Model rocket shock cord mounting

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rbeckey

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I found a way to mount shock cords in Estes type rockets that have been flown and need a new cord. I have several that were built prior to discovering this forum and kevlar mounts. I had remembered from the old days to put in lots of extra elastic, and generally thicker than Estes includes in a kit, but eventually they get scortched. Luckily I caught a couple that were almost fried BEFORE the last launch. I recall comments in previous threads about difficulty mounting new cords in used tubes due to smoky ejection residue in the tube.
What I did was use alcohol on a rag to scrub the loose stuff off the side of the tube, then wrap 220 grit sandpaper around a dowel rod and lightly sand the inside of the tube to roughen it.
I cut a piece of kevlar cord approx. 1/8 inch in diameter and layed it down the length of the tube where I had sanded, then ran polyurethane glue down the length of the cord, starting BELOW WHERE THE NOSE CONE SHOULDER will sit. Make sure the cord is wet all over with glue. Let the tube sit level as the glue cures and the cord will be secure.
I now mount cord in new kits this way also. I put in enough cord to extend past the end of the body tube, and at least two body tube lenghts of elastic. No Estes dent, and no sign of cord failure after many flights.
 

Quasar

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I guess great minds do think alike. :D I'm experimenting with something similar, only I use a 3-4" strip of 1" wide kevlar ribbon called "tape" that I found at an R/C airplane shop. I glue one end where the standard Estes shock cord mount would be attached, then I glue the shock cord to the other end. The ejection gases never touch the shock cord and there's less chance of the 1" wide ribbon causing a zipper. Right now, I've only used it on an Alpha III, so I need to try it on something heavier before I proclaim it to be zipper-proof.

Mike Fields
 

rbeckey

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The ribbon sounds better. I didn't know that it was available. do you know who makes it?
 

BBB

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I am having problems with the small tubes, like BT-5 and BT-20. In addition to the burn-through problem mentioned above, sometimes the tightly packed parachute or streamer will hang up at the shock cord mount. I was thinking of attaching the cord, probably kevlar, on the outside of the tube then looping it into the body tube via a small groove in the nose cone's shoulder; the kevlar would attach to the elastic. This way the interior of the tube would be a smooth uninterrupted bore from the motor to the nose cone. Has anyone done anything similar to this?
 

kuririn

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Wow, 18 year old necro thread.
If you are building new, most of us anchor the kevlar leader at the motor mount and attach the elastic shock cord to that. No tri-fold mount to obstruct the recovery laundry.
For minimum diameter tubes cut a groove on the outside of the thrust ring and tie the kevlar to that.
For an already assembled model try laying a coat of glue over the tri-fold mount, paying close attention to the hump at the bottom. The glue should smooth over the hump and lessen the chance of a hangup. Also helps to lengthen the life of the shock cord mount and prevent scorching and tearing.
An external shock cord has been used in some older models that are rigged to come down horizontally. The disadvantage is the unsightly external cord. But it can be done successfully.
Good luck!
 

MidOH

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No thrust ring at all in my Bt20's. But I could add one half way down for Kevlar cords.

A tightly packed streamer or chute can't get hung up on my tri fold mount. I put the mount way down the tube under the chute. It's unlikely that my chute will unravel and slide that far down, due to launch G's. I use a dowel rod coated with TBiii to get the tri fold down in there.

I haven't tried bonding multiple inches of the Kevlar to the tube to spread out the load. Thanks, that's a good idea. But I'm still going to stay under the chute area.

I use the least amount of Kevlar that it takes to get a bowline loop out of the tube. Then I attach the Rubber band shock cord to that. There's no point in using $4 of Kevlar on a $7 Estes rocket. Sometimes I just use simple twine for those models that tend to fly away.
 

BBB

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Thanks for your replies. I am just getting back into rocketry; I last flew in the 1970s and then we did not have kevlar available. If attached at the engine block, the kevlar will be exposed to the full temperature and force of the ejection charge. Will even kevlar hold up for multiple launches?
 

kuririn

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Will even kevlar hold up for multiple launches?
No, kevlar is fire resistant, not fire proof.
But it should hold up for many launches.
If you want to go the extra mile there is a method of mounting the kevlar to make it removable and replaceable/inspectable.
Newsletter338.pdf (apogeerockets.com)
And for a minimum diameter rocket
Newsletter343.pdf (apogeerockets.com)
Both articles are authored by Chris Michielssen.
Check out his blog for great building tips and techniques.
Model Rocket Building
 

MidOH

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Someone said they cook free eventually. Generally my rockets don't last long enough for it to matter. Trees, powerlines, excessively fast decent onto pavement because streamer only.

I only want to work my way up to HPR so I can take advantage of huge club fields, and plenty of others to test the winds first. "Nah, it's OK. You can launch first.''. Lol.

I lit my frayed Kevlar cord on fire for minutes and it hardly changed at all. Softened up a bit. Might be easier to glue down.
 
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