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tjtx

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Being a natural born "lurker" and a Beginner Old-Gray-Haired Rockateer, the more I search and read about setting up recovery systems, the more confused I get. I have several Estes/Cox LPR's in various stages of construction and one QModeling MPR on my building table. Through reading, I have scrapped the trifold/rubber band technique. I have purchased Kevlar thread, 1/8th" and 1/4" elastic, snap swivels, barrel swivels, etc. for use .
Several questions come to mind regarding the recovery system. After I attach the Kevlar thread to the CR and allow 3 times the length of the rocket in front of the BT:
1. Is it necessary to use elastic in the system?
2. If so, what length?
3. Do I attach the elastic to the NC or the parachute?
4. If I tie a loop just below the BT, do I use (3 times the rocket length) of elastic?
5. Why use elastic anyway?
As you may see, I am like a dog chasing it"s tail-too old and too confused to ponder each ramification! Please an old soul who wants to learn. I thank you in advance for any or all points of view.
Respectfully, tjtx
 

powderburner

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First, welcome to TRF! We are glad you joined in here, hope we can be of some help too

The sort of things you are asking are the same qstns lots of other people ask. The short answer(s): there are usually several ways to do just about any of this.

1) Elastic v.s. none --- For many low-power rockets, with a long enough piece of kevlar (or other good-quality, high strength fiber) for a recovery system anchor line, like about 3 x the rocket's length, the ejected nose cone will probably slow down enough that when the line is fully extended and the nose cone "snaps" to a stop, you won't have much of a problem. Why is it so long? You don't want the "snap" to be so severe that it yanks the anchor line from the nose cone (separated NCs are seldom ever found) or that it yanks the other end loose from the attach point inside the rocket. If you have lots of ballast weight on the NC to trim your rocket for flight, the "snap" load on the attachment will be more severe. It's just an easy thing to do to add a section of elastic (some folks prefer the rubber-band-like material) to give a bit of shock-absorber-action to the entire system. Possible problem with this approach: if you select an inappropriate ejection delay and the recovery system deploys at high airspeeds, the opening jerk of the parachute can tug hard on the kevlar anchor line, which can cut into the front cardboard edge of the body tube and cause a lot of damage (this is called a "zipper").

2) What length --- You will get many opinions on this one. Some people like to use elastic for the full length of the anchor line. I happen to like to use a kevlar anchor line attached to the rocket, with a long enough length to reach well outside the front (so if I have to make repairs later I can reach the knot where the kevlar connects to the elastic), usually using a piece of kevlar (or similar) that is about 2 to 3 rocket lengths (this cord usually packs easily into a small volume). I tie on the elastic band, and you may want to add a snap swivel at this connection for clipping on a streamer or parachute. The elastic band should be about 1 rocket length (or not much more), with the NC attached to the far end, so that when the whole mess is in descent mode (hanging under the recovery device) the NC is suspended at an intermediate point and is not banging into the rest of the rocket.

3) What goes where -- I hope I covered this in the explanations above.

4) 3 x length elastic --- If there is room inside the forward BT, more elastic (and kevlar) can be helpful. At some point it probably becomes just dead weight to add more length of elastic (like, it doesn't really help much to add more), probably a length of elastic around 3-4-5 x rocket length becomes wasted effort. If your rocket only has a small volume available to pack in the recovery system, you might have to skimp a bit and try a shorter length of elastic.

5) Why elastic at all? -- Again, if you just used a long piece of basic anchor line (like kevlar), as long as it will pack into the rocket and deploy without getting all tangled in a giant knot, you might be able to skip the elastic part completely. If you try using a line material like nylon (woven cord, like surveyor's line) it will have a little stretch already (kevlar tends to have very little stretch or "spring"). However, with materials like nylon you will have to begin worrying about the anchor line being damaged by the high ejection gas temperatures.

Some rocketeers like to build (especially on heavier model rockets) with stainless steel woven wire that attaches to the rocket and has a loop on the free end to tie on the rest of the recovery system. If you would like to try this, check out your local sporting goods store for fishing leaders. This might be over-kill, but at least you won't ever ever ever have to worry about ejection charges roasting and severing the anchor line.

The problem with the shock cord materials supplied with many kits from "big" manufacturers is that this is the only anchor line material in the kit (they don't include a length of kevlar) and what they do provide is too short. This results in an ejection event where the NC runs out to the end of the stretched elastic and is immediately pulled straight back into the front end of the BT, giving you a nice dent in the cardboard. You can fix it by straightening out the BT and reinforcing with a few drops of superglue (dollar-store kind is OK) applied on the inside. Most of the newer kit manufacturers have listened to the customer complaints and designed their kits with better materials, and you should not have this problem.

The most important point that I would like to make is that the attachment at the rocket-end needs to be a little more carefully thought out than many kits show in the instructions. A certain major manufacturer of kits likes to use a "tri-fold" system glued inside the front end of the BT (deep enough so you can still seat the NC). I don't like this system because it means adding a thick wad of folded cardboard in the path of the recovery system where something can get snagged, torn, or completely plugged and stopped. (This is not a good thing.) If you have kevlar available, even a minimum-diameter design can be "fixed" by tying the kevlar anchor line to the thrust ring before the ring is inserted and glued in place. The kevlar anchor line then runs the length of the rocket and extends out the front and goes on to the rest of the recovery system. If your rocket has a larger BT, with a motor-mount tube (MMT) held in place with centering rings, you can tie the anchor line around the MMT and hold it past the outside edge of the forward centering ring as you insert and glue the MMT assembly in place. This gives you a solid attachment for the recovery system and lets you pack the anchor line behind the parachute, which I think is a much better approach.

And then there are always the practical considerations, like if you only have certain supplies on hand and want to be able to launch tomorrow (without waiting for a mail-order), you use what you have. Hey, this is supposed to be fun, not work.
 
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tjtx

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powderburner. I really appreciate the time that you took to make sense out of this for me.There is not a club nearby here in west Texas, so my sole source of information is the internet. I thank you very much! TJ
 

jflis

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Welcome to TRF. I am glad you found us :) As you can tell, this is one place on the internet to get your questions answered :)

Enjoy and don't be a stranger!
 

Patch

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I lurk also and also a beginner. I learn alot just reading. Welcome to the obs---hobby.;)
 

adrian

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Just to be different... :D

I like the tri-fold paper system and use it often in my scratch-builds. The down side with tying a piece of kevlar to the thrust ring is that even kevlar is not entirely proof against ejection charges, and when it eventually snaps, good luck putting in a replacement. The tri-fold paper mount is above the recovery wadding, protected from the ejection blast. Mind you, I prefer slightly larger models, e.g. BT-55 or BT-60, so there's plenty of room to pack the parachute and also plenty of room for the whole recovery system to get past the tri-fold mount.

Whether the shock cord is kevlar, elastic or a combination of the two, another way to tie things is to have the nose cone half way down and the parachute at the far end from the rocket. Maybe tie the nose cone half way down a single shock cord, maybe tie the nose cone to the cord which is fixed to the rocket and then tie another cord to the nose cone, the parachute going to the other end of this second cord. This makes it even less likely that the nose cone will dangle and bang into the rocket, and also means that if the rocket lands in long grass and there's enough cord between the nose and the parachute, the parachute may sit on top of the grass and be easier to see.
 

MarkII

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Just to be different... :D

I like the tri-fold paper system and use it often in my scratch-builds. The down side with tying a piece of kevlar to the thrust ring is that even kevlar is not entirely proof against ejection charges, and when it eventually snaps, good luck putting in a replacement.
You would still have many shock cord mounting options, including attaching Kevlar or steel cable to the inner wall of the tube, inserted deep in the tube where it is out of the way. The tri-fold paper mount, placed up high as you describe, is useful if you are just using elastic or rubber as a shock cord, with no additional "leash." You obviously want to keep those materials as far away from the ejection charge as possible. But your shock cord can eventually break with that type of mount, and probably sooner, too, if you use heat intolerant shock cords. So what do you do then? Stick another paper mount into the top of the tube? Eventually all of those accretions to the upper end of your tube add up.

