Space Elevator, Can we build one? Options

[imran]

Not so good news in the space elevator world.

The space elevator: going down?

[Marz]

Not so good news in the space elevator world.

The space elevator: going down?

I can't access the article (without paying). Can someone provide an "executive summary" what the issues are?

[Guest_Richard Trigaux_*]

The problem described is that, while microscopic individual nanotubes have the required theoretical strength to build the space elevator, large clusters of nanotubes don't have, due to the accumulation of small defects.

Why is it so? Two possible reasons:
-the fabrication process is not efficient, but we could improve it and obtain perfect large scale nanotube wires with the same properties than the small scale individual tubes
-there is a thermodynamical reason for this, in this case it is impossible to build large nanotubes structures.

A similar problem happenend with monocrystalline metals used into turboreactors, which have to sustain heavy centrifugal forces at high temperatures. Finding that usual metals break in the joints between crystals, the idea was to build monocrystalline blades. But thermodynamical reasons make that defects appear into metal crystals, so that ususally only microscopic crystals can exist. As far as I know mettalirgists found a fix, but these monocrystalline blades are very difficult to make, and thus very expensive and are used only in this case.

[ljk4-1]

For those who can recall and/or have the novel handy, what was
the Space Elevator in Arthur C. Clarke's The Fountains of Paradise
made of? Was it something at least based in physical possibility?

Now there's another work that should be made available online.

[edstrick]

"...monocrystalline blades are very difficult to make, and thus very expensive and are used only in this case...."

High performance jet engines use superalloy <nickle based, usually> blades that have increasingly complicated internal passages for cooling and reducing blade mass. The state-of-the-art has been directionally solidified ORIENTED single-crystal blades. The molding and casting technology is more than *not trivial*, and the performance requirements keep getting more severe, as is the inspection requirements. The single-crystal technology is widely used and mature, however.

The whole report on the impossibility of nanotube cable being strong enough is probably pure b##s**t, based on off-the-cuff assumptions. In reality, while nanotubes have to be strong enough, what's critical to the design is that the strong-enough tubes have to be long enough that inter-tube friction, gained by relatively weak bonding to the tube and mechanical interweaving of tubes, adds up over millimeters to meters or whatever to enough tensile strength to make cable strong.

[Guest_Richard Trigaux_*]

probably true.

At least this report is not a STUDY of what could be theoretical/thermodynamical limits into nanotube length. It seems that they just picked samples of state-of-the-art "ropes" and tested them. And they found they had much less strength than individual nanotubes. But this is a known result: we don't know yet to make nanotubes larger than say 1mm. But this don't mean that we cannot do this, and more and more people are working on this. So, with no foreseable theoretical limit, we can hope we will be able to build lengthy nanotubes, which will be very useful for the space elevator, or at least for many earthy purposes, such as much lighter cars or aircrafts, large building beams, super strong fabrics, etc.

And this is not the end. While boron nitride showed harder that carbon diamond, perhaps boron nitride nanotubes, or other compositions, will be even stronger than carbon nanotubes. At least it is worth trying.

Anyway today nanotube fabrication is something very rough: to evaporate carbon and hope it will solidify into nanotubes. Some do, but of course there is a lot of rubbish, and no quality product. It is a bit like throwing bricks at randon in hope they do a wall by themselves. Maybe there will be some more clever process, some catalyser able to build continuous and perfect nanotubes of whatever length we want. Or a nanomachine similar to the biological ones working in cells, but operating at a higher energy required to work with carbon bonds. It would rotate while adding carbon atoms, while being fed in energy by chemicals. As carbon atoms are usually not free alone, the nanomachine would start from things like methane molecules, which can be dissolved into a liquid medium.

The only real limit on nanotubes would be it they show toxic like asbestos. Probably they will be, for the same reason (mechanically breaking cell membranes and chromosomes). In this case, of course, their use could be strictly forbidden or restricted only to very special areas. Like the space elevator...

[alan]

There is not a specific minimum tensile strength needed to build a space elevator. The space elevator is tapered so it gets thicker as it goes higher. I believe the minimum of 63 GPa is what would be required for an untapered space elevator without a margin of safety. The formula for the taper required (area at geosynchronous orbit/area at sea level)is in this wikipedia article
http://en.wikipedia.org/wiki/Space_elevator
A design with taper of 1.5 for 130 GPa (includes a safety margin of 2) was described here
http://www.isr.us/Downloads/niac_pdf/chapter2.html
If the strength was 30 GPa (same safety margin) then it would require a taper of 1.5 * e^4.33 or ~113, if it had a round cross section it if would have 10x the diameter at geosyncronous orbit.
Theoretically it could even be built using kevlar but the taper would be 10^8, obviously not practical.

[Cugel]

For those who can recall and/or have the novel handy, what was
the Space Elevator in Arthur C. Clarke's The Fountains of Paradise
made of? Was it something at least based in physical possibility?

"...What is it?"

"The result of two hundred years of solid-state physics. For whatever good that does, it is a continuous pseudo-one dimensional diamond crystal - though it's not actually pure carbon. There are several trace elements in carefully controlled amounts. It can be mass-produced only in the orbiting factories, where there's no gravity to interfere with the growth process."

"Fascinating ... I can appreciate that this may have all sorts of technical applications. It would make a splendid cheese cutter."

Quoted from the book, without permission of Arthur himself.