Phonon (virtual particle) theory is an atomic-scale quantum explanation of how kiteline "tug" transmits force over distance. Model a single bond in a kiteline molecule can quantify how much tug is phonon transfer and how much energy dissipates as convective and radiative energy for total thermodynamic transmission efficiency. At macroscales most kiteline behavior is better characterized by classical mechanics of waves in a physical medium (stress-waves) although atomic properties still influence critical behavior.  In fact, phonons and stress waves are merely different views of the same solid-state physics. Electron transmission, the basis of many AWE schemes, is discussed downpage.

A dancing kite feels tether-force as quasi acceleration, a dynamic virtual gravity much like a science fiction "tractor beam". Unlike gravity, tether-force is capricious, truly chaotic, with slack and jerk in odd directions. Since aero-towing began almost 100 years ago, glider pilots (and later hang-glider and paraglider pilots) have dreaded "lock-out", where control is mysteriously overwhelmed and the towed aircraft hooks and dives into the ground. Gradually pilots developed heuristics (guidelines) to avoid lock-out, without fully understanding the phenomenon. In collaboration with KiteShip in '07 in Alameda I got to meet Don Montague (a kitesurfing buddy of the Google Founders and CEO of Makani Power). At that time I raised lock-out as a fundamental kite failure mode. Both companies ordered copies of Towing Aloft, by Dennis Pagen, an empirical start at understanding and eliminating lock-out.  Its taken a couple of years of further study to formally identify multiple causes for lock-out emerging from the complex harmonic interaction of wind, kite, and tether. 

Wind is mostly uncontrollable partially predictable input
to which a kite is roughly reactively matched.     Experienced kiters match the weight of a tether to wind strength.  Altitude along the windspeed gradient can be somewhat selected. Tunable critical-damping of the kite's inputs across all conditions is a key to reliable flight.  Tether selection and setting is the easiest most flexible tuning means.

Modern polymer tethers are very reliable if well attended.  Large kite designer and showman Peter Lynn gets about 200 days of tether life without worry; and some sport fliers get away with years of almost unlimited use. I have been flying the same tethers for hundreds of hours to see how they age and only once in recent years has a tether parted unexpectedly, burnt out in electric hail.  As Mario Milanese has observed, flying multilines is pretty much guaranteed to prevent runaway.   [[Log tether use, incidents, inspections. A tether is as ready for a use as its weakest station. Smart tether monitoring?]]

Two broad classes of AWE tether are Electrically Conductive Tethers for FlyGens and Polymer Tethers for tractive applications like GroundGens.  Conductive Tethers suffer from higher mass and aerodrag than polymer tethers of equivalent power transmission, but may be favored on small scales and when mechanical transmission proves less practical. One overlooked problem with high altitude conductive tethers is enhanced corona discharge at lower atmospheric pressure. This limits transmission voltage or entails more insulation mass.

Electrical failure mode of a tether is important: If the circuit opens, then the load gets spiked and the turbine might overspeed.  If the circuit is shorted, then the flygen will brake suddenly, possibly burning out and breaking turbine blades. Conductive tethers are considerable hazards around power lines and lightning. In non-saline atmosphere, polymer tethers are not direct shock/short hazards when downed on power lines and are not a major lightning risk.

The strongest fibers allow the thinnest and lightest tethers. High performers have a lower ROI if too pricey. Primary factors are yield/breaking strengths, elasticity, aerodrag, and mass. Other considerations include UV/abrasion resistance and melting point. The best performing common kiteline is Ultra High Molecular Weight PolyEthylene (UHMWPE; Dyneema/Spectra) which rivals in strength early carbon nanotube samples. Polyester and Nylon are cost-to-performance competitive with UHMWPE when stretch and thicker cross-section is allowable. Nylon is favored when some stretch is desirable but has low UV resistance. Elastomer, usually synthetic or natural rubber, is used for shock absorbers and compliance mechanisms. For terrain enabled applications, where weight limits are relaxed, wire rope (galvanized steel) is hard to beat. Its possible biomaterials like silk, hemp, linen, and cotton will find a place in advanced kiting due to aesthetic or environmental grounds. There are many curious tether interactions, for example, cotton will saw, or rather melt, a stronger UHMWPE tether due to its higher heat resistance.

Like weight, tether aerodrag is fundamental limiting factor to kite performance. Line rake is a great drag reduction mode, the more rake angle the more simple round line wins by unbeatable strength-to-drag. Crosswind tethers are high drag. No real-world tether is truly worst-case crosswind as some catenary angle is always raked in along most of its length. Angled upward downwind they generate downforce and must be longer for a given altitude. Angling a conductive tether upwind against a downwind angled polymer tether may help flygen applications.

Faired tethers is a well known and obvious idea. Good data has existed for nearly fifty years since MIT first did experiments. Faired tethers do have considerably less drag but the balsa TEs MIT tested don't survive normal usage and suitable material hardly exists to this day. Line twist and strumming drag is uncontrollable without weight and complexity penalties. Faired line entails handling and wear issues with pulleys, fairleads, and reels. Round line of the same strength has less drag than ribbon sectioned line due to lessened strum. Twisted fibers with a fine enough texture have an aero advantage, by shedding strum canceling vortices and acting like golf ball texture, postponing detached flow.

A graded tether made up of stronger lower sections and thinner upper sections, with swivels in between, out-performs a long monotether. To assemble a tether in sections with hardware like swivels and shackles is classic art from the Victorian Era. High wear sections are sleeved or overspecified without much added weight. A multi tether is runaway resistant, but many lines adds snag risk and other operational challenges. A kitefield is best purged of all snags.

Dipping booms (think fishing-pole) and elastic sections ("snubbers") absorb peak loads to maximize tether reliability and performance. One use of a snubber is at a pilot-lifter bridle to insulate it from yanking by a power element downline, like a looping foil.    [[line snubber, mechanical snubber, electrical snubber]]

KiteLab has observed Sudden Tether Sag (STS) experimentally, most dramatically in heavier lines such as electrical conductors. An inclined tether that first slacks at the kite creates a transverse wave that moves quickly downward (faster and at higher amplitude the more massy the tether). As such a wave races groundward its hard to retract fast enough to prevent touchdown. An engineer at HAWP09 questioned the massy faster sag observation by citing Galileo, to wit, a heavy tether must sag as fast as a light one. A less elastic and less draggy (by increased strength growing faster than cross-section) tether sags a bit faster, but this is not the main sudden sag effect. Dave Lang, the AWE's tether guru, has seen similar sag effects in his simulations. STS is a major challenge in large high-altitude flygen schemes, as McNaughten & Co. suggest.

Powerful kite tethers are quite hazardous. They have lassoed and pulled aloft kiters and dropped them dead. Thin tethers easily cut flesh and dejoint like a machete, while a thicker line might only burn. Joe Hadzicki taught me that when all hell breaks loose and killer kiteline is whistling about, you should crouch slightly, chin tucked, clasping your ears with elbows tucked. Thus poised jump-rope nimbly over any ground sweeping lines. The idea is to protect your throat, jugular, and all other major arteries and veins, not to mention your balls, and your ears too, since getting them sewn back on is a hassle. Better too for a line to saw at bone than cut right thru a joint. Wear gloves with gauntlets to handle fine line; carry a hook knife; dress to minimize snags; write your will; and don't freak.