Kevlar Shock Cord Anchoring

One of the first Rocket kits I built was the Semroc Vega. Semroc includes a length of kevlar thread, and instructs the the builder to tie an overhand knot, and anchor the kevlar under the front of the engine hook, right where it goes into the motor mount tube. This seems like such a simple and effective method that I started copying it on all my low power builds. Other places I've checked, though usually recommend tying the kevlar around the motor tube, anchoring it to a centering ring:

[video=youtube;IrrcNk4mmLU]https://www.youtube.com/watch?v=IrrcNk4mmLU[/video]

Any problems with the Semroc method, or should I switch to knotting around the tube?

Hopefully the motor isnt going anywhere. Thats a good bet!

gna said:

One of the first Rocket kits I built was the Semroc Vega. Semroc includes a length of kevlar thread, and instructs the the builder to tie an overhand knot, and anchor the kevlar under the front of the engine hook, right where it goes into the motor mount tube. This seems like such a simple and effective method that I started copying it on all my low power builds. Other places I've checked, though usually recommend tying the kevlar around the motor tube, anchoring it to a centering ring:

[video=youtube;IrrcNk4mmLU]https://www.youtube.com/watch?v=IrrcNk4mmLU[/video]

Any problems with the Semroc method, or should I switch to knotting around the tube?

Click to expand...

Gna & Jeff:
Not really a problem initially but over time the edges of the metal motor hook can damage the Kevlar threads eventually resulting in shock-cord seperation.
tying around the motor tube then sending the Kevlar out to the outer edge of the forward centering ring move the Kevlar out to the coolest area. There is always the fact the Kevlar is NOT flameproof. Kevlar is Heat resistant but degrades a bit with each flight where the line is exposed to temperatures at and above 900degs f ( our BP motor ejection charges are WAY hotter then that. So over time our Kevlar shock-cord mounts will break at or very near the forward centering ring.

What I'm now doing is extending that time by using either .024" or .038" 7x19 strand Stainless steel aircraft cable with crimp sleeves from around the motor mount , out to the outer centering ring/body tube joint with a small eye crimped about 5"-6" above the forward centering ring. I then attach my 70-130lb braided Kevlar to the stainless cable eye with 3 half hitch knots. By moving the Kevlar out of the Highest Heat region inside the body tube I've all but eliminated the degrading of my Kevlar shock-cords. Since I've started using this Stainless Steel cable Shock-cord anchor system I've not had a Kevlar shock-cord failure. Been using this system now about 15years on everything from Micro models using 49strand .018" stainless beading wire to .0469" 1x19 strand Stainless Aircraft cable on Mid-power birds.

Another easy to get Stainless steel anchor already made-up with spliced eyes are 30lb x 12" stainless fishing leaders. I've used these in many LPR models that have longer then 12" body tubes. By the time the leader is wrapped around the motor tube and extended forward of the centering ring the length is about 8-9in giving plenty of room for tying on the Kevlar well inside the model body. Below are a couple photos of these cable options and one or two attachment options
Hope this info helps.

I came up with a replaceable Kevlar line attachment.
It is written up in an Apogee article:

https://www.apogeerockets.com/education/downloads/Newsletter338.pdf



The Kevlar is looped and tied around the rear ring on the engine mount
With tweezers you can pull the line out of the back after a few flights and check it's condition.
The line is run through a small tube along the side of the engine mount tube.
I use this method on most all of my BT-55 and larger models and have had
great success with it over the past two years.

For smaller BT-20 and BT-50 sized models I did a follow up article:

https://www.apogeerockets.com/education/downloads/Newsletter343.pdf

hcmbanjo said:

I came up with a replaceable Kevlar line attachment.
It is written up in an Apogee article:

https://www.apogeerockets.com/education/downloads/Newsletter338.pdf

View attachment 274221

The Kevlar is looped and tied around the rear ring on the engine mount
With tweezers you can pull the line out of the back after a few flights and check it's condition.
The line is run through a small tube along the side of the engine mount tube.
I use this method on most all of my BT-55 and larger models and have had
great success with it over the past two years.

For smaller BT-20 and BT-50 sized models I did a follow up article:

https://www.apogeerockets.com/education/downloads/Newsletter343.pdf

Click to expand...

Very nice write ups! I will use these on some future builds.

hcmbanjo said:

I came up with a replaceable Kevlar line attachment.
It is written up in an Apogee article:

https://www.apogeerockets.com/education/downloads/Newsletter338.pdf

View attachment 274221

The Kevlar is looped and tied around the rear ring on the engine mount
With tweezers you can pull the line out of the back after a few flights and check it's condition.
The line is run through a small tube along the side of the engine mount tube.
I use this method on most all of my BT-55 and larger models and have had
great success with it over the past two years.

For smaller BT-20 and BT-50 sized models I did a follow up article:

https://www.apogeerockets.com/education/downloads/Newsletter343.pdf

Click to expand...

While an interesting anchor system your slip tube rear ring anchor system does nothing to address the Kevlar Heat degradation which occurs with each flight. Further placing the shock-cord against the Motor tube puts the Kevlar in the worst possible position with regard to heat. A better position for your small tube would be at the outer centering ring edge inside and against the inner surface of the Air-frame body tube.

hcmbanjo said:

I came up with a replaceable Kevlar line attachment.
It is written up in an Apogee article:

Click to expand...

Smart, very smart.

Thanks for the replies. Very clever ideas.

Micromeister said:

While an interesting anchor system your slip tube rear ring anchor system does nothing to address the Kevlar Heat degradation which occurs with each flight. Further placing the shock-cord against the Motor tube puts the Kevlar in the worst possible position with regard to heat. A better position for your small tube would be at the outer centering ring edge inside and against the inner surface of the Air-frame body tube.

Click to expand...

The rear ring tie loop does address when Kevlar breaks down. It allows you to pull it out the back, inspect and replace it if needed.
I purposely keep the Kevlar to the side of the engine mount tube. Running the tube to the outside makes the tied loop too large and loose.
I don't want it slipping down and off during boost.

Putting the Kevlar only 1/4" to the side of a centered ejection charge won't make that big a difference in the life of the Kevlar.
At 1/4 away the ejection charge heat isn't that much different.
I know, you'll argue that point.

The whole purpose to this style of mount is I check can the removable Kevlar line every four or five flights and replace it if needed.
You do your mounts using crimped wire under the upper centering ring.
I have a lot of Kevlar and simply prefer this method.

I think that min dia version slip knot around the notched engine block technique might also work if you are anchoring to an eye hook on an ejection baffle. You would need some kind of long thin tweezer clamps and a hook to thread it in there depending on how deep it is from the end of the tube opening, but then you could also hook it to pull loose and replace after time. Cool ideas, thanks for sharing.