So many questions about Kevlar

[techrat]

Why is Kevlar cord the way to go for attaching the shock cord to? It's obviously lightweight and strong, but also seems thin, which makes me believe that it would cut through the shock cord when under stress. I've used thin paracord, which is many times thicker than kevlar, but doesn't seem like it would cause the shock cord any damage. I'm doing a build now which includes kevlar line, I'll be interested to see how it performs.

Another question I have, is that looking at HPR videos on youtube, everyone's shock cord is about a mile long. Are the ejection charges that powerful? Or is there some other reason that you need to pack that much into a small diameter tube?

 

[heada]

Flame resistance. The ejection charge can singe/burn most shock cords and require them to be replaced. Kevlar based cords are not damaged by the ejection charge. It is non-elastic (doesn't stretch) and *can be* thin so can cut into the BT more often. There are a few methods to get around this such as using a much longer section of cord and adding a wide section of tape on the cord where it exits the BT to prevent zippering. Best of the options I've seen is a Kevlar sleeve that goes over a nylon cord. You get the flame/heat protection of the Kevlar and the stretch of the nylon. That is normally only used in HPR setups though.

 

[waltr]

Biggest advantage of Kevlar for rocket is its heat & flame resistance.
Biggest disadvantage is its lack of stretch to absorb energy and causes 'shock' when rocket parts pull it tight. This is the main reason for longer lengths of Kevlar. It is hoped that the parts will slow from air drag before fully extending the Kevlar cord.

Yes, it is thinner than a Nylon strap and can/does cut into the end of the BT, known as a Zipper.

Note: heada posted while I was typing.

 

[mtnmanak]

Good answers so far. Once you convert to Kevlar, you won't go back.

- Check out Emma Kites kevlar strings on Amazon, you can get some as small as thread diameter up to heavy weight kevlar suitable for Level 1 and 2 HPRs
- Kevlar is stupendously strong for it's weight and size, which makes it ideal for all rockets, but more so in LPR and MPR. You can ditch a lot of weight if you reduce that elastic shock cord. Estes still insists on using elastic cords, but many other makers of LPR and MPR rockets have long ago switched to Kevlar. It makes more sense with a smaller rocket - do you really want 20-30% of your weight being shock cord?
- Kevlar won't burn. It can handle many, many ejection charge events (unlike elastic, which can handle zero ejection charge events if it is directly exposed to the charge)
- You can use a swivel between the kevlar and elastic in LPRs and MPRs to give you two advantages - the kevlar won't cut the elastic and the swivel keeps your rocket from spinning and twisting the shock cord during descent - bonus benefit - size your kevlar so the swivel is even with the lip of the body tube - that will help prevent zippering. For LPRs and MPRs, fishing swivels are fine.
- Never attach a shock cord using the 3 tab paper attachments Estes recommends. Even on the smallest beginner Estes rockets, I don't use that method. I get why Estes uses the method - it is easier for beginners. Once you have progressed past beginner, attach your base shock cord to the motor mount during the build. If you use kevlar to do this, it will not break due to stress or burn through and the shock cord won't come out. See above about attaching a swivel to the other end of the base shock cord. Also, if you attach the shock cord at the top of the tube using the Estes method, it has the possibility of blocking your parachute from ejecting properly. Attaching the shock cord to the motor mount eliminates that.
- For HPR, forget elastic. Seriously, don't use it. It WILL snap. Maybe not on the first or second launch, but, eventually it will snap when the main opens and your booster will come screaming to the ground. Elastic shock cord is fine in LPR and most MPRs.
- The longer the shock cord, the more it will help to bleed off descent speed before the shock of the main parachute opening can do damage to the rocket. For HPRs, I use a shock cord about 5 times the length of the rocket as a rule of thumb. Yep, I have shock cords as long as 100 feet. For dual deployment, the drogue cord should be even longer. On some of my bigger rockets (over 100 pounds), total shock cord lengths are 150 feet or more. For any rocket over 50 pounds, I just use Onebadhawk kevlar cords. A 1/2" Onebadhawk kevlar cord can be used on rockets up to 150 pounds and folds neatly into a very, very small bundle.

 

[neil_w]

For my LPR builds I use the relatively common Kevlar/elastic combination. It has worked well for me so far, although I might experiment with pure Kevlar at some point.

