Broken Tether

MicroMeteors

For objects large enough to be tracked, some sort of collision avoidance (see below) can be used. A more difficult problem is the micrometeors that are not tracked and so can not be avoided. An advantage of a rotating tethers over hanging tethers is that they can be much shorter for the same tip speed, and so have much less risk of being hit by a micrometeor. However, it is necessary to plan on any tether being hit from time to time. The basic options are to design a tether that is fault tolerant, or be ready to replace the tether if it fails.

Fault Tolerant Tether Design

There has been research on fault tollerant design. Some references are listed with the word failsafe. One has some good sketches. Robert Hoyt designed one which is now known as the "Hoytether". The basic idea is to have a webbing such that some strands can be broken without the whole tether failing. The expected life can get to 100 years.

If the tether can only withstand limited damage, you may also have something that goes up and down the tether inspecting and repairing damage. If the design can withstand lots of damage, repair may not be necessary.

Replacement

The final tether has a mass of 56 tons. It could be replaced with 14 payloads. The SSTT could launch every 100 minutes, but the tether can only handle one or two payloads per day. One of thing to bring up is the parts to make another tether. While it is sitting in reserve it adds to your ballast. A tether can always use more ballast.

When you are growing your initial space tether complex you will use a smaller tether to bring up a larger tether. The old small one is a backup to bring up more parts for the larger tether.

Collision Avoidance

To avoid known space objects, a collision avoidance process is needed. In a spinning tether the rate of spin can be changed. This can be done by using thrust, or by shifting weight during rotations. You can reel in and let out the tether, or move a mass along the tether.

The orbit of the tether can be changed. The tether is moving at about 7.5 km/s. If the tether adds 2 meters per second to it's velocity, in one hour it will be 7.2 km away from the predicted path. It will also be in an orbit that reaches 8 or 10 km higher, and the period will be 20 seconds or so longer. If this is done days in advance, the tether can provide a large margin to avoid a danger.

To avoid known objects, it is necessary to know where they are. Governments with secretive military satillites may not want to reveal where these satellites are now or where they will be. For these cases it will probably be necessary to publish the tether's expected position and accept requests from people like the US military to change the tether's orbit or spin from time to time.

Major breaks

If a piece of space debris larger than the tether hits the tether, it may overwhelm the fault tolerant design. Depending upon the rotation of the tether, a piece of tether could fall into the atmosphere or go into an orbit as far out as GEO.

If part of the tether breaks and falls into the atmosphere it will burn up, so it is not much of a danger. Most of the equator is water or sparsely populated land. The grapple assembly has some risk of surviving re-entry and landing someplace. The tether itself would burn up.

A bigger danger occurs if the tether breaks and flies out where it can hit other satellites. Robert Hoyt and Robert Forward have a design for a Terminator Tether that can de-orbit a satellite using passive means. A short conducting wire interacts with the Earth's magnetic field to convert momentum into electricity, creating a drag on the satellite. A Terminator Tether on the grapple end of the large tether can rapidly de-orbit that end if it breaks.

If the break leaves the fragment in an orbit that is always above 1000 km altitude, the Terminator will not be effective. To minimize this danger, an ion-drive and a winch can be at the end of the tether. The ion-drive could be used to either lower the orbit so the Terminator is effective or to avoid objects. The winch can roll in the tether. There would have to be some solar power as well.

We could send a space-tug after a broken tether to either collect it or make it reenter the Earth's atmosphere and burn up. These could catch it and deal with the problem even if the built in system failed. We could have any number of such backups.

This issue is very important because it could provide grounds for others to block the tether politically if it is not handled well.

We have not read information about fatigue properties of Spectra-2000 yet. A spinning tether has more stress when vertical than horizontal, so there is a bit of stretch and relax every rotation, which is a few times per hour. We suspect this is not a problem, but still need to check.