Three shock cord anchor designs: Which would you choose and why?

Which of these would you chose?

I want to solve the issue I had with my Mariah where the shock cord cut at the lip of the body tube. I also want to make sure I can remove my cord if I want to. So I want a harness where there is something permanent but strong attached to the eyebolt in my engine mount and I want that same harness to stick out the body tube where I can attach my shock cord to it.

First idea; use a single loop of 5/16 Kevlar woven tube.

I tied a square knot in the Kevlar tube and I passed the 1/8 in line through the sewn loop at the end. The 5/16 tube should never break on the edge of my body tube. It is long enough to stick past the end of my body tube and it will take the abrading, not the shock cord. This is strong as hell but bulky.

Second idea; use a double loop of 1/8 Kevlar line.

Here I have a 40 inch long line tied in a single square not. I fold that loop in half and pass one half through the eye bolt. I then tie my shock cord by passing it through the sewn loop as in the previous design. This design has four times as many cords passing over the lip of the body tube. It should be less bulky than the first design.

Finally, I tie a butterfly knot 18 inches from the end of my shock cord and I just use the shock cord with no other cord.

Here I show off my Boy Scout skills. A butterfly knot is strong, can be tied anywhere along the length of a rope, and it takes load in any direction. However, now that I think of it I can't use this knot as shown. I had to pass the motor retainer through the knot twice to get the sewn loop to attach to the knotted loop correctly. I will have the retainer glued into the body tube by the time I ever want to take this knot off. I would be forced to cut the shock cord. This design has another drawback. This is the same size shock cord as broke in my Mariah. I am not much better off. The one thing this has going for it is there are two lines passing over the lip of the body tube instead of one. If I do want to use this design I would have to use a quick link to attach the loops. Quicklinks are heavy and bulky. It's OK to do but I want to avoid it.

I am leaning towards the second design. What do you think?

If your MD retainer is removable then consider having the harness sewn directly onto the eye-bolt. I had Teddy at One Bad Hawk sew my hardness directly onto my eye-bolt for my 54mm MD retainer, which is removable.

What Teddy provided me was a 25' harness with the same eye-bolt sewn in, so it was a quick swap. No additional harness to fail, no quick link to bother with and carry the extra weight, it is a simplified design.

mpitfield said:

If your MD retainer is removable then consider having the harness sewn directly onto the eye-bolt. I had Teddy at One Bad Hawk sew my hardness directly onto my eye-bolt for my 54mm MD retainer, which is removable.

What Teddy provided me was a 25' harness with the same eye-bolt sewn in, so it was a quick swap. No additional harness to fail, no quick link to bother with and carry the extra weight, it is a simplified design.

Click to expand...

The cylinder of the retainer will be glued into my body tube. The idea of a removable eyebolt is interesting. I would need a small right angle wrench. That is likely custom. Some builders glue their eyebolt in so it won't come undone. I am using a lock washer and a lock nut both. It should stay without glue. It would also be removable. Interesting idea.

I looked on McMaster Carr and found a crows foot that might do it.
https://www.mcmaster.com/#1213a1/=12evny6
It might just fit. I might have to grind off part of the back of the crows foot if I bought one to get it to clear the wall of the body tube.
It costs $15. The shock cord I show in the pictures above I already own so no additional expenditure. However, for a 54 mm I would probably consider it.

TomSmith58 said:

The cylinder of the retainer will be glued into my body tube. The idea of a removable eyebolt is interesting. I would need a small right angle wrench. That is likely custom. Some builders glue their eyebolt in so it won't come undone. I am using a lock washer and a lock nut both. It should stay without glue. It would also be removable. Interesting idea.

I looked on McMaster Carr and found a crows foot that might do it.
https://www.mcmaster.com/#1213a1/=12evny6
It might just fit. I might have to grind off part of the back of the crows foot if I bought one to get it to clear the wall of the body tube.
It costs $15. The shock cord I show in the pictures above I already own so no additional expenditure. However, for a 54 mm I would probably consider it.

Click to expand...

