Attaching a shock cord to a plywood centering ring

I'm building an Estes Partizon - 2.5 inch airframe with 1/8 inch plywood centering rings.

The standard build uses a teabag mount with 1/2 inch elastic. I've read that the teabag mount is fine in these rockets, but I'm just not crazy about it - and more importantly, I'm using these kits to learn how to upscale my building techniques for when I get into high power.

Consequently, I've put an eye bolt through the forward centering ring.



Now I've realized that if I merely attach the included elastic to the eye bolt, I'll lose a ton of length on the shock cord. The rocket is 56 inches long.

Also, don't know if it matters, but the elastic is a little wide for the eye bolt.



So I'm trying to figure out how to attach my shock cord to the eye bolt.

I know that people replace the elastic in high power kits with nylon webbing. I don't mind doing that, if I can just run out to a store and get a long enough piece of it. But I'd prefer not to throw out the elastic band. I'm wondering what I can attach to the eye bolt to extend the length of the cord.

I do have some Kevlar, but it's the 138 pound test I got from JonRocket.com, which I think is only for smaller rockets. I was thinking maybe I'd do a double loop of that, and then either tie or sew the elastic to the end of the loop (if I have enough Kevlar).

Any ideas? I'll order something online if I need to, but I'd prefer to use something I can go out and buy.

On my Ventris, I used 100# kevlar. I drilled holes in the centering rings and epoxied in a piece of 1/8" automotive brake line tubing. The kevlar is tied in a lariat loop around the motor tube at the retainer and run up the tube. The kevlar and the elastic are tied to a metal ring. The ring is so the Kevlar will not cut the elastic.
If you want to stay with the eyebolt, just tie your kevlar to that.

mikemech said:

On my Ventris, I used 100# kevlar. I drilled holes in the centering rings and epoxied in a piece of 1/8" automotive brake line tubing. The kevlar is tied in a lariat loop around the motor tube at the retainer and run up the tube. The kevlar and the elastic are tied to a metal ring. The ring is so the Kevlar will not cut the elastic.
If you want to stay with the eyebolt, just tie your kevlar to that.

Click to expand...

What size Kevlar did you use?

When I built my leviathan, I used a loop of 1/8" Kevlar through the eyebolt. This was one long continuous loop tied off to a length so that the loop of Kevlar extended from the eyebolt almost to the forward lip of the body tube. Then I tied the elastic to that. The nice thing is you can detach the elastic from the Kevlar and you can inspect the Kevlar for damage by pulling it length by length through the eyebolt. You can even replace it if necessary.

When you get into 4" airframes, you can reach your hand all the way down the airframe to the eyebolt and attach/detach the cord with a quick link.

I've also seen a few suggest using a stainless steel fishing leader to attach to the eyebolt. It gives you 12-18" of flameproof line closest to the point of the ejection charge which comes in handy on smaller models where you can't reach in and easily replace the cord should it become singed by the ejection charge. Right now in my MPR rockets I use a length of Kevlar that runs most of the length of the body tube from the motor mount and then attach a sewing elastic cord from there and it seems to hold up OK.

I like the idea of using a continuous loop of Kevlar too.

Whatever you decide, DON'T tie your elastic that close to the flame and heat of the motor (as you showed in the original post's photographs). That's a very good way of assuring separation... The nosecone and parachute go one way.... The body tube comes straight down. :shock: :y:

Another lighter idea for the attachment of your shock cord has you drill a small hole or cut a small notch through each of your centering rings near the outside edge of them (to keep your line as far away from the heat of the ejection as possible). Then you fish your line through the holes, tie a loop that will go over the engine's retainer (thus securing it (add a small piece of tape to prevent it from falling off if it's too loose), and a loop to attach your elastic, or long cord at the other end of the rocket. to inspect it , you just pull the shock cord back through the hole, and you can see all but the topmost section from the back end. Remember that you want to keep the holes in line with each other, and only as big as needed to pass the shock cord through so the ejection gasses don't escape, and cause the ejection to fail. You'll also need to be careful not to fill the holes with epoxy when doing your internal fillets (a straw would help with that).

