Zipper Prevention
Zipper prevention is mechanism or technique to prevent the rocket's shock cord from tearing
or damaging the end of the body tube. When the rocket's parachute deploys at a high
speed, the shock cord can cut a large slit into the end of the body tube.
The most common "anti-zipper" technique is to put the 'chute into an upper section of the rocket
as described at http://info-central.org/construction_antizipper.shtml.
For my EZI-65, I used a "Fireball" which I purchased from Giant Leap Rocketry (http://www.giantleaprocketry.com/products_recovery.asp#fireball). The
Fireball works by spreading out the force using a spongy ball which is attached to the shock cord at the end of the body tube.
References: