Let the Debate Begin

Two questions here:

1) Having read pages and pages here on knots, is there any definite reason why I can't just use a good 'ole (Improved) Clinch Knot on my Sumo build?

I've replaced the standard elastic with 20' of 1/8" TK. I've used it in fishing for years, and have never lost anything with it.

2) Also, how to you "finish" off the TK so that it doesn't unravel?

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Originally posted by akpilot
Two questions here:

1) Having read pages and pages here on knots, is there any definite reason why I can't just use a good 'ole (Improved) Clinch Knot on my Sumo build?

I've replaced the standard elastic with 20' of 1/8" TK. I've used it in fishing for years, and have never lost anything with it.

2) Also, how to you "finish" off the TK so that it doesn't unravel?

Click to expand...

1. In reality, no. It will reduce the strength of the cord/strap/line/whatever but I'm betting that with that rocket and the 1/8" TK you're still good to go.
2. Only two ways I've finished of the ends of TK are an overhand knot very close to the end and let it fray to there; not the prettiest but works very well.
The other is to smear a bit of epoxy on about the end 1/4 - 1/2 inch and form into a nice finished end and let dry.

Greg

Originally posted by Rock_It
Here is the proper way to secure TK. Anytime you use a knot you basically cut the strength in half.

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You're basing that claim on what, exactly?

A good knot doesn't cut the strength of the line in half, but a bad knot does.

If I remember the numbers correctly, some of the climbing knots retain 80% - 85% of the original strength of the line.

-Kevin

I use the same knot PML recommends for their tubular nylon... It reminds me of a knot I've used for fishing line before, with good results.

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However, with many of my rockets, the shock cord (kevlar/beefy nylon) even at 85% strength is far from the weakest link. So the method of attachment is kind of irrelevant.

Oh - and I use sewn loops and quicklinks/u-bolts.

Even at 50%, it's still probably not the weakest link in the recovery system of most rockets.

Robert Dehates "chinese finger trap" is the only "knot" using kevlar not to reduce the strength of the joint. For small/mid stuff likely not an issue but as I built my L3 and looked into the issue, it was clearly superior and has dozens of testimonials to support its use. Luckily my GF sews and sewing a loop into all harness materials was just simpler, but on the field or thoise w/o a sewing machine, no other way to join TK IMHO. Streses always pile up at sharp curvatures, why do it at all if it can be avoided.
JS

I gotta ask this one. Why does anyone really cae if TK with 1600# breaking strength loses even half of its strength

If the shock cord is long enough, I just don't see anywhere near that amount of force applied to the cord. during the ejection process.

I'd be interested in hearing of anyone whas ever seen TK fail due to overload and if it failed at the knot. TK weakened by the solvents in epoxy, CA, or by ejection gas do not count.


Oh yes I normally have sewn loops and use quicklinks. However in a pinch I use the PML knot at the ends and a butterfly knot for the parachute. To secure the ends I just fuse it with an open flame.

BTW - Robert Dehate does not use a knot. The technical term is an eye splice.

A

See, I told you about the debate . . .

Now, getting back to the topic at hand, for a Level 1 attempt, will the (Improved) Clinch Knot be feasible for a Sumo using 20' of 1/8" TK?

From what I gather, the clinch knot's strength should still be, mostly, intact as it still lies along the main cord and is secured by tension (e.g. little if any weakening).

Originally posted by Hospital_Rocket
I gotta ask this one. Why does anyone really cae if TK with 1600# breaking strength loses even half of its strength

If the shock cord is long enough, I just don't see anywhere near that amount of force applied to the cord. during the ejection process.

I'd be interested in hearing of anyone whas ever seen TK fail due to overload and if it failed at the knot. TK weakened by the solvents in epoxy, CA, or by ejection gas do not count.


Oh yes I normally have sewn loops and use quicklinks. However in a pinch I use the PML knot at the ends and a butterfly knot for the parachute. To secure the ends I just fuse it with an open flame.
Robert Dehate does not use a knot. The technical term is an eye splice.

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The flip side is why knowingly compromise the strength of a material you paid high dollars for? For an optimal vertical flight when apo deploys at apo and the mains are not asked to deploy at high velocities, you could probably use a 100 feet of string for the drogue and some decent shroud line for the main shock line.
But rockets and their motors don't always behave and its one way to try to keep them out of the sick bay.

The other reason I see is the cargo volume dept. One doesn't always have room for 5 AF lengths of shock material, esp in min dia projects. So high, albeit brief, stresses may be present. Again if its avoidable why mess around?

I have seen plenty of separations--what causes them? Seems to me the strongest and simplest recovery chain used eye bolts/nuts from motor to NC with no transfer to bulkheads, and no knots.


BTW, Thats why the term knot was in quotes in my post.
JS

Originally posted by akpilot
See, I told you about the debate . . .

Now, getting back to the topic at hand, for a Level 1 attempt, will the (Improved) Clinch Knot be feasible for a Sumo using 20' of 1/8" TK?

From what I gather, the clinch knot's strength should still be, mostly, intact as it still lies along the main cord and is secured by tension (e.g. little if any weakening).

Click to expand...

You'll be fine using that setup.