Nylon and even less so Kevlar have limited shock absorbing ability, the long shock cords allow the ejection forces to dissipate before the shock cord reaches the maximum extension (which puts shock loads on the mount points if it reachs max extension). All my 1/4" Kevlar shock cords are 25' long, and I can Z-fold them with masking tape wraps every 5 folds. As the shock cord extends the tape breaks creating a decelerating action, the length really takes up very little space and is just insurance. I also size my ejection charges to extend the nose cone/payload bays apart by about 3/4 the length of the shock cord when ground testing.
+1...
Everything. ...
I also size my charges to about 3/4 the length of the harness. ...
Remember, a harness should not be taking a shock load..
If it is there's something fundamentally wrong with your set up..
Usually too energetic of an ejection charge,,
Or too short of a harness....
The harnesses long length is to allow the energy of separation to dictate before reaching full extension and transferring the shock to the attachment points....
Teddy
longer shock cords usually puts one section(typically the fin can) closer to the ground in the event of a tree landing .
Rex
longer shock cords usually puts one section(typically the fin can) closer to the ground in the event of a tree landing .
Rex
Good comments in this thread. I'll just add my two cents.
Until we change the terminology there is going to be misunderstanding on just what the part does that connects all the pieces of the rocket together as it is recovered.
In the early days of model rocketry it was indeed a shock cord and used as such. I have had many experiences when a 10 or 12 inch rubber band shock cord has been stretched to nearly it's limit and then brought the pieces flying back together to snap off a fin, crush the body tube, or at least scrape off paint to show where the nose cone and rocket have collided. The first improvement was to go to elastic which made the cord stronger but did not prevent the above from happening. Sometimes, it seemed to me, it would be better if the cord broke. It would cause less damage. When shock cords absorb shock they redirect it. Watch Wile E. Coyote and his giant slingshot. It never works out well for him.
We should be using recovery harnesses not shock cords. If you think of them differently you will use them differently. Use strong material. Make them longer so they do not have to somehow dissipate the shock of the ejection charge but just hold the weight of the parts and the shock of the chute opening for a safe recovery.
I like my harnesses just long enough that one or two people will announce, "separation" before everything reaches it's limit. Slow and graceful.
I always say,,
If your harness is taking a shock load something is fundamentally wrong with your set up...
Usually too short of a harness or too energetic of an e charge....
Teddy
I totally agree!!! I think most times it's too energetic of an e charge... especially with the apogee charge on DD.
Another way is with an elastic bungie cord across a non-elastic recovery harness. Let's see if a picture is still in my stuff...View attachment 273893
I agree that what you show is an alternate method of reducing the shock on the recovery system, but it really is a band aid. If you don't use too much powder to begin with, you don't need the extra elastic.
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