Kevlar bridal in 4 inch fiberglass

[Oberon]

Teddy,

One thing I think would make a great standard item for you: flat Kevlar bridles. Basically a length (maybe 6ft or so?) of Kevlar tape, plain on one end, with the other end ending in a sewn loop with a SS D ring. End user would cut the plain end to desired length and epoxy it to their motor mount, then attach their shock cord to the D ring. You could offer in 1 arm and 2 arm (Y harness) versions. I think I've seen you do custom work like this, but a standard item would be great to have around for on-a-whim build days.

I've been using this approach (with a tied loop because I'm not the seamstress you are) and it's nice because it lets me share my favorite Onebadhawk harness bewtween rockets, remove and clean/inspect them between flights, etc. Also I prefer braided Kevlar for the harness but flat tape works better for attaching to motor mounts - this gives the best of both worlds.

I don't like using eyebolts for attaching the harness at the centering ring, because it's often tight down there and the attachment is effectively permanent anyway... why put weight aft if you don't need it? And as the thread here notes, putting stress on thin CRs can feel dodgy.

 

[Nathan]

Tape between the kevlar and the CR to prevent cutting?

Yes. Permatex self-sealing silicon tape is good for protecting areas of Kevlar harnesses that are subject to abrasion. I use it a lot.

 

[LawnDart2020]

Teddy,

One thing I think would make a great standard item for you: flat Kevlar bridles. Basically a length (maybe 6ft or so?) of Kevlar tape, plain on one end, with the other end ending in a sewn loop with a SS D ring. End user would cut the plain end to desired length and epoxy it to their motor mount, then attach their shock cord to the D ring. You could offer in 1 arm and 2 arm (Y harness) versions. I think I've seen you do custom work like this, but a standard item would be great to have around for on-a-whim build days.

I've been using this approach (with a tied loop because I'm not the seamstress you are) and it's nice because it lets me share my favorite Onebadhawk harness bewtween rockets, remove and clean/inspect them between flights, etc. Also I prefer braided Kevlar for the harness but flat tape works better for attaching to motor mounts - this gives the best of both worlds.

I don't like using eyebolts for attaching the harness at the centering ring, because it's often tight down there and the attachment is effectively permanent anyway... why put weight aft if you don't need it? And as the thread here notes, putting stress on thin CRs can feel dodgy.

The attachment point being at the MT definitely makes it almost impossible to attach the shock cord. Moving the attachment point up makes sense and is more convenient. Making a bridle out of steel cable seems like it would never wear and could be potentially lighter in the aft end due to the lack of epoxy on the motor tube. I drew up some different scenarios, that show some ideas I'm toying with. Sorry, I'm not an artist and I'm not at a computer that I can design something on. I think they get the idea across, but they are far from perfect. I think the kevlar one has the advantage of laying flat in the confined space but I do like the idea of steel cable. The one where the same steel cable runs through both u-bolts had the advantage of being replaceable once the rocket is together. In any case I think I will use 2 attachment points, one on either side of the CR.

 

[SpaceBound2020]

Those are interesting ideas. I think using steel cable gives a lot of flexibility. Another idea could be two u-bolts on each side of the centering ring, like you've done, but then a steel cable could be ferreled into one loop, then the loop could be pushed under the u-bolt and both ends could be hooked into a quick link. This way would allow it to sit parallel to the motor tube, similar to your last drawing. Then this would allow the steel cable to be replaced and you could have different lengths based on the motor used.

 

[LawnDart2020]

Those are interesting ideas. I think using steel cable gives a lot of flexibility. Another idea could be two u-bolts on each side of the centering ring, like you've done, but then a steel cable could be ferreled into one loop, then the loop could be pushed under the u-bolt and both ends could be hooked into a quick link. This way would allow it to sit parallel to the motor tube, similar to your last drawing. Then this would allow the steel cable to be replaced and you could have different lengths based on the motor used.

That's a great idea! Right now the two loops I have are too bulky and they can't be replaced so I've been racking my brain trying to figure out a better way to attach the cable. But the u-bolts are rock solid so I want to keep them as attachment points. I'll give the loop a shot this weekend and let you know how it goes

 

[LawnDart2020]

Alright, it took longer than planned, but I have arrived at what I hope is the final answer. I checked and the cable with ferrules sits nicely between a motor and the inside of the motor tube. The connection is secure, replaceable, and very resistant to motor ejection charges. Plus as an added bonus the wrap on the u-bolts absorbs shock pretty well and distributes the load evenly over the 2 u-bolts. Pretty happy with it overall, but I'm sure some edits will come up for the next one.

 

[SpaceBound2020]

Alright, it took longer than planned, but I have arrived at what I hope is the final answer. I checked and the cable with ferrules sits nicely between a motor and the inside of the motor tube. The connection is secure, replaceable, and very resistant to motor ejection charges. Plus as an added bonus the wrap on the u-bolts absorbs shock pretty well and distributes the load evenly over the 2 u-bolts. Pretty happy with it overall, but I'm sure some edits will come up for the next one. View attachment 368509

That looks great. I like what you did at the top for the connection between the steel cable and the quick link.

 

[Handeman]

I used a steel cable as a shock cord mount in my scratch built L1. Never again. That BT was only a 2" ID so I had to have a special tool to twist up the cable to get it into the tube. PITA.

