New to using Kevlar shockcord, looking for advice

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neil_w

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I have taken to using thin CA for BT reinforcement. I apply the CA directly (and CAREFULLY) to a cotton swab, then wipe it around the inside of the tube. Depeding on tube size I might do a second application to get about 1/2” coverage at the tube end. I find the thin CA tends to soak into the paper better, rather than coating it, as medium CA tends to do. Still need to sand it out, though, if for no other reason than you want it as smooth as possible.
 

beeblebrox

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My thoughts: Attach kevlar loop thru upper motor mount centering ring. Build rocket with ejection baffle system. put a length of heat shrink tubing over the kevlar from the motor mount ring to an inch above the baffle. use long piece of kevlar, no need for elastic...The kevlar will outlast the rocket.
 

Red7Fifty

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I make removable shock cord mounts from Cable Clamps, easy to mount/replace any type of shock cord. Recently bought (3) rolls of black elastic.....1/4", 3/8", 1/2"......with 144 yds on each roll, will last a long time.
 

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GaryT

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Another topic that is directly related to your SC length is proper sizing of your BP Charge. IMO the old adage of blow it up or blow it out is ridiculous, unnecessary over sized charges reeks havoc all over your rocket. Obviously to much is always better than to little but do your research and try to figure out what the perfect amount is for each rocket.

I also use different length SC's for my main and drogue, especially if say its a windy day and you want to pop your main at say 300,400' it takes less time to unfurl giving your main a better chance of opening successfully. Same theory behind a longer apogee/drogue SC, a longer SC here "Acts" as a sort of drogue, it has less cd as an actual drogue but more cd than a shorter SC say the same size as your mains length.
 

jqavins

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Keeping Kevlar short ends badly... The cord would jerk violently when to short and snap at the glue joint on body tube.
And that's what the elastic is for. There are good reasons not to use it, but it is really good at mitigating that shock so you don't need lots and lots of Kevlar.

I looked over the article in the Apogee Newsletter; awesome stuff. I do wonder though, if anyone has put the tube that encircles the Kevlar on the outside of the centering rings so as to keep the Kevlar away from the exhaust?
The main reason to use Kevlar is its flame resistance. The fact that it's not perfectly resistant is why you're using this technique to make it replaceable. Given all that, why would you want it out at the edge? Since you started the thread with a concern over zippering, you should want it in the middle.

The only thing that is a hassle is when I use superglue to toughen the parachute end of the body tube (Krazy glue brand, which is thin and comes with a brush), it requires a lot of sanding to get rid of the extra thickness on the tube lest the nosecone fit way too tight.
I have taken to using thin CA for BT reinforcement...
To clarify (I don't know if you really need this, so forgive me if I'm accidentally talking down to you) Krazy Glue only seems thin compared to the thick or gel stuff next to it on the rack, but isq% actually medium. Grocery stores, hardware stores, and the like sell basically only the two kinds. In hobby stores you'll find thin CA, which is as thin as (I think thinner than) water, and soaks right in. Does a better job of hardening stuff as well as not needing so much sanding.
 
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neil_w

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To clarify (I don't know if you really need this, so forgive me if I'm accidentally talking down to you) Krazy Glue only seems thin compared to the thick or gel stuff next to it on the rack, but isq% actually medium. Grocery stores, hardware stores, and the like sell basically only the two kinds. In hobby stores you'll find thin CA, which is as thin as (I think thinner than) water, and soaks right in. Does a better job of hardening stuff as well as not needing so much sanding.
I surely do wish that the hardware store CA (again, I'm partial to Loctite) was labeled "medium" somewhere, just for consistent terminology's sake.

Medium CA generally soaks in only partially, so you end up with a hard coating. Thin CA soaks in much more thoroughly, until you've really saturated the material and then it starts to build up (like when coating a balsa nose).
 

GaryT

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Don't use CA on your Kevlar SC or any other SC for that matter, it'll weaken it to the point where it can Crack in half.
 

jqavins

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I surely do wish that the hardware store CA (again, I'm partial to Loctite) was labeled "medium" somewhere, just for consistent terminology's sake.
And I wish they'd carry thin, but I don't think either of those is going to happen.
 

GaryT

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I have taken to using thin CA for BT reinforcement. I apply the CA directly (and CAREFULLY) to a cotton swab, then wipe it around the inside of the tube. Depeding on tube size I might do a second application to get about 1/2” coverage at the tube end. I find the thin CA tends to soak into the paper better, rather than coating it, as medium CA tends to do. Still need to sand it out, though, if for no other reason than you want it as smooth as possible.
Neil,
Sanding sealer works great for sealing and coating the inside of your BT, apply let soak in dry then sand. Does a great job at helping preserve you BT against moisture and repeated BP charges going off in the same spot.
 

Nytrunner

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I surely do wish that the hardware store CA (again, I'm partial to Loctite) was labeled "medium" somewhere, just for consistent terminology's sake.
This stuff? I've used it before and it seems crazy thin
 

neil_w

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No, I use this as my medium:


See, that's the problem. The only kinds that get tagged for viscosity are the gels; for all the others you're left to guess.
 
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