Max rocket weight for 3/8" Tubular Kevlar

[rocketgeek101]

I'm currently building an Ultimate Wildman and have used 3/8" tubular kevlar to construct a Y-harness that attaches to two eyebolts on the forward CR using hand stitched loops (sewn with 250 lb kevlar thread). This cord has a strength rating of 3600 lbs. I'm looking for some feedback on if this arrangement is sufficient. Assuming a pad weight of 45 lbs and a 50G max load on the cord during recovery, that should result in a force of 2250 lbs which is 63% of the rated strength.

I've recently spent some time looking at various build threads for large rockets and I've found in pretty much every single case that folks are opting for thicker/stronger shock cords (making me second guess my choice). The MMT has not yet been epoxied into the airframe so things can still be easily changed (though I'd need to order some new kevlar).

 

[3stoogesrocketry]

The most important thing you can do is radius the inside edge of your fiberglass body tube . I will do almost a full thickness radius ( 6 inch BT is about .080 thick IIRC so a .060 radius ) to prevent what happened in the other form . The inside edge of a fiberglass tube is razor sharp.

 

[rocketgeek101]

The most important thing you can do is radius the inside edge of your fiberglass body tube . I will do almost a full thickness radius ( 6 inch BT is about .080 thick IIRC so a .060 radius ) to prevent what happened in the other form . The inside edge of a fiberglass tube is razor sharp.

By design my Y-harness doesn't quite reach to the top of the booster tube so that any potential damage from the side of the tube will occur on the main portion of the shock cord (which is easily replaceable). That said, I'm also planning to wrap something around the main portion of the cord were it meets the lip of the tube to try and prevent any wear on the cord.

 

[Handeman]

Don't forget the ball bearing swivel on the fin can. I had my swivel get hung up and not spin on one flight. The spinning fin can twisted the shock cord so bad it twisted the drogue chute right up inside the shock cord.

 

[3stoogesrocketry]

By design my Y-harness doesn't quite reach to the top of the booster tube so that any potential damage from the side of the tube will occur on the main portion of the shock cord (which is easily replaceable). That said, I'm planning to wrap something around the cord were it meets the lip of the tube to try and prevent any wear on the cord.

The end of the cord where it connects will not be where it cuts / breaks . The cord virtually never fully extends before it contacts the BT . When the two sections of rocket separate , the booster still fly forward and the payload bay will tumble off to the side. The shock cord will act as a rope saw and either create a zipper or part the harness . Then when the load is applied by the shock of the two ends pulling tight , the cord breaks where it parted on the way out.

 

[manixFan]

Keep in mind that most termination methods and/or any knots in Kevlar reduce the strength, sometimes considerably. I’ve posted several charts in other threads that show a loss of as much as 50% based on the knot. I only use splices to make loops, which is a bit of a pain. You don’t say how you made the Y, but my guess is that joint will fail well before the rated load.

And as mentioned, the cord needs to be protected any place it comes into contact with a sharp edge. On small rockets I use a couple of layers of heat shrink. On larger ones I use the commercial nomex shock cord protectors.


Tony

Here’s a link to thread about knots and Kevlar:

https://www.rocketryforum.com/threads/tying-knots-in-kevlar-cord.169359/

 

[rocketgeek101]

Keep in mind that most termination methods and/or any knots in Kevlar reduce the strength, sometimes considerably. I’ve posted several charts in other threads that show a loss of as much as 50% based on the knot. I only use splices to make loops, which is a bit of a pain. You don’t say how you made the Y, but my guess is that joint will fail well before the rated load.

And as mentioned, the cord needs to be protected any place it comes into contact with a sharp edge. On small rockets I use a couple of layers of heat shrink. On larger ones I use the commercial nomex shock cord protectors.


Tony

Here’s a link to thread about knots and Kevlar:

https://www.rocketryforum.com/threads/tying-knots-in-kevlar-cord.169359/

I used 250 pound kevlar thread and a sewing awl to stitch loops into the ends of the cord directly around the eyebolts affixed to the motor mount (my understanding is that this method should preserve the majority of the strength of the cord, and will be a lot stronger than a knot). No knots used anywhere.

 

[tfish]

here's a quick link slightly weaker then your kevlar cord.



For about the last 8 years..I'm just doing knots to join things..Mostly a bowline..sometimes 2 or 3 overhand knots..just depends on what I'm doing...for loops..a bite and an over hand knot..sometimes with a safety. It's sort of like a water knot..in a way.


Tony

 

[heada]

Forgive me if I get the names wrong or the limits, I'm not a rigger and I don't play one on TV.

Ropes, cables, connectors, etc. have yield limits and working limits. For example, I recently purchased some 2.5mm braided kevlar that was listed as "700lbs" That is the yield or breaking limit but the working load limit (WLL) is 125lbs or ~1/5th of the yield limit. The 3200lbs limit is more than likely the yield limit and the working limit is closer to 1/5th of that, or ~640lbs. Same goes for a u-bolt or a quick-link or an eye-bolt (forged, non-forged is WAY lower). Lowest WLL of all the components is the max for anything in the entire recovery system. Knots, all knots, will lower the WLL of all cords/cables/etc.

 

[manixFan]

here's a quick link slightly weaker then your kevlar cord.

View attachment 508246

For about the last 8 years..I'm just doing knots to join things..Mostly a bowline..sometimes 2 or 3 overhand knots..just depends on what I'm doing...for loops..a bite and an over hand knot..sometimes with a safety. It's sort of like a water knot..in a way.


Tony

Thats exactly why I don’t use quick links of any kind. I use a splice around my attachment points. I just started using soft links (also known as soft soft shackles) and they are exactly the right kind of solution to joining a Kevlar spliced loop to a terminal point.


Tony

PS: in spite of its popularity, the bowline is a terrible knot for Kevlar. I know lots of folks use it successfully, but it costs about 40% of the cord strength.

 

[Buckeye]

PS: in spite of its popularity, the bowline is a terrible knot for Kevlar.

Some people also complain that a bowline can come undone in Kevlar. It depends on the Kevlar. The hard, stiff, flat "tubular" Kevlar will not come undone. The soft, slippery, flexible tubular Kevlar may be more prone to loosening, but I have never seen it come completely undone. I also use nothing but knots, mostly bowline. My rockets max out at 15 lbs or so.

Back to the OP's question, your analysis makes sense to me. 3/8" would work in that scenario.