Coop,I usually use both... Slip the shock cord through, then make a couple quick loops on either end. Then, I will anchor the streamer in place with whatever's handy--quick clips, zip ties, short lengths of Kevlar.
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The twisting around the shock cord is odd. I've only ran across it a couple of times and that was with the triangular-shaped streamer, as I have previously mentioned.
I have a few questions for you... First, are your problems with this all on the same rocket, or does it occur over multiple models? Second, does the streamer wrap itself around the shock cord from the bottom or the top? Third, where do you mount the swivel you mentioned using? Fourth, could you tell me how large the streamer is, and how you pack it?...
I'm sorry if I gave the impression that the streamer was twisting around the shock cord, it's not. It's twisting itself into a long, very thin rolled up piece of fabric. This has been with both commercial and homemade streamers. As you can imagine, once they have twisted up they no longer ofter either of their benefits - visibility and a slowed descent rate. I've tried several swivels to no avail. I attach the streamer in much the same place I would attach a drogue. When using a swivel I generally use a short length of Kevlar between the swivel and the streamer.
But I think one issue might be is that several of the streamers were much longer than the 10:1 or even 12:1 ratio that you mention. I'll have to double check though.
Tony