanotube



[Emphasis added.]

If this is correct, CNTs would not be a good material for building a structure like this.

Well, tensile structures (http://en.wikipedia.org/wiki/Tensile_structure) are common. At least a large fraction of a structure could be based on tensile materials. One of the simplest examples is an inflated shell, with the gas inside providing pressure to keep the outer wall under tension. Of course, tension is very important in many bridge designs. But, I suspect tall structures tens of miles high would use some amount of active stabilization. Also, if it is possible to make super materials, don't forget that diamond does great under compression.