Shock cord material for lpr/mpr

[icyclops]

Oh man, first @jqavins wants to stop us from speculating, now you want to stop us from overthinking. WHY DOES THIS FORUM EVEN EXIST???

I would think that if the lug ended several inches below the end of the body tube, the Kevlar wouldn't be able to pull at a sharp enough angle to zipper it.

That is a great technique but not all designs allow for it.

I don’t want to stop you from anything…..sorry you feel that way. Just seemed that a lot of worry when there is a simple fix is all. And haven’t had any zippers on the designs I have needed to do this on….as I do not due this on all of my builds…never said I actually did on everything I build….only the ones I may think things may get crispy a bit. But hey, that’s the beauty of opinions and suggestions/comments….use them if you wish - or don’t.

 

[neil_w]

I don’t want to stop you from anything…..sorry you feel that way. Just seemed that a lot of worry when there is a simple fix is all. And haven’t had any zippers on the designs I have needed to do this on….as I do not due this on all of my builds…never said I actually did on everything I build….only the ones I may think things may get crispy a bit. But hey, that’s the beauty of opinions and suggestions/comments….use them if you wish - or don’t.

No worries, just joking. I was hoping it was obvious but I guess not.

 

[lakeroadster]

I would not normally compromise a design just to include in this feature.

Compromise? Ugh. What's the compromise?

 

[neil_w]

Compromise? Ugh. What's the compromise?

Some designs simply don't allow it easily or at all. For instance, those with a tail cone that ends at the tip of the motor mount (no centering ring at rear). I have many more examples. To accommodate the replaceable Kevlar, I would have to materially change the design. For me, that would be the tail wagging dog.

 

[lakeroadster]

Some designs simply don't allow it easily or at all. For instance, those with a tail cone that ends at the tip of the motor mount (no centering ring at rear). I have many more examples. To accommodate the replaceable Kevlar, I would have to materially change the design. For me, that would be the tail wagging dog.
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Sure, I get your point. But form follows function and with all due respect you're selling yourself short. We've seen your designs. If you put your mind to it, not only could you do it, you could make it look awesome.

Something that mimics cable tunnels that fit the contour of the tail cone.. or a series of internal tubes that are tapered on the ends to match the tail cone, one of which is used for the Kevlar chord. Kind of like the barrel of a revolver.

 

[lakeroadster]

I don’t want to stop you from anything…..sorry you feel that way. Just seemed that a lot of worry when there is a simple fix is all. And haven’t had any zippers on the designs I have needed to do this on….as I do not due this on all of my builds…never said I actually did on everything I build….only the ones I may think things may get crispy a bit. But hey, that’s the beauty of opinions and suggestions/comments….use them if you wish - or don’t.

OK @icyclops ... I'm calling you out. Post up some photo's of those crispy prone rockets you've designed and photo's or sketches of their internals.

I'm not from Missouri.. but I've been through there on numerous occasions.

 

[Ronz Rocketz]

And I thought I had a pretty good one - Kevlar in a tri-fold so the lower end is far away from the ejection charge with that tied to the elastic element of the shock cord, and a bit of heat-shrink tubing over the Kevlar where it rubs on the upper lip of the body tube to keep it from wearing away (and a for a little zipper protection). But I have had that fail now, so am trying something else.... Time will tell.

I'm also using the heat shrink at the upper lip of the BT. I'm surprised the Kevlar failed below the heat shrink. Some don't use heat shrink and tie the elastic to the Kevlar at the rear then pull the line through the MM, so only the elastic touch the tube lip. I have yet to successfully do this.

Currently I'm addicted to Qualman baffles and connect the Kevlar to the eyelet. That's the main reason why I extended the Goblin so I can insert a baffle. I discovered yesterday it fits the jLCR very nicely.

Since many folks tend to not follow links... I feel compelled to show the diagram that's in the link above that @BABAR posted above.

It's easy peazy and slicker than Opossum snot. FWIW: I use coffee stir sticks as the guide tube..

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This baffles me. Is securing the Kevlar to the rear of the MM any better than connecting it to the front centering ring (no guide tube needed)? I've used a 12" drill bit to modify old rockets and attach the Kevlar to the rear to eliminate the trifold cord attachment at the front of the BT. The hard part is threading the Kevlar through the new holes (thin wire helps).