Having a removable retainer is not that difficult and provides the benefit of ease of serviceability. Have you considered not gluing the MD retainer in?

View attachment 291002View attachment 291003View attachment 291004

That is maybe the best idea yet. I will think about it.

TomSmith58 said:

That is maybe the best idea yet. I will think about it.

Click to expand...

I believe this is one of those decisions that you may regret if you were to permanently glue the retainer in, and here is why.

I slightly bent the eye-bolt on one flight and decided to replace it (see below). This was before I had Teddy sew a recovery harness to an eye-bolt. I am not sure how I bent it, possibly the quick link had something to do with it, but I am glad I was able to replace it. Now I disassemble the retainer after each flight, do a visual inspection, as well as clean, which is also much easier using this method.

View attachment 291032

If you decide to pursue this method I would recommend using self clinching nuts as they have groves on the face to help adhere to the retainer. When I epoxied the self clinching nuts onto the back/inside of the retainer, I also roughed up the spots quite a bit to give the epoxy something to adhere to on the retainer side.

Measuring for the placement of the retainer is simple. Assemble the biggest motor you plan to fly with the retainer attached and screwed half way in, make sure the bottom of the airframe lines up with the thrust ring then carefully measure this outside the airframe and double check before you drill. I made sure everything was okay, marked my first hole then inserted the assembly into the aft end of the airframe and drilled the hole though the airframe and the retainer. Then I removed the retainer epoxied my first nut on let dry over night. Once dry you can reinsert the assembly into the airframe and make a witness mark on the inside of the airframe to match a witness mark inside the retainer, then screw it in. Now double check everything lines up and if so go ahead and drill the last two holes.

Once this is done then you can countersink the holes until the head of the screw is flush. This may open up the hole a bit depending on the thickness of the airframe so do it slowly and be careful. Also make sure you use a decent countersink so it does not wobble in the hole and misshape it off center, a drill press helps if you have one.

I don't see why a single strap attached to the retainer wouldn't work. Reinforce it where it hits the end of your BT. Heat shrink, nomex, more Kevlar, something. Personally I hate seeing two pieces of "harness" tied together like that, I am talking about your harness to cord connection. No reason in particular really, just the iron head and rigger in me. We never rigged two straps together, always used a shackle in between.

A few companies, Owner being one, makes "indestructible steel rings" for fishing. I'm not sure on all their sizes but maybe tie one of those to the end where the harness leaves the tube. So that the ring hits the BT lip and not the harness. Just a thought.

Mike

Mpitfield, what self clinching nuts did you use? Do you have a part number or a link?

TomSmith58 said:

Mpitfield, what self clinching nuts did you use? Do you have a part number or a link?

Click to expand...

I sourced mine from Fastenal, likely not the least expensive but it was convenient for me. Here is a link to one that you could consider. I would use stainless and you could go with either 4-40 or 2-56, I use both depending on the application, one thing you want is a low shank for this application. The shank is the height of the self-clinching nut that sticks up, you don't need a high shank so the nut in the link below has a very low shank.

https://www.fastenal.com/products/details/0127351?term=0127351

If you look up PEM nuts it is the same thing as a self clinching nut, PEM is just a brand name...just pay attention to the shank height.

The screw you would use should be a stainless machine screw flat head screw and your screw head preference. I used a torx t-10 head and I cannot recall the length but it was likely 5/16.

mpitfield said:

I sourced mine from Fastenal, likely not the least expensive but it was convenient for me. Here is a link to one that you could consider. I would use stainless and you could go with either 4-40 or 2-56, I use both depending on the application, one thing you want is a low shank for this application. The shank is the height of the self-clinching nut that sticks up, you don't need a high shank so the nut in the link below has a very low shank.

https://www.fastenal.com/products/details/0127351?term=0127351

If you look up PEM nuts it is the same thing as a self clinching nut, PEM is just a brand name...just pay attention to the shank height.

The screw you would use should be a stainless machine screw flat head screw and your screw head preference. I used a torx t-10 head and I cannot recall the length but it was likely 5/16.

