Attach harness to forward closure: why "knot?"

[Buckeye]

Why not simply tie kevlar to the motor forward closure as the recovery anchor point? My application is a 54mm min diameter (Wildman Mach 2). 1/8" Kevlar (750 lb, I think) should be plenty. Here are three knots I am toying with. Shown is an Extended Forward Closure for clarity. Photo on left is a limp cord, and the photo on right is with tension on the cord.



Honestly, I don't think any of these will come loose. Anybody try this?

 

[tfish]

I do that. I put electrical tape over the knot and around the FC to help hold everything place. I also use 'shock cord protection' for the first foot or so. (which can also be electrical or duct tape)

Tony

 

[Buckeye]

I do that. I put electrical tape over the knot and around the FC to help hold everything place. I also use 'shock cord protection' for the first foot or so. (which can also be electrical or duct tape)

Tony

Cool. I figured this would be a tfish or Crazy Jim thing to do! Tapped closures, hardpoint anchors, etc. seem so unnecessary.

 

[tfish]

Cool. I figured this would be a tfish or Crazy Jim thing to do! Tapped closures, hardpoint anchors, etc. seem so unnecessary.

They sort of are..unless your also using that "threaded point" for motor retention, bulk plate retention, rail button anchor point, ebay and sled attachment, or shock cord anchor. All those are hard to do with knots and tape..you might need some bailing wire too!

Tony

 

[cautery]

Why not simply tie kevlar to the motor forward closure as the recovery anchor point? My application is a 54mm min diameter (Wildman Mach 2). 1/8" Kevlar (750 lb, I think) should be plenty. Here are three knots I am toying with. Shown is an Extended Forward Closure for clarity. Photo on left is a limp cord, and the photo on right is with tension on the cord.

View attachment 417851

Honestly, I don't think any of these will come loose. Anybody try this?

Nice idea. But intuitively, it seems to me that knot choice and execution are key to function/durability.
Kevlar does NOT like to see sharp bites or bends. Over-hand knots are really hard on it, for sure. Ask a rigger.

That constrictor knot has a lot of promise. Is this for a "Y-harness" loop to the 3-loop harness?
I would consider taping the two running ends together for about 1-2" directly above the knot when pulled reasonably tight.... for obvious reasons.

Note: I am NEW to HPR, and my opinions are based on life experience in other related (and unrelated) fields.... so perhaps, "What do I know?"

 

[manixFan]

Nice idea. But intuitively, it seems to me that knot choice and execution are key to function/durability.
Kevlar does NOT like to see sharp bites or bends. Over-hand knots are really hard on it, for sure. Ask a rigger.

That constrictor knot has a lot of promise. Is this for a "Y-harness" loop to the 3-loop harness?
I would consider taping the two running ends together for about 1-2" directly above the knot when pulled reasonably tight.... for obvious reasons.

Note: I am NEW to HPR, and my opinions are based on life experience in other related (and unrelated) fields.... so perhaps, "What do I know?"

I’ve spent way too much time learning what kind of knots work best with Kevlar, and the answer is… none of them. A splice or ‘finger trap’ is the best way to form a loop. But that constrictor knot is interesting. Kevlar also does not play well with any kind of sharp transition, and running past an edge is as bad as an overhand knot. If you have enough margin you can get away with it, but it should be replaced often. In some of my setups I replace it every flight.

Actually, in many ways Kevlar is really not a very good choice for our usage, but as long as we have huge margins, like 2000 lb stuff in a 10 lb rocket, we’ll be fine.

Most of the time.

Tony

 

[Buckeye]

Yep, I am using the constrictor knot in twist form as shown above. Very hard to untie, but it is one-time use anyway. Yes, we probably have good margins.

Constrictor Knot (Twisting Method) | How to tie a Constrictor Knot (Twisting Method) using Step-by-Step Animations | Animated Knots by Grog

I have not experienced these kevlar issues you guys describe. I use the soft, supple, tubular kevlar. Some vendors sell an alleged "tubular" kevlar that is flat, stiff, and twists like Jamaican cornrows. I avoid that stuff.

 

[manixFan]

in the mid-2000's, I bought thousands of feet of surplus mil-spec Kevlar cord of various weights, from 200lb to 2000lb. It was part of a club purchase but I ended up not reselling nearly as much of it as I thought I would. It came with spec sheets that listed the test results to satisfy the military specifications and that started me on my journey of learning about aramid fiber based rope. I found several industry and military studies into the use of Kevlar, and came away with three main points: it does not like knots, it does not like sharp bends - like running over an edge, and once 'shock loaded', it can then fail without warning.

One of the biggest issues with Kevlar initially was 'self friction', or heating and wear from the fibers rubbing against each other or against an edge. So the fibers were coated with a substance to make them very slippery, which is one reason knots in Kevlar can come undone. The other issue is the huge loss in strength when the fibers are bent, like in a knot. The initial terminations for Kevlar were either potted in a special end fitting or simply sewn to form a loop. No knots at all. The fingertrap done correctly, is not really a knot either, and maintains a significant portion of the original strength. I've posted links in several other threads that show that knots like a bowline can lose up to 70% of the original strength of the cord.

The flat, stiff form is much better for sewing loops, but that's a different story altogether.

One of these days I should summarize all those test results so folks have a better understanding of how kevlar should be used.


Tony

 

[kenstarr]

I used to be into caving years ago and I purchased the book "On Rope" super cool stuff about knots and how some knots weaken rope more than others...
Ken