So we’ve got a length of Kevlar shock cord and a fixed anchor point on our rocket. Unless our shock cord has a sewn loop, I guess we’ll need to tie a knot. Certainly we know how to tie the simplest of all knots, the overhand knot, sometimes called a granny knot. During ejection of a chute and when that chute opens, this knot is stressed in tension, exerting hundreds of pounds of force or more, and after the chute opens, the knot will be stressed in torsion. In all likelihood, a simple overhand knot will fail, probably resulting in the anchor, and whatever is attached to it, going one way and the shock cord going the other. Adding a couple of half hitches is an improvement but still not very reliable.
Most rocketeers who’ve been at it for a while have a “go to” knot they use to attach shock cords to anchors. Three knots seem to excel for this application: fisherman’s, bowline, and figure eight. All three of these work well with rigid anchors like screw eyes or U-bolts. All can be untied even after being subjected to a load. And all three are fairly simple.
As its name suggests, the fisherman’s knot is normally used to attach a hook to the end of a line. It is good in tension and torsion.
Notice that the fisherman’s knot tightens its loop as the line is loaded.
The bowline knot is used to make a loop on the end of a line. It is used in sailing and is recommended by the FAA for lashing down light aircraft. There are a number of variants but the standard knot forms a loop that will not close when loaded.
My personal favorite is the figure eight. Most technical climbers trust their lives to figure eight knots. We should be able to trust our rockets to them. The figure 8 also produces a non-closing loop, even when the line is loaded.
Although knots may be small, they are not trivial. Maximizing the reliability of your knots will make your rockets a little bit more reliable, more likely to be recovered intact and ready to fly again.