Zippering...Too long & too short a delay?

Yes. I've done it both ways.

Probably Kevlar, or some sort of string.

It will tell you the optimum for delay for whatever motor you simulate. And you'll find there are some motor-rocket combinations for which there is no suitable delay available. (Unless you trim your own :)

I put a screw-eye on the forward centering ring, then Kevlar to just below the opening, then elastic or flat nylon the rest of the way. I see some who let the Kevlar/string come out of the tube.

In a high speed deployment from an early or late ejection, there will be quite a jerk to the harness and to the airfram opening. If the force is all concentrated on a small spot, such as a string, you're more likely to get a zipper. Concentrated in one spot is the definition of a knife by the way.

Having elastic or nylon webbing spreads the shock over a wider section of the circumference of the opening reducing the chances of zippering.

Doug

Not a lot of experience, but I've seen a few of these recently in building bigger scratch rockets. I hate them.

The tube zippers because it is being rapidly rotated or abruptly restrained in flight by the force on the shock cord, usually from an intentional deployment at high speed or an unintentional separation/deployment at burn-out. A thin shock cord concentrates the force where it contacts the body tube.

The anti-zipper design Joe mentioned works like a champ. The drawback to the InfoOnline design is that the recovery system weight is moved aft, a plus is that the two rocket sections are usually unstable if separated. I have a couple rockets with the same design but they break apart near the nose, not back aft. They require a short payload section below the NC for coupling and recovery storage. In this case, my "booster" sections are stable and one came in ballistic on a shock cord separation.

Another technique which works for me is a restrained piston; the piston does not leave the airframe. The piston skirt is ported to vent ejection gases and the motor-side shock cord stops the piston before it leaves the airframe. It makes the same sort of setup as the antizipper design, but you can mount the recovery stuff all the way forward and pop the NC normally. I make the piston longer than normal and glass the inside of the skirt to add strength. It stops about halfway out. If you do this, consider how to detach the piston from the motor-side cord (or steel cable in my case) so you can clean the body tube. I've used quick links (make sure a small wrench fits through the ejection gas ports) and an upside-down U-bolt. This method puts quite a strain on the motor-side piston components as the piston is really moving when it suddenly stops, with the airframe still under some pressure, I'm sure. Static tests generated some ugly sounds. I try to minimize piston travel and weight.

Yes, both situations can cause a zipper. It's the result of deploying at higher than optimal speed, either on the way up or on the way down.

I've built a couple of rockets according to Stu's suggestions and will never do so again, especially for smaller diameter rockets. First, you lose a lot of room in the rocket when you put the bulkhead in. Second, if you have a stuck motor, you can't use a dowel to push it out from above because the bulkhead is there. Third, If you use motor ejection, the ejection charge pushes the shock cord and chute further up into the top section before it yanks it back out. I've had this cause several chutes to fail to deploy after being jammed up into the upper body tube.I'd imagine that this wouldn't be a problem with larger diameter rockets with lots of room for a chute and shock cord. But I still don't like the loss of access to the motor mount from above.

Mark Simpson NAR 71503 Level II God Bless our peacekeepers

I had the same experience with a 2 stage maniac mod I built a few years ago. After 20+ flights the chute opened correctly about twice. Luckily the damn thing was so tough that it usually fell flat without much damage.

Jon