What weight kevlar is suitable for my rocket?

[jrap330]

Regarding: “I don't think the shock-cord can be too long.” In post 22.

rocket internal space permitting, this may be true. I can only speak for low power, but some of Estes designs are exceptions that prove the rule that Estes makes great rockets. Specifically sometimes there is very little space available, and trying to stuff too much shock cord in there may make for a failed deployment (for low power, the rule, “if you can’t blow it out with your breath, something is too tight” applies.). Longer cords also need to be packed with a bit more care, they tangle easily, and a knotted long cord may have a very SHORT effective length.
Stronger (and thicker) cords are accordingly tougher to pack.

so I might say “put in as long a cord as you can easily pack.”

Here’s a nice trick for getting a lot of cord into a small space by Tim Van Milligan. It is a bit time consuming but it works quite well

Just jump to 3 minute in video.

Regarding: “I don't think the shock-cord can be too long.” In post 22.

rocket internal space permitting, this may be true. I can only speak for low power, but some of Estes designs are exceptions that prove the rule that Estes makes great rockets. Specifically sometimes there is very little space available, and trying to stuff too much shock cord in there may make for a failed deployment (for low power, the rule, “if you can’t blow it out with your breath, something is too tight” applies.). Longer cords also need to be packed with a bit more care, they tangle easily, and a knotted long cord may have a very SHORT effective length.
Stronger (and thicker) cords are accordingly tougher to pack.

so I might say “put in as long a cord as you can easily pack.”

Here’s a nice trick for getting a lot of cord into a small space by Tim Van Milligan. It is a bit time consuming but it works quite well

Just jump to 3 minute in video.

I saw this a few months ago and I gave my dame opinion, too complicated and hard especially for " non-coordinated" types like me so I will stick with elastic for low power and have trouble removing them off my finger.

 

[afadeev]

Regarding: “I don't think the shock-cord can be too long.” In post 22.

rocket internal space permitting, this may be true. I can only speak for low power, but some of Estes designs are exceptions that prove the rule that Estes makes great rockets. Specifically sometimes there is very little space available, and trying to stuff too much shock cord in there may make for a failed deployment

The above initial statement was mine, and its application should be appropriately constrained by common sense.
Comically excessive lengths of shock cords are a waste money and space. But I doubt anyone here is actually thinking of stuffing 100-foot-long shock cord into a LP Estes rocket.
A few of the times I was pressed for interior space (e.g.: 18mm motor Estes X-wing, or SR-71), my biggest challenge came from getting the chute to fit in. Second-most problematic item was the Nomex blanket. And lastly, came the considerations of the shock cord tangling.

In tiny rockets, I ended up resorting to the following two space saving measures, roughly in order:
1). Give up on my regular preference for Nomex blankets (4x4" work great otherwise), and stuff a few sheets of Estes disposable wadding - takes less space.
2). Keep trying smaller and thinner-mil chutes to see if they fit. If none do, fall back on the semi-disposable Estes plastic chute - takes less space. Bring a spare to the launch site (Estes plastic chutes have a very short life span, and otherwise suck).

(for low power, the rule, “if you can’t blow it out with your breath, something is too tight” applies.). Longer cords also need to be packed with a bit more care, they tangle easily, and a knotted long cord may have a very SHORT effective length.
Stronger (and thicker) cords are accordingly tougher to pack.

I just wrap my shock cords around the wrist, then stuff the oval bundle into the tube without too much trouble or any worries. They always come out and extend just fine.
Maybe it's just me, but I've never EVER had difficulty packing shock cords or harnesses. Ever.

Here’s a nice trick for getting a lot of cord into a small space by Tim Van Milligan. It is a bit time consuming but it works quite well
Just jump to 3 minute in video.

I'm sorry, but that's just nuts!
Crochet-ing shock cords with a needle/hook?
Having "hard time stuffing shock cord in" the rocket? Really?
Maybe after one too many?