The attachment between the two is totally fine if done correctly. I use a square knot arrangement that is quite frankly fantastic (I have a picture somewhere but can't find it at the moment).

Because it has no stretch, Kevlar shock are longer so more of the ejection energy can be dissipated before the nose cone hits the end of the cord.

 

[techrat]

Ok @mtnmanak, that has answered my questions!

So, what I have learned is that I'm currently doing it wrong.

Interestingly, the kit I'm currently building (Skyferry), has kevlar thread wrapped around the motor mount as the shock cord mount point and then I guess you're supposed to loop it into an elastic cord for the shock cord that then ties into an eyeloop for the nosecone/payload bay... I have ordered some Emmas Kites kevlar via Amazon (might even be delivered today), so we will see how it goes. I still like the idea of using paracord, but I see why people prefer the kevlar as it does save weight.

 

[Scott_650]

Ok @mtnmanak, that has answered my questions!

So, what I have learned is that I'm currently doing it wrong.

Interestingly, the kit I'm currently building (Skyferry), has kevlar thread wrapped around the motor mount as the shock cord mount point and then I guess you're supposed to loop it into an elastic cord for the shock cord that then ties into an eyeloop for the nosecone/payload bay... I have ordered some Emmas Kites kevlar via Amazon (might even be delivered today), so we will see how it goes. I still like the idea of using paracord, but I see why people prefer the kevlar as it does save weight.

If the “Skyferry” you’re building is the ASP Sky-Ferry then yes, the instructions show tying a loop in the Kevlar cord followed by tying the elastic shock cord to the Kevlar loop - it’s shown (partly) in Figure E and explained in step 4. My Sky-Ferry has flown many, many times - it’s one of my favorite LPR rockets - with no damage from the Kevlar cord to the body tube. Enjoy your Sky-Ferry - it’s a great kit!

 

[techrat]

Estes uses bare elastic, but I also see there is fabric covered elastic, which I'm wondering is any better or worse than bare elastic for LPR and MPR builds. I do a lot of scratch builds, but so far, I've been following the "estes" method for my original creations, although, as I work my way up to HPR, I'm obviously going to have to change my method of construction to much more robust practices.

 

[neil_w]

I don't know the exact details of how ASP specifies its shock cord arrangement, but this is a fairly typical Kevlar motor mount:


The Kevlar is looped around the motor mount, and filleted into the front centering ring. It then passes the ring (I usually do it on the outside but others do it inside; the ring is notched in the appropriate location).

A loop is tied in the Kevlar so that the end of the loop is just shy of the end of the body tube, so it's pretty short. A longer piece of elastic is then tied to the end of the Kevlar. This is usually done with the cord pulled out the back of the rocket (before the nose cone is attached) where it is much easier. After the two are attached, everything is fed back out the front.

There are countless variations on this theme.

 

[neil_w]

Estes uses bare elastic, but I also see there is fabric covered elastic, which I'm wondering is any better or worse than bare elastic for LPR and MPR builds. I do a lot of scratch builds, but so far, I've been following the "estes" method for my original creations, although, as I work my way up to HPR, I'm obviously going to have to change my method of construction to much more robust practices.

Braided elastic, available from Michael's or Joanne's or Amazon or whatever, is a good choice. I usually use 1/8" for BT50 and below, 1/4" for BT55/BT60, and 3/8" for larger (although that's getting close to where the switch to pure Kevlar is good).

USE BRAIDED ELASTIC ONLY, NOT KNITTED.

 

[techrat]

My Sky-Ferry has flown many, many times - it’s one of my favorite LPR rockets

As long as no trees try to eat my rockets, they all come back to me mostly undamaged because I try and employ MPR and HPR techniques to even LPR kits.

For example, I use two-part epoxy in areas I think need it (fillets for example) or I paper-coat the balsa fins for added strength. This does add weight, which makes even my LPR builds push the limit on 18mm motors (I have a rocket that uses a C6-5 as the minimum, and after that I have to go composite). My Big Daddy is built like a tank and won't even get off the pad without a E12.

So again, I am thinking Sky-Ferry as an "E and above" 24mm flier, although I'm going to try and keep the weight down as I recently acquired a stash of D12-5 engines and need to use them up somehow!

As a fellow Sky-Ferry owner though, I have to ask if you've put a payload into the bay, and what can you fit into a BT-55?

 

[mtnmanak]

So, what I have learned is that I'm currently doing it wrong.

