How to repair shock cord inside rocket?

[JRThro]

Can anyone suggest a way to repair the Kevlar section of the shock cord in my Quest Intruder? It looks slightly scorched and has also unraveled so it looks like 3 separate threads instead of a single Kevlar string.

What *do* you do to repair parts of recovery systems that are mounted deep within the bowels of a rocket? The only thing I can think of doing is to cut the BT apart just above the motor mount and then see what's what, fix it if I can, and then couple the pieces back together.

It just now occurred to me that a way to make this a more repairable system would be to eject not just the nosecone, but the entire body forward of the motor mount. Then after every flight, you'd be able to look at the condition of the shock cord that's nearest the motor and that's most likely to be damaged by the ejection charge.

Does this make sense?

Now, how many questions have I asked, and how many can you answer?

 

[graylensman]

The simplest answer is to cut the kevlar as far in as you can and mount a new shock cord just aft enough from the nose cone shoulder, using the standard tri-fold mount technique.

Separating the body immediately fore of the motor mount reduces the space you have for recovery device, and may impact CG/CP relationship.

Personally, I don't see the logic of attaching a shock cord right at ejection charge ground zero, no matter how "heat-proof" the thread is.

 

[powderburner]

I assume that the existing anchor for your tether is to the MMT?
If the inside of the BT is clear (unobstructed) ahead of that (nothing glued to the insides?) you could just tie a new tether to a short section of coupler, slide it in from the front, drip in some glue, and you are back in business.
I like to coat the 'hot' end of my tethers with a liberal coating of glue. It seems to help with heat/flame resistance. Dunno, maybe it adds just enough mass to help resist the momentary thermal peak when the ejection gas flashes by?
Anyway, another thing you can do is to pull the existing tether snug, out the front of the BT, in line with the attach point so that it is laying along the inside wall of the BT. Coat most of the length of the tether in glue or epoxy, concentrating on the rear half, making sure to leave the forward several inches un-glued (so you can still get the NC on).
If you used an Estes-style tri-fold mount, use a launch rod to reach inside the BT and poke/peel off the old one.

 

[JRThro]

Originally posted by graylensman
The simplest answer is to cut the kevlar as far in as you can and mount a new shock cord just aft enough from the nose cone shoulder, using the standard tri-fold mount technique.

Separating the body immediately fore of the motor mount reduces the space you have for recovery device, and may impact CG/CP relationship.

Personally, I don't see the logic of attaching a shock cord right at ejection charge ground zero, no matter how "heat-proof" the thread is.

That sounds like a good simple idea.

But I'd still have the whole body tube for the parachute. I wasn't planning to install a bulkhead at the "bottom" of the body tube, but instead to connect the two sections of the rocket with a new shock cord and attach the parachute to the new shock cord.

Is that why Estes uses the tri-fold mounting technique, to get the entire shock cord far away from the ejection charge?

I'm guessing that Quest uses the technicque they use because it gives a really secure mount to the shock cord, although yeah, it obviously exposes more of the cord to the ejection charge.

 

[powderburner]

FWIW, I don't like the Estes tri-fold system because I don't think it is smart to take a folded-up wad of paper (or cardboard) and elastic (or rubber band) and glue it inside the front end of the BT. You are effectively putting an obstacle in the path where you want your recovery system to slide out . . . freely, reliably, quickly.
The only good thing that this system has going for it is you can usually reach it pretty easily inside the front end of the BT.

 

[shreadvector]

Scorching which is on the surface only is purely cosmetic and will not affect the strength.

Unraveled Kevlar string is as strong as 'raveled'. It is just not as pretty.

Quest tells you to add a drop of glue to the free end of the Kevlar to help prevent it from unraveling. It can still unravel in the middle and is no big deal.

If you must replace it, how about epoxying the anchor end to a fin root on the outside and then just running it up the outside and then inside to the shock cord/nose cone.

 

[JRThro]

Originally posted by shreadvector
Scorching which is on the surface only is purely cosmetic and will not affect the strength.

Unraveled Kevlar string is as strong as 'raveled'. It is just not as pretty.

Quest tells you to add a drop of glue to the free end of the Kevlar to help prevent it from unraveling. It can still unravel in the middle and is no big deal.

If you must replace it, how about epoxying the anchor end to a fin root on the outside and then just running it up the outside and then inside to the shock cord/nose cone.