On my Leviathan, I epoxied a piece of 1/8 tubular Kevlar to the MMT and the aft face of the forward CR. Kevlar goes around the CR along the BT. It was about 5 ft long. I just tied the elastic to that. That and rail buttons were the only modifications I've done to the Leviathan and its flown fine on 3 & 4 grain H motors.

Leaving out the eye bolt also lightens it some.

lcorinth said:

I'm building an Estes Partizon - 2.5 inch airframe with 1/8 inch plywood centering rings.

<snip>

Consequently, I've put an eye bolt through the forward centering ring.

Click to expand...

That eye-bolt (even without being welded closed) is probably *far* in excess of the strength of the 1/8" plywood. While I understand this is only a 2.5" vehicle, you might want to consider backing up the plywood ring with some G-10 fiberglass (looks like the nuts on the eyebolt are still able to be disassembled). This doesn't have to be a complete 'ring' (impossible as that would be to put in place at this stage of construction), but a 'crescent' shaped ("washer", if you will) would spread out the recovery loads over a much wider area - which would somewhat mitigate the 'punching shear' that the standard washer is going to impose on the back (interior) side of the centering ring. You might could also consider, alternately, using a 'fender' washer (much larger in diameter than a standard washer) to do the same thing - which may involve a little grinding to clear the motor tube.

But, basically, I'm envisioning a 'half-ring' (+) of some 1/16" or 1/8" G-10 fiberglass - epoxied to the plywood ring - centered on the eye-bolt. Should be on the interior to properly strengthen the plywood ring.

Again, I'm aware that this is a smallish, 2.5" vehicle - but I'm kind of a believer in 'balancing' things - structurally - and the size of that eye-bolt kind of overwhelmed me (compared to the 1/8" plywood ring it's mounted in).

byoungblood said:

I've also seen a few suggest using a stainless steel fishing leader to attach to the eyebolt. It gives you 12-18" of flameproof line closest to the point of the ejection charge which comes in handy on smaller models where you can't reach in and easily replace the cord should it become singed by the ejection charge.

Click to expand...

I'm a pretty big believer in this approach, as well. While Kevlar is pretty good at flame *resistance*, it is NOT flame *proof* - and will start to decompose at around 700-800 degrees (maybe double what nylon can handle). However, that ejection charge is going to have portions probably pushing 2000 degrees and putting Kevlar in that environment just isn't exactly what the creators at DuPont were thinking about when they came up with Kevlar.

In anything that is going to be permanent once closed up, I simply don't believe it's exceedingly rational to put a 700deg material in a 2000deg environment - regardless of how many times it may 'dodge the bullet' of those few burning particles of black powder that may land just right (i.e. on the surface of that 700deg material). Solve the problem... completely - and then you don't have to solve it again. (This not to ignore Thirsty's approach of being able to rotate the length around to inspect/replace -- which is smart).

I've got several scrap pieces of some 3/32" braided stainless steel cable with a swaged eye in one end -- which is really good in an application like this - but I think a better (and easier) solution is simply to get a length of cable that is twice (+) what you would need and some 'open swage' fittings (i.e. uncrimped), fold the cable in half and slide the fitting up near the mid-point/loop and crimp (if you desire - or epoxy if you don't). Run the tag ends down through the upper centering ring and wrap the two ends in opposite directions around the motor tube (at least one complete revolution each) and epoxy to the motor tube (could even wrap a little bit with some fiberglass tape/scrap material) - all bedded in epoxy. Solid anchorage there and you don't have to worry about the swage fitting pulling out (since most of those swage crimpers are expensive, it isn't even important in this application). The fitting is only to keep the two runs of the braided cable together (forming the loop - which also doubles the strength of this). It's also quite a bit smoother this way (no ragged end of a swage fitting to catch on part of the recovery system). If this is routed right up next to the interior of the body tube, it is out of the way with the recovery system and gets any part of the textile materials *away* from the most severe areas of the ejection charge gasses. Obviously, this is 'zipper city' if the cable assembly were to extend past the end of the body tube - but stopping it short (2-3") allows quick link connection to whatever textile material constitutes the shock cord. But the main thing here is that the permanent portions of the recovery harness is now a rated material that exceeds whatever temperature the ejection charge could ever generate.

