Shock Cord Length

[Dad of Sapling]

Before I go any further I need to check with the brain trust. I'm building a Cherokee E. I've replaced the coupler with an ejection baffle and swapped the tubes so the shorter one is near the nose of the rocket. I know the recommended length for a kevlar shock cord is 3-4 times the length of the body. But is that assuming the attach point is at the motor mount? If I use that number with the attach point closer to the nose I will have 4+ feet of shock cord between the end of the body tube and the nose cone. It seems to me that this can be much shorter. Thoughts?

 

[JimByrne]

A little long won't hurt-----better long than short

 

[mh9162013]

Whether the attachment point is the baffle or the engine mount, with a shock cord recommendation length range of 3-4 times the length of the rocket's body, the difference will be negligible.

 

[MidOH]

It's totally your call. I ain't taking that away from you.

 

[Bill S]

I usually don't count the portion that is inside the body of the rocket and doesn't leave it (say you attach the kevlar at the motor mount and run it up to the front of the rocket, count only the part that extends out of the tube). No real basis for it other than "why not"?

 

[jjoos99]

Not sure if this is correct but from the launches Ive been to the shorter shock cords seem to corkscrew more while drifting down. I put almost 10ft kevlar cord on my nike x and it decends beautiful and straight down.
Jeff

 

[Back_at_it]

Before I go any further I need to check with the brain trust. I'm building a Cherokee E. I've replaced the coupler with an ejection baffle and swapped the tubes so the shorter one is near the nose of the rocket. I know the recommended length for a kevlar shock cord is 3-4 times the length of the body. But is that assuming the attach point is at the motor mount? If I use that number with the attach point closer to the nose I will have 4+ feet of shock cord between the end of the body tube and the nose cone. It seems to me that this can be much shorter. Thoughts?

More shock cord is better. I wouldn't worry about an extra few inches or even a foot in the rocket the size of the Cherokee E. I would need to check mine but I'll be willing to bet there is 6" or 7" of cord in it.

 

[Jacktango22]

I learned a lesson recently. Scratch built BT 70 30 inches launched with E 12 -4, ejection charge pulled out the Kevlar knot on the nose cone. Twice! 100 lb Kevlar with an Albright knot. my conclusion is cord was way too short … about twice body length (measured from the top). Lost the nose cone on the second flight.

 

[Hobie1dog]

Whether the attachment point is the baffle or the engine mount, with a shock cord recommendation length range of 3-4 times the length of the rocket's body, the difference will be negligible.

+1 what he said.

 

[jjoos99]

One thing I always do with my kevlar knots is add a drop of glue to the knot itself. Ive had a couple come untied before I started to glue the knots.
Jeff

 

[jahall4]

A Couple of axioms that have served me and others well…

#1 Harness length is the “great redeemer” it cures lots of ills. IOW use as long a harness as you can. Old saying… if you have room left in the rocket the harness is too short.

#2 If the harness is too narrow to stitch use a knot like a Uni-Knot, they are easy to tie, round”, and untie-able. Never glue a Kevlar knot or harness in general., makes it inflexible and prone to breaking were the glue stops.

 

[Jacktango22]

I hear different theories about adding glue to a Kevlar knot. From what I can tell CA glue is not a good idea, but are there glues that are advisable? TB II for example?

 

[jahall4]

I hear different theories about adding glue to a Kevlar knot. From what I can tell CA glue is not a good idea, but are there glues that are advisable? TB II for example?

Certainly not theory and since there is no reason to glue a knot why do it?

 

[Tractionengines]

One thing I always do with my kevlar knots is add a drop of glue to the knot itself. Ive had a couple come untied before I started to glue the knots.
Jeff

A better option is a short piece if heatshrink over the knot and tail. It will keep it from working loose. But doesn't make a brittle transition that is a stress concentration.

If you use glue something that stays a bit flexible is better and something that sets very hard. (ie. Avoid epoxy and CA; Woodglue or silicone adhesive are better.)

 

[jdsmith39]

For LPR/MPR, I'll do 1 to 3 times the length of the rocket. LPR/MPR I will often use some elastic in addition to Kevlar, which allows me to keep it shorter. For HPR's I generally do 3 to 5 times the length of the rocket. Bigger the rocket the closer I get to 5 times. If using only material that doesn't have shock absorbing (like elastic/rubber band) you need more length.
Another thing to keep in mind is if your rocket lands in a tree and you have a long shock cord, you may be able to still reach the rocket. This has saved me on my Darkstar 4 Dual Deploy.
As far as knots, I use a poacher's knot for the ends.
https://www.animatedknots.com/poachers-knotFor mid loops I use Alpine Butterfly Loop: https://www.animatedknots.com/alpine-butterfly-loop-knot

These knots have never failed me...so far. I don't do anything special on the knots. No glue or shrink wrap (though I do like that idea) on the knots. Poacher's knots, tighten if pulled.

