Choosing elastic size for shock cord

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neil_w

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I use braided elastic + Kevlar shock cords for my LPR builds. It occurred to me that I have no particular formula for deciding which size to use for any given build.

The extremely vague heuristic I've been using is:
small rocket -> 1/8" elastic
larger rocket -> 1/4" elastic
something large bordering on MPR -> 3/8" elastic (only used once so far)

I would like to be a little smarter about this. How can I intelligently determine the most appropriate elastic size?
 
Well, here's how I look at it. Say it's an Alpha. That would get an 1/8". A Big Bertha would get a 1/4". Anything bordering MPR would get Kevlar. Consider the weight factor also.
 
I agree with those two data points. I'm having trouble with the in-betweens. :)

Current case in point: the IRIS-T. Because I have a small-ish laundry compartment, I was thinking 1/8" would help things fit more easily. But then I was having a hard time convincing myself that it would be sufficient... and that's when I realized that I don't actually have any sound basis for my thinking.

Possible variables to factor in would seem to be:
1) Mass of rocket
2) Mass of nose cone
3) Motor class (=> ejection charge strength)
4) Size of BT (practical consideration)

In Starship Avalon I used 1/8" because I had no choice, needing to fit into a BT-50. In a BT-60 rocket I would always use 1/4" because there's plenty of room. BT-55 rockets seem to be the gray area, and of course that's where I spend most of my time. :)
 
Consider a combination of elastic & Kevlar. You can get more Kevlar in a space than you can the elastic. Learn a knot or two, and I like to glue/shrink wrap that knot joint as well.
 
Consider a combination of elastic & Kevlar. You can get more Kevlar in a space than you can the elastic. Learn a knot or two, and I like to glue/shrink wrap that knot joint as well.
Come on, man, I've only just mastered tying my shoes.

I do use Kevlar inside the BT between motor mount and the elastic, but haven't yet tried using Kevlar for the external cord. In that case, length of the elastic piece would become another variable. Although if the elastic where short I'd probably go right to the 1/4".
 
I just use Kevlar for everything. My first rocket snapped the elastic I used, so I switched to Kevlar and haven’t looked back.

My ONLY gripe with Kevlar is that the 63kg (140lb) kevlar I get from my LRV is epoxy coated to prevent fraying and it makes it quite stiff - it can be challenging to stuff into a small rocket. The solution was simple, I switched to 3mm (1/8in) braided Kevlar (400lb from memory) which is softer and stuff easier. I even used in on my 29mm MD quite successfully...
 
What do you do for zipper prevention?

My luck has been pretty good with elastic so far (no snappages yet); my biggest issues have been when the fit is too tight in the BT and it gets jammed.
 
I use the softer braided 1/8" Kevlar inside to just above the airframe, and elastic for the remainder. I may also choose to go with all Kevlar. All this providing it's necessary to go that distance. If it's a smaller rocket (Alpha as example) I'll go with all elastic.
 
Elastic can go all the way up to a 7.5" tube. While I don't personally use it in anything larger than a 3" tube his has it's benefits and faults. Elastic is lightweight, wider and lower cost than kevlar. The width helps prevent zippering which is a bonus over kevlar. But it will wear out and need to be replaced where kevlar usually last the life of the rocket. 54mm -2.56" I'd go with 3/8", and 1/2" for 3 and 4". 3/4" or 1" for 5.5-7.5". But only if you're staying low and slow. It won't handle overly aggressive separations. As stated above, a kevlar shock cord mount is a great idea where hardware may not work.
 
What do you do for zipper prevention?
I usually use Kevlar on my LPR and MPR builds. I always use the 1/8" braided stuff because it is thicker and less likely to floss the edge of the tube. The breaking strain is ridiculous overkill of course, but that is only one of the parameters worth considering.
 
I start using 1/4" elastic when a BT is 60 or better. Usually on a longer model. Squatty rockets made with a BT 60 might get 1/8" depending on how close it is to the motor. Then splicing in some Kevlar may be in order.
 
