LPR Kevlar retrofit

[Crash-n-Burn]

I have a bunch of rockets with the teabag-style attachment for a rubber shock cord. I've had a couple fail but generally this technique has been reliable for me. In some applications, though, I've wanted to change out the teabag-style recovery system. I do this when:
- Narrow bt or want more space in bt for chute
- Heavy residue in bt from ejection charges makes new teabag gluing suspect

This works best with cardboard CRs, I have not tried it with a sleeve style CR.

1. I bought a 1/8" standard Drill bit from Irwin that is 12" long. It is not a concrete-style bit, those wouldn't work as well. $5.
2. Pick the spot you want to drill a hole in the aft CR, I aim for halfway between the MMT and BT but not interfering with motor hook.
3. Hand-spin the bit to cut a hole in CR.
4. Carefully slide bit further through aft CR until contacting the foreward CR.
5. Ensure the bit is plumb with the rocket - you want the holes in line with each other and the BT.
6. Spin the bit again to cut a hole in the forward CR.
7. Ream the hole with the bit, clear fibers and remove.
8. Put some CA glue on your kevlar thread and roll out straight. The goal is to stiffen the kevlar thread into a needle. I found 6" to be enough.
9. Once thread is dry and straight, thread through the two holes. Patience is key.
10. continue to feed kevlar through the rocket, trim the CA'ed section of kevlar off, tie a loop to accept some elastic.
11. Pull kevlar from the aft end of the rocket until the kevlar line is below the top of the BT. This eliminates the kevlar causing a zipper on ejection.
12. Cut kevlar to appropriate length at aft end of rocket. Tie a loop over a spare motor tube, remove spare motor tube, and slide the looped kevlar over the motor tube on your rocket.
13. Put a dab of glue ove rthe aft 1/8"hole with the kevlar running through it to control ejection gases.

So that's it. I found this retrofit to be helpful, hope someone can benefit from this.

 

[caveduck]

Nice! I like the part about stiffening the kevlar line for threading it through the CRs. Never had occasion to do a retrofit (been building them new with kevlar wrapped around the forward CR for a few years now) but I need to remember this tactic.

 

[MALBAR 70]

I usually run a hollow coffee stirrer between the centering rings to make sliding the Kevlar through easier.
If you have kids the straw from a Capri Sun juice pouch works good too.:2:

 

[Crash-n-Burn]

I usually run a hollow coffee stirrer between the centering rings to make sliding the Kevlar through easier.
If you have kids the straw from a Capri Sun juice pouch works good too.:2:

Right, I do the same on new builds sometimes, but getting the straw glued in place on a retrofit would be difficult and risks getting glue in the hole of the straw. How would you get glue on the forward CR to anchor the straw?

 

[MALBAR 70]

It's more of a guide than anything else. If the hole is drilled slightly smaller than the stirrer friction should be enough to hold it in place. I usually cut the stirrer about a 1/2" longer than the motor mount so when its flush with the rear CR it extends past the forward CR, after a few flights the straw will melt around the Kevlar and create it's own seal.

 

[hcmbanjo]

Here's the info from an article I wrote for the Apogee POF Newsletter from 2013:
https://www.apogeerockets.com/education/downloads/Newsletter338.pdf

The small cotton swab straw is smaller, stiffer and thicker walled than the juice box straw.
A wrap of electrical tape over the straw and engine hook will keep it from slipping.
The straw should be extending a little outside, beyond both of the centering rings.
I haven't had any melting of the guide straws with them recessed below the top of the mount.
All are clear and allow the line to be checked after many flights.
In the article, Figure eight shows the straw and tape wrap.

 

[Crash-n-Burn]

Chris, the technique in your PoF article is the gold standard for replaceable kevlar, but the original intent of this thread was a post-build retrofit. Some of the items in your article (taping straw swab in place) don't appear possible in a built rocket. Maybe I'm mistaken on that.

Do you have any ideas to improve on a retrofit process?

I think MALBAR's point was that a friction fit would suffice on a retrofit since it isn't under load. Tension would keep the straw in place. I haven't tried it but it seems plausible.

 

[Solomoriah]

I use thin wire picked up at the dollar store to feed Kevlar through centering rings, or through the paperclip anchors I use in some of my rockets.

I need to post pictures of those anchors, actually...