Attaching the shock cord with a paper mount high up in the tube does have its place, though. It is the preferred method in rockets that are quite long (like the Estes Mean Machine) and which have been built to separate for recovery high up on the airframe. In those cases, mounting the shock cord deep down in the tube would require the use of a very long cord, and it not really be necessary anyway, because the ejection gas will have cooled somewhat by the time it reaches the recovery device up near the separation point.

The only occasions in which I have seen shock cords burn through and part in any of my rockets were when the cords were attached using tri-fold mounts. And the only occasions in which I had recovery devices fail to deploy due to blocked tubes were in rockets that had the shock cords mounted to the inner wall of the tube up high, near the nose cone, with paper mounts.

Look, it might dozens, maybe even hundreds, of exposures to ejection charges to finally break down the Kevlar. Much depends on the type of motors flown, the size (diameter) and quality of the Kevlar, and where and how it is mounted. Tubular or braided Kevlar probably holds up better than simple twisted Kevlar twine, which no doubt holds up better than thin Kevlar thread. I say "probably" because I have never seen any form of it break from repeated exposure to ejection charges. If you have a rocket that you can fly enough times (and not lose it, crash it, or cato it) to break down a decent Kevlar mount, then you are one pretty darned lucky (or conservative) flier and your rocket has had a charmed life.

Thin stainless steel cable can also be used as the shock cord "leash" and can also be mounted deep in the tube, where it will stay below the recovery device (and not get in the way). I have never heard of anyone complaining that their steel cable shock cord mount burned through.

MarkII
 
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MarkII

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On a related note:

I was wondering why the rubber band-style shock cords that I had in my Estes rockets in the 1960's seemed to hold up really well in comparison to the rubber or thin diameter elastic shock cords that are in use today. At first I wondered whether the rubber was somehow just "better" back then. But then I realized that it isn't because that today's rubber and elastic shock cords are less durable; it is because the ejection charges are much "hotter" now, and they cause shock cords to break down much more quickly. I won't use 1/8" braided elastic in small rockets anymore because it just doesn't hold up very long to those hot charges. Instead, if I want to use a small diameter stretchable shock cord in a rocket that is too small for 1/4" elastic, I use either oval elastic or small diameter mini bungee cord, both of which last much longer.

MarkII
 

Micromeister

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Have to say Ditto to just about everything Powderburner, Jim and Mark and have posted on this subject.
Personally I haven't used a teabag shockcord mount since finding Kevlar back in the stone age late 60's.
I've posted here many times Lots of different ways to retrofit ANY size model rocket with a Kevlar or Stainless leader/kevlar shockcord mount that outlast the entire model. Some are in the old TRF-1 archive others here.
It's been my experience as well that Oval elastic with kevlar or Kevlar/stainless leaders work best for just about any models under BT-60. Larger models generally get a Stainless Anchor with Kevlar/1/4" elastic combination. 1/8" Flat braided elastics just does not do well with the heat.

Mark your assumption the Rubber was better back in the day is for the most part correct. Whatever the compound formula Estes was using back than did/does hold up well though atmospheric contaminates degrade just about all rubber compounds over time. Some of the Really OLD kits in my collection contain the old Rubber shockcord that seem to be as strong as the day they were made....as Long as the bag is still sealed;) There has been alot of talk over the past couple years by Estes and others about the "New" rubber formula they are using today that is said to outlast the elastic lines they were offering for awhile.
 

adrian

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Look, it might dozens, maybe even hundreds, of exposures to ejection charges to finally break down the Kevlar. Much depends on the type of motors flown, the size (diameter) and quality of the Kevlar, and where and how it is mounted. Tubular or braided Kevlar probably holds up better than simple twisted Kevlar twine, which no doubt holds up better than thin Kevlar thread. I say "probably" because I have never seen any form of it break from repeated exposure to ejection charges. If you have a rocket that you can fly enough times (and not lose it, crash it, or cato it) to break down a decent Kevlar mount, then you are one pretty darned lucky (or conservative) flier and your rocket has had a charmed life.
I do have a rocket which, though not having such a charmed life (it has crashed a few times and been damaged a few times for other reasons, but always been repaired), has broken its kevlar cord after 24 flights. This was twisted twine tied, as per recommendation, to the thrust ring. It's broken pretty close to the thrust ring, and repairing it is going to be a good trick. Just for once I don't want to use a tri-fold mount because this rocket has a piston, not only to help eject the parachute but also to help eject various other objects (a parachuting teddy bear being favourite).

