Tech Briefs

Secondary lines take up the slack when primary lines are severed.

Hoytethers™ are failure-resistant multiline tethers named after their inventor, Robert Hoyt. They were originally intended for use in outer space to provide reliable tethering of satellites in the presence of orbital-debris and micrometeorite impacts that can sever single-line tethers. Hoytethers™ may also be adaptable to terrestrial use as tensile structural members in situations in which high reliability is required, such as climbing ropes, suspension bridges, and towering cables.

Secondary Lines Redistribute the Load around a cut in a primary line. The angles of the secondary lines are exaggerated in these views for the sake of clarity; in reality, the secondary lines are more nearly parallel to the primary lines.

The basic failure-resistant multiline tether structure (see figure) 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 on a ring or bar that enforces the circumferential spacing (for a tubular tether) or lateral spacing (for a tape tether). The primary lines are connected diagonally at regular spatial periods by 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, a small flying object that could sever a single line in the tether is still unlikely to cause total failure of the tether, even if it strikes in the vicinity of a previous cut.

Numerical simulations and experiments have confirmed the expectation of the inherent ability of these tethers to redistribute loads around severed primary lines and to maintain spacing between primary lines without need for rigid braces. The simulations and experiments have shown that these multiline tethers can withstand multiple cuts while retaining structural integrity and sustaining only mild pinching, which is limited to the vicinities of the cuts. Thus, the lifetimes of these multiline tethers can be orders of magnitude longer than those of single-line tethers with comparable masses.

This work was done by Robert P. Hoyt and Robert L. Forward of Tethers Unlimited, Inc., for Marshall Space Flight Center. For further information, please contact the company via e-mail at This email address is being protected from spambots. You need JavaScript enabled to view it. or visit their web site at

Hoytether is a trademark of Tether Unlimited, Inc., for their interconnected multiline space tether design. MFS-31305

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