While the above description sounds more high power -- there are all manner of strengths for braided stainless fishing leader (and the crimp fittings) that should cover any conceivable size model. Personally, I pretty much set a minimum of 1/16" braided stainless (7x28 is pretty good and flexible) and it's easy to work with - pretty widely available (McMaster-Carr is good).

I just don't 'cotton' to putting something like Kevlar (or any textile material) in such a hostile environment as a black powder ejection charge is. There are better and more permanent solutions to that problem.


-- john.

jcato said:

That eye-bolt (even without being welded closed) is probably *far* in excess of the strength of the 1/8" plywood. While I understand this is only a 2.5" vehicle, you might want to consider backing up the plywood ring with some G-10 fiberglass (looks like the nuts on the eyebolt are still able to be disassembled). This doesn't have to be a complete 'ring' (impossible as that would be to put in place at this stage of construction), but a 'crescent' shaped ("washer", if you will) would spread out the recovery loads over a much wider area - which would somewhat mitigate the 'punching shear' that the standard washer is going to impose on the back (interior) side of the centering ring. You might could also consider, alternately, using a 'fender' washer (much larger in diameter than a standard washer) to do the same thing - which may involve a little grinding to clear the motor tube.

But, basically, I'm envisioning a 'half-ring' (+) of some 1/16" or 1/8" G-10 fiberglass - epoxied to the plywood ring - centered on the eye-bolt. Should be on the interior to properly strengthen the plywood ring.

Again, I'm aware that this is a smallish, 2.5" vehicle - but I'm kind of a believer in 'balancing' things - structurally - and the size of that eye-bolt kind of overwhelmed me (compared to the 1/8" plywood ring it's mounted in).



I'm a pretty big believer in this approach, as well. While Kevlar is pretty good at flame *resistance*, it is NOT flame *proof* - and will start to decompose at around 700-800 degrees (maybe double what nylon can handle). However, that ejection charge is going to have portions probably pushing 2000 degrees and putting Kevlar in that environment just isn't exactly what the creators at DuPont were thinking about when they came up with Kevlar.

In anything that is going to be permanent once closed up, I simply don't believe it's exceedingly rational to put a 700deg material in a 2000deg environment - regardless of how many times it may 'dodge the bullet' of those few burning particles of black powder that may land just right (i.e. on the surface of that 700deg material). Solve the problem... completely - and then you don't have to solve it again. (This not to ignore Thirsty's approach of being able to rotate the length around to inspect/replace -- which is smart).

I've got several scrap pieces of some 3/32" braided stainless steel cable with a swaged eye in one end -- which is really good in an application like this - but I think a better (and easier) solution is simply to get a length of cable that is twice (+) what you would need and some 'open swage' fittings (i.e. uncrimped), fold the cable in half and slide the fitting up near the mid-point/loop and crimp (if you desire - or epoxy if you don't). Run the tag ends down through the upper centering ring and wrap the two ends in opposite directions around the motor tube (at least one complete revolution each) and epoxy to the motor tube (could even wrap a little bit with some fiberglass tape/scrap material) - all bedded in epoxy. Solid anchorage there and you don't have to worry about the swage fitting pulling out (since most of those swage crimpers are expensive, it isn't even important in this application). The fitting is only to keep the two runs of the braided cable together (forming the loop - which also doubles the strength of this). It's also quite a bit smoother this way (no ragged end of a swage fitting to catch on part of the recovery system). If this is routed right up next to the interior of the body tube, it is out of the way with the recovery system and gets any part of the textile materials *away* from the most severe areas of the ejection charge gasses. Obviously, this is 'zipper city' if the cable assembly were to extend past the end of the body tube - but stopping it short (2-3") allows quick link connection to whatever textile material constitutes the shock cord. But the main thing here is that the permanent portions of the recovery harness is now a rated material that exceeds whatever temperature the ejection charge could ever generate.

While the above description sounds more high power -- there are all manner of strengths for braided stainless fishing leader (and the crimp fittings) that should cover any conceivable size model. Personally, I pretty much set a minimum of 1/16" braided stainless (7x28 is pretty good and flexible) and it's easy to work with - pretty widely available (McMaster-Carr is good).

