A zipper is when deployment occurs at high speed (and sometimes not so high) where the shock cord or recovery harness slams up against the side of the airframe and either splits or rips down through the airframe, Zipperless designs tend to limit zippers, at least of the booster section by capping the the booster section with a permanently installed coupler and bulkhead with a shock cord attachment point. The shock cord and other recovery items reside in the next section forward and are kicked out to the rear instead. Because the booster is designed such that no shock cord runs over the edge of airframe in the booster, it at least prevents zippering of the booster section of the bird. Zipperless design does NOT prevent a zipper of forward recovery section or sections, just the booster.

Warren

Oh, thats how the one im working on now is. 8)

I'm planning a dual-deploy setup with a small avionics bay as the coupler between the booster section and the top (main chute) section. I was planning to attach the coupler section permanently to the top section, which will also protect it from zippering. The booster section, though, would be unprotected. If I were to do it differently I could protect the booster section and the top section would be unprotected. Is one more likely to need the protection than the other?

I'm not experienced enough to know the answer to Adrian's question, but I want to point out that I've read that a long shock cord helps lower the risk of zippers. Specifically, I've read that a shock cord (when using nylon or kevlar as opposed to something bouncy) is suggested to be 3 to 5 times the length of the rocket. The reason for the length is that it gives the 2 bodies some time to slow down before the cord becomes tight and zippers the frame.

Could anyone with more experience confirm or deny this info?

Similarly, I've been converting over to kevlar thread for shock cords in my small models because I've had many tubes dented by the rebounding nose cone. I love little rockets but I hate getting a half-moon dent around the mouth of the frame. Am I taking the correct approach in converting over?

As a recent victim of said zipper, let me say I've done a bit of reading and there is no 'perfect' solution... It's just you against physics. I had a reasonably long harness (~20'), but with an early deploy at about 150mph, Kevlar wins over LOC paper tubing, even with a fireball. Now the top of the airframe is glassed and should hold up a bit better. I've heard of people even glassing in a metal band to reinforce the end.

The best solution I've heard is reasonable reinforcement + electronic deployment. That's what I'm doing next with 'Excess in Moderation'.

Next bird is a PR Competitor 3 in glass. 😈 Can you say L2?

Ken

I think you are right on target on both counts. I use the long side of the range you mentioned with Kevlar, because really doesn't have much "give" at all.

The other thing to remember when discussing zippers is the width or diameter of the harness. The problem with thread is it takes much less force for it to cut through the top of a tube. The wider the harness material, the more it spreads the load along the lip of the airfarme which also helps prevent/reduce zippers.

For my part, I can say that I have *never* had a zipper. Part of that is because I use a lot of glass tubing (less prone to zipping) and I used L-O-N-G shock cords (though not as long as Bruce - yipes!).

Hey Warren, any way you can shrink this obnoxiously large avatar? perhaps a wee too much of a good thing 😉

JW

A combination works for Me.

A longer cord has helped the most.

Also less ejection charge, or the right amount of charge has
helped a lot also.

But I like your avitar John. 😉