I expect the tether to recoil violently if broken. True, the amplitude of the compression wave created by snapping a Spectra cable with a given tension is less than that of snapping a bungee cord with the same tension. However, if the potential energy due to tension is the same in each case, then the energy released by snapping the tether is the same either way, too. If the energy can't go into a compression wave, then it will go into a transverse wave instead -- like a whip, rather than a slingshot. Now, that energy may be insufficient to carry that wave all the way up the tether despite damping, but it still goes somewhere. If the tether breaks, the resultant wave is headed straight for the platform. It doesn't really have anyplace else to go.

The same thing is observed in steel chains and cables. They don't elongate much either, but you don't want to be standing next to one when it goes. Of course, they also have more inertia than Spectra and are therefore less subject to damping forces. So the recoil of the VBP tether should be less than with a heavier rope breaking under the same load.

Also, I'm coming to the conclusion that tapering the tether is a bad idea. There will be brief periods, especially during the initial ascent, when we can expect the line tension at the bottom to exceed the line tension at the top of the tether. As the platform approaches the tropopause, wind drag will cause more tension than weight will. It doesn't dramatically affect the wind drag to increase the tether diameter by a few millimeters, as long as we can lift the extra weight. Also, we will find it cheaper to have the tether manufactured with a uniform thickness.

My current estimate for best tether thickness is 20mm, not 16mm. This is able to resist 45 tons of force, which is my computed worst case scenario. (To reach this kind of drag, the VBP has to start out in 7m/s ground winds, which increase exponentially all the way up to 20km, with no fall-off in wind speed past the tropopuase or anywhere else. This is the equivalent of deploying in a 16 mph wind with a 20T platform and encountering artificially exaggerated winds all the way up. Physically impossible, but not too many times worse than we may ask of it from time to time.) The 20mm tether has more than twice the minimum strength of the 16mm, enough to increase our safety margin to respectable levels. Of course, at twice the strength it also has twice the weight: 19 tons.

It's a shame the Technora-blend stuff wears better. I was looking at the New England Ropes page and noticed that the straight, single braid Spectra has a higher modulus and lower weight than the T-900 double braided rope we'll be using. With that stuff, we could get by with the 18mm rope, which would only weigh 11 tons. Of course, the folks at New England Ropes say it's unsuitable for winches because it abrades too easily. I do have an estimate for the brittleness temperature of high density polyethylene: -100C. I think Spectra rope should remain intact at higher temperatures, being chemically the same material, but don't quote me on that. I'm still looking.

I don't think we're going to have much luck shaving weight off of the rope, so we should start toying with ideas to take it off the platform. Reducing the platform weight could also lower the drag, allowing us to thin out the rope again, so it's important to consider.

I'm looking at supporting the platform and tether mount in much the same way as the gondola is supported beneath a conventional hot air balloon. Ropes do all the work.