Click to expand...

Odds are there is a Fastenal local to the OP, they will probably have to order the PEM nuts but their price should still beat Mc-Master Carr because of shipping.

I like a water knot on a single cord with the loose end taped to the main for all my attachment points. If space is an issue, Teddy's sewn loops are very space efficient.

The loops, double loops, and Y harness methods are good, just be careful that it doesn't loop around your chute / extra cord and prevent a proper deployment.

With an eyebolt, make sure a spinning chute can't unscrew it on the way down. Never made a min diameter, but those are the things I'd watch out for.

rharshberger said:

Odds are there is a Fastenal local to the OP, they will probably have to order the PEM nuts but their price should still beat Mc-Master Carr because of shipping.

Click to expand...

Sadly I would not know about Mc-Master Carr's shipping as they will not ship to Canada, at least they never used to...I have not checked in a while.

cavecentral said:

With an eyebolt, make sure a spinning chute can't unscrew it on the way down. Never made a min diameter, but those are the things I'd watch out for.

Click to expand...

Good point.

If you look at the top of the eye-bolt you will also see a locking nut in place. These Aeropack retainers come with these just to to prevent the eye-bolt from backing out. Because I like to overbuild everything, I also added some red loctite to the screw to give it a git more resistance.

After having a "strong enough" piece of kevlar fail, I'm a HUGE fan of removable, inspectable, replaceable kevlar/nylon. Whatever is lightest/easiest for you is the right choice. If you fly it and don't like it, change it. (Lather, rinse, repeat). :wink:

PEM NUTS: https://www.missileworks.com/store/#!/PEM-Nuts/c/5760492/offset=0&sort=normal or Ebay

I would use a piece of 1/8" with a loop on both ends. Either a sewn loop or one formed by threading the end back inside the tubular Kevlar (forgot the name for that technique). Both loops should extend far enough out of the BT that the doubled part of each loop meets the end of the BT. I would feed one end through the eye bolt and then both loops through the lower ring of a ball bearing swivel. Put the loops over the swivel and pull tight so they both lock on the lower ring of the swivel. Then attach your shock cord to the other end of the swivel in any manner you wish.

I've found that with DD the fin can is what tends to spin and twist up the lower shock cord. The higher you fly the longer you fall to main deploy and the more that lower shock cord twists up. A good ball bearing swivel on the fin can works great and unless you put a huge charge in for apogee there is very little load on the swivel during flight. Make sure you use a good ball bearing swivel with welded rings. The cheaper ones don't work well and don't hold up.

You can always use one of these on the retainer. You can install/remove with a socket. Also makes it easy to install cord.

Rocketry Nut

I've never included a swivel in any recovery system. I've seen far too many failures because of swivels. Because both parts are free to rotate, you typically don't get tangles. I've been flying for 15 years and haven't ever had anything come down with the parachute shroud lines twisted.

Edward

AlphaHybrids said:

I've never included a swivel in any recovery system. I've seen far too many failures because of swivels. Because both parts are free to rotate, you typically don't get tangles. I've been flying for 15 years and haven't ever had anything come down with the parachute shroud lines twisted.

Edward

Click to expand...

I am only in my 3rd - 4th year of HPR and although I started off seeing a swivel as a way to mitigate issues due to the components rotating in opposite directions, I am slowly coming to the conclusion that they may not be necessary and just a point of failure. Keep in mind this is based on limited experience compared to a lot of rocketeers who use them and those who don't.

My current thought or working theory is that if I have a long enough shock cord then any apposing rotating would be taken up by the slack in the harness. This may not be true for all flights and conditions, such as higher altitudes, however so far I have tested this up to 16,000 without any issue. Again this is with a limited number of flights so YMMV.

I would be interested to hear both sides from those with a much wider base of experience.

AlphaHybrids said:

I've never included a swivel in any recovery system. I've seen far too many failures because of swivels. Because both parts are free to rotate, you typically don't get tangles. I've been flying for 15 years and haven't ever had anything come down with the parachute shroud lines twisted.

