Space Architecture

Wednesday, September 24, 2008

The Space Elevator, and Nanotechnology

Slashdot had an article from the Times Online UK about the Japanese Space Elevator Association, and their attempt to make the space elevator a reality. The quoted price for such an elevator is one trillion yen, which is about $9.4 billion. To put that in contemporary perspective, the Bush Administration has proposed a $700 billion plan to bail out various Wall Street firms. The price for the space elevator is premature, and most likely much lower than what the actual cost will be. I won't even touch the Wall Street issue.

The reason, that the price for the space elevator is premature, is because the material necessary for the cable does not yet exist. A great deal of this material will be needed, and is the critical component of the space elevator. Any price tag at this point is meaningless.

The most commonly looked-at material for the cable is carbon nanotubes. I wrote about nanotechnology in general for my Master's Thesis, and trying to summarize the current state requires more than a typical blog post.

Let us take one thing at a time, starting with the Slashdot commentary. Some of the best reader comments are more informative than the articles to which they are responding.

Poster "Rei" says, "The problem is that even the *simplest* form is way beyond what we can produce in the present day, and you're wanting to do a form that's far harder.

"In a space elevator, the tether has to be long. Very, very, very long. So much that even if you could build a cable with the density of graphite and a tensile strength of 100GPa, it'd still have to taper severalfold as it reaches toward the earth. With the taper requirement, pulleys are simply right out (can't have the pulley's cable change shape as it goes, now can you?), as is *anything* that can increase the weight of the fiber. You need elevator "climbers", powered by beamed power transmission.

"The problem remains the cable. 100GPa with the density of graphite is just so far beyond anything that we can achieve today it's really just a sci-fi concept that people like to dream about. The last I checked, the strongest *individual single-walled carbon nanotubes* that people had directly measured the strength of broke at just over 60GPa. This is for single tubes, let alone bundles of tubes, let alone a bulk fiber, let alone an entire tapered cable. Tubes theoretically can be stronger, but I haven't seen any measurements confirming such extreme theoretical strengths. The strongest SWNT bulk fiber I've read about was planar sheets that were about 10GPa.

"Yes, you can build a space elevator with a tensile strength of less than 100GPa. But your taper factor for the elevator rises *very fast* with decreasing tensile strength or increasing density, which means that its mass increases *very fast*, which rapidly puts it outside the realm of possibility. Honestly, something more like 120GPa would be much easier to build, but that's even further from what we can achieve today. I'm not even sure it's physically possible to achieve. SWNTs are pure graphene SP2 structures; how can you get stronger than that? The only thing I can think of that could help us best today's best strengths are complete perfection, every atom of the fiber all the way up, and I'm not sure that would do it". [Rei's website]

That is the problem in a nutshell. The cable must go out past Geostationary Orbit, which is about 35,800 km above the surface of the Earth. GPa is a measure of tensile strength. Tension is the critical issue here, because to keep the cable aloft, the cable needs to be pulled away from the surface of the Earth as much as Earth's gravity pulls it down.

This 2001 paper describes the tensile strength of carbon nanotubes (CNTs) as ranging from 0.14 - 0.177 TPa. If you know metric, T is for Tera, which is 1,000 times greater than G (Giga). However, those numbers are theoretical, as the research team in the paper notes, testing the CNTs is quite difficult, due to their very small size. A more recent 2003 paper shows that strength for both single-wall CNTs and multi-walled CNTs to range from 40 - 50 GPa, far below the requirements for a space elevator.

So, you have two issues: strength and size. I expect strength to vary as the length of the cable grows, and is exposed to conditions different than that of a laboratory. I also expect that when someone grows a CNT to some decent length (say, 1 meter), they will take it outside and test it under ambient-environment conditions. The longest CNT is currently about 2 cm.

What is the hold-up on length and strength? According to "Carbon Nanotubes -- the Route Toward Applications" (2002), "All currently known synthesis methods for SWNTs result in major concentrations of impurities. Carbon-coated metal catalyst contaminates the nanotubes of the HiPco route, and both carbon-coated metal catalyst and, typically, ~60% forms of carbon other than nanotubes are formed in the carbon-arc route (11). These impurities are typically removed by acid treatment, which introduces other impurities, can degrade nanotube length and
perfection, and adds to nanotube cost".

What is HiPco? High-Pressure carbon-monoxide. The HiPco route appears to be one of many methods of creating carbon nanotubes, using gas-phase growth procedures.

Essentially, the length and strength problems are problems of production. The above quote from the 2002 article is but one example of the many problems one runs into when trying to produce nanotubes. The machines seem to suffer an analogous problem to that of the ones used to measure the effects of quantum mechanics. Trying to detect the motions of electrons by throwing objects the same size or bigger than the electron itself.

With nanotube production, you are producing atomic-sized creations not necessarily by throwing atoms together, but by creating chemical environments optimal for producing the chemical reactions that lead to the tubes (or fibers, plates, dots, etc). These same environments produce other compounds that you may not want, so another process is required to purify your batch, with the side effect of weakening your nanotubes.

Despite this, as one reads the literature over the years, there has been a gradual improvement. Perhaps the Japanese Space Elevator Association is banking on continued improvement. As the Times Online article mentions, there will be an international conference in November to create a timeline.


Post a Comment

<< Home