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Astron Sprint - Replaceable kevlar shock cord?

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David_Stack

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Afternoon All;

Amassing parts so that I can clone an Astron Sprint. Would like to use kevlar for the shock cord (at least up to the top of the body tube but not above to minimize the 'zipper risk', but I've been racking my brains without success to arrive at a solution that will allow me to install the shock cord in a way that it would be replaceable.

I've seen the newsletter article on Apogee which describes notching the 20-50 centering rings to allow the kevlar to pass through from bottom to top, but the Sprint's boattail means there is no way to access the lower end of the kevlar (which normally would be looped around the bottom of the MMT below the lower CR).

Thought about putting two notches in the upper CR spaced some distance apart, with teflon tubing on the backside that would serve as a guide, and creating an endless loop with the kevlar. If the kevlar ever wore out I could cut the loop and use it as a pull string to feed a new length of kevlar through, but I am doubtful I could affix the new to the old securely enough (yet retain a small diameter and be flexible) to not have it come apart as it makes the 180 degree turn down at the upper CR.

Soliciting suggestions from this esteemed group...

Thanks,
Dave
 

David_Stack

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Assuming you are referring to the video on the Apogee home page, in that instance the shock cord is anchored to a screw eye which is mounted to the plywood centering ring.

In the Sprint, you are talking about a BT-20 engine tube mounted in a BT-50 using wrapped paper 20-50 rings (similar to a thrust ring, just mounted externally). Not much free space between that MMT and the body tube. Perhaps I could find a very small screw eye and thread it into the 20-50 centering ring, 'indexed' so it is parallel to the body tube inner wall...

I'll have to see if there is room once the parts arrive and I can mock it up.

Thank you for the suggestion.

r/
Dave
 

GlenP

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Clarification: How-To-Video #314, not newsletter #314
which deals with an metal eye hook in a plywood centering ring, which I agree is not really appropriate construction methods for this LPR forum.
Screen Shot 2020-09-23 at 1.59.51 PM.png


Newsletter #338 shows the method by @hcmbanjo
Screen Shot 2020-09-23 at 1.48.29 PM.png




You can use the LPR method with a straw that extends to the bottom of the engine mount tube, it will have to go through an elliptical hole in the boat tail. Then, just allow for about 1/8" of that engine tube to extend just past the boat tail, rather than have it flush. So, the Kevlar loop will show on the outside of the engine mount tube on that 1/8" extension just below the boat tail. Alternately, it can just wrap around the engine.
Screen Shot 2020-09-23 at 1.52.22 PM.png
 

GlenP

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another option, is to make two little triangular "fins" that match the external profile of the boat tail, run them adjacent to each other to allow the Kevlar to pass through, but get held in place by a good sized knot. The knot could fit in between these fins, but get stopped by the lower centering ring, might need some reinforcement or CA to stiffen it. Then when you make the boat tail, slot it in between those two formers. You will have kind of a conformal slot to access the straw, but instead of wrapping the Kevlar loop around the engine mount, it is held in place by a knot that can hide in that slot. you could even tape over it, or put a card stock "hatch" over it, that you can cut and remove later to replace the Kevlar.
 

rklapp

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Clarification: How-To-Video #314, not newsletter #314
which deals with an metal eye hook in a plywood centering ring, which I agree is not really appropriate construction methods for this LPR forum.
View attachment 432673

Newsletter #338 shows the method by @hcmbanjo
View attachment 432674



You can use the LPR method with a straw that extends to the bottom of the engine mount tube, it will have to go through an elliptical hole in the boat tail. Then, just allow for about 1/8" of that engine tube to extend just past the boat tail, rather than have it flush. So, the Kevlar loop will show on the outside of the engine mount tube on that 1/8" extension just below the boat tail. Alternately, it can just wrap around the engine.
View attachment 432675
Someone suggested this for converting a side BT mount to a MM Kevlar connection. Last week, I apparently successfully drilled a 1/8" hole through the outside center ring and punched through the inside centering ring. I then passed the Kevlar through the holes and attached to the outside of the MM. No more paper connector to the inside of the BT. Now I'll try it with my Nike Smoke.
 

GlenP

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Someone suggested this for converting a side BT mount to a MM Kevlar connection. Last week, I apparently successfully drilled a 1/8" hole through the outside center ring and punched through the inside centering ring. I then passed the Kevlar through the holes and attached to the outside of the MM. No more paper connector to the inside of the BT. Now I'll try it with my Nike Smoke.
Does the rocket you performed this on have a boat tail?
 