I always contemplate of it's better to connect the Kevlar close to the motor tube or against the BT? Against the motor tube seems more secure but it's closer to the election charge.

Of course most of my rockets end up in the canal or drift away into the next zip code before they fail. I've learned to launch "disposable" rockets to check the wind before launching the heavier rockets.

 

[Hobie1dog]

thanks to all who have replied I am learning more and more.

 

[neil_w]

I'm also using the heat shrink at the upper lip of the BT. I'm surprised the Kevlar failed below the heat shrink. Some don't use heat shrink and tie the elastic to the Kevlar at the rear then pull the line through the MM, so only the elastic touch the tube lip. I have yet to successfully do this.

I'm curious about your use of the word "successfully" there, which implies "tried but failed". That method is exceedingly easy, not sure exactly what can go wrong. Please elaborate, if you can.

For BT60 rockets, I don't really find it necessary to do the attachment out the back of the rocket; I can do it inside the edge of the body (using the linkage I showed previously). But out the back usually is easy.

This baffles me. Is securing the Kevlar to the rear of the MM any better than connecting it to the front centering ring (no guide tube needed)?

Because it's not attached, it's just looped around. Therefore it is replaceable if/when the Kevlar burns through.

Of course most of my rockets end up in the canal or drift away into the next zip code before they fail. I've learned to launch "disposable" rockets to check the wind before launching the heavier rockets.

Important point. @BEC estimates about 25 flights on one piece of Kevlar... I sincerely doubt any of my rockets with parachutes* will ever reach 25 flights, since I fly so little relative to the number I build. So it is unlikely to ever be an issue for me.



*I can imagine hitting 25 flights or more on my Quinstar.

 

[Greg Furtman]

Important point. @BEC estimates about 25 flights on one piece of Kevlar... I sincerely doubt any of my rockets with parachutes* will ever reach 25 flights, since I fly so little relative to the number I build. So it is unlikely to ever be an issue for me.
*I can imagine hitting 25 flights or more on my Quinstar.

It's nice to know that someone else makes more rockets than they fly.

 

[BEC]

mportant point. @BEC estimates about 25 flights on one piece of Kevlar... I sincerely doubt any of my rockets with parachutes* will ever reach 25 flights, since I fly so little relative to the number I build. So it is unlikely to ever be an issue for me.

Streamers, too . The little Checkmate two-stager is one I've had kevlar failures on...

*I can imagine hitting 25 flights or more on my Quinstar.

My Quinstar was eviscerated by a C6-0 CATO. I need to at least set it up so that I can use D12-0s in it, and smaller boosters with the 18/24 Estes adapters.

 

[Lee]

My experience is limited, but I must say that I've had zero failures with trifold mounts or elastic shock cords - for both LPR and MPR. I'm now tinkering with braided Kevlar. My experience with braided fishing line explains the zippering, it can cut grooves in fishing rod guides, spinning reel bails and hands. When I install braided Kevlar in a trifold mount I plan to anchor it with a fiber washer inside the trifold card stock.

 

[lakeroadster]

This baffles me. Is securing the Kevlar to the rear of the MM any better than connecting it to the front centering ring (no guide tube needed)? I've used a 12" drill bit to modify old rockets and attach the Kevlar to the rear to eliminate the trifold cord attachment at the front of the BT. The hard part is threading the Kevlar through the new holes (thin wire helps).

@neil_w answered this... but I'll comment further. Being able to inspect the Kevlar for damage is the "better" approach, and being able to replace it if it is damaged is also the "better" approach. Your comment about "threading the Kevlar" is answered by installing the coffee stir stick as previously discussed.

 

[teepot]

I also don't expect any shock cord failures. My rockets usually only fly once. I build way more than I fly. At the rate I'm going if I stopped building today I would still have months worth of rockets to fly. But, to comment on the question. Regardless of the rocket size or weight I use kevlar only. I also use shock cord protectors. With 15' to 20' of shock cord I don' feel I need elastic. In the past[ 2 years ago ] I did use elastic and kevlar. On one rockets first flight the heavy elastic failed 50 feet above the ground. That was the last rocket that had elastic. If your kevlar is long enough you shouldn't need rudder or elastic. My opinion. Yours may be different.

