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-   -   those pesky shock cords (http://www.oldrocketforum.com/showthread.php?t=18345)

dholvrsn 03-03-2020 11:15 AM

those pesky shock cords
 
I guess that I'm going to have to relearn something simple at my old age. I had more shock cords break off in the last season than I had in all the previous years in this hobby going back to '73. So what is the best way to mount a shock cord in a little BT-5 or BT-20? These are going to be for little work horse rockets like pop-pods.

ghrocketman 03-03-2020 12:45 PM

Kevlar cord tied around the thrust ring, tie the other end to the shock cord.
The old-style Estes fold-mount is very passé.

BEC 03-03-2020 12:56 PM

The tri-fold may be "passé" but, when used with kevlar, works just fine in small tubes.

Kevlar around the thrust ring works great for awhile....but somewhere between 10 and 20 flights it is going to fail from being cooked by ejection charges. Been there, done that. Kevlar in a tri-fold in the usual place below the nose cone is immune to this problem.

astronwolf 03-03-2020 03:22 PM

For pop-pods? I'd use kevlar and use the lariat method. Tie a loop around the nozzle end and run the line up alongside of the motor. Tape the motor in. Done. The line comes out when you remove the motor, so it's replaceable.

jetlag 03-03-2020 03:40 PM

I try to set up the pop pods to come down sideways so as lower the risk of kinking the pop pod tube. Just balance with an empty motor casing.
Works well for me. I just tie around the pod's cg, then tape into place. Easily replaceable, if needed.

Allen

dholvrsn 03-24-2020 11:36 AM

I was wondering if I was using inferior quality kevlar that keeps burning through? Can anybody recommend a good brand of the stuff that doesn't?

jetlag 03-24-2020 11:45 AM

I use the kevlar from Emma Kites. Lots of sizes available and much cheaper by the spool. Can't give you a website, but they have a strong eBay presence. That's how I found out about them. Shipping takes a while, coming from Japan, though.

Allen

LeeR 03-24-2020 02:20 PM

Quote:
Originally Posted by astronwolf
For pop-pods? I'd use kevlar and use the lariat method. Tie a loop around the nozzle end and run the line up alongside of the motor. Tape the motor in. Done. The line comes out when you remove the motor, so it's replaceable.


I use this method on an old Dragonfly pop-pod boost glider a friend gave me. It works great. My kit had an engine hook, so I place the loop around the hook.

And another shout out to jeffyjeep!

This was several years ago, and Jeff identified this glider (it was mostly built but had no instructions, and I didn’t know what it was.) He sent me the decal for it.

tbzep 03-25-2020 12:39 AM

Just remember that these materials aren't fire proof, but are resistant. In other words, they can burn, but they stop when the heat source is removed. I've had gear over the years that have been made with just about everything DuPont and their competitors could come up with and they all have ended up with burn holes. Some of the fabrics have been Kevlar, Nomex, PBI, PBO, Lenzing, etc. in various blends.

LeeR 03-25-2020 12:25 PM

I’ll add a suggestion from Chris Michielssen (“hcmbanjo” here on YORF). He probably has 4-5 builds going on and is ignoring us ... ;)

One technique of his that works well, if you have a rocket with motor tube mounted with centering rings, is to place a small tube, such as a launch lug, or perhaps small piece of brass tubing, through both rings. A replaceable Kevlar anchor/shock cord can be run thru the tube, with a knot at the aft end of the tube, or loop around the motor tube.

I had forgotten about this method, but the post reminded me to consider the technique more often, if I think I might ever get 20 flights on a rocket, like Bernard. :)


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