Posted: Aug 08, 2008  
To the cosmos by nanotechnology 

(Nanowerk Spotlight) Our title today refers to the 1960 article by Yuri Artsutanov in Pravda: "To the Cosmos by Electric Train" (pdf download, 132 KB). This article is the granddaddy of all 'space elevator' concepts and first to propose the idea that a cablebased transport system could become an alternative to rockets for launching people and payload into space.  
Artsutanov wrote: "Take a little piece of string and attach to it a stone. Begin to rotate this primitive sling. Under the influence of centrifugal force the stone will try to pull itself away and tightly stretch the rope. Well, what will happen if one fastens such a 'rope' to the Earth's equator and, having flung it far into the cosmos, one hangs on it an appropriate load? Calculations show ... that if the 'rope' is sufficiently long, then centrifugal force will also pull it out, not letting it fall to Earth, just like the stone stretches out our string. Indeed, the Earth's force of attraction lessens in proportion to the square of the distance, and centrifugal force grows with the increase in distance. And already at a distance of about 42,000 kilometers centrifugal force turns out to be equal to the force of gravity."  
The single most difficult task in building the Space Elevator is achieving the required tether strengthtoweight ratio – in other words, developing a material that is both strong enough and light enough to support the up to 100,000 km long tether. Thanks to nanotechnology, this material has become available in the form of carbon nanotubes (CNTs). The challenge ahead is to weave these raw CNTs into a useful form – a space worthy climbable ribbon. Assembling carbon nanotubes into commercially usable fibers is still one of the many challenges that nanotechnology researchers are faced with when trying to exploit the amazing properties of many nanomaterials.  
Before any physical construction of a cable can begin, and notwithstanding the huge technical hurdles of building a space elevator cable, researchers must develop models and perform simulations to identify the optimal size, shape and defect concentration for such a cable.  
Scientists at the Politecnico di Torino in Italy have now developed a new multiscale numerical approach for simulating the mechanics of macroscopic cables composed of carbon nanotubes.  
"We carried out thousands of multiscale stochastic simulations in order to perform the first insilico tensile tests of CNTbased macroscopic cables with varying length" Dr. Nicola Pugno tells Nanowerk. "The longest treated cable is the spaceelevator megacable but more realistic shorter cables are also considered in our bottomup investigation."  
Pugno, an Associate Professor of Structural Mechanics, together with colleagues Dr. Alberto Carpinteri and Dr. Federico Bosia, have demonstrated for the first time that a computer model can simulate a kilometerslong CNT cable. They reported their findings in a recent paper in Small ("Multiscale Stochastic Simulations for Tensile Testing of NanotubeBased Macroscopic Cables").  
In their work, the Italian researchers simulate different sizes, shapes, and concentrations of defects, resulting in cable macro strengths not larger than ∼10 GPa, which is much smaller than the theoretical nanotube strength of ∼100 GPa.  
Schematic image of the adopted multiscale simulation procedure to determine the space elevator cable strength. Overall, the cable comprises a total number of nanotubes given by Ntot=(Nx Ny )k and, in order to obtain the correct number in the spaceelevator cable, which can be estimated as Ntot= 1023, k=5, Nx= 40 and Ny= 1000 was chosen. (Reprinted with permission from Wiley)  
Pugno explains that there are no bestfit parameters present in their multiscale simulations: the input at level 1 is directly estimated from nanotensile tests of CNTs, whereas its output is considered as the input for the level 2, and so on up to level 5, corresponding to the megacable. "Thus, five hierarchical levels are used to span lengths from that of a single nanotube of about 100 nanometers to that of the spaceelevator megacable of about 100,000 kilometers."  
To numerically evaluate the strength of the spaceelevator cable, the authors adopted the SE3 code, formerly proposed by Pugno in a 2006 paper ("On the strength of the carbon nanotubebased space elevator cable: from nanomechanics to megamechanics"). With this model, stress?strain curves, Young?s modulus, number and location of fractured fibers, kinetic energy emitted, fracture energy absorbed, and so on, can be computed, in addition to the cable failure stress.  
"In this paper, preliminary simulations were carried out on a small piece of the spaceelevator cable – basically our level 1 results in the current paper – postponing a detailed and hierarchical investigation as the main topic of a subsequent, that is, the present, paper" says Pugno.  
Multiscale simulations are necessary in order to tackle the size scales involved, spanning over 15 orders of magnitude from nanotube length to spaceelevator cable length, and also to provide useful information about cable scaling properties with length.  
With regard to the concept of the proposed spaceelevator cable, the Politecnico team's results are sobering news. "Our simulations imply that the megacable strength is predicted to be much smaller than the theoretical nanotube strength (∼100 GPa), erroneously assumed previously in the spaceelevator design," says Pugno.  
"Accordingly" he continues, "the multiscale simulations suggest a taper ratio (the ratio between the maximum crosssectional area, at the geosynchronous orbit, and the minimum, at the Earth?s surface) larger than 613, in spite of the current naïve proposal of 1.9."  
The results attained by Pugno and his colleagues strongly confirm the previous theoretical predictions on the dramatic role expected to be played by even small defects in the megacable.  
"Our predictions are conservative, since we have assumed perfect junctions between the nanotubes" Pugno points out. "The junctions are expected to be the weakest links, even if advanced nanotechnology – which doesn't exist yet – could lead to nearly perfect interconnects, i.e., junctions with a strength that is larger than that of the nanotubes themselves."  
Rather than reaching for the stars with a space elevator cable, the Politecnico team is now focusing on the scaling of nominal properties such as strength, dissipated energy density, etc., which is is a major issue in developing realistic models for the behavior and attributes of nanofibers and nanotubes.  
By
Michael
Berger
–
Michael is author of three books by the Royal Society of Chemistry: NanoSociety: Pushing the Boundaries of Technology, Nanotechnology: The Future is Tiny, and Nanoengineering: The Skills and Tools Making Technology Invisible Copyright © Nanowerk 
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