Attaching recovery shock cord to stuffer tube centering ring?

[Bill S]

I'm working on a new mid-power rocket, having only built and flown a Super Big Bertha, and an Executioner. Both of those, I put in a BT-60 stuffer tube with cardboard centering ring. I ran a kevlar cord from the base of the rocket through some hollow tubing that goes through both motor mount rings, and all the way to the front of the stuffer tube ring, and then attached 1/4" cotton elastic for the shockcord. Both rockets were 9-11oz empty.

The new rocket, its looking to be 18-22oz, and having looked at some comparable weigh rocket kits, they seem to be using heavier kevlar/elastic than I am used to using (normally use 100# kevlar and 1/4" cotton swimsuit elastic. Body tube will hopefully be LOC 2.56 tubing.

I am debating whether to use #200lb kevlar from the rear of the rocket to just below the front end of the body tube, then attach the elastic there, or to use an plywood front centering ring at the front of the stuffer tube and use some sort of screw-eye and attach the shock cord directly to the plywood ring. The disadvantage of the screw-eye method is that if the elastic shock cord ever wears out (have yet to see it with this type of shock cord) it'll be very hard to attach a replacement to the screw-eye due to not being able to easily remove it or get my hand in there. At least with the other method I can untie the kevlar at the rear end and pull it through.

Even if I did somehow attach the shock cord to the front plywood ring, if we're talking a nearly 1.5lb rocket, is it likely that the ring will take the stress and not damage the body tube?

 

[waltr]

For MPR I do not use any elastic.
I use much heaver Kevlar from mount to a few inch out of the main BT, then a long piece of light Kevlar. Rational is the heaver Kevlar is hard to replace so make sure it stands up to many flights.
For 1-2 pound rockets I go with at least 400# Kevlar since this is not the one you want to break.
100# Kevlar would then be fine as the long (3-5 rocket length) shock cord.
Taped bundles or Braided works well to prevent cord tangles and absorbs some shock as the nose flies off.

With stuffer tubes I have secured the heavy Kevlar to MM the feed through a hole in the top center ring for the stuffer tube. No need for a tube and has been working well.

If there is a Eye-bolt on the MM or top ST CR I have done a loop of heavy Kevlar to the top of the BT. This would allow replacement by tying or taping a new piece on to the old and carefully pulling it down and through the eye.

An eye-bolt on the stuffer tube's top center ring should also work. Just ensure the CR is thick enough and then well glued to BT and ST.
1-2# rocket should be ok with an Eye Screw if epoxyed into the CR.

 

[Bill S]

I've been trying to avoid using kevlar near the front of the tube due to persistent issues with the kevlar damaging the front of the tube slightly. I even used some black grip tape (used on tennis rackets, etc to give good grip) to increase the diameter of the kevlar and hopefully avoid zippering.

 

[jmasterj]

I know I overbuild, but I'd just use heavier Kevlar (500 to 1000 lb) from the motor all the way to the chute and nose cone. Make it 12-15 feet long and you won't have to worry about zippers unless something goes terribly wrong.

 

[Tractionengines]

I don't use elastic. For smaller rockets like this I REALLY like the LOC 3/16" tubular nylon. A 700# or 1000# kevlar leader. Then 6-15 feet of this nylon. It is light, has just a bit of give. I bundle it into taped groups. NOT for shock absorbing, as it doesn't take much to tear the tape, BUT it does a great job of organizing the deployment. So no tangled lines. I have also chain knotted it, and it releases well.

https://locprecision.com/products/shock-cords?variant=39778658025663
This is a 1.6" LOC EZE Park Flyer (extended with a 10" payload bay. It has 9ft. of 1/8" kevlar glued to the 29mm motor mount, then Piston, then 12ft of the 3/16" tubular nylon, 24" chute, with JLCR. Can fit a H135W-14A, too. (1) 2-56 nylon shear pin holds it together, since it's a tight fit.

 

[waltr]

Good pictures of your deployment setup Tractionengines
.
This is very similar to what I do (minus piston).