Downline

Downline, often abbreviated to 'line', is the common trade name for cables made of morphic polyxylene. Although it has uses in cleaning, electronics and more recently in hydroponics, for most people line is a term synonymous with orbital engineering.

The advanced complexing structure in a length of line results in a cable whose physical properties are effected by the presence of strong electrical fields. Typically, a field perpendicular to the line causes it to become rigid. A field along the direction of the cable causes the normally strong fibres to stretch and snap like strings of unspun cotton. However, both of these changes are found to be negated by a current flowing along the wire, which increases both its flexibility and its (already impressive) tensile strength.

If one end of a length of current-carrying line is immersed in certain ionic solutions, anions may be carried along the length of the cable and form protective complexes at any weakpoints where the cable has been damaged. This self-repairing property makes downline almost uniquely suited for tough conditions such as use in ground-to-orbit elevators and scattered world moorings.

Line Elevators

In 1984, noted inventor Jonathan Greebe found an unforseen use for downline fibres. Due to the cable's incredible tensile strength when carrying current, it had already become common practise to use loops of line for applications where more resilience was needed, allowing a generator to be easily connected across the ends. Greebe's breakthrough (which, although it may seem obvious now, was quite a stroke of lateral thinking) was to build modified cable-cars which could be run from the current on the line itself.

These cars, originally known by the trademark name Greebe Elevators but now widely referred to by the generic term Line Elevators, revolutionised ground-to-orbit transport. A satellite could retain the centre of a line, and drop both ends to the ground. The ends would then be attached to a stable power grid, allowing elevators to climb and descend the cable and simultaneously strengthening the line.

In more modern times, the rock-end system developed by Greebe is virtually unseen. In light elevator systems, especially those on class 2 planets, it is more common to have the middle of the cable anchored to the ground by sinking it into ceramic or rock, while the current between the ends is driven from a satellite's solar wings (a possibility which is often referred to as a sky-end cable). Meanwhile, more advanced worlds distribute the electrical load on the system by running elevators on a 2-up/3-down system. This generally involves a single-length line, one sky-end pair and one rock-end pair. This system strengthens all cables as necessary with the least power wastage, and provides redundancy in case of failure in any one line. Power from the satellite drives ascending cars, with any excess power (as well as the energy generated by the weight of a descending elevator) being channeled into the planet's electrical grid.