It will probably work better in a 4" tube, but I would be shocked if the whole cable assembly and links would be lighter then a Kevlar harness and a few grams of epoxy. One u-bolt probably weighs as much as the epoxy need for a Kevlar harness. Have you weighed the components?

 

[LawnDart2020]

I used a steel cable as a shock cord mount in my scratch built L1. Never again. That BT was only a 2" ID so I had to have a special tool to twist up the cable to get it into the tube. PITA.

It will probably work better in a 4" tube, but I would be shocked if the whole cable assembly and links would be lighter then a Kevlar harness and a few grams of epoxy. One u-bolt probably weighs as much as the epoxy need for a Kevlar harness. Have you weighed the components?

I haven't weighed the components. I'll weigh them and also weigh some kevlar. I would like to see how much epoxy would be used when attaching a bridle to a motor tube, then we could do a side by side to know the difference for sure. I was thinking of doing something similar with kevlar strapping, but I'm not much at the sewing machine. Maybe doing this same thing with kevlar would be the best of both worlds.

 

[WILDMANRS]

This thread is one of the things I love about this hobby.
There is always a better way to build a mouse trap.
The problems I see with your solution is it will zipper the tube as pictured and it will weigh twice as much a piece of Kevlar dong the same job.

 

[LawnDart2020]

This thread is one of the things I love about this hobby.
There is always a better way to build a mouse trap.
The problems I see with your solution is it will zipper the tube as pictured and it will weigh twice as much a piece of Kevlar dong the same job.

It would definitely zipper if I flew it the way it's pictured. Fortunately it will sit way further back in the rocket and the steel won't be in contact with the edge. I think before I make it all permanent I'm going to try something similar with 1" kevlar.

I was originally concerned with it being replaceable. If I could figure out a way for the kevlar to be replaceable then I would be happy with it.

 

[blackjack2564]

Should you ever need to replace [which is highly doubtful, I have over 110 flights on 3 in rocket] such as a cato or burn through,

Just get a coupler section 3-4 in & another 2in..... slice the smaller one to fit inside the normal coupler sandwich the new harness between the 2 and slather glue.

Slide it down on top of CR ...glue and done. New harness.

[For pics one inside airframe is a dry fit, why the harness ends are sticking out]

This one is for a 4in. airframe .

 

[LawnDart2020]

Should you ever need to replace [which is highly doubtful, I have over 110 flights on 3 in rocket] such as a cato or burn through,

Just get a coupler section 3-4 in & another 2in..... slice the smaller one to fit inside the normal coupler sandwich the new harness between the 2 and slather glue.

Slide it down on top of CR ...glue and done. New harness.

[For pics one inside airframe is a dry fit, why the harness ends are sticking out]

This one is for a 4in. airframe .

View attachment 368797 View attachment 368799 View attachment 368798

That is nice. I'm not sure why I have such a phobia of irreplaceable recovery attachment, but seeing that definitely makes me realize that where there's a will there's a way. I like the u-bolts attachment but I'm starting to think kevlar is the smarter choice. Worst case scenario after I epoxy the MT in place I can remove whatever I settle on and then change it around to something different. This weekend when I have a little more time I'll mess with the kevlar and weigh both scenarios along with the u-bolts. Get some numbers on which is better.

 

[Buddy Michaelson]

Check out my Kevlar Y-Harness’s
https://the-rocketman.com/harness/

 

[Wallace]

Those are interesting ideas. I think using steel cable gives a lot of flexibility. Another idea could be two u-bolts on each side of the centering ring, like you've done, but then a steel cable could be ferreled into one loop, then the loop could be pushed under the u-bolt and both ends could be hooked into a quick link. This way would allow it to sit parallel to the motor tube, similar to your last drawing. Then this would allow the steel cable to be replaced and you could have different lengths based on the motor used.

I've been using stainless steel fishing "stuff" for quite a while now. Obviously it has it's limitations, but used properly it's a very cool system. I "discovered" it a few years ago when I received a Bass Pro gift card for my birthday. Since I no longer hunt or fish I just stopped in to wander around and see all the latest/greatest gear and found this stainless leader material (they also have titanium but it's a bit pricey). I've used it on at least a half a dozen rockets ranging up to 4 pounds without a single failure. It's lightweight, flexible and impervious to heat/bp fouling. Many different ways to attach it to whatever you need to constrain. The key is to used double ferrells with double crimps....

 

[LawnDart2020]

Well I finally weighed the the components and just one steel cable bridle weighs 74 grams, so the pair is almost 150 grams. I then weighed 15 feet of 1" Kevlar strapping and the entire 15 feet that I have on hand weighed 64 grams. If I made the bridle in house I would use about 3 feet per side. this means that the Kevlar complete with Nomex would weigh a little more than 30 grams this is about 1/5 of the weight for the same job, so WILDMANRS your estimate of just double was a little off. I am looking into getting a 9/16 bridle made, and if I get it done I'll be sure to post the weight and share just how replaceable it is. I didn't have a spare U-bolt to weigh but on the next run to the hardware store I'll be sure to get one one and let you all know the results, so maybe someone else can let us know how much JB Weld they use to epoxy a Kevlar Bridle to the MT, then we could compare these side by side. This has been tedious, but thanks to everyone for participating and sharing your secrets.