Edward

Click to expand...

I've been flying 13 years and I've never had anything come down with the main parachute shrouds twisted either.

My last L3 flight didn't have a swivel on the fincan and even after coming down under main from a 1000 ft, the shock cord between the av-bay and fincan was twisted so bad from the fincan spinning on the drop from apogee that it was less then half its normal length and over half the shroud lines of the drogue chute were twisted up in the shock cord.

I only recommend a high quality ball bearing swivel on the fincan on DD flights, never on the upper shock cord or on the main chute. Not all fin cans will spin while dropping, but many spin quite fast and that is where a swivel works great. Putting the swivel on the fincan also puts the swivel at the point where shock loads should be lowest, during the apogee charge. The only time you'll see large loads on the swivel is if you fly drogueless and the fincan is above the payload when the main deploys and the fincan drops all the way to the end of the shock cords. If that happens, the swivel can be a weak point, but so can the regular shock cord, quick links, or eye-bolts when you allow that kind of excessive shock loading.

I've had the opportunity to fly a wide range of rockets so far. Minimum diameter to 27k and scale Delta II rockets and everything in between. Our club is also decent sized, and on a good weekend launch you can see a couple hundred HPR rocket flights.

In general, recovery seems to be the last planned part of a rocket. Motor, airframe, let's stuff the rest in there and make it fit. Good recovery, in my opinion, takes careful planning and is not a one size fits all solution. I have a very different approach if the rocket is 5.5" and goes to a 1000' vs. a larger project that weighs hundreds of pounds. I do have some things that I will always do. I always open the parachute in a strong wind or behind a car and see how it behaves. If it spins, twists or acts odd I'll re-balance the parachute. Since I started I've been sewing my own parachutes. I use a toroidial style parachute currently, but sometimes when something is a simple a flat circular sheet will do. I also go very slow on descent - 5-10 fps. I don't like hard landings. I also use a positive system to get the parachute out. This is either a typical piston or other cylinder. I learned from a very successful flier how simple pistons were, and how well they work. I minimize connections to things and typically use fid loops (chinese finger trap) or tied loops. I trust sewn harnesses, but just don't like being tied down to a certain length and move my harness' around my rockets.

One thing on spinning and tangling is I have a parachute that I use on a 4 pound rocket that is a rotofoil. It is made to spin and rotate while coming down. Even with this parachute, I don't have lines twist, everything just spins. I have long-ish harness lengths. 5" and under are typicall 25-30'. Larger is a case by case basis. Maybe I've been lucky, but having the parachute come out nicely, unfurl easily and inflate in a predictable sequence has always worked. I see people do the 'stuff the bag with the parachute wad' and hope for the best. I'm too detail oriented for that.

Edward

No matter what you do, please do not use a square knot to join the two ends. Square knots are not good at taking sustained loads, and are especially lousy for shock loads. They're ideal for temporarily holding stuff (like reefed sails, hence their other name: reef knot). Use a sheet bend instead of a square knot in anything you want to hold a load.

Photo 3 could also be done removably by tying a bowline to the butterfly knot instead of the splice or whatever is there under the black tube.

None of this is to say you shouldn't do the removable MD retainers like everyone else said.

Guys
Thanks for all your advice. I will show you what my final design is when I get to it this weekend

Presuming that you use an eyebolt, my idea works like this... tie a loop on one end of your kevlar (I use a bowline knot). push that through the eyebolt, pull it up almost to the top of the parent body tube. Mark the other end just short of the top of the body tube, and tie a loop on that end. Now both ends should be just short of the top of the tube. Tye a loop into your other shock cord material (tubular nylon, elastic, whatever). Attach all three loops with a quick link.

With this method, you can pull the kevlar around to inspect it/replace it quick and painlessly, and the edges of it will be too short to zipper the body tube.

I don't fly MD, usually, but if it were me, I'd use the removable anchor modification listed above, and perhaps a small length of a Nerf Dart at airframe contact point to distribute the force should the harness come into contact with it...


Later!

--Coop