What Warren and JW are saying is that even Kevlar can be cut. Not easy when you WANT to cut it. But, if you send a rocket up with epoxied Kevlar, you've added a knife to it... by the time it gets down with all the stress and spinning, etc., or waaaaaay before, it will be cut and separate. Do not go there, Batman. I have a few 1/4-20 eyebolts you can have/use.

Again, thanks for all the suggestions guys. Just to redirect back to the originl post, I was looking for methods that work with 1/2" or less space between motor tube and airframe. Certainly if you have a full inch or more for a welded eyebolt and washer that's easy. Eyebolt in threaded closure certainly seems to be the best for what I am looking to do, I was just under the impression that when you got to L3 caliber motors having multiple attachment points was preferred.

I'm curious if anyone has actually had a failure on an epoxied webbing anchor. I only ask because I have at least a dozen PML kits that' I have built over the years that were done that way per the instructions. I never really worry about flying them, but maybe I should. To date, I have never had one fail. On the more recently built rockets, I used tubular kevlar for the piston strap so I didn't have to worry about frying the nylon one.

The epoxy can (and does) also change the physical properties of the Kevlar. I'll bet if you soaked the flat Kevlar in epoxy and took it out later you could actually break it in two - it becomes very brittle.

Kevlar is - far and away - the most superb material there is for shock cords... but like anything else, it just needs to be handled appropriately. I'd never dream of using anything else for a shock cord - but I'd never saturate it with epoxy.

I think you might be surprised what you can fit in a 1/2" gap between motor and body tubes. A 38mm tube in a 3" rocket is just over 1/2". Split the distance for the mounting, and at 1/4" in from the inside of the body tube you get a chord of 2 7/8", leaving plenty of room for a good 1/4" eye bolt/nut. For a 3" motor in a 4" rocket, the chord is 3 7/8". That might even give enough room for a U bolt on either side. I've personally done this with a 3" body and 38mm motor tube.

There are many good options. I have to agree that epoxy and kevlar shouldn't be one of them. I did it once for a mid-power bird, never again.

Hi everyone,

How about..... Beef up your most forward centering rings (group of 3 for great surface contact), set two 1/4-40 eye-bolts, one on each side of the motor tube. Dead center, should leave room for the 7/16th nut. Then you can do your shock cord, doesn't anchor the motor though....hmmmm. Guess I'll stop there then, 2-cents.

Very tricky to get to later, trick fishin.

My L3 has a 3/8" (or, just a step—or two--above a 1/4-20) All-Thread through the center of one ebay--don't make me go measure it, with two Kev harnesses, one 60' and the other 40' long. No problems. The rocket is 12 foot long, flight weight about ~50+ pounds. In model rocketry we always used Elmer's Glue to glue the shock chords, but this is not model rocketry. Your PML kits will not stress epoxied Kevlar enough. Again, Bret, going higher AND HEAVIER is all about construction techniques and materials. AND, I made all my mistakes by not knowing or considering that. Does epoxy (which one?) change the physical property of Kevlar--thoroughly saturated; yes, in the way that freezing something puts a liquid into another "state." Though not literally. While I never had a broken arm--yet, imagine putting your arm in a cast, then having someone, a big someone, trying to snap it over their knee. Our bones have flexibility, but if they are held in place, they will break. I remember a story my Dad told me... he fell down a ladder and caught an arm in a wrung--instead of breaking it, it bounced back (his arm, not the wrung), no constraints. Surrounded by crystals--sharp, jagged crystals of epoxy. But unless I find out what Kevlar is actually made out of chemically and chemically which epoxy is used, I won't speculate what the physical or chemical changes are/or are not. There are two things missing here. One, Bret, I only used one CTI motor for highest power, a 98mm casing, and it had a mounting for the Eye-Bolt; I honestly don't know about the CTI 75mm casing, but I can't imagine them NOT having a threaded forward closure. If they don't, go with AeroTech. Although... I REALLY liked the CTI 98mm M1060 and prefer CTI. For the L3, I used a large--again, don't make me measure it--U-bolt on both plates of the ebay. No problems. G10 is stronger than you may think. Doggone it, Batman, it's late, I woke up and checked the site... can't remember what the second point was that I was going to make. 😉 Bret, I remember an Oktoberfest or so ago when Jon Scuba and I were sitting next to each other just before Doug Gerrard’s movies were about to start. He was laughing, I asked why. He told me that I was the most nervous L3 cert he’d ever seen… then we both started laughing. The thing that Joe Hinton has said to me—and everyone else--all along was have FUN… and for that moment in time Jon and I did… and not at each other’s expense but at each other’s journey.

