I still am startled that it is possible to discuss a space tether as a possibility. Thirty years ago, all calculations merely underlined the sheer impossibility. Now we get to worry about the need to make improved electrical motors. We are now discussing problems that we believe can be resolved sooner or later.
This is putting numbers on paper that allows one to go deeper into design issues.
We need to produce a fabricating device that produces a fiber made from nested carbon nanotubes. We have them in the lab. Continuous production is quite a trick but one that at least sounds solvable. A bundle of such fibers would make an excellent tether material that we all believe in. Flaw testing should be as simple as sending current down the tubes.
While we are at it, that current could handily drive a simple mechanism that causes the lifting craft to spin around the tether. If the craft then had short wings, it would be able to auger in and out of the lower atmosphere, which is the most difficult part of the trip. It would be a lousy ride for a human being but a definite way to accommodate large tonnage lifting needed for near space manufacturing while lowering the load on the tether itself.
Current Space Elevators Tether Expectations and Engine Power Density
http://4.bp.blogspot.com/_VyTCyizqrHs/SgIcRYk-viI/AAAAAAAADo4/7rMgr27CK00/s1600-h/spaceelevatorfeasible.JPG
Spaceward has a page on space elevator feasibility that looks at tether strength and power to weight ratio for the power system for the space elevator climber.
Based on a gradual convergence of experimental and theoretical results, the specific strength of raw CNTs will not exceed 50 MYuri [1 Mega-Yuri = 1 N/Tex = 1 GPa-cc/g]. A failure mechanism known as the Stone-Wales causes spontaneous defects in the Nanotube structure and limits the possible strength. Using 45-50 MYuri CNTs, we can expect a near-flawless spun tether to perform at 40 MYuri, and with a 33% safety margin, we can load the tether at a TSL of 30 MYuri.
Reaching a power mass density of 1.5 – 2.5 kWatt/kg is difficult. The best electric motors today achieve just under 1.5 kWatt/kg, leaving no margin for the PV panels. In order for the complete power system to reach 1.5 kWatt/kg, electric motor weight needs to be reduced by a factor of at least 2
Assuming extropolated technology space elevators are still feasible but space elevators are pushing what might be possible on several levels.
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