 

[Sooner Boomer]

I hear different theories about adding glue to a Kevlar knot. From what I can tell CA glue is not a good idea, but are there glues that are advisable? TB II for example?

I use Elmer's Craft Bond. It stays soft when cured (I use it to repair spines in paperback books, also, for this reason). I think any white glue would work as well. You might want to thin the glue just a little, so it soaks into the fibers, not just sits on top. If you can't get anything else to work, RTV silicone glue might work. I've used it. Let the knot cure/outgas completely for several days before you put it in the rocket. It will still be giving off acetic acid (vinegar). You don;t need extra sources of corrosion (rocket engines/ejection charges make plenty).

 

[Sooner Boomer]

One thing I always do with my kevlar knots is add a drop of glue to the knot itself. Ive had a couple come untied before I started to glue the knots.
Jeff

I lost a JLCR because the knot came undone. A dab of white glue worked into the middle of the knot made it secure.

 

[MidOH]

A bowline is the one where you need to secure the tail. The other knot I use is permanent, might be a poachers. But someone called it a fisherman knot.

I did glue the JLCR's knot. But only glue the ends of my Kevlar with wood glue.

 

[prfesser]

I use Elmer's Craft Bond. It stays soft when cured (I use it to repair spines in paperback books, also, for this reason). I think any white glue would work as well. You might want to thin the glue just a little, so it soaks into the fibers, not just sits on top. If you can't get anything else to work, RTV silicone glue might work. I've used it. Let the knot cure/outgas completely for several days before you put it in the rocket. It will still be giving off acetic acid (vinegar). You don;t need extra sources of corrosion (rocket engines/ejection charges make plenty).

FWIW RTV silicone caulk is available in several varieties. The most common gives off acetic acid during cure, but there are also types that give off ammonia (less corrosive) and I think there are some that given off methanol (not corrosive).

 

[SolarYellow]

Polyurethane adhesives and sealants are much better than silicone for a variety of reasons if you want it to stay flexible.

 

[Jacktango22]

This is all very helpful guys. This forum has been very helpful to my learning curve as I come back to this hobby in retirement. Great source of practical help. Combined with Open Rocket I am having a blast!

 

[teepot]

I am just finishing a 2.6" rocket that is 56" long. Dual deploy. There is 20' of 1500lb Kevlar on each side of the av bay. Longer is best IMHO. It's going to fly on an I 500. The rocket should weigh in at about 40 ounces. The nose cone is balsa and the eye bolt is secured with JB Kwik Weld. I don't want to stress that attachment point. The av bay is only held together with # 6x32 threaded rods and nuts. I don't want to stress that either. To secure the knots I use high temp engine compartment friction tape. After it gets heated it can't be unrolled.

 

[Sandy H.]

I fly LPR, MPR and a bit of HPR. On the LPR's if they are small, shock cords might be limited by space and patience in packing. For all others, I tend to do very long shock cords, rarely less than 10-15 ft. I have a rocket or two that have fairly heavy nosecones and I put the parachute about 1/3 the shock cord length from the nosecone end. On those, they seem to develop a helicopter-like rotation/oscillation on the way down. It seems to wind the chute up a bit, likely a negative impact for performance of the chute, but also that rotation seems to slow the rocket down a bit. No empirical data, just visual observation. I should try moving the chute either up or down the shockcord to see if that changes anything. . .

Anyway, I seem to fly long shock cords, sometimes wrapped with a piece of tape etc., to slow down the extension process. I rarely have zippers, but not 'never.'

Sandy.

 

[SolarYellow]

Try a swivel on the chute.

 

[TomBroad]

I lost a JLCR because the knot came undone. A dab of white glue worked into the middle of the knot made it secure.

The knot provided by JL is absurd -- see my post about tying a knot that will NOT ever come untied. https://www.rocketryforum.com/threads/jolly-logic-chute-release.159948/post-2206554

 

[BABAR]

One thing I always do with my kevlar knots is add a drop of glue to the knot itself. Ive had a couple come untied before I started to glue the knots.
Jeff

Deleted.

