I did not know the strength of the kevlar shock cord supplied with my tiny triskelion so I looked on Fliskits web site...they do not specify but sell 16 lb in the micro Max recovery section...I have never used kevlar as have only built Estes kits but occurred to me that if the kevlar fails it would be difficult to repair! When the stock cord fails is it usually from heat damage? Or abrasion? Or knot failure...I just want to understand what the advantage of kevlar shock cord use is?...my first thought is weight savings.
Kevlar Line (Twisted or Braided) it a heat resistant (NOT FlameProof) material. While it does not burn it degrades each and every time it is subjected to temperatures over 900DegF. Our BP motors ignition temperature is 909degs and goes up for there
That said: our kevlar shock lines are generally tied to or around our motor mounts. Over time I've discovered that a miminum of 30" of 70lb kevlar has enough length to allow the Nosecone and laundry to deploy and slow down before hitting the end of the line. On really long Micro models I'll add the length of the model body to that magic 30" line length. The less force placed on the kevlar at ejection the longer the line will last.
BUT and I say this Unless using some from of Shock-line protective shield or Stainless Steel Beading wire shockline Anchor you will have to retorfit a new shockline in your models after a few flights. I've fallen in love with 70lb Braided Kevlar from the Kevlar Store on-line. One 100yd spool goes a really long way.
My Tiny Triskelion broke the supplied very light-weight line on the first flight. I Replaced it with a very short .018"dia. 26lb test 49strand Stainless steel Beading wire shock-line anchor with 30" of 70lb kevlar. Haven't had any more trouble with it at all.
Bead Stringing Wire and tiny crimp sleeves are a Beadalon product you'll find in most any Hobby arts or Craft stores like Micheals or AC Moore. The stuff is NOT Cheap (last I bought was 13.49/30ft roll) but you only need a few inches so the roll goes a long way.
Another approach is to use a heavier shock-line of 90-100lb kevlar using the same 30" length. Yes it's very possible to stuff 30 inches of 100lb kevlar in a Micro Minimum diameter model with a recovery streamer and still have room to put the nosecone on
On a related note! To cut down on the bulk and need for FP Wadding or dog-Barf wadding. I now use nothing but TEFLON Plumbers Tape or a bit thicker Military Grade PTFE Plumbers sealing tape for my Micro model Streamers. Why? Teflon or PTFE is FlameProof eliminating the need for any other wadding so the Streamer surves both as FP Wadding and Recovery Streamer. The only trick to using Plain Old 1/2" or 3/4" White Teflon Plumbers tape is that it must be talc baby powdered well on both sides before the first fold/rollings or stuffing. This helps keep the teflon from sticking to itself. Re-powder these teflon streamers after every other flight or as needed. Military grade PTFE tapes come in a variety of widths from 1/4" to 2" and in muted colors Yellow, Green, Red, & Orange. PTFE sealing tapes are availble from just about any industrial supply (McMaster-Carr is where I get mine) on 43ft rolls. Current Cat. No's 44945K11, K12, K13, K14, K15 and 44945K16 from 3.48/roll to 15.93/roll. I've found 1/4", 3/8", 1/2" and 3/4" are the most useful widths for micro models. 3/8" and 1/2" getting most of the work.
So to answer your question a bit more directly:
Supplied Kevlar shock-lines will NOT last very long. they can either be strengthened by using heavier Kevlar, a kevlar/Stainless combination or adding additional length and LB test to the lines. That said Any Kevlar shock-line is bettery then any other single shockline source out there.
To Replace a broken shock-line isn't all that difficult but does require adding another anchor point from the forward end of the model. This can be a bit of a headache but I have pictorials of how to do it if needed
Hope all this isn't to much info at one time.