Kevlar braid, Dacron, quick connectors, and knots

I have been searching and reading on the forum about the lines and connectors that are used in the rockets. Lots of new info since I dropped out a while back!

I have been using 135# braided Dacron and connecting it with the same quick links that I use on my large control line airplanes, along with some large swivels from BassPro. I make a loop with an overhand knot and trim the tag end of the line close to the knot. I use Lark's Head hitches on some of the connections. The loops and quick links make changing things pretty easy and I would like to keep it that way. The Big Daddy is set up with about 5' of Dacron from the rocket to the swivel, about 1' to the nose cone. the chute is connected with about 3' of 1/4" black elastic. The line coming out of the rocket is covered with some industrial heat shrink tubing and is epoxied to the motor mount.






The Dacron is a bit singed in some spots. Not bad, however, this is with only 2 flights on the Big Daddy. After reading about the Kevlar properties, I ordered some.
The 100# braided Kevlar I ordered from the Kevlar Store arrived today and I was thinking about replacing the lines on the Big Daddy and the few other rockets I have.




I'm thinking about using the 100# Kevlar in a similar fashion as the 135# Dacron.
Would a simple overhand knot covered with heat shrink tubing to capture the knot work for the 100# Kevlar?






Anyone have any thoughts, suggestions, improvements on this setup?

Aero Antics said:

I'm thinking about using the 100# Kevlar in a similar fashion as the 135# Dacron.
Would a simple overhand knot covered with heat shrink tubing to capture the knot work for the 100# Kevlar?

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For a low power rocket? Sure.

Most knots create weak points in the line. That said, let's say your knot takes away 60% of the strength of the line, leaving it at 40lbs. For a Big Daddy, 40lbs is plenty.

If you move to midpower and up, you'll need to learn different knots. The figure-8 knot is a good one, that's relatively easy to tie, and good information is available online, on how to tie it.

-Kevin

+1 on the figure-8 knot.

I tend to prefer variants of the bowline knot for loops. https://www.animatedknots.com/photos/bowline/bowlineR7.jpg

I do not consider elastic to have any place in MPR or HPR for chute attachment or pretty much any other purpose. I use a chain stitch https://0.tqn.com/d/crochet/1/0/S/-/-/-/chainstitches.jpg if I want some shock absorption in a section of line. It is also useful in that it makes the length of the line 1/3 the original length, making packing easier. Another way is to gather a few folds of line and tape together with masking tape. Under load, the tape breaks.

Elastic is not the sort of material to depend on. Assume chutes will be deployed at the max speed the rocket is capable of achieving. The load can be quite high.

Gerald

Thanks for the answers!
I have never used braided Kevlar, or any kind of Kevlar line before. Worst thing I have had to deal with up to now was Dyneema. Proper knots were a pain with that stuff and I had to tie them wet to get any strength. Just tying the knot dry created enough friction heat to weaken it.

I did some more research the suggested Kevlar knots. I thought this was an interesting instruction set from Amphenol Aerospace where they replace steel cable with Kevlar and using bowline knots.
https://www.amphenol-aerospace.com/servicedrawings/L-40451-260.pdf
I will order some larger Kevlar as the rockets go up in size, too.....

Another thought on the elastic shock cord, I remember years ago seeing a shock cord set up with a section of elastic that was about 2' long tied into two loops about 3' apart on the main shock cord. I assumed it was in case the elastic broke.

Aero Antics said:

I have never used braided Kevlar, or any kind of Kevlar line before. Worst thing I have had to deal with up to now was Dyneema. Proper knots were a pain with that stuff and I had to tie them wet to get any strength. Just tying the knot dry created enough friction heat to weaken it.

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I'd have concerns about using anything that heat-sensitive in rocketry.

-Kevin

troj said:

I'd have concerns about using anything that heat-sensitive in rocketry.

-Kevin

Click to expand...

Dyneema and Spectra have the same properties. I use it for lines on my small control line models and for bridles on stunt kites.

Won't get it near a rocket!

Using elastic in that fashion creates a spring. It gets stretched, then pulls the masses back towards each other. Absorbing some energy on the outward travel is a good thing, but returning it by accelerating the masses back towards each other is a bad thing. It can contribute to smashing parts together. It is better to use a technique which absorbs some energy on the way out but is incapable of returning any energy.

Folded sections of line and masking tape, or knitting the line as a series of knots (https://0.tqn.com/d/crochet/1/0/S/-/-/-/chainstitches.jpg - chain stitch) which pull out as one pulls on the line. This absorbs some energy as well, and shortens the line to 1/3 the original length. Stowing is easier.

Gerald

G_T said:

Folded sections of line and masking tape, or knitting the line as a series of knots (https://0.tqn.com/d/crochet/1/0/S/-/-/-/chainstitches.jpg - chain stitch) which pull out as one pulls on the line. This absorbs some energy as well, and shortens the line to 1/3 the original length. Stowing is easier.

Gerald

Click to expand...

Thanks Gerald
I tried the chain stitching with a weight tied to one end. I see how it works. Sometimes it takes me a minute.....

I would recommend against the chain stitching, except for storage purposes -- as the loops pull out, it generates a tremendous amount of friction, which generates heat, and damages your harness.

A lot of the work in designing professional parachute rigs is in figuring out how to prevent friction, and the associated damage. I have a 28 foot parachute in the garage which is trash due to friction damage -- you can easily put your finger right through the material.

Instead, accordian the line, then using masking tape to hold sections together. As the tape tears, it uses up some of the energy.

-Kevin

troj said:

Instead, accordian the line, then using masking tape to hold sections together. As the tape tears, it uses up some of the energy.

-Kevin

Click to expand...

I will try that too, Kevin.

Everyone, I really appreciate the help. I am looking forward to going to some launches next year to get the benefit of seeing how others do everything.
Looks like there are two clubs within 80 or so miles of the Louisville area. Seems odd that there are no clubs actually in Louisville with it being a large city.

Thanks again!