Model Forum / General / Rockets / January 2005

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

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