This paper is a survey paper whereby we have considered the practicality and the state of art regarding ribbon design for Space Elevator system using Carbon Nanotube (CNT).
The pioneer of modern science Sir Albert Einstein once said, “If at first, the idea is not absurd, there is no hope for it” and for supporting this quotation what else could be the best example other than the idea of “Space Elevator”? In order to make space missions easier and cost-efficient “Space Elevator” is an idea of connecting earth surface with GEO (Geostationary Earth Orbit) through Carbon Nanotube (CNT) made ribbon which will be used to carry the load up to GEO altitude (Approximately 35,786 Km).(1)
Chemical Rocket vs. Space Elevator
The mammoth cost (Between the time period of 2005-2010 its approximately $ 1.5 billion per launch (2)) of conventional chemical rocket launching and the question of it’s reusability triggers the urge of Space Elevator. Even today’s reusable chemical rockets throw away almost 80% of the mass at the launch pad as consumed fuel. (2) Regarding the cost efficiency, because of the low energy consumption to move up and down the payloads it will be possible to send 1Kg of load in the orbit at less than $10 dollar! (3)
Obviously there are hundreds of questions to make this idea sounds absurd at the first hearing and the first question would be nothing but “Is there any material exists which is strong enough to hold massive load
of a space mission and lift it up to 35,786 Km altitude?” The scientists have discovered the required material for this mission. In this article we would like to discuss why this particular material is fit for this project and how we can implement it in Space Elevator Project.
Ribbon Material for Space Elevator
The total required length of ribbon depends on the available counterweight in the space elevator system. If no counterweight is used the total length of ribbon would be 150,000 Km. (1) However, 100,000 Km is considered as the optimum length for this system. (1) First two requirements for this ribbon material are – Strength and light-weight. The required strength for the ribbon material in Space Elevator is approximately 62.5 GPa (Giga Pascal). However, an allotrope of Carbon known as Carbon Nanotube or CNT has shown approximately 200 GPa of strength which is much stronger than the requires strength. If we use CNT-made ribbon the weight of total tether will be around 9.2 tons. And the diameter would potentially be 0.15 mm at the earth surface and 0.26 mm at the GEO. (3) One thing is to be mentioned here that in 1978, the legend of science fiction literature Arthur C. Clarke in his famous book “The fountain of paradise” not only proposed the idea of space elevator even he set up 2 stage plan to implement it 1. He invented super strong monofilament carbon fibers, 2. Built a cluster of cables, then he built a solid tower around the skeleton cables.
From the above discussion it becomes quite obvious that CNT is perhaps the strongest candidate for being used as ribbon material in Space Elevator project. Now in proceeding part of this paper we will see how this CNT is formed and the possibility & challenges of using CNT as ribbon material for Space Elevator project.
CNT for Space Elevator’s Ribbon
CNT is a member of Fullerenes family which is the 3rd all allotrope of Carbon. There are two types of Carbon Nanotubes – Single-Walled nanotubes (SWNT) and Multi-Walled nanotubes (MWNT). SWNT is made by wrapping a single-atom layered praphene into a seamless cylinder. On the other hand, MWNT is made by several grapheme sheets rolled around inside each other.(1) These nanotubes are made in microscopic level hence MWNT is a bit challenging to produce as it requires putting one rolled layer of CNT into another rolled CNT. Therefore, SWNT is the better option in Space Elevator project. We can have our ultimate ribbon by making carbon nanotube bundle of SWNTs. Most materials show their strongest forms when they are in fibrous shape. (4) The proposed ribbon will be composed of 3 mm2 cross sectional area of 10 micron diameter fibers; approximately 30,000 fibers at the anchor. (1) The interval between the tiny fibers will be 10 cm or more. (3) However, this is not economically and perhaps also technically feasible to make the entire ribbon with only CNT. In this case the best idea could be using CNT is composite. But for a number of reasons making CNT composites is not a simple task. However, In University of Kentucky, they produced 5 Km long CNT composite fiber using 1% CNT doping. This CNT doping increased the strength from 0.7 Gpa to 1.14 Gpa however, for achieving the ultimate purpose of required strength the ratio of doping has to be 80%. (1) A good example of CNT composite could be PAN/SWNT composite fiber. Even though the tensile fracture surface of Polyacrylonitrile (PAN) fiber shows extensive fibrillation however this fibrillation could be decreased significantly using PAN/SWNT composite. (5) A study showed, using 10 wt% (mass percentage) SWNT it’s possible to increase PAN fiber’s breaking strength up to 100%. (5) It is to be mentioned here that other alternative ribbon designs have been also proposed for space elevator project. (1) We have the opportunity of experimenting SWNT composite with wide range of other materials (like PAN). Still it’s a long way to go to find out the right combination of ribbon design and SWNT composite. It’s also important to consider the challenges like lightning, meteors, space debris etc are the important risk factors in carbon designing.
The barrier between producing CNT in laboratory and implementing this CNT to build ribbon is quite challenging. However, theory says it’s quite possible what a matter here is to invent the right technique. The rapid progress on nano-techonology is quite cheering and we can hope that it is just matter of time to overcome the Space Elevator’s first hurdle: Ribbon material with nothing but carbon Nanotubes!
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