I just don't 'cotton' to putting something like Kevlar (or any textile material) in such a hostile environment as a black powder ejection charge is. There are better and more permanent solutions to that problem.


-- john.

Click to expand...

You wouldn't have any pictures, would you? I've never worked with braided steel wire. I'm not sure what the crimp fittings would look like, nor where to get them. Are they meant to go with fishing leader? Could I get something like that in a sports store?

I agree that the eye bolt is large for the centering ring (well, it looks large to me. I don't know, really - it's my first time doing this). This may be a "learn it the hard way" experience for me. But I'm not too worried. For one, the rocket is designed to use a paper teabag mount, so I don't think the weight will really damage the centering ring.

But I may be wrong. And if I'm wrong, I do have another one. I got several of these PSII rockets during the Estes holiday bundle sales. This is my first try! I want to build it well, but if I make a critical mistake - well, that's why I got doubles.

lcorinth said:

You wouldn't have any pictures, would you? I've never worked with braided steel wire. I'm not sure what the crimp fittings would look like, nor where to get them. Are they meant to go with fishing leader? Could I get something like that in a sports store?

Click to expand...

Yes, I should be able to get a picture or two - but it'll have to wait until tomorrow. Here's some links to McMaster-Carr's stuff:

Cable:
https://www.mcmaster.com/#stainless-steel-cable/=vub7xw

Swage fittings (ignore the crimpers, unless you have a spare $200+ to spend):
https://www.mcmaster.com/#wire-rope-compression-sleeve-tools/=vuctag
(scroll down to the 'oval sleeves')

Most of this should be available in smaller sizes at fishing supply places - like Bass Pro Shops or Outlet Bait and Tackle. Here's a link to some BassPro offerings:
https://www.basspro.com/Offshore-Angler-Mini-Double-Sleeves/product/8565/

(Don't forget to look at the 'alternates' on the right margin to the page)

(Note this is the 'Offshore Angler' division - which is their saltwater division (bigger stuff), which may not be necessary. If you can find some fishing leader rated, say, 20-50 times the vehicle (recovered) weight, you should be fine). I would imagine, also, that any moderately decent fishing place should have the crimp sleeves as well.

lcorinth said:

I agree that the eye bolt is large for the centering ring (well, it looks large to me. I don't know, really - it's my first time doing this). This may be a "learn it the hard way" experience for me.

Click to expand...

I wouldn't worry about that, Daniel. Those who "learn it the hard way" are those who make it too *small* - and this is not your problem (here) (unless we're talking about the centering ring). And, even there, you could fly this rocket for years and never have the slightest problem - my comments were more about the (apparent) disparity between the eye-bolt and the centering ring it is in - coupled with the fact that I generally try to think 'worst case' on recovery issues. As I noted, backing up the eye-bolt with a fender washer could easily solve the issue quite simply.

The large part of what prompted my reply was the issue of using Kevlar down near the rocket motor. Lots of people do it - many have no problems - but it just seems like a less-than-thought-out solution. If it is installed such that it can be inspected/replaced (like Thirsty talked about), then that is a different situation than a near permanent installation that can't be either inspected nor replaced. A stainless steel cable will last the life of the rocket and never cause a worry. That seems sensible to me.

I'll try to get you some pix tomorrow.

-- john.

I used steel cable like that in my L1 rocket. I bought it at Home Depot and the swage fittings. The guy that cut the cable crimped it for me on the spot. Make sure you know how you want it crimped when you go in and you can probably have it crimped for you without having to buy the crimping tool.

With that said, the cable worked pretty well, but in a 2" ID rocket I had to notch a dowel and use that as a special tool to twist and push the cable down into the tube. Personally, I wouldn't bother using it again. I'll stick with Kevlar. The Kevlar may not last as long, but it seems to last as long as the rocket does. The steel cables seems like a 50 year solution for a rocket that has a 5 year life expectancy.

This is what I have done... on a 3" rocket with 38mm mount:



I found a very long screw bit to attach or detach eyebolts from the bottom. You will need to drill a large hole or cut a pie section out of the centering ring so you can get the bit inside every time you need to remove the shock cord.