Look for a detailed article in the next 24 hours regarding my L3, vacuum-bagging and recovery systems in the Composite Construction forum.

Warren

Hi everyone,

How about..... Beef up your most forward centering rings (group of 3 for great surface contact), set two 1/4-40 eye-bolts, one on each side of the motor tube. Dead center, should leave room for the 7/16th nut. Then you can do your shock cord, doesn't anchor the motor though....hmmmm. Guess I'll stop there then, 2-cents.

Very tricky to get to later, trick fishin.

If you have enough area on your rings - say, putting a 54mm motor inside a 4" airframe - then it works famously. On a rocket that size you can reach in just fine.

How about..... Beef up your most forward centering rings (group of 3 for great surface contact), set two 1/4-40 eye-bolts, one on each side of the motor tube. Dead center, should leave room for the 7/16th nut. Then you can do your shock cord, doesn't anchor the motor though....hmmmm. Guess I'll stop there then, 2-cents.

Speaking of beefing up rings, for whatever paranoid reason, I used aluminum or steel plates on my ebay both top and bottom. I believe that Ken Plattner, our treasurer, made them for me. I wanted to spread the stress over the whole plate. Worked. Was it needed? I don't know. I flew my Intimidator 5 without them--no, just checked, it has them, too. However, strengthening any component is never wrong the higher up you go, unless you're seeking altitude. Where's the beef? 🙂 Pay close attention to what Warren posts in the next 24 hours. It will be dead-on. He had one of the most perfect L3 flights I've seen. If you want anyone's paperwork, just ask.

Here's what I did on a 4" to 54mm for shock cord attachment as well as beefed up centering rings. I figured this design would give me plenty of glue-joint area as well as spread out the area of force. Also there is room for redundant attachments.

Plenty of room.

I wish I could say how well this works, but I have never flown this bird yet.

WHAT A PRETTY ROCKET! Ed, is that the Madcow BB2? I'm not normally a big scale modeling geek, but the MC Black Brant II is the real deal. My hunch is that is what we are looking at in your photo? Or did you build it from scratch?

JW

I built this mostly from the Scotglas components - nosecone, tailcone, and fin set. I bet the finished product is pretty much the same as the Mad Cow version with the exception of the molded fins vs. the MC's G10 fins.

From 10 feet away you would never know the difference.

I don't want to hijack the thread, I just wanted to chime in on the beefed up centering rings.

Ed - I like the double centering ring approach for the booster retention u-bolts, but do you think it is stronger than just stacking a pair of rings together? Surface area of the glue joint would be the same and therefore of equivalent strength - unless you did something to ensure an epoxy fillet on both sides of both rings.

W

I could get twice the glue area and filets on the body tube side of the centering ring assembly. I could not filet between the rings where they meets of the motor mount - only on the fore and aft-most joints.

I think this will be much stronger than one G10 plate that is twice as thick. But, since it's been many, many, years since a mechanical engineering class, I don't think I can articulate why.

On the last two rockets I built, I positioned the upper centering ring so that it coincided with the upper rail button. Doubling the thicknes of the centering ring allowed enough material for a threaded insert, which was used to secure the rail button. Combining the OD surface area of the centering ring for epoxy, the fillet on top and the screw for the rail button provides plenty of strength and keeps the centering ring very secure.

Ken