David_Stack

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Good Morning All;

As I read through the thoughts and suggestions, it dawned on me this morning that I am attempting to create a solution to a problem that likely won't exist... I've read the horror stories about aging Estes rubber shock cords, and that is the point of failure I am trying to head off through the user of a Kevlar shock cord (hoping to make that replaceable as well).

As I thought about it, I suspect the kevlar cords end up damaged primarily due to exposure to the ejection charge, so attaching it down at the MMT so it can be replaceable actually exposes it to greater 'wear' than would be the case if it were secured toward the top of the body tube. The issue with the latter is that the traditional tri-fold or equivalent mount does not allow for easy replacement.

What I now envision is a tri-fold mount with a short 'pig tail' of kevlar, terminated in a loop just below the top edge of the body tube. To that loop I will secure a length of elastic shock cord. I can replace the elastic as needed, and the kevlar in the mount will likely outlast the rocket being a fair distance from the initial ejection charge...

Again, thank you all for the contributions and 'virtual conversation'.

r/
Dave
 

GlenP

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If you are not that concerned with adding weight, you could also consider adding a baffle and anchoring the Kevlar leader line to that.

I like to use a very long Kevlar line with elastic shock cord at the end that attaches to the nose. Where the Kevlar exits the end of the tube put a small “tea bag” of paper folded on the line, or a short section of a launch lug, to help prevent zippering at the tube opening.
 

David_Stack

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Thanks for the suggestion of a baffle Glen.

One of the cleverest anti-zipper protective measures I saw while exploring the archives (can't recall if it was here or in YORF); someone used a foam earplug through which then reamed an awl on the long axis, then slid a length of small diameter tubing over the awl and through the earplug. The tubing was the conduit through which the kevlar line was run.
 

tlainhart

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What I now envision is a tri-fold mount with a short 'pig tail' of kevlar, terminated in a loop just below the top edge of the body tube. To that loop I will secure a length of elastic shock cord.
Dave
That's what I do for LPR. I've had kevlar mounts on the fore end of the MMT burn through.

I learned this from some LOC kits built some years ago.
 

BABAR

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You could use a wire fishing leader for the first 6 inches forward of the motor mount.
 

mikewrt

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Good Morning All;
As I read through the thoughts and suggestions, it dawned on me this morning that I am attempting to create a solution to a problem that likely won't exist... I've read the horror stories about aging Estes rubber shock cords, and that is the point of failure I am trying to head off through the user of a Kevlar shock cord (hoping to make that replaceable as well).

As I thought about it, I suspect the kevlar cords end up damaged primarily due to exposure to the ejection charge, so attaching it down at the MMT so it can be replaceable actually exposes it to greater 'wear' than would be the case if it were secured toward the top of the body tube. The issue with the latter is that the traditional tri-fold or equivalent mount does not allow for easy replacement.

What I now envision is a tri-fold mount with a short 'pig tail' of kevlar, terminated in a loop just below the top edge of the body tube. To that loop I will secure a length of elastic shock cord. I can replace the elastic as needed, and the kevlar in the mount will likely outlast the rocket being a fair distance from the initial ejection charge...

Again, thank you all for the contributions and 'virtual conversation'.

r/
Dave
That's what I do for LPR. I've had kevlar mounts on the fore end of the MMT burn through.
I learned this from some LOC kits built some years ago.
I do this too.
Here is a kevlar leader on my Big Bertha rebuild to replace an estes dry rotted rubber shock cord.
Full thread here: https://www.rocketryforum.com/threads/big-bertha-memories.160213/post-2015901

IMG_20200628_161213.jpg
 

rklapp

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I was able to wrap the Kevlar around the outside of the MM. I'm doing this for the rockets after the MM was installed and I want to get rid of the glued shock cord connector.

A1D81B99-C310-4408-89C9-4382537E3ACB.jpeg
 
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rocketguy101

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This is what I'm doing to my Sprint clone...the idea came from my Semroc Lil' Huster...the nose is a lot heavier and the rocket flies on "F" motors and has worked fine...

Edit: and I put a piece of masking tape around the Kevlar to prevent damaging the edge of the body tube...this has worked for years on many different size LPR & MPR rockets
 

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