Handeman said:

I used steel cable like that in my L1 rocket. I bought it at Home Depot and the swage fittings. The guy that cut the cable crimped it for me on the spot. Make sure you know how you want it crimped when you go in and you can probably have it crimped for you without having to buy the crimping tool.

With that said, the cable worked pretty well, but in a 2" ID rocket I had to notch a dowel and use that as a special tool to twist and push the cable down into the tube. Personally, I wouldn't bother using it again. I'll stick with Kevlar. The Kevlar may not last as long, but it seems to last as long as the rocket does. The steel cables seems like a 50 year solution for a rocket that has a 5 year life expectancy.

Click to expand...

We don't have a Home Depot near me, but I'm going to see if I can find it at Lowe's or Menard's. I might go with the Kevlar option, but I'm not sure I have enough. I bought a 10-foot thread from JonRocket.com, but I've used some of it.

When I have them crimp it, should I take in the motor mount, so they can attach it to the eye bolt for me? I assume crimping it closes a loop, right?

Handeman said:

I used steel cable like that in my L1 rocket. I bought it at Home Depot and the swage fittings. The guy that cut the cable crimped it for me on the spot. Make sure you know how you want it crimped when you go in and you can probably have it crimped for you without having to buy the crimping tool.

With that said, the cable worked pretty well, but in a 2" ID rocket I had to notch a dowel and use that as a special tool to twist and push the cable down into the tube. Personally, I wouldn't bother using it again. I'll stick with Kevlar. The Kevlar may not last as long, but it seems to last as long as the rocket does. The steel cables seems like a 50 year solution for a rocket that has a 5 year life expectancy.

Click to expand...

We don't have a Home Depot here, but I'll see if I can find it at Lowe's or Menard's. I don't know if I have enough Kevlar. I bought 10 feet at JonRocket, but I have used some of it.

If I have them crimp it, should I bring in the motor mount so they can attach it to the eye bolt? Crimping makes a loop, right?

Handeman said:

<snip>
I'll stick with Kevlar. The Kevlar may not last as long, but it seems to last as long as the rocket does. The steel cables seems like a 50 year solution for a rocket that has a 5 year life expectancy.

Click to expand...

What, you don't 'repurpose'?? If you scrounge any usable item (like I do far too often) off of old vehicles, it means you only have to fabricate/purchase the cable assy once for every 10 rockets.

I'm sure you're right, and I won't deny I can be a little OCD on things like this. That 600 ft spool of some braided stainless steel cable I had lying around is probably what did it for me.

That said, I think Thirsty's idea of looping the Kevlar through the eye-bolt (for inspection/replacement) is a truly smart idea. As for your note about having to 'scrunch' down the cable (with that notched dowel), I'm hoping to get my pix uploaded this evening and what I did there was (as I noted previously) have the cable assy routed right up against the inner surface of the body tube. In this application, it never was necessary to scrunch it down -- it simply ran beside the recovery system (the parachute was bag-deployed, but that was in an outer 'bag' (the same size as the recovery bay) made from old ironing board cover - such that there was no worry about anything hanging on the recovery harness. It simply slid past it.) Many times, I wouldn't even connect the shock cord to the harness until *after* the parachute was already loaded in the vehicle (since the harness kind of 'peeked out' from beside of the outer parachute bag). Once I get the pix uploaded, that'll become clearer.

-- john.

lcorinth said:

We don't have a Home Depot near me, but I'm going to see if I can find it at Lowe's or Menard's. I might go with the Kevlar option, but I'm not sure I have enough. I bought a 10-foot thread from JonRocket.com, but I've used some of it.

When I have them crimp it, should I take in the motor mount, so they can attach it to the eye bolt for me? I assume crimping it closes a loop, right?

Click to expand...

Daniel, I would agree that the Kevlar approach is probably the most sensible on this vehicle - at this stage of construction. All these comments (at least mine) are simply meant to give you a few ideas to mull around and try if you see any sense and purpose in them - and discard if you don't.