For me, this is just a needlessly complicated solution to a problem that doesn't exist.


a
P.S.: I mostly enjoy Apogee videos and subscribe to Tim's channel, but some of his advice is a bit questionable, or obviously self-serving (buy my gizmo XYZ). I've thrown out a few things I bought from him over the years, so this observation reflects my first-hand experience.

 

[MidOH]

Do any of ya'll Z fold your shock cord, and then tape some of it? That would soak up hard hits.

That's all that the shock damper on my climbing harness (err, falling harness?) is. It's just 6 feet of heavy strap, z folded into a shrink wrapped sleeve. The energy required to rip the strap free softens the fall.

 

[bjphoenix]

I've had problems in the past with the elastic rubber bands and with various white elastic cloth straps. When I resurrected some of my old LPR recently I switched to Kevlar. At first I used some smaller single strand Kevlar because that's all that I had, I tested it to around 75#. Then I bought some 150# kevlar and use that. I think for most LPR retrofits the kevlar will be stronger than the attachment. Regarding length- people say 2 or 3 times the body tube length, but this makes no sense to me. So my Executioner would need 3 times the length that my Big Daddy would need? I just figured an average BT length and used twice that- about 10'. In a dozen launches this has worked well.
I don't think there is much load at ejection- everything blows out a little bit and stops before reaching the end of the cord. The biggest problem is that people watching the flight think you've had a separation before it is clear that airframe and nose cone are descending at the same rate.
Now if you had a very long delay and things were going down pretty fast- the load on the cord would be from the chute opening. Maybe this could still cause a zipper, I just try to have delays that aren't real long.
I'm trying to devise a method to hold the long shock cord in the body tube. I did an experiment with a small spool I made- wrap the cord around the spool and let it unroll on deployment. This has promise, I need to try it some more.

 

[waltr]

Yes, use a longish length of kevlar cord.

With smaller cord, <150#, I do the chain stitch (Apogee rocket has a video showing this).
The chain makes inserting the long cord into the BT easier and prevents tangles. It also absorbs energy as a stitching is pulled out.
On thicker cord I do 3 Z-folds holding them with masking tape.
This works very well for getting the cord into the BT and absorbing energy.

Motor eject Delay time is also important. Ejecting just past apogee is best since the rocket has minimum velocity at that time.
I have seen at club launch rockets with too long a delay. They were trying to have the rocket lower before deploying chute to reduce wind drift distance (we have lots of tall trees around).
The longish cord was not Z-folded and taped. The rocket was low enough to clearly see the chute deploy, the cord pull tight and the booster section abruptly flip around. The BT was Zippered.

 

[Back_at_it]

I made the switch to Kevlar and Elastic when returning to the hobby this time around and I've played around with various sizes. For a while I use to used only 100# Kevlar as it was stronger than any connection point on a LPR rocket. I though this was a good idea but I ran into a problem. The very thin cord was cutting through the connection points. I had one slice completely through the connection point on the nose cone.

I don't use the stock loop on the cones. I cut that off and drill holes and loop the Kevlar through the nose cone so I figured that this would be plenty strong. Lets also not talk about what it will do to a body tube. This was a Mini Honest John flown on a B6-6 with about 6ft of 100# Kevlar.

Shortly after that incident I started doing a Kevlar leader then attaching Elastic to that. For a while I thought I had found a solution. My recoveries were good and I was happy.... For a while. Then I had an Estes AeroSpace Plane separate. The body body tumbled to the ground and the nose cone, elastic and Kevlar came down under the parachute.

Upon inspection I found that my 100# Kevlar had burned through on it's 6th flight. I looked at the cord on that and a few other models and saw the same pattern. The cord was getting burnt within the first two inches above the motor mount. My theory was that this is the part of the cord that was below the wadding and taking the brunt of the heat.

I decided to use heavier Kevlar anywhere I could and protect the first few inches of that with some eat shrink tubing. In short BT50 rockets I'm comfortable using 300# Kevlar. In longer models, 500# and up. Since doing this I haven't had any issues with burn through or separation. I've also moved to adding baffles to just about everything so that helps. Just be sure that the Kevlar stops below the end of the body tube to prevent zippers.