Does anyone have a good method of attaching a shock-cord in such a way that it can either be replaced when it becomes brittle or that uses a material that is less subject to decay than rubber or elastic.

I was thinking of using a short length of a more durable material (???) that would get folded into the shock-cord mount or tied to the screw eye mount and attach a small link (tiny washer perhaps) that in turn you could attach a length of rubber or elastic shock cord to.

That way when the shock cord becomes brittle, it can simply be replaced without having to remove the mount.

Has anyone else out there dealt with this issue?

Thanks,
Don

Quest method: Kevlar cord attached to the thrust ring and long enough to entend out of the top of the body tube. Then simply tie your elastic shock cord to the top of this. No need for a link - just a simple knot. Easy to replace the elastic when worn or damaged.

I just pulled a 20 year old box of saucers out of the rafters of my garage. All the elastic was mummified. I will fly them as pop apart instead of installing new elastic simply because I will be using them to fly official NAR Old Motor Testing Program FSI E5-0 and D20-3 motors. In other words, they may not be saucers for long.....

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-Fred Shecter NAR 20117 (L2)
Southern California Rocket Association, NAR Section 430

I have a couple of smaller rockets (a Big Bertha and an Estes V3) that I built using a small version of North Coast Rocketry's Gorilla Mount. Using thin gauge steel-core fishing line, looped and clamped around the engine mount tube, ending in a loop near or at the exit of the body tube. I then tie the elastic/vinyl/rubber/twine/whatever, to this loop. Works great, except that you have to play with it a little while packing the chute. It can sometimes be so stiff that it pushes the nose cone off just sitting there.

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Roy
nar12605

I use something like Fred's technique, except:

I attach the cord to a centering ring, or a tube coupler if the body has one. I know Kevlar is supposed to be heat resistant, but it does fray and get cooked and I have had it break on me.

The end of the Kevlar anchor cord is short of the front end of the body tube.

The anchor and shock cord are connected via a barrel swivel. Reduces "wind up."

To attach the swivel, pass the anchor cord out of the rear of the model via the motor mount tube.

I've used both the aforementioned methods, kevlar and metal cable/fishing leader. The latter is probably more durable, but the problem mentioned (stiff, pushes nose cone, also can get in way of chute or streamer packing) does occur and is especially troublesome in smaller diameter birds, where there's not as much side to side room to buckle the wire.

I was recently (within the last year or two) BARred and have been experimenting with these "new" techniques.

I put a fishing leader in a Midget (very short, BT-20). That was not a good idea. Should have used Kevlar in that one. On the other hand, I've put fishing leaders on several Alphas (BT-50) and that worked great.

The leaders typically have a small metal loop at one end and a snap, a la a snap-swivel at the other end. I often flatten the metal loop with a pliers and then slide it over the engine hook below the bottom ring, then assemble the engine mount rings over the cable. Then the snap hoop ends up at the nose cone end. With leaders, the difficult part is picking a proper length, because they come in fixed lengths. You can adjust this an inch or two by looping the leader around the engine mount or not.

I think the optimum length for either method is to end up with a hoop or loop just inside the nose cone end of the body tube. If the cable or kevlar extends beyond the end of the tube, and strong sideways force is exerted on the rocket or recovery system, then the narrow cable or kevlar may cut through the body tube at the opening. If the loop or hoop is too deep in the body tube, then it is difficult or impossible to attach the shock cord.

I usually tie my shock cord to a snap swivel and then attach the snap swivel to the fishing leader or kevlar loop. The snap part of the snap swivel makes a convenient hook with which to catch the hoop/loop just inside the end of the body tube. And it's an easy way to replace the shock cord when it expires, rather than trying to untie the old cord and tie a new cord inside the body tube.

I use good quality microbraid Kevlar (not the thin thread stuff) as my shock cord leash. I tie it to the motor mount and pass it up between the upper centering ring and the inner wall of the body tube. I make it just long enough to reach almost to the upper end of the tube, and then I tie a small split ring to the end. I tie my elastic or nylon shock cord (or an elastic/nylon combination) to the split ring. If the shock cord would ever need to be replaced, I can use a dowel to push the Kevlar out through the motor mount and easily reach the split ring that way.

I have also used steel cable instead of Kevlar for the leash, but I don't use fishing leader. Instead, I use nylon-coated 49 strand stainless steel bead cord. It can be found in the beading section of most craft stores, like Michael's, usually sold under the brand name Beadalon. Although there are 7-strand and 19-strand varieties, I recommend getting the 49-strand version. The higher the strand count, the more limp and flexible the cable is. The stuff is more expensive than Kevlar, though, and it isn't as strong; you will mostly find it in the 12-15 lb. test strength range. But for most LPR, this is strong enough. Although it is limp enough to be knotted, I prefer to make my attachments using crimp beads. For those of you who are unfamiliar with bead crafts, a crimp bead is a small metal tubular bead that is used to create loops in bead cable. To do so, slide the tiny crimp bead onto the cable near one end, form the loop and then insert the tag end of the cable through the same hole in the crimp bead. Then crush the bead with a pair of flat jawed pliers so that the flattened bead grips the cable and the tag end. With a firm crimp on the bead, the tag end will not pull out and the loop will be very strong. I have often used this type of shock cord mount with my scratch-built Micromaxx rockets, but it will also work with just about any type of rocket that is powered with A to C motors.

MarkII

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SAM # 0011

Here are two other ways to attach a Kevlar shock cord. These are especially good for "retrofitting" a rocket if its existing shock cord has broken. They also move the end of the thread farther away from the motor so it's less likely to burn through.

1) Use the Estes folded paper method. Cut a piece of paper in the clipped triangle shape. Put a spot of glue in the middle. Make a small loop in one end of the Kevlar thread and lay it in the glue. Fold the paper over and add another spot of glue then fold again. Glue the assembly down about an inch inside the body tube.

2) Cut two small parallel slits in the body tube starting about an inch down the side. Press the tube in between the notches. Feed the Kevlar inside the tube and through the little tab created where the tube bends inward. Feed enough in so that you can invert the tube and pull the end back out. Tie a big knot at the end of thread then pull it tight against the tab. Add a drop of glue on the outside of the tube between the slots.

I like to use a long length of Kevlar thread tied to a short bit of elastic shock cord.

-- Roger

I ran into this situation with a couple older Estes models that I have had since the '70s. The old "rubber band" had dry rotted, and the mounts were a little deteriorated. In both cases I carefully removed the old folded paper mount and went to the Kevlar cord method. In the Sprint I was able to notch the outside of a 24mm engine block and push it back to the CG with the cord attached. The solution for the ARCAS was a Semroc BT55 baffle kit, a really neat item, which even comes with a Kevlar cord. (Thanks again to Carl and his nifty laser cutter!)

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Steve Coker
NAR#13865

Thank you for that. That's cool to know and now I want to try that.

Ah, Google Images came up with this brief tutorial in case anyone wants slightly more detail.

Crimp Bead Tutorial

And Google turned up this vendor. I don't know if these prices are any good, but it shows a wide variety of beading supplies including the threads and crimp beads that Mark reported. The 49 strand thread is available in several thicknesses. The .024 is rated up to 40 lbs according to this website. This link is to the crimp beads and tubes page. You'll have to use the links at the bottom if you want to see threads.

The Bead Peddler