By contrast, another couple of rockets with elastic cords in tri-fold mounts each have almost 30 flights logged. They too have needed the occasional repair, but a replacement shock cord wasn't among them.
 

Micromeister

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I do have a rocket which, though not having such a charmed life (it has crashed a few times and been damaged a few times for other reasons, but always been repaired), has broken its kevlar cord after 24 flights. This was twisted twine tied, as per recommendation, to the thrust ring. It's broken pretty close to the thrust ring, and repairing it is going to be a good trick. Just for once I don't want to use a tri-fold mount because this rocket has a piston, not only to help eject the parachute but also to help eject various other objects (a parachuting teddy bear being favourite).

By contrast, another couple of rockets with elastic cords in tri-fold mounts each have almost 30 flights logged. They too have needed the occasional repair, but a replacement shock cord wasn't among them.
Adrian:
While I'm a very big Kevlar & Kevlar/Stainless anchor advocate, Mark and I have had this discussion before, Kevlar alone is not flame or even heat proof, it is heat resistant according to it's manufacturer Dupont. I have had braided first quality 100lb kevlar lines BURN OFF about 1/2 to 1" above the motor mount before I moved the line to the inteior OUTSIDE airframe/forward centering ring joint. since then I haven't had a failure on those type shockcord mounts. I have still had a few on minimium diameter models where the motor block is used as the kevlar anchor. Particularly with twisted Kevlar lines in the 30-50lb range. As Mark mentioned Braided or as it's more commonly known tubular Kevlar seams to handle these situations a bit better.
To be perfectly Honest, since I bit the bullet and made the initial investment in a number of different size Stainless Steel Avaition Cables and Beading wire rolls I haven't used a straight Kevlar/Elastic Shockcord or line on any recent builds. The addition of a Stainless or If you have the funds Titanium anchor, which completely removes our Kevlar from most of the ejection heat threat, we no longer have to worry about any kind of shockcord replacements until the elastic end need an easy un-tie re-tie LOL!!!;)

Retro-fitting a new kevlar or Stainless/Kevlar mount in our model isn't that difficult. All that's needed is a coupling or centering ring the size of our model, a small round needle file and the Kevlar or Stainless cable/kevlar shockcord. I generally run a large diameter dowel wrapped with 220 grit sandpaper down the inside of the model to clean out any builtup ejection soot and girt. stuff a tack-rag all the way to the forward centering ring and remove the debris. Then apply either a wood glue or epoxy in the area the new ring is to sit. Slide the retro-fitted sanded couple/shockcord into place in the glue and let is cure. Sometimes if the model has a long body a setting plunger is needed. A plunger is easily made with a 1/2" or larger dowel build-up to the ID of the tube with Duct tape... works like a champ.
Here how I set up a reto-fit motor block/Shockcord for a BT-50 size model.

Shock-Cord Mount-h1_Retro mount 8pic pg1of2_09-28-06.jpg


Shock-Cord Mount-h2_Retro mount 6pic pg2of2_09-28-06.jpg
 
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hardinlw

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I have recently begun using a combination of 1/8" braided Kevlar and twisted Kevlar for shock cords. The larger braided Kevlar is attached to the motor mount or front centering ring (near the OD is best) and terminates in a loop a couple of inches outside the front of the body tube. That is really a leader and serves the same purpose as the stainless steel leaders others have mentioned, but it's more flexible. I have been making removable shock cords by attaching fishing snap swivels to the ends of the twisted Kevlar, but it is not all that hard to just tie knots and cut them when you need to do a replacement. The braided leader also helps prevent "zippers" which are tears in the body tube where the shock cord cuts it if the parachute pops while the rocket is moving fast. We had a TARC rocket using this technique deploy the parachute on the way up (altimeter indicated 200 ft/sec when the charge fired) and there was no damage at all. I think that's a combination of the braided leader and the way they were packing the chute to get a soft deployment.
 

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