Damage Protection

The protection of a space elevator ribbon against damage caused by micrometeorites and man-made space debris was described earlier (see Out of Order in Chapter 6), but also other types of tethers will be susceptible to such impacts.

As shown by the Small Expendable Deployer System (SEDS-2) experience, the 19.7-km-long (12.2-mile-long) tether that was severed after only 3.7 days in orbit, the chance of a tether being cut by space debris or a micrometeorite is a major mission risk. Joseph Carroll and John Oldson of Tether Applications have used micrometeorite and debris impact data from the Long Duration Exposure Facility (LDEF) satellite to model the risk of such impacts in low Earth orbit. Obviously, the thinner and longer the tether, the less time it will survive. The time it will take for a tether to be cut is a function of the tether diameter in millimeters plus 0.3, to the third power. The result is in kilometer-years. For example, a tether with a diameter of1 millimeter (0.04 inch) will be severed within (1 + 0.3)3 = 2.2 kilometer-years. That means that if the 1-millimeter-diameter tether has a length of 2.2 km (1.4 miles), it will likely survive only 1 year in space (and if it would be 1 km long, it would probably survive about 2.2 years).

To limit the risk of debris and meteoroids severing a tether (increasing its kilometer-year survival index), its diameter can be increased. However, this quickly leads to thick and heavy cables. A better idea is to construct a tether out of multiple strands. A simple example of this is a caduceus tether as used in the YES mission, which consists of two or more intertwined strands. Such a tether can be designed so that if one line gets cut, the remaining lines are still strong enough to handle the loads.

A more complex but also more robust option is the so-called Hoytether, developed in 1991 by Robert Hoyt of Tethers Unlimited. This tube-shaped tether is composed of multiple primary strands that are interlinked by diagonal secondary lines, making the tether look like a fishing net. Support rings ensure a cylindrical spacing between the primary lines. In the normal, undamaged situation the secondary lines are slightly slack and do not carry any loads. However, when a primary line is cut, these secondary lines divert the tension around the damaged area. A cut in one or more strands is thus effectively bridged by the interconnections. Unlike for simpler multistrand tethers, where a cut line is completely lost, in a Hoytether a severed line still helps to carry the load in most of the tether; only at the location of the cut do the other primary strands have to compensate for the damage with the help of the local secondary lines (Fig. 7.1).

Hoytethers could be made by hand with the primary and secondary lines connected with knots. However, knotted connections severely limit the strength of the strands, so it would be better to use some other kind of interconnection technique. Moreover, as the tethers would need to be tens of kilometers long, it would not be economical to make them by hand. Fast and inexpensive, knotless mechanical methods are thus required for their practical fabrication.

If a piece of rubber band is stretched and then cut, the pieces snap back violently. This may also happen when a thread in a multiple-strand tether breaks, causing additional damage to the rest of the tether. Freeman Dyson pointed out that the elastic energy stored in a stretched carbon nanotube is so high that a snapping strand could cut neighboring tether strands as well, annihilating the entire ribbon through a cascade of snapping strands. Dyson stated, "If it tears in one place, it is likely to be a disaster" However, it turns out that carbon nanotubes are very good thermal conductors, so the elastic energy in a snapping strand may quickly dissipate in the form of heat, leaving insufficient mechanical "snap" energy to do serious further damage. Moreover, the mass of a piece of snapping strand of carbon nanotube is rather small, so even if all the elastic energy would be converted into kinetic energy and thus speed, it would probably do little damage; the strand would hit the rest of the space elevator ribbon at high speed but with little power due to its low mass. All these assumptions and considerations need to be verified in a laboratory, so that a space elevator ribbon may be constructed in such a way that breaking strands would not damage other lines (for

example, by reducing the stress on the ribbon or building in sufficient separation spaces between the strands).

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