Special electrically conductive, failure-resistant rope/wire combinations are being developed for use as electrodynamic spacecraft tethers. These or similar combinations could also prove useful on Earth in applications in which there are requirements for strong, reliable, lightweight tethers that can conduct electric currents of moderate size: such applications might include tethering of aerostats and of wind-powered electric generators.
The electrically nonconductive, tensile-load-bearing, main structural portions of these tethers are of the type described in “Failure-Resistant Multiline Tethers” (MFS-31305), NASA Tech Briefs, Vol. 24, No. 4 (April 2000), page 50. To recapitulate: The basic failure-resistant multiline tether structure is an open net comprising primary and secondary lines that, together, provide redundant linkage. Each primary line runs the full length of the tether and is anchored at both ends. The primary lines are connected diagonally at regular spatial periods to the secondary lines, which are attached firmly by braiding, twisting, clamps, or other knotless interconnection methods, so as not to slip. The secondary lines remain slack as long as the tether is undamaged. When a primary line is severed, the secondary lines assume the tensile load and redistribute the load in such a way that the effects of damage are localized to the vicinity of the cut. Although the secondary lines are diagonal, they lie nearly parallel to the primary lines. Therefore, the structure necks down only slightly when secondary lines take up the load from a damaged primary line and, as a consequence, the structure remains an open net, even in the vicinity of a cut.
Thus far, failure-resistant multiline tethers have been made from various high-tensile-strength polymer fibers that, in general, are electrically nonconductive. The tensile-load-bearing main structural portions of the present developmental tethers are made of ZylonJ poly(p-phenylene-2,6-benzobisoxazole) [PBO]. To provide electrical conductance, a mesh of fine-gauge stranded wire is loosely interwoven with the primary and secondary PBO lines (see figure). The interweave prevents accidental separation of the wire mesh from the main PBO structure.