Five. That's how many attempts I needed to earn my Level Two certification. Hardly the type of accomplishment that will earn me any bragging rights, it does make me a minor expert on things that can go wrong during a high-power rocket flight. An attribute of this hobby that appeals to me is that every aspect of a successful flight can and should be addressed before you press the launch button - from the design preferences to the selection of materials to the choice of motor, every decision you make will influence the success or failure of your flight.
The overwhelming preponderance of failures occurs during the recovery phase of the rocket's flight, and a recurring factor there is extreme velocity at ejection. A rocket can easily be moving hundreds of feet per second at parachute deployment if ejection occurs well before or after apogee, or even at apogee if the flight trajectory is relatively flat. Many undesirable consequences can result - you may strip a chute, sever your shock cord, or most likely - zipper your airframe.
I recently shot some informative
in-flight video
that clearly demonstrated what happens when ejection occurs at high velocity. When the rocket separates, the booster section of the airframe becomes instantly unstable and begins to tumble sideways while the forward section continues to rocket forward like a bullet. The booster is yanked forward again when the tether between the two sections becomes taut, with the main force being born against the lip of the open booster tube. This is zipper #1. About this time the chute opens violently and the booster gets yanked about for the second time, either lengthening the previous zipper or creating a new one.
Many rocket builders use an anti-zipper design that moves the tube opening (and parachute compartment) to the front section instead of the booster section - a sensible decision, since the front section is less likely to turn sideways after ejection. A coupler is mounted at the top of the booster and the recovery harness is attached to hardware mounted in the forward bulkhead. This design can take quite a violent thrashing without sustaining any in-flight damage.
This design is particularly friendly to electronic deployment configurations. The avionics bay can be located in the center of the forward section, and can blow the drogue chute out backwards and the main chute out the nose. You may still sustain some damage in the event of a high-speed deployment, but those forward tubes are very simple to replace. (In fact, my younger brother recently employed this design in a nifty J-powered rocket. The drogue tube sustained a nasty zipper at deployment - but he had planned ahead and built an extra parachute tube as a replacement. So he had the option to just swap the tubes and continue flying.)
I personally prefer to have the option of using motor ejection, either primary or backup. This can still be used with an anti-zipper design if you a number of vent holes in the booster bulkplate to allow the ejection pressure to pass through. Build some baffles inside the coupler and you eliminate the need for ejection wadding. As an alternative to baffles, fasten a wire-mesh scouring pad inside just behind the vented bulkplate. This allows the pressure to pass through while catching all the hot particulates. The pads I found were 3 inches in diameter, but a 4-inch pad can be made by partially unrolling one pad and nesting a second one inside of it.
Drilling vent holes in the bulkplate can weaken it substantially, so some design modifications are recommended. First, double the thickness of the bulkplate - use thicker plywood or glue two bulkplates together before drilling the vent holes. Second, don't mount a screw eye in the center of the vented bulkplate (I've seen the center pulled right out of a vented bulkplate). Instead use a pipe clamp slightly narrower than the tube diameter. This moves the mounting points out next to the edge of the bulkplate and distributes the stresses more evenly.
It only seems right to come clean and say that, not only didn't these ideas originate with us, we aren't the first to post articles about it on the Internet. There is some great information to be found on the subject at Rocketry Online's
InfoCentral:
Stu Barrett has a great article on
Anti-Zipper Design
and Giacomo Bosso has a very detailed article on the construction of
Anti-Zipper Baffles.