The core of the Space Elevator is the physical connection between Earth and space. In our baseline design this connection is in the form of a ribbon though earlier designs discussed a round cable.
The primary technical challenge to building the Space elevator has always been the availability of material with sufficient strength. The availability and strength of this material drives all aspects of the space elevator.
|Material||Carbon nanotube||SWNT or MWNT two centimeters in length. General status of material see TetherDataTable.jpg also see the general file archive for this Wiki, numerous papers on CNT developments are located there. CNTs have been measured with tensile strength as high as 200GPa (+-30%) but more consistantly around 110 to 150 GPa. Incorporating CNTs into a matrix or spun thread will reduce the strength by 20 to 90% depending on the technique.|
|Width||variable||10 cm wide in the lower atmosphere, widens to a meter at LEO and maintains this width for remainder of ribbon. To improve on resilience to orbital debris it has been suggested that the ribbon my increase in width to 2m between 500 and 1200km altitude and reduce the risk of damage. This has implications on the climber and other components.|
|Thickness||Ill-defined||Not a single mass, composed of axial fibers (see small scale design below). The thickness is determined by the required taper profile for the cross sectional area of the ribbon File:Cable worksheet.ods and the width variations mentioned above.|
|Length||100,000 kilometers||The length was selected based on engineering parameters such as relative mass of climber (counterweight) and cable and on the destinations of interest (Mars, asteroids).|
|Small scale design||Spun CNT thread||The number and diameter of the axial threads are yet to be determined and will be set by production limitations and ribbon robustness. Thread diameters of 10 to 50 microns are being considered.|
|Interconnects||Tape sandwich||Millimeter to several millimeter wide strips of gold-coated polymide tape with CNT reinforcing fibers roughly 10 microns thick. Spacing varies with the closest interconnects (10 cm) at and below LEO (500km to 1200km altitude). Interconnects must be able to hold axial threads at 1% to 3% of their tension if broken allowing slippage at higher tension.|
These images show the proposed basic design though not completely to scale. Many small axial, load-bearing fibers are held in a ribbon configuration by orthogonal epoxy, adhesive tape or woven interconnects. increasing the size of the ribbon is accomplished by splicing additional ribbons to the sides or face of the initial ribbon.
Discussions on the ribbon can be found File:Cable write up.odt and the basic spreadsheet for cable calculations for different planets, lift capabilities, lengths, etc. can be found in this File:Cable worksheet.ods
Insert Sketchup graphic and related Sketchup file http://sketchup.google.com/3dwarehouse/details?mid=bd063afd8c6594bb86852bf1a7913ebc&prevstart=0
Dynamics of the Ribbon
A number of simulations have been done for the ribbon and how it responds to various forces and events. The most complete set has been done by David Lang and most of it can be found on the GTOSS and Elevator simulation page. Work has also been done by Blaise Gassend but it is not yet published here (Blaise?).
Major Development Issues Related to this Component
- Stored Energy Release in Ribbon
- Splicing Ribbons During Construction
- Optimizing Ribbon Design
- Atomic Oxygen Effects on Ribbon
- Spooling of the Ribbon for Deployment
- Degradation of the Ribbon in the Space Environment
- Attaching the Ribbon to the Anchor
Additional Work Needed
The ribbon is the greatest technical challenge facing the space elevator. It must be extremely strong, fault-free and damage-tolerant, lightweight, and resistant to radiation, ultraviolet and atomic oxygen.
|Item||Notes||Completed and Ongoing Efforts|
|Production of carbon nantubes||To develop a process and begin producing CNTs is a major project that requires resources, time and determination. However, following the developments and piecing together what is being done has incredible value here. We need someone to pull together the best information on CNT production techniques, the current and expected cost of production, and the final characteristics of the CNTs produced. How they are produced, the cost and the various characteristics will have a bearing on the ribbon and climber designs directly and everything else indirectly. The additional enticement is that I (Edwards) believe that there are several techniques being developed that when combined will produce an incredible production system - fast, low cost production of high-quality CNTs. The different aspects have been demonstrated but not together.|
|Develop woven interconnects||As an alternative to tape sandwiches interconnects that are essentially CNT fibers woven around and through the axial fibers might perform better and be more resistant to the environment. The ribbon would look like a woven cloth where the interconnects are and bare axial fibers between. One issue is whether splicing can be done in the same way or if this can only be done with ribbon made on the ground. What we need is someone to find the right weaving technique to make this possible, the required characteristics of the fibers, determine roughly the performance expected and then produce a FEA of the woven ribbon. First stage can be thought experiment second stage needs to be quantitative.|
|Develop ribbon designs and optimize components||There are several ribbon designs that have been proposed and an objective quantitative analysis of the various designs need to be done. The mass to strength needs to be determined for each, the degradation properties, the viability for production, and an FEA model needs to be completed. At the next level the specific thread sizes need to be determined, the large and small scale design and finally tests of possible ribbons. Initial work has been done on several ribbon designs and those are good starting points.|
|Make and test various ribbon designs||This is a hardware task to begin gathering data to feed the other tasks. Initial ribbon need to be made and tested in terms of their performance and degradation. Initial ribbons can be done with a test fiber like nylon instead of CNTs. A wide ribbon many meters long with interconnects spaced by centimeters to meters needs to be constructed and stretched to near breaking then individual threads cut and the result recorded. Keep going on the middle and edges until the ribbon blows apart. For those who like to see things break this one is for you! The results would be extremely valuable for those studying ribbon design and building FEA models. I (Edwards) did initial tests like this on a small ribbon and it was impressive to see it work.|
|Test CNT threads to destruction||When we get CNT threads we can try to make them really come apart and test the energy storage issue... can't test this yet but...|