First,
Nylon is fine, many love it, it just has to be protected from the e charge heat far more than Kevlar does..
As far as stretch I have never been one to accept that helps much or at all..
Neither material stretches very much, and then only when you really load them..
They're all quite strong, if you're putting enough load on either material to stretch then something is wrong with your set up and something is going to eventually fail..
Don't forget, generally speaking, these harnesses are stronger then everything else they're attached to..
If the harness isn't damaged, something else will fail before the harness does..
Personally, I have found that when you're done satisfactorily protecting Tubular Nylon from the heat of the e charge..
it can become bulky enough to be a problem fitting everything in the cargo space you have for the recovery equipment..
The one place there is a gain with Nylon is if you're knotting the material..
Here the difference in the stretch properties of the two materials makes enough of a difference to speak about, ( still not a drastic difference though )..
The reason Tubular Kevlar doesn't do well with knots is it's lack of elasticity.
If you stress Tubular Kevlar to failure and there's a knot in it it will fail every time at the knot.
As the knot wraps tightly around itself one side of the material comes to tension before the other side,
this can be difficult to explain, when the knot wraps tightly like a turn around a race track, there is a longer way around and a shorter inside track around the turn.
The fibers that take the long way around come to tension first and then fail first. If the whole piece of material isn't coming to tension at the same time
and doesn't fail at ( near ) the same time then you don't get the full capacity of the whole width of the material..
This is the reason that a knot in Tubular Kevlar will make the whole length of material fail at an average of 1/2 the fail of the material elsewhere in the length.
So by not knotting the material you can use material of an average of half the weight and heft and have the same fail capacity..
All of this speak for me is neither here nor there.
If you're putting this kind of tension on your harness then you have a problem with your set up..
The answer is not a stronger harness as something else will fail.
The answer is to figure out why you're stressing your harness this way and correct the problem..
Kevlar weight ratings-
Tom said it,
I've seen Tubular Kevlar from different manufacturers that are similar in size and far off each other in weight ratings..
For that reason I don't put as much stock in the # a manufacturer specs as much as the heft of the material you can see and feel..
( Caveat -- the type of weave used in manufacturer does make a difference in overall strength )..
Any attachment point is built into the rocket, it must be.
It is not generally serviceable or replaceable ( unless you have the room on the top centering ring for links of course )..
It should not reach out of the booster…
Something is going to wear against the sharp edge of
the booster,, that can be the built into the rocket “Y” attachment point
that is very difficult to access/service,,
or what wears can be the very easy to access/service booster harness...
This is why I keep these attachments short enough so they
don’t reach out of the booster...
This is a 4" airframe with a 75mm motor mount..
First drill the centering ring 180 degrees apart on either side..
Then mount the long shank eyes..
Then grind the inside of the washers so the ring will go back on the motor tube..
Then epoxy the ring in place..
Then grind the outside of the washers so the motor tube assembly will go back into the airframe tube..
Then continue your build as you would ..
I also bury this in epoxy with a syringe taped to a dowel after the motor mount is in the airframe..
Teddy