Darn it. I’m stumped. OneBadHawk sells 3/8” Kevlar for MPR uses rates at 1800 lb. Apogee sells #300 Kevlar they claim works on G motors. I have a 24mm MD Scratch Build and am seeking opinions in another thread. Will use HPR motor.

If I could just find a nice wall to bash my head into.
 
Darn it. I’m stumped. OneBadHawk sells 3/8” Kevlar for MPR uses rates at 1800 lb. Apogee sells #300 Kevlar they claim works on G motors. I have a 24mm MD Scratch Build and am seeking opinions in another thread. Will use HPR motor.

If I could just find a nice wall to bash my head into.

No head bashing required. On a 24 mm rocket the Apogee stuff should be fine, or even the elastic like Aerotech packages with their kits, which might be 3/8 or 1/2 inch wide.
 
On a 3.34 pound HPR I picked a shock cord that was 1500 lb rated Kevlar. Approx 450x the Force rating divided by the weight.

If I Scale that from 0.635 pound rocket multiply by 450 I get 285 lb rating cord. And I don’t think it scales necessarily. That’s why I’m struggling with picking cords. I’m wondering if the only way to know is ground test.
 
I am using the the 1/8" braided stuff (or aroundabout that) from Apogee for my 38mm MD. If you are concerned about the windmilling cutting the cord on the way down just add a bit of masking tape or something around the rub point.
 
Possible variables to factor in would seem to be:
1) Mass of rocket
2) Mass of nose cone
3) Motor class (=> ejection charge strength)
4) Size of BT (practical consideration)

The other major factor is the cord length and the elasticity of the cord.

When the ejection charge goes off that energy forces the cone/body pair to fly apart.
Then aerodynamic drag slows the two parts down with respect to each other.
If you had a long enough cord (with sufficient altitude) then the cord would not need to be very strong - just enough to support the weight of the pieces.
However, we typically cannot put in such a long cord.
So now the cord needs to absorb some of the energy from the ejection charge.
The shorter the cord, the more energy. More energy, the stronger the cord needs to be.
In addition, a kevlar cord has virtually no elasticity so when you get to the end of the cord you'll have a sudden shock as the two pieces need to bring their relative velocity to 0 "instantly". Hence you need a longer cord to allow the aero drag to slow the pieces down some first.
Whereas an elastic cord will stretch (to its point of breakage) to gradually absorb the energy of the two pieces flying apart. And then pull the pieces back together again (someone say Estes "smile"?). Again, a longer cord will allow both the aero drag to slow the pieces down before needing to stretch and to slow back down from the elastic pulling the pieces back together. You can trade off a longer 1/8" elastic vs a shorter 1/4" elastic in terms of strength vs space.

Then you also need to factor in strength of the material vs bulk, type of elastic (straight rubber, braided, knit), safety factors (both to include special cases like a super shotgun ejection charge and wear/degradation over time), etc.

Bottom line - I don't think there is a simple "For situation X, use Y length of material Z" solution
 
Nosecone mass 31.8 grams.
Motor CTI G-145 with 0.6 grams ejection charge.
24mm MD. (cramped)
I’m thinking of stepping it up to #500 cord.
 
You've got an ounce in the nose and maybe 2-3 in the butt.

Query: ballpark the speed, acceleration, and jerk necessary to exceed the downrated WLL for the skinny stuff.
 
When the ejection charge goes off that energy forces the cone/body pair to fly apart.
Then aerodynamic drag slows the two parts down with respect to each other.
If you had a long enough cord (with sufficient altitude) then the cord would not need to be very strong - just enough to support the weight of the pieces.

I've watched a lot of highpower ejections where the parts slowed down by the time they reached the end of the cord just like this, 30' of cord seems to help. I have been trying to figure out how long this needs to be for small models and I'm thinking about trying 8-10' of thin kevlar.
 
Neil, I'm in your elastic camp as well. I prefer stretchier cords like that (nylon for hi power).

Consider the comparison of the QCC explorer to your Iris-T, it uses a 1/4" elastic and I can measure it when I get home. For some reason I've always associated the IRIS-t as being comparable to the QCC.
 
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