It does only appear to be surface scorching. And when I cut off the end that's knotted to the elastic cord that's tied to the nose cone (so I could add another length of elastic), it was still hard to cut through with my relatively sharp scissors.

The elastic looks fine, btw.

 

[JRThro]

Originally posted by powderburner
I assume that the existing anchor for your tether is to the MMT?
If the inside of the BT is clear (unobstructed) ahead of that (nothing glued to the insides?) you could just tie a new tether to a short section of coupler, slide it in from the front, drip in some glue, and you are back in business.

I like to coat the 'hot' end of my tethers with a liberal coating of glue. It seems to help with heat/flame resistance. Dunno, maybe it adds just enough mass to help resist the momentary thermal peak when the ejection gas flashes by?

Anyway, another thing you can do is to pull the existing tether snug, out the front of the BT, in line with the attach point so that it is laying along the inside wall of the BT. Coat most of the length of the tether in glue or epoxy, concentrating on the rear half, making sure to leave the forward several inches un-glued (so you can still get the NC on).
If you used an Estes-style tri-fold mount, use a launch rod to reach inside the BT and poke/peel off the old one.

Using a coupler as a mount for the shock cord is a technique I hadn't seen or thought of before, but it sure sounds like a good one. So do your other suggestions.

 

[jflis]

I wish we had them to sell at this point, but a clean solution would be one of our (soon to be announced.... ) baffle kits.

These come with a short coupler, baffle rings/plates and a length of #90 kevlar cord.

Assemble it, slide it down the tube to the desired location (recommend as near the top as possible and still fit your recovery device) then apply glue to the forward edge with a long stick.

No more recovery wadding and your kevlar cord protected from the bulk of the heat at the same time...

Send me an email and I'll see if I could get you going with one.

jim

 

[Stymye]

Originally posted by powderburner
FWIW, I don't like the Estes tri-fold system because I don't think it is smart to take a folded-up wad of paper (or cardboard) and elastic (or rubber band) and glue it inside the front end of the BT. You are effectively putting an obstacle in the path where you want your recovery system to slide out . . . freely, reliably, quickly.
The only good thing that this system has going for it is you can usually reach it pretty easily inside the front end of the BT.

theres a better way to do almost anything in this hobby
It makes a big difference in how you fold it , if you lay the cord onto the paper at an angle it will be a third less bulky when you fold it
I must be lucky, never had a chute stay in the tube due to the trifold mount.

 

[JRThro]

Originally posted by stymye
theres a better way to do almost anything in this hobby
It makes a big difference in how you fold it , if you lay the cord onto the paper at an angle it will be a third less bulky when you fold it
I must be lucky, never had a chute stay in the tube due to the trifold mount.

Neither have I, but I've only had maybe....[thinking here]... 30 or 40 flights total on all of my rockets. Of which 3 have the tri-fold shock cord mount, with a total of maybe 15 flights.

Also, keep in mind that the Intruder BT's inner diameter is just 0.944", so it's a pretty tight fit anyway.

That's a good idea about laying the cord at an angle like that.

 

[prowlerguy]

I often use a tri-fold made of paper, but instead of elastic I embed a loop of kevlar thread. This keeps the mass of the mounting small. The loop comes to just below the top of the BT, with the elastic tied to the loop. This keeps zippering to a minimum. Plus, if I ever need to replace it, it is easily reachable.

Another alternative is a loop of kevlar with the bottom anchored in a puddle of epoxy. Harder to replace, but a nice smooth surface in the BT.

 

[powderburner]

If you want to use a tri-fold mount, and want to keep it thin, you can use cord (kevlar? nylon? etc) for the first part of your tether and fold *that* into the tri-fold. That makes it even thinner.

You can run the length of tether out the front of the rocket and attach your elastic cord out there.


Edit: Prowlerguy beat me----great minds think alike?

 

[rbeckey]

I have repaired a failed shock sord by laying a length of kevlar down the inside of the tube and gluing it with polyurethane. Don't use lots of glue, just enough to wet the cord. Also, do NOT get glue on the tube where the NC fits or on the part of the cord not attached to the tube. If the tube is a little sooty, wipe the inside with a cloth dampened with alcohol.

 

[Mad Rocketeer]

I'm thinking that after I've tried everything a time or two, just for the experience and variety, I'll build pretty much all my scratch stuff and some of my kit modifications with baffles. They seem to solve a lot of issues, albeit with a slight mass increase, but that's on the right end of the rocket.

My old Centuri Long Tom had a baffle, and I loved it, lightweight though it was. Never scorched a chute with it.

