kevlar shock cord lengths

I've decided to use 1/16" (or 1/8"for larger models) kevlar for shock cords instead of elastic for my low power rockets.

My question is how long should the kevlar shock cord be?

We are talking about rockets in the 18" to two foot long range, flying on D through G motors (mainly 24mm, but maybe some lower impluse 29mm).

I've heard three times the body length, but this seems a bit short to allow air friction to slow the nose cone at ejection.


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Use light tubular nylon such as 1/4", 3/8", or 1/2" snowshoe lacing, rather than Kevlar - it's a little bit stretchy, which cushions peak loads and reduces the risk of zippering. (Kevlar is good for a "leader" down inside the airframe near the motor mount, where the line mainly exposed to ejection charge.)

6-12 ft. of 1/4" snowshoe lace (depending on what fits easily in the airframe) should be a good starting point for the size range of the rockets you mention.

-dave w

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David Weinshenker

More is Better. I routinely include a length of Kevlar yarn that is three times the length of the tube in my rocket kits, most of which are intended for A-C motors. You can get by with less if your chute or streamer starts catching wind early in the process of being shoved out of the tube. Since my kits have Nomex Heatshields threaded onto the shock cord, that adds a bit to the early-drag scenario. It also helps to be a bit sloppy when folding chutes and streamers...a rare example of my natural tendency to be a slob being a good thing!

My Super Six kit is 12 inches long and has a 32-34 inch shock cord. The Polaris is 24 inches long and has a 38 inch cord. Both are streamer recovery, and both get a lot of action at schools and summer camps. I haven't had any reports of shock cords tearing out. Unless you count the kid who forgot to put any glue on the motor mount tube in his Polaris. When he ignited it, the motor mount slid halfway up inside the tube, scorching the heck out of the tube and providing an interesting (I'm told) test of the Krushnik effect. It wobbled up to about 100 feet on a B4-4. At ejection the motor came out one end and everything else came out the other. Somehow the tube held together. The really amazing part of the story is that he flew it again! Doug Pratt dad-at-pratthobbies-dot-com

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