You are right about closing the loop (through the eye-bolt) - but you might want to slide a piece of heat shrink tubing over the cable assy before you close the upper loop and then slide up over the swage fitting and shrink down over it - as the ends of the cable can be rather nasty with a 'rat's nest' of fine wires peeking out from the fitting and is something that'll catch on practically everything (including your parachute). Smoothing that over with a short piece of heat shrink will neaten things up a little and preclude that from happening.

Understand that my (initial) approach ran both ends of the cable back down below the forward centering ring - such that there were no loose ends to catch on anything. (I'll hopefully get those pix uploaded this evening and it'll be clearer with them out here).

-- john.

lcorinth said:

You wouldn't have any pictures, would you? I've never worked with braided steel wire. I'm not sure what the crimp fittings would look like, nor where to get them.

Click to expand...

OK, here's some photos of some of this 'cable harness' approach:


Here you can see a selection of the various crimp sleeves I have (from left to right): 1/16" Stainless Steel (SSt), 3/32" SSt, 1/16" Tin (plated) Copper (SnCu) and 3/32" SnCu. The SSt sleeves are basically oval in cross section (probably because it may overstress (i.e. embrittle) the material if it were shaped into the 'double-barreled' cross section that the SnCu are (which is similar to a short section of a double-barreled shotgun)). The SnCu ones are thicker in cross section and heavier (due to the copper), but one could probably crimp these on an anvil with a hammer relatively easily (albeit with the caveat that the load rating may be reduced over what a proper-fitting crimper would do).

(NOTE: the way I have been using these is NOT to actually put a load on the connection -- they are simply used to gather/hold a twin cable together, so they are, effectively, carrying no load.)


Here is a mockup of how this all goes together - with a SSt sleeve on a scrap piece of 1/16" cable. The SnCu sleeve is below and to the left. The item on the right is a 1/16" SSt Thimble that is used to reinforce (and lessen wear) on the loop - available at McMaster-Carr here:

https://www.mcmaster.com/#wire-rope-thimbles/=vvckju

(The one you see here is the 'Light Duty - 3/64",1/16",5/64",3/32" 304 SSt')


Here I am holding the whole assembly together (without a decent fold in the cable, the tension in the cable tends to 'snap' back the sleeve down the length of cable, so it needs to be held in place). If actually assembling this, one would fold the cable enough to prevent that.) Upon assembly, you can lightly crimp the sleeve in place - tack with super glue - and flood the interior of the sleeve with some epoxy. If the sleeve is tight up against the thimble, the thimble will be securely held within the loop - but the sleeve *does* need to be rather tight up against it. Putting some heat shrink over this once assembled would be recommended. Again, please remember this would only work if *both* ends of the cable are tied back down to the motor tube, such that the recovery loads are equally carried (and transferred) to the airframe through the motor tube. You *can't* expect the sleeve to actually carry a load unless properly crimped - but this approach makes that unnecessary.


Here's another view of that cable assembly (lower one - showing both the SnCu and the SSt sleeves) - and the upper one is the 3/32" 'factory' crimp cable assembly I mentioned the other day. Note the approx. 3/8" 'tag end' of the cable peeking out from the left edge of the crimp sleeve. All the individual wires of the cable will come unravelled with use and that can catch on practically anything in the vicinity (like your parachute) - which is why I strongly recommend smoothing that whole crimp with some heat shrink tube down over it.


Here's an actual installation (this a 5.68" airframe). The red is some Vulcan epoxy that fills up the SSt crimp sleeve (again, this area should have a bit of heat shrink on it - actually, there's nothing preventing running the heat shrink all the way back down to the forward centering ring.) The loop projects about 6" above the centering ring and, as you can see, exits the ring such that it can lay up against the airframe once the parachute (in it's bag) is placed in the vehicle. As long as the length of the harness doesn't project past the end of the body tube (for zipper protection), it can be any convenient length - but would obviously need to be close to the upper edge of the airframe for smaller diameters - such that connecting the shock cord is easier.

There are no photos of how the cable is anchored back into the airframe (i.e. below the forward centering ring), but, as described the other day, once below the ring, it turns over to the motor tube and the two 'tag ends' are wrapped (in opposite directions) around the motor tube (at least one full wrap). With a little extra length, one could put another crimp sleeve thru those two tag ends (entering from opposite sides) and crimp to secure those ends of the cable assy (structural here). I don't believe I did that -- just wrapped the cable with some fiberglass tape bedded in a healthy bed of epoxy.

