10-19-2010, 09:03 PM
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Master Modeler
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Join Date: Jan 2009
Location: Fairbanks, Alaska
Posts: 6,507
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Quote:
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Originally Posted by cas2047
-SNIP- I find that longer shock cords help to prevent damage to the rocket and cone (if it's balsa) from the nose cone whipping back and hitting the rocket. It also seems to prevent zippering. Although I can't back that up with anything more than my own personal experiences.  -SNIP-
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With elastic and Kevlar/elastic shock cords, that's been my experience as well. Because the rocket is usually still moving slowly at ejection, when the nose cone snaps back the rocket body isn't in the same location it was in when the nose cone popped off, and it usually misses the rocket body. Watching ejections on low-altitude flights, I can see the whole rocket body/shock cord/nose cone assembly begin to perform a slow end-over-end somersault when the nose cone snaps back (until the parachute inflates or the streamer unfurls and "takes hold").
For lightweight BT-5/ST-5 to BT-50/ST-10 size rockets, I've found that (as Chan Stevens' NAR R & D Report indicated) a non-elastic shock cord of Kevlar works just fine. I usually use 18" to 24" of 100# Kevlar cord. Thick cotton string also works well for this application, although it does get singed and has to be replaced sooner than Kevlar. |