 

[Joshua Smith]

You can use both Kevlar and elastic.

part of the trick is where and how you mount the rocket body attachment.

I am not a fan of trifold mounts, but they do have the advantage of being far forward, not directly in line of ejection blast.

lots Of people like to attach a Kevlar line to the motor mount, I like this as I hate the trifold mount as in narrow diameter rockets it can catch the chute and prevent deployment.

problem with motor mount attachments is puts it right in front of flame, and Kevlar is flame resistant but not flameproof.

@hcmbanjo came up with this
https://www.apogeerockets.com/education/downloads/Newsletter338.pdf
Which lets you easily check and replace a burned out Kevlar mount.

I am thinking that the replaceable mount, with a Kevlar rear section and elastic forward gives you best of both worlds.

I might put in some wadding, pack the Kevlar part, then maybe a couple piece of additional wadding before the elastic. Hopefully the cone, chute, and elastic are out before the Kevlar gets hit with the flame. Maybe I am overthinking.

wire fishing leader would work, but for a LPR rocket especially narrow diameter doesn’t seem to pack well.

Oh, that is clever!

 

[Joshua Smith]

Since many folks tend to not follow links... I feel compelled to show the diagram that's in the link above that @BABAR posted above.

It's easy peazy and slicker than Opossum snot. FWIW: I use coffee stir sticks as the guide tube..

View attachment 473142

View attachment 473155

This is very clever. I'm already a fan. I've been doing the heat shrink tubing thing to protect the cord nearest the ejection charge (and I usually encourage the tubing to be out of the direct way of the ejection charge by, e.g., gluing it lightly against the inner wall of the body tube or similar), but I might have to try this next

 

[neil_w]

This is very clever. I'm already a fan. I've been doing the heat shrink tubing thing to protect the cord nearest the ejection charge (and I usually encourage the tubing to be out of the direct way of the ejection charge by, e.g., gluing it lightly against the inner wall of the body tube or similar), but I might have to try this next

Have you experienced a failure of the shrink-tubing-protected Kevlar?

 

[Joshua Smith]

Have you experienced a failure of the shrink-tubing-protected Kevlar?

No, but I don't remotely have enough launches on those rockets to come to any conclusions

 

[Ronz Rocketz]

I'm curious about your use of the word "successfully" there, which implies "tried but failed". That method is exceedingly easy, not sure exactly what can go wrong. Please elaborate, if you can.

For BT60 rockets, I don't really find it necessary to do the attachment out the back of the rocket; I can do it inside the edge of the body (using the linkage I showed previously). But out the back usually is easy.


Because it's not attached, it's just looped around. Therefore it is replaceable if/when the Kevlar burns through.

Important point. @BEC estimates about 25 flights on one piece of Kevlar... I sincerely doubt any of my rockets with parachutes* will ever reach 25 flights, since I fly so little relative to the number I build. So it is unlikely to ever be an issue for me.



*I can imagine hitting 25 flights or more on my Quinstar.

Got it. Makes sense now. I figure if the Kevlar does break, I'll use my 12" drill bit and loop the MM, if it breaks.

By successfully, I mean too lazy to try.

It's moot now because I've become addicted to Qualman baffles, including the BT-50. I don't imagine the Kevlar will wear out on the front side of the baffle in the perceivable future.

 

[Joshua Smith]

Got it. Makes sense now. I figure if the Kevlar does break, I'll use my 12" drill bit and loop the MM, if it breaks.

By successfully, I mean too lazy to try.

It's moot now because I've become addicted to Qualman baffles, including the BT-50. I don't imagine the Kevlar will wear out on the front side of the baffle in the perceivable future.

I'm a big fan of all of Qualman's stuff. I have about 6 baffles built, but none of them are actually in a rocket yet

 

[Ronz Rocketz]

I'm a big fan of all of Qualman's stuff. I have about 6 baffles built, but none of them are actually in a rocket yet

The question is always how far down the tube to insert the baffle. I want a lot of forward room for the laundry but don't want it up against the MM. With the Kevlar attached to the eyelet, it's easy to reposition it.

 

[Joshua Smith]

The question is always how far down the tube to insert the baffle. I want a lot of forward room for the laundry but don't want it up against the MM. With the Kevlar attached to the eyelet, it's easy to reposition it.