You are definitely not doing anything wrong! In my opinion, the only thing you can really do "wrong" in this hobby is be unsafe. Using the Estes method is not unsafe, so it is a perfectly acceptable method.

As you progress, the decisions you make on everything are about a degree of "better" or "worse" (usually couched in a financial decision). I personally think using Kevlar is a "better" option, but other methods are not wrong at all. Experiment, be safe and have a whole lot of fun!

 

[Scott_650]

As long as no trees try to eat my rockets, they all come back to me mostly undamaged because I try and employ MPR and HPR techniques to even LPR kits.

For example, I use two-part epoxy in areas I think need it (fillets for example) or I paper-coat the balsa fins for added strength. This does add weight, which makes even my LPR builds push the limit on 18mm motors (I have a rocket that uses a C6-5 as the minimum, and after that I have to go composite). My Big Daddy is built like a tank and won't even get off the pad without a E12.

So again, I am thinking Sky-Ferry as an "E and above" 24mm flier, although I'm going to try and keep the weight down as I recently acquired a stash of D12-5 engines and need to use them up somehow!

As a fellow Sky-Ferry owner though, I have to ask if you've put a payload into the bay, and what can you fit into a BT-55?

We do have some different techniques in play here - I very rarely use epoxy on LPR kits (definitely not on wood and/or paper joints) - the mess and odor just aren’t worth it to me, not that it’s “wrong” - nobody ever had a coupler or motor mount seize up going into a body tube using epoxy - just not the way I do it. As for elastic shock cord or trifold mounts - neither are wrong (not even, necessarily, inferior to other techniques) sometimes the only way to re-do a failed Kevlar motor mount shock cord is a traditional trifold mount. Because even Kevlar can fail after enough launches. Any kind of elastic will work fine especially if it’s long enough. My one consistent complaint about Estes kits is the sometimes ridiculously short shock cords they provide.

I paper balsa fins more to avoid sanding (I dislike sanding!) than for strength or appearance since you can get a stronger fin with plywood and a smoother one with lots of filler and sanding - but a papered fin is stronger and plenty smooth enough to suit me. One of the most satisfying aspects of rocketry for me is trying the variety of techniques available to find out which ones work…for me. The experimentation is so much fun.

So I tend to build as light but strong as I can - sort of the same philosophy as using the smallest chute I think will work, I’d rather repair a rocket than watch it float away (or crash from a less than optimal launch ). But that’s what works for me - I understand building strong to minimize damage and increase lifespan! (Having boxes of unbuilt kits makes trying to get each rocket to last tend to drop lower on my list of priorities).

I have a couple of small altimeters that fit easily in a BT55 tube and a little recording thermometer that just fits (the sample rate is measured in full seconds so it needs to get up there a ways for any “meaningful” data). The Flight Sketch Mini altimeter - my other little altimeter is OOP - works great and if they ever get any more in stock I highly recommend it.

 

[tsmith1315]

So, what I have learned is that I'm currently doing it wrong.

other methods are not wrong at all.

My first HPR was an old LOC kit that included a short loop of nylon cord. Instructions (yes, there were some instructions) said to epoxy that loop to the inside of the airframe below the nose cone shoulder for shock cord attachment.

In essence, they replaced the Estes trifold with a puddle of epoxy. It's still there and still feels solid.

Experiment, be safe and have a whole lot of fun!

That's what it's all about right there!

 

[techrat]

OKAY, I'm *not* doing it wrong. I need to be more precise in my language. What I mean to say is that I would prefer to apply best practices to all of my builds, be they LPR, MPR and HPR, and I have much to learn. I enjoy the construction part of this hobby even more than the launches, although that part is also quite fun, but I find the act of taking bits and pieces and making something that looks and operates as a fully functional rocket, and I would like that rocket to be the best that it can be.

 

[mh9162013]

What I mean to say is that I would prefer to apply best practices to all of my builds

What's best depends on your objectives. For example, are you going for Durability? Convenience? Altitude?

For example, I use snap swivels to attach my nose cones and parachutes to my rockets. Why? So I can use the same nose cone or parachute on more than one rocket. Does this hinder performance? You betcha, but the added convenience is worth it to me. But if I was building an "X,XXX feet or bust" type of rocket, I'll pass on the snap swivels.

Like @mtnmanak said, as long as you're being safe, don't worry.