Can Carbon Nanotubes Make Space Elevators a Reality?

[russ_watters]

Russ I seriously expect that you are pulling some numbers out of your A$$.

Does making up numbers make this a meaningful post? Perhaps you should stick to generalities.

Yes, I'm making them up and yes, they most certainly make for a meaningful post if they are educated guesses and illustrate the magnitude of the problem.

Ivan used the example of a translantic cable for comparison, but translantic cables are only something like 2,000 miles long, while geostationary orbit is 22,000 - and a translantic cable doesn't hold up its own weight and uses cheap, mass produced materials. It is important for him to understand that his example is wrong by many, many, many orders of magnitude. It doesn't actually matter he's off by 5 orders of magnitude or 10 (or 50), the point is the same: the position that a space elevator is anything but Star Trek level science fiction is absurd.

 

[russ_watters]

The only number that really requires estimation is the power required to produce the nanotubes.

Just for argument sake, let's say that a plant could make 1 nanotube the required length to get something into stationary orbit, let's say 1000 km, in 6 months. Let's also say that the power required is no more than a common household usage, let's say 20 kWh/day. Let's also assume that a bundle required for the space elevator requires 1 million individual tubes.

According to my calcs, that would equate to an energy (and only energy) usage of 3.6x10^9 \frac{kWh}{bundle} which equates to $360 M/bundle. In terms of the space program, that cost is not too exorbitant. However, does anyone have a handle on the cost for materials and equipment required to produce this stuff? I have no idea there.

However, looking at the time to produce, if there were 1000 factories churning out 1 tube 1000 km in length every 6 months, that still equates to 492 years to produce one bundle.

Like all engineering endeavors, we're already behind schedule.

Well, you are also making big assumptions about the capacity of a factory and the power usage. These look pretty energy intensive to me:

Carbon nanotubes can be manufactured using a variety of methods:

· Laser ablation uses a high-power laser to vaporise a graphite source loaded with a metal catalyst. The carbon in the graphite reforms as predominantly single-wall nanotubes on the metal catalyst particles.

· Arc discharge involves an electrical discharge from a carbon-based electrode in a suitable atmosphere to produce both single and multi-wall tubes of high quality but in low quantities.

· Chemical vapour deposition (CVD) is where a hydrocarbon feedstock is reacted with a suitable metal-based catalyst in a hot furnace to ‘grow’ nanotubes which are subsequently removed from the substrate and catalyst by a simple acid wash.

http://www.azonano.com/details.asp?ArticleID=1108

I'm also not sure how it is even possible to speculate how we could go from the current production levels to 40,000 km (not 1,000 km) every 6 months. Some more research, though:

NASA scientists have developed an SWCNT manufacturing process that does not use a metal catalyst, resulting in simpler, safer, and much less expensive production. Researchers used a helium arc welding process to vaporize an amorphous carbon rod and then form nanotubes by depositing the vapor onto a watercooled carbon cathode. Analysis showed that this process yields bundles, or “ropes,” of single-walled nanotubes at a rate of 2 grams per hour using a single setup.

NASA’s process offers several advantages over metal catalyst production methods. For example, traditional catalytic arc discharge methods produce an “as prepared” sample with a 30% to 50% SWCNT yield at a cost of approximately $100 per gram. NASA’s method increased the SWCNT yield to an average of 70% while significantly reducing the per-gram production cost. Additional research is needed to determine the physical characteristics of NASA’s SWCNTs.

http://www.fuentek.com/technologies/carbon-nantubes.htm#techdet
It doesn't say what "significantly" means, but for a starting point, we can try $10 per gram. (that's actually more production for less than I would have expected).


http://www.isr.us/Downloads/niac_pdf/chapter2.html
Here's a space elevator site that says carbon nanotubes are around 100x stronger than steel and 1/4 the density. I really should let the people proposing it can be done do the math to prove it (unless that site calculated the mass and I missed it), but maybe I'll come back later and do a cost estimate with what we now have...

 

[FredGarvin]

What I was trying to show was that, even with extremely "advantageous" number estimates for the production costs (which I know are way too low) that the production still has no hope of getting up to speed any time this century. Even if the process uses the power that a common household uses, the energy requirement is pretty darned large.

 

[Benzsk8]

PBS had a good special on it in layman's terms. Just google PBS + space elevator and you should find it. One of the big problems noted is the carbon nano tube structure which is the heart of the elevator.