Note, also, due to the fact that the cable penetrates the forward centering ring at the outer edge, it could easily zipper it's way through the ring back to the motor tube (with a hard deployment). I reinforced for that possibility with a piece of 1/8" G-10 gusset (in a 45-45-90 triangle shape) directly from the cable entry point back to the motor tube. I also was using both a bag deployment and a reefing cutter on this (in it's latter flights), so my deployments were generally quite gentle. I never saw any sign of stress on the anchorage (as this photo shows).

I am aware that this is a larger, high power vehicle - but the concepts can be scaled to pretty much any size.

'Holler' back with any questions.

-- john.

Use a Quick-Link. Tie the elastic to the Quick-Link. Use lots of dog barf. Works for me!

Lose the elastic...likely failure point. Replace with 1/8" tubular kevlar. I would also replace the screw eye with a cast one as the type you have can open under load.

markkoelsch said:

Lose the elastic...likely failure point. Replace with 1/8" tubular kevlar. I would also replace the screw eye with a cast one as the type you have can open under load.

Click to expand...

That is all very good advice, especially for large rockets.

I've found that for most rockets, the materials supplied by the manufacture usually handles any loads quite well. That includes the elastic and eye-bolts. My opinion is that changes to increase longevity, like Kevlar instead of nylon near the ejection charge are usually good ideas if they don't increase weight, but changes to increase strength are usually overbuilding, especially if it increases the weight.

That is just my opinion, so if you decide to ignore all of it, I will NOT be upset in any way. Good luck and have fun.

Handeman said:

I've found that for most rockets, the materials supplied by the manufacture usually handles any loads quite well. That includes the elastic and eye-bolts. My opinion is that changes to increase longevity, like Kevlar instead of nylon near the ejection charge are usually good ideas if they don't increase weight, but changes to increase strength are usually overbuilding, especially if it increases the weight.

Click to expand...

That's not a bad way of looking at it. While I tend to look worst case on recovery issues (mainly due to the wide array of problems I've seen folks experience), my structural training causes me to constantly investigate ways to improve the 'robustness' of the recovery system - due almost exclusively to the fact that there is almost no hard and *rational* data to actually *design* the system to handle the forces it is to handle (unlike designing a steel beam to resist 50,000lbs of load over a 20 ft span -- there, you know what you have, you know how much steel can handle, so the design is rather straightforward). In a rocket recovery, there are so many unknowns - and very close to nothing as to just how close one is to that 'ultimate strength' ("would 10 more lbs of load cause failure? - or could it handle an additional 500?? 1000??"). If one is 10 lbs from failure, that's too close because of the myriad other variables involved (inflation time, unusual descent profile, etc) that can easily push one over the edge. But, when one doesn't know these things, using the term 'design' is being a little generous - it's more like 'a wish and a prayer'.

With all these flight computers flying today, we have a mechanism to get a little more precise handle on just what those forces are -- which is good. We need that.

-- john.

Elastic can be a real issue, if for no other reason than after a few years, it will simply stretch and become brittle, even if it's not flown that often. I'd use Kevlar of tubular nylon instead (the Kevlar will be easier to pack in smaller rockets). Another thing about eleastic, is that it does stretch, and if you're charge is a bit large or the elastic a bit short, you can have "snap back", where the NC flys out and then bangs back into the AF, damaging the AF.

I just built a Ventris and used 1/8" Kevlar which is rated for 1500#. Good rule of thumb is that a knot reduces the rated strength by 50% at the point of the knot. I drilled an 1/8" hole in the top CR and threaded the Kevlar through it. I then took a 1/2" - 1" piece of 1/8" dowel and tied the Kevlar around the middle. I then pulled it tight against the lower side of the CR and added a dollop of 5 minute epoxy. I know 1/8" dowel doesn't seem strong, but when crossing a small 1/8" gap, it is very strong. Even if it did break, the knot and epoxy would keep it in place. Very strong and simple.