Cluster Mount Shock Cord Placement?

[DigBaddy]

I'm new to the whole kevlar shock cord game, so thanks for bearing with me. I started a new project that uses a 3x18mm cluster mount from Rocketarium for a BT-80 tube. The front centering ring has a rather convenient hole in it and I suspect that could be used for passing the cord through to tie off to the beefy printed locking ring joining the tubes or mounting a small eye bolt to tie the cord to. But, I worry that the central position may not end up with good enough life for even a kevlar cord. Should I use that location with a bare cord, maybe add some shrinkwrap tubing near the motor mount, cut a small slot at the edge of the ring instead or just give up and tea bag it? Thanks for any insight!

 

[dhbarr]

How are you planning to retain your motors? If the middle is open in the back ring as well, I'd pass the cord through both and knot on the bottom so that the leader can eventually be replaced as needed.

 

[DigBaddy]

How are you planning to retain your motors? If the middle is open in the back ring as well, I'd pass the cord through both and knot on the bottom so that the leader can eventually be replaced as needed.

Mount uses hooks on all tubes. Passing the cord through the rear ring as well is a good idea. Hadn't thought of that. Rear ring isn't glued to anything yet, so drilling a small center hole is easily doable.

 

[K'Tesh]

If it was me, I'd cut a notch (or drill a hole) on the outer edges of the centering rings, big enough to add a straw that the shock cord can pass through. Through the straw I would pass a length of the shock cord through the rocket. I'd tie a loop in the upper end of the shock cord that, with careful tying of a loop on the lower edge, ends just short of the edge of the body tube. I'd pass the line through the lower loop, and secure it around one of the motor tubes. To hold it in place, I'd use a piece of tape, or perhaps (for plywood CRs) a screw (or two) with a washer to prevent it from getting loose. I'd attach the rest of the shock cord to the loop at the top of the tube with a quick link that is half in, half out of the body tube (to prevent zippering). After flights, I can pull the shock cord down from the back to inspect it for damage, and if needed, replace it.

[EDIT] Added the image below after reading BigDaddy's following post... A picture is worth a thousand words. [/EDIT]

 

[DigBaddy]

If it was me, I'd cut a notch on the outer edges of the centering rings, big enough to add a straw that the shock cord can pass through. Through the straw I would pass a length of the shock cord through the rocket. I'd tie a loop in the upper end of the shock cord that, with careful tying of a loop on the lower edge, ends just short of the edge of the body tube. I'd pass the line through the lower loop, and secure it around one of the motor tubes. To hold it in place, I'd use a piece of tape, or perhaps (for plywood CRs) a screw (or two) with a washer to prevent it from getting loose. I'd attach the rest of the shock cord to the loop at the top of the tube with a quick link that is half in, half out of the body tube (to prevent zippering). After flights, I can pull the shock cord down from the back to inspect it for damage, and if needed, replace it.

After reading that a few times, I can picture it now and that makes a lot of sense. I can even go elastic attached via that quick link on the upper loop. But, it's easy to swap out the kevlar if it shows excessive wear and it's really easy to check. The body tube is short (21cm) so seeing it all without pulling it out the rocket's um, rear-end, may not be needed all the time. But, that capability to swap out is key. Easier with edge notches/straws than small small center holes, and it's more out of the way of the 3 ejection charges too. Thanks for the idea! Clearly you've done that before

 

[K'Tesh]

After reading that a few times, I can picture it now and that makes a lot of sense. I can even go elastic attached via that quick link on the upper loop. But, it's easy to swap out the kevlar if it shows excessive wear and it's really easy to check. The body tube is short (21cm) so seeing it all without pulling it out the rocket's um, rear-end, may not be needed all the time. But, that capability to swap out is key. Easier with edge notches/straws than small small center holes, and it's more out of the way of the 3 ejection charges too. Thanks for the idea! Clearly you've done that before

One thing I forgot to mention... be careful when gluing the motor mount in. You don't want to clog the straw (a skewer would probably help keep it clear.

Actually, I haven't done that before. What I have done is a lot of research, and thinking about rocketry. When I see a good idea, I'll ponder it, and/or modify it to suit my needs. That said, when the rockets I tend to build get that big, I use baffles, and attach the the shock cord to the baffle. I've never built a cluster (I own one, but I can't bring myself to put a 1969 era Centuri Saturn V through that (my teacher built it and flew it "back in the day")). If I can't use a baffle, often I'll use an eye bolt, and my U Loop method of attachment.



Your rocket seems to fall into a place where the method I posted about would be the most effective method (too small for baffle, too small for eye bolt).

 

[mjennings]

K'Tesh what are you using for fire resistant sleeves?

 

[teepot]

What ever method you use to secure the shock a Nomex sleeve is a good idea. Several vendors sell them or you can get some Nomex fabric and make your own.

 

[K'Tesh]

K'Tesh what are you using for fire resistant sleeves?

Nomex

 

[DigBaddy]

Good info, thanks! Today I learned Top Flight Recovery is just down the road...had the card from my BD's Nomex blanket sitting on my desk and noticed the address I like buying local; so I'm going to have to put them on top of the heap for recovery equipment.

 

[mjennings]

Thanks. I'll have to get some!

 

[DigBaddy]

Perusing other old threads this morning and noticed some folks mention using fishing leaders for the first bit of harness off the motor mount. Interesting approach. With a short body tube like I have here, a leader could even be removed given a sufficiently long needle-nose pliers if connected to an eye bolt.

 

[teepot]

I've used metal leaders in some rockets and wire rope in others. Now I just use Kevlar and shock cord protectors.

 

[DigBaddy]

@K'Tesh This seems like it'll work nicely. Rear ring isn't glued in yet. Knotted, and screw/washer to pinch it down a bit.

 

[K'Tesh]

@K'Tesh This seems like it'll work nicely. Rear ring isn't glued in yet. Knotted, and screw/washer to pinch it down a bit.

Nice! That's what I was trying to explain.

What knot are you using for the loops? Me, I'm a big fan of the Bowline.

 

[DigBaddy]

Bowline up front and will probably use that in the rear as well (just a slip knot for now).