 

[adrian]

Originally posted by stymye
theres a better way to do almost anything in this hobby
It makes a big difference in how you fold it , if you lay the cord onto the paper at an angle it will be a third less bulky when you fold it

... as shown in the Estes instructions.

I have also never had any problem with tri-fold paper mounts. One rocket had its shock cord, attached close to the motor mount, blown out by a particularly violent D12-5 ejection charge; the shock cord is now re-attached with a tri-fold mount, and has since flown safely. I've also used the method of kevlar attached to a trifold mount and elastic attached to the kevlar on a few of my rockets. It works very well - it makes for a smaller mount and also means all the elastic can be kept above the wadding, out of harm's way.

When fitting a tri-fold mount, make sure it's not too near the top of the body tube. It must be far enough down that it does not obstruct the nose-cone's shoulder. But of course, if you're following Estes instructions, you'll already know that.

I'd be wary of putting the parachute into a tube ahead of the joint, e.g. by cutting the body tube just above the motor mount and putting the 'chute into the forward half. The ejection charge may blow the parachute further into the tube and the parachute may not come out again until you pull it out after the rocket is back on the ground...

 

[JRThro]

Originally posted by adrian
... as shown in the Estes instructions.

I have also never had any problem with tri-fold paper mounts. One rocket had its shock cord, attached close to the motor mount, blown out by a particularly violent D12-5 ejection charge; the shock cord is now re-attached with a tri-fold mount, and has since flown safely. I've also used the method of kevlar attached to a trifold mount and elastic attached to the kevlar on a few of my rockets. It works very well - it makes for a smaller mount and also means all the elastic can be kept above the wadding, out of harm's way.

When fitting a tri-fold mount, make sure it's not too near the top of the body tube. It must be far enough down that it does not obstruct the nose-cone's shoulder. But of course, if you're following Estes instructions, you'll already know that.

I'd be wary of putting the parachute into a tube ahead of the joint, e.g. by cutting the body tube just above the motor mount and putting the 'chute into the forward half. The ejection charge may blow the parachute further into the tube and the parachute may not come out again until you pull it out after the rocket is back on the ground...

I've never had a problem with tri-fold mounts either, based on however many flights I claimed earlier in this thread.

Yep, I know that they have to be in the tube below where the nose cone's shoulder will be.

If I did have the body come apart just above the motor mount and therefore have the parachute in the forward half of the BT, the chute would be attached to the shock cord such that when the forward BT popped off, it would pull the chute out of the BT. Kind of like now, where I have the chutes attached directly to the nose cone with snap swivels on most of my rockets. If the nose cone comes off, the chute *does* get pulled out of the BT. It's worked really well so far.

 

[DynaSoar]

Originally posted by JRThro
It does only appear to be surface scorching. And when I cut off the end that's knotted to the elastic cord that's tied to the nose cone (so I could add another length of elastic), it was still hard to cut through with my relatively sharp scissors.

The elastic looks fine, btw.

I had one burn through, so I didn't wait for the merely scorched to get worse. I cut them all off as far down as possible, tied the kevlar into several knots, laid a puddle of epoxy inside the tube below the level of the nose cone shoulder, and embedded the knotted kevlar in the epoxy, LOC style. Since then I've done several scratch builts the same way.

 

[Mister Rogers]

Hi,
I have built rockets using a variety of mounting methods for the shock cord including kevlar and wire leader. They anchor superbly, but on the other hand, I've only had a problem with the old tri-mount system once and I was too young to do a good failure analysis.
One BIG advantage in my book to the tri-mount system is when the worst happens and your rocket winds up high in a tree.
It usually is because the shock cord and/or parachute have snagged the branches. The rocket body is usually hanging by the shock cord straight down because of the weight of the expended motor and mmt and fins/fin can. I go back and check after every good rain and each time I have used the tri-mount system, I am able to recover the rocket body because the mount failed in the rain and let go, releasing the body to fall to the ground. This won't happen with the kevlar or wire leader anchors. I have then been able to salvage at least some of the rocket. Twice, everything was fine after it dried out, just add new nosecone and recovery system. Twice more, I was able to cut off everything above the motor mount and install a new bt and nc on the rear.
For me the fins are the most time consuming part of building any rocket and I'd like to get that back. BTW, if you are using a plastic fin can, there is no damage at all even if the mmt has to be scraped out and replaced (happened once). Just an angle to this thread that I thought worth mentioning.