Very strong bungee/hi-start rubber
This is great stuff, but not for the faint of heart. I probably wouldn't use it with a 3 meter built-up balsa glider, as it's awfully strong.

It's most commonly cut into shorter sections (about 15-20 ft.) without adding a braided line, and used to bungee launch EPP slope foamies at 100 mph. Be sure to use welded metal rings at each end, and several stakes or a screw-in dog run anchor. You don't want this flying back at you with a metal ring at the end of it.
I give this product 5 stars. Review by Thompson, Jeff