Had a very similar thought. The commercial product is this:
http://yatesgear.com/climbing/screamer/
Pick a thread with a break strength similar to the force you want imposed, and
then put in as many loops as necessary to absorb the total energy you want
absorbed.
Depending on the energy you want to absorb, can be a pain to sew abd tie off
all those loops.
-------- Original message --------
From: Keith and Mary Stormo <kstormo@xxxxxxxxxx>
Date: 01/31/2016 11:54 PM (GMT-07:00)
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: HPR recovery help...
I will chime in with a technique that has worked well for me in the past.
Weigh the nosecone and main chute and then sew
some loops in the webbing with a calibrated shear
thread. If the main and nose cone section weighs
10 lbs, I would use a thread that breaks at about 10 pounds. Then I would sew:
A. two loops with 4 passes of the thread (40 lb
breaking strength) holding the loop together
B. two loops with 3 passes of thread
C. two loops with 2 passes of thread
D. four loops with 1 pass of thread
When I was done my 30 foot of webbing would be
about 5 feet long with all the rest of the webbing sewn into the 10 loops.
This method would usually break all the single
passes and all of the double passes but not all
of the triple passes and usually none of the four
passes. It is somewhat similar to the sewn fall
arresters in human harnesses that keep us from
being dismembered when we fall and the harness
stops us from falling. Depending on how far you
fall the harness threads will unzip different lengths.
The nose cone/main chute section sees 4 jerks at
1 g, 2 jerks at 2 g, and so on each time
releasing the the webbing from the loop that just
broke and lengthening the webbing until the next higher strength loop breaks.
For smaller rockets I do the same thing but
instead of thread I use masking tape and tape a
number of loops with 2 layers of tape, then some
with 3 layers, some 4 layers.
I used these when I was testing long delays and
trying to just save a lawn dart! The breaking in
sequence would also nearly always re-orient the
webbing and body tube so there was no zipper in the body tube.
Back to lurking,
Keith
At 09:54 PM 1/31/2016, Paul Breed wrote:
I've been using my L3 box stock dual deploy HPR
rocket to fly GPS experiments...
I have several issues and was looking for advice from the group...
The rocket has flown probably 25 times and the
nose cone and  dual dploy electronics and
recovery bay probably 35 or 40 times.
Until the last three flights this has been 95% flawless.
I've added a bunch of weight to the nose cone in
the form of GPS recorders batteries etc...
So the nosecone and main chute has been deploying at apogee....
For the last flight I doubled the number of
shear pins (now 4X 4-40 nylon shear pins)
and it still deployed when the drogue deployed at apogee...
2nd problem is that the webing and heat shield
covers for the deployment are getting a little bit tired looking......
I was going to replace all the webbing with
brand new kevlar, but I think this will probably
 make the apogee deploy worse as the kevlarÂ
has very little give in it and will bring the
main and noscone to an abrupt stop probably
shearing the pins and deploying the main at 18K ft AGAIN....
So my specific questions are :
So what is the cleanest way to incorporate some
shock absorption in the system?
The kevlar straps are about 25ft long  I was
thinking that I would fold a few feet of them acordian styleÂ
and then put some heat shrink over the folded
sections this will provide some shock absorbtion asÂ
the kevlar pulls out of the heat shrink and extends....
i could just add a parallel shock cord with the kevlar?
Thoughts and ideas?
I want something that is clean and just works
with minimal hassel for the next 15 to 20 flights.
Paul