 

[bjphoenix]

Before I go any further I need to check with the brain trust. I'm building a Cherokee E. I've replaced the coupler with an ejection baffle and swapped the tubes so the shorter one is near the nose of the rocket. I know the recommended length for a kevlar shock cord is 3-4 times the length of the body. But is that assuming the attach point is at the motor mount? If I use that number with the attach point closer to the nose I will have 4+ feet of shock cord between the end of the body tube and the nose cone. It seems to me that this can be much shorter. Thoughts?

The Cherokee E is several feet long so we're talking around 8' of shock cord, not 4'. I use 10' of kevlar for my rockets of that length. I attach a piece of kevlar inside the rocket and let it stick out at least 6" with a loop in it, then I connect my 10' to that, so I have more like 10.5' of cord between nose cone and rocket body. I put heavy snap swivels on both ends of the shock cord so I can move it from rocket to rocket.

A bowline is the one where you need to secure the tail. The other knot I use is permanent, might be a poachers. But someone called it a fisherman knot.

I did glue the JLCR's knot. But only glue the ends of my Kevlar with wood glue.

Bowline is the only knot I can easily remember so I continue to use it. (I need to try the Palomar knot someday.) I put a small bit of TitebondII on the knot, it never gets real hard and brittle so doesn't seem to affect the kevlar. Without glue it will tend to loosen with handling.

 

[Hobie1dog]

I'm sticking with my 3/8" wide elastic for all my LPR needs.

 

[SolarYellow]

Whether the attachment point is the baffle or the engine mount, with a shock cord recommendation length range of 3-4 times the length of the rocket's body, the difference will be negligible.

Just pondering this a little. Much like "calibers" for stability factor, this seems to be a rule of thumb that's based on an assumption about an "average" rocket, but in its conception switches from the relevant variable to another one that doesn't directly bear on the objective.

If the goal is to give the kinetic energy of the ejection event some time to dissipate before the distance between the parts reaches the length of the shock cord, then the key variables are how much KE is there at the moment of separation and how rapidly will the parts dissipate that energy.

It's mentioned that a Cherokee-E is a longer rocket, so it should have a longer cord. However, just to make a point, I pulled up an OR sim of the Cherokee E. Reduced the overall length of the airframe by 50 percent and added 10 grams of clay to the tip of the nose cone to get stability back. Same basic rocket. Airframe volume pressurized by the ejection charge is less than half (because the pressurized volume excludes the volume occupied by the motor and mount). So the KE at the moment of separation should be considerably greater than with the unmodified model. Yet the rule of thumb relating to body length (or overall length, if one prefers that formulation) will have us using a much shorter shock cord.

In other words, if 10 ft. plus a leader the length of the body tube plus six inches is the right length of shock cord for the Cherokee E, it's probably also the right length, if not possibly too short, for, say, a Goblin.

Similar concept: The calculations for sizing BP ejection charges are discussed as targeting pressurizing the internal volume to be opened to ~15 psi. A longer rocket will need more powder, but so will a short/fat rocket of increased volume. The amount of BP to be used isn't just determined by looking at the length of the cavity.

 

[mh9162013]

Just pondering this a little. Much like "calibers" for stability factor, this seems to be a rule of thumb that's based on an assumption about an "average" rocket, but in its conception switches from the relevant variable to another one that doesn't directly bear on the objective.

If the goal is to give the kinetic energy of the ejection event some time to dissipate before the distance between the parts reaches the length of the shock cord, then the key variables are how much KE is there at the moment of separation and how rapidly will the parts dissipate that energy.

It's mentioned that a Cherokee-E is a longer rocket, so it should have a longer cord. However, just to make a point, I pulled up an OR sim of the Cherokee E. Reduced the overall length of the airframe by 50 percent and added 10 grams of clay to the tip of the nose cone to get stability back. Same basic rocket. Airframe volume pressurized by the ejection charge is less than half (because the pressurized volume excludes the volume occupied by the motor and mount). So the KE at the moment of separation should be considerably greater than with the unmodified model. Yet the rule of thumb relating to body length (or overall length, if one prefers that formulation) will have us using a much shorter shock cord.

In other words, if 10 ft. plus a leader the length of the body tube plus six inches is the right length of shock cord for the Cherokee E, it's probably also the right length, if not possibly too short, for, say, a Goblin.

Similar concept: The calculations for sizing BP ejection charges are discussed as targeting pressurizing the internal volume to be opened to ~15 psi. A longer rocket will need more powder, but so will a short/fat rocket of increased volume. The amount of BP to be used isn't just determined by looking at the length of the cavity.

Yup, that's why it's a rule of thumb rather than a set rule.

I currently make hybrid shock cords using elastic from Estes kits and a comparable length of Kevlar. Together, they don't follow the "3-4 times body length" rule of thumb, but they work for me.