Indeed. I'm in this dilemma now with an up-motored Estes bull pup. Dicey, tho I could maybe claim some of the nose cone space for laundry

 

[Ronz Rocketz]

Indeed. I'm in this dilemma now with an up-motored Estes bull pup. Dicey, tho I could maybe claim some of the nose cone space for laundry

It is a very long nose. I also made a 24mm version. No baffle though. Gonna try the JLCR in it soon.

 

[Joshua Smith]

It is a very long nose. I also made a 24mm version. No baffle though. Gonna try the JLCR in it soon.

Yeah, I feel like this is gonna scream on an E, tho we're gonna need a cement mixer in the nose for stability

 

[Joshua Smith]

My experience is limited, but I must say that I've had zero failures with trifold mounts or elastic shock cords - for both LPR and MPR. I'm now tinkering with braided Kevlar. My experience with braided fishing line explains the zippering, it can cut grooves in fishing rod guides, spinning reel bails and hands. When I install braided Kevlar in a trifold mount I plan to anchor it with a fiber washer inside the trifold card stock.
View attachment 473572

Are you going to tie the end of the Kevlar to the washer? I'm assuming the washer is to add some surface area to help anchor the Kevlar to the trifold?

 

[Joshua Smith]

Yeah, I feel like this is gonna scream on an E, tho we're gonna need a cement mixer in the nose for stability

Altho not as much cement mixer as my 24mm Mini Honest John needs

 

[lakeroadster]

On my Rocket Sled Rocket I did a tube in a tube design, and ran the chord between the 2 tubes.

This chord is not replaceable, I figured it would be protected enough that simply looping it around the front motor mount would be good enough.

The hexagonal thing is my hi-tech shipping tape anti zippering device.

More details on this rocket, and it's sled, can be found in the link, in my signature line below.

 

[Joshua Smith]

It is a very long nose. I also made a 24mm version. No baffle though. Gonna try the JLCR in it soon.

what do you consider the min spacing between the top of the motor and the baffle? I thought I had asked Dave Qualman that and I thought he said something like roughly 3 body diameters, but I couldn't find that in my email, so I guess not.

He did say if there is a good air gap between the engine and baffle - 3 to 6 inches I'm guessing - then wadding probably isn't needed. But, I go back and forth with wadding or not, depending on the parachute. Homemade thin ones, use dog barf, otherwise none. He also felt that the biggest value of the baffle is less wear and tear on the shock cord...separating it from the hot gasses and ejection particulates makes me feel better.

I also told him that I painted the "fire side" of the baffle w/a thin layer of wood glue or epoxy (depending on size/weight) and high temp paint just...because and he agreed with putting some protection on the baffles, especially on heavy fliers.

 

[Ronz Rocketz]

what do you consider the min spacing between the top of the motor and the baffle? I thought I had asked Dave Qualman that and I thought he said something like roughly 3 body diameters, but I couldn't find that in my email, so I guess not.

He did say if there is a good air gap between the engine and baffle - 3 to 6 inches I'm guessing - then wadding probably isn't needed. But, I go back and forth with wadding or not, depending on the parachute. Homemade thin ones, use dog barf, otherwise none. He also felt that the biggest value of the baffle is less wear and tear on the shock cord...separating it from the hot gasses and ejection particulates makes me feel better.

I also told him that I painted the "fire side" of the baffle w/a thin layer of wood glue or epoxy (depending on size/weight) and high temp paint just...because and he agreed with putting some protection on the baffles, especially on heavy fliers.

That sounds about right. I use a lot of TB2 in constructing the baffle plus soak the joints with thin CA. Didn’t think about protecting the fire side with more adhesive but haven’t seen damage to it yet because of the tough construction.

I agree with protecting the shock cord. I don’t imagine the Kevlar connected to the eyelet will ever give out. The eyelet will probably fail before the Kevlar does, maybe after a couple more dozen launches. It might end up in the canal before then.

I extended my Goblin clone to fit the baffle. Really flew well with the JLCR.

 

[icyclops]

OK @icyclops ... I'm calling you out. Post up some photo's of those crispy prone rockets you've designed and photo's or sketches of their internals.

I'm not from Missouri.. but I've been through there on numerous occasions.

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Ok, let me take a few pics to post for you….sorry but I just got back from a trip and didn’t check this site until recently.