A lot of the soft shackles offered are made from Dyneema. Super strong, but not really good with high temperatures of ejection charges. So my plan for a couple of forthcoming rockets are to use a wide kevlar strap out of the fin can extending out of the body tube (wide reduces zipper likelihood), fastened with a Kevlar soft shackle to a smaller shock cord. Like a 1/4 strap out of my Argent, fastened to a 1/8" kevlar shock cord. For the larger DX3, there's space for an eyebolt, so I'll use a stainless shackle to allow the flat tubular kevlar strap a place to mount without bunching (why go to the effort of using super strong materials, and then derate them with a knot? ). Eventually I'll probably use all soft shackles instead of quick links. Stronger, lighter, and they don't rattle around and damage paint and stuff.
Dyneema was not usable IMHO, and small kevlar shackles were hard to find so as a test I made a couple of kevlar shackles. From Emma's kites MBR 500 (2.1mm) kevlar. Shown in the picture. The tool I used was a length of old guitar string (a "B", or somewhere in the 0.013" range) bent in half with a reasonably sharp bend. One can pierce the outside of the cord with the bent end, snake the tool up the center of the cord, emerge from the cord at a desired point, and then feed the other strand into the wire loop. The loop can then be used to pull the second cord through the center of the first. I will probably get a small (2 or 3mm) hollow paracord "fid" to facilitate pulling tapered end sections of kevlar cord into the center of other cords.
The open loop shown uses a diamond knot, the closed one uses a rather artless figure 8. I think both work. In working shackles, my plan is to use an oysterman's stopping knot, as it is larger than a figure 8, and because one can adjust the knot more easily to get exactly the same size for each loop in a tightened shackle. BTW, the little fluff on the open shackle is not a snag, it's just a poor job dressing the end of the line left over after the diamond knot was tied.
The guitar string tool worked well enough that I'm going to likely make a couple and figure out how to put handles on them that will allow me to pull more easily. Also, I'll probably get some 750lb test (like 2.6mm?) kevlar for future shackles. The rule of thumb is that the shackles are rated at between 1.5 and 2.1 times the MBR of the cord itself. So I figure that shackles like the ones below (after some practice to ensure better, more uniform results) should be good for 750-1000lbs. The larger line will not only give a lot more strength (more than needed, probably), but also is easier to work with.
I was able to cut the kevlar with a razor, but not easily and not without dulling the razor (X-Acto knife). So I ordered the Jonard Optical Cable and Kevlar shears that
@Tractionengines showed in the post above.
