Zippered Initiator - how to fix

[mitaccio]

I am really confused how this happened in the first place. F40-10T and it still zippered 2". This is my first rocket. How do I fix? I am also making an AT Mustang. Can I do something to prevent this (Besides getting the motor right every time)?

 

[dixontj93060]

Do you have a picture so we can assess the damage and then recommend the fix?

 

[mitaccio]

It'll be tomorrow before I can post a pic of the damage. The tubing 'popped' back into place and the nosecone can still slide same as before the damage. The outside is a bit rough but from more tha 5' away you can't see it.

 

[NjCo]

Seems your delay was a bit long. Most of the recommended motors for the Initiator in the E and F range have only a 4 sec. delay. A zipper happens when the chute deploys while the rocket is still traveling fast, either on the way up or the way down. The chute pops and essentially stops the nose cone but the body keeps going. The shock cord then stretches along the lip of the body tube and once the force is great enough it cuts through the body tube.

To make zippers less likely you can do a couple things:

A) Use the right motor. Using the recommended motors for the kit is a good start. Or run your rocket configuration through RockSim to make sure you are ejecting close to apogee. Not all ejection charges are accurate though but this is always a good start.

B) Harden up the lip of the body tube. Apply very thin CA around the lip of the body tube (when using cardboard tubes). On a rocket like the initiator I'd put maybe a 3/4" band all the way around. Then sand it down so it doesn't interfere with the nose cone. This hardens up the body tube so it can take more punishment.

C) Change the shock cord. The initiator comes with a wide elastic shock cord. Is this the shock cord you were using? A wide shock cord stretched over the lip of the body tube will not zipper as often as a skinny one (like Kelvar cord). Of course, even a wide cord when stretched will have a small surface area so you still have to be careful.

D) Look into using Giant Leaps Fireball. This device keeps the shock cord away from the lip of the body tube.

https://giantleaprocketry.com/products_recovery.asp

scroll about half way down.

I'm sure other members will add more ideas.

As for fixing it, I don't have any experience with that - luckily.

 

[mitaccio]

Here is the pic of my zippered rocket.

 

[dixontj93060]

It doesn't look that bad. Likely you could align and hold the zippered area together while applying CA glue on the inside of the body tube. For a beefier repair use some 30 minute epoxy and a bit of cloth. Apply very thin and sand down after completely dry.

Then as for the outside trim away excess ripped body tube with an Exacto knife leaving a small indent/groove at the zipper location. Fill gap with Bondo or wood filler. Sand smooth. Shoot sparingly with some primer, then when dry repaint.

 

[dpower]

Agreed that the -10 is too long of a delay - the F40-7 is the best delay for a stock Initiator. It was travelling over 100 fps and accelerating (down) at 10 seconds. Motor choice is the #1 way to prevent a zipper. In addition to RockSim, there are some simple free simulators that will help you pick motors (wRasp, wRasp32, and pRasp are the ones I use).

dixontj recommends a good fix. If you're lazy (like me), a quick fix would be one layer of clear packing tape completely around the outside of the tube. Not real strong, or pretty, but good enough for quite a few launches. Consider your Initiator Initiated.

 

[blackjack2564]

Clear packing tape.........The duct tape of midpower.


Truly a great quick fix that I also use. Use a red magic marker to color the raw edges and carefully wrap. No on will ever notice!

 

[El Cheapo]

I had the same thing happen to my initiator due to a non-existant delay on a G71 Redline. I cut the tube down, added a coupler and stretched my Initiator by about 8". I think it's actually better looking a little long than in stock length.

 

[MaxQ]

I've reinforced the airframe below the internal nose cone shoulder with tube couplers glued inside, then some filler outside and repaint.
- or simply a curved patch from a piece of body tube.

https://www.info-central.org/?article=325

This repair "how to" is for bigger rockets, but it brings to mind an old subject:
Anyone remember the Stu Barrett "zipperless" designs from the old Rocketry Online forum?

I did a few of those concept mods on some kits - and my own designs...
Works.
Info Central used to have the design illustrated......I found this picture online, which is I believe, is one of Quickburst's kits utilizing it...

(tell me what you see in the picture that is different than what usually is done..then you have figured it out).

 

[Breeze1913]

Can't tell what you've done differently from most "anti-zipper" designs.
Give us a hint!

 

[Gillard]

coupler is on the bottom half of the rocket, so if anything zips, its a coupler not a BT.
i like pistons, so not a fix for me.

 

[NjCo]

coupler is on the bottom half of the rocket, so if anything zips, its a coupler not a BT.
i like pistons, so not a fix for me.

Never used a piston but I'd like to try it. So is this design close to a piston? Wouldn't you just convert the coupler to a piston and anchor it to the motor mount? And of course keep the body tube the same length.