Length of shock cord?

[pjbatglen]

I was wondering how to calculate the length of the schock cord. I want to design my on rocket and I was unable to locate a thread that explaned how to calculate the length. I would like to used the kevlar string to anchour the elastic shock cord to the chute.
The rocket will be powered by E engines.

Thanks
Paul

 

[Austin]

Nylon and Kevlar shockcord should normally be 3-5 times the total length of the rocket. Elastic I make normally 2 times the model length, just for safety...too long and it can tangle.

Hope this helps,

Carl

 

[sandman]

go to a fabric shop and buy a couple of extra yards of "good" elastic shock cord.

It's really cheap!

sandman

 

[trogdor]

Nothing against Carl's response, this has been a general rule of thumb for as long as I can remember, but I've always felt that the "rules" for efficiently and sufficiently installing shock cord are way too general!

For example, I've used kevlar only in an astromcam test subject and even though I only used an Estes C11 with its associated BP ejection charge AND over 6 ft of kevlar, I still had the weighted (exactly the astrocam film-loaded weight) nosecone break (on ejection) a decent quality swivel right in half...thus losing the nosecone. So this cord was about 3 times the body length...

Then I've had BT80 size rockets (say the Estes Optima/Shadow) with relatively short (2.5-3 feet) 1/4 inch nylon elastic only survive many many flights fine (except the core sample that was my failed attempt at a new improved and cheap baffle!!

I've had more success with kevlar/elastic nylon combos in shorter smaller diameter rockets (I think the bungee effect saves from having to use a mile of kevlar to avoid the astro test above) and kevlar avoids the cord cooking inside...

and for large LPR/small MPR (24mm reloads and SU motors, Estes/LOC parts) just using 1/4 inch elastic 4-6 feet or so is fine... also can depend on how much stuffer tube on BT80 type rockets... like old Shadow had only 10 inches or so of parachute tube with the rest of the 4 foot rocket having a 24mm stuffer tube. Then theres TLP kits that don't use stuffer tube but are equally as long... so BP from a D12 say can't push nose out as hard...

I guess what I'm trying to say is that there is no "unified theory" on this that really works in my experience and a few model types should be referenced for much better guesstimates..

i.e.

small diameter non-superroc types = kevlar tied to 1/8 inch nylon

BT80 or so with/without stuffer tube

anything with extra heavy nosecone gets special treatment...

etc...

The problem seems to be not so much length related but internal volume to pressurize for the designed motors and the weight of nose and/or recovery system....

what do you all think? How to better categorize rockets for this? Just my thoughts.....

 

[EMRR]

I agree with a kevlar to Elastic combination for mod-roc. I like my kevlar to stop just below the top of the body tube. I then like 2-3 lengths of the rocket of elastic.

This is funny though, because if I had a 48" rocket flying on B and C's I wouldn't use 8 feet of elastic!

So here's my gut feeling, the distance between an A-C motor and the end of elastic (that attached to the nose cone) needs to be 36-48".

If flying multiple A-C's, or D's and E's then it needs to be 6-8' (feet) from the top of the motor.

If using RMS 24mm you are still okay, but you will not want to use all of the powder for shorter/narrower rockets.

If your nose cone is weighted, then you'll need to make a judgement call. Say add a foot for every 1 ounce of weight (hey, that's my stab).

BTW, why hasn't anyone wrote a program for this considering we can determine the amount of energy created with the exiting nose cone and the amount of shock energy that can be disappaited by air, elastic, and kevlar. I personally couldn't do this but there are some real mathimaticians in this group that could.

Nick

 

[EMRR]

Originally posted by EMRR
So here's my gut feeling, the distance between an A-C motor and the end of elastic (that attached to the nose cone) needs to be 36-48".

Hmm, after reading my own post, I guess there should also be a minimum length incase your rocket is > 36"!

So perhaps a minimal length is 36"?

This should really confuse the thread originator (which was not the intention).

Nick

 

[Austin]

Hmmmm...powered by E Engines...guess I missed that one, so...

On smaller rockets, I run Kevlar cord/thread just outside the bodytube, then tie it to the shockcord and cover it with heatshrink. The heatshrink keeps the Kevlar from zippering. Most smaller rockets I use a 2-3' piece of elastic...Mid power I still use elastic 2-3 times the BT length.

...use my 3-5 times the length on HPR birds, not small stuff. Also, to avoid seperation/zippering, bring down your heavy nosecones on a seperate parachute.

Carl

 

[rstaff3]

I seem to be having a hard time organizing my thoughts this AM (time for another cup of java?), so here's my 2 cents in an unorganized fashon:

For HPR and MPR which use non-elastic cords (tubular nylon, Kevlar, etc) I pretty much agree with Carl's assessment. There are ways to verify this more rigorously, which involve the strength of the cord, the amount of 'stretch' of the material, weight of the rocket, assumptions about the worst case free fall, etc. I prefer to just use the common wisdom and don't worry about it. You can get by with a lot less if ejection is perfect each time, but these lengths allow for some miscalculation in this regard.

Now for the other extreme. On modrocs, elastic only, you can't go wrong with a 2x length...unless you have a small BT and packing problems. Kits generally don't come with this much. My favorite is the kevlar/elastic combo previously mentioned. I have used Kevlar leaders sized from about 1x the rocket length down to where they reach to just inside the top of the BT. I find if they are on the longer side, it is easier to protect the elastic portion from ejection. On the otherhand, if they extend past the BT you increase the chance for a zipper. The combined length is nominally 1x to 2x the length of the rocket. But then I occasionally fly the old, short estes rubber bands successfully

On midpower, I am also leaning to the kevlar/elastic combo. I have several kits that use a wire leader and an elastic cord (NorthCoast and Thoy). The length of the elastic is probably about twice the rocket length. These have shown some deterioration, so I recently added 1/8" kevlar leaders.

I bet you are thinking maybe I've had too much coffee?

 

[Micromeister]

I'm a little torn on this one also Dick:
I don't fly HRP so I have no clue on that catagory. But I've stopped using elastic only on Model Rocket and LMR's up to the 3.3lb limit. I've switched to Kevlar/elastic combinations on everthing except Micro-Maxx. 10.5 to 24mm models I go with 70 lb kevlar tied to the motor mount or thrust ring, positioned against the OUTSIDE of the mount or ring and ending even or just short of the forward end of the bodytube, a 2" piece of 1/2" masking tape about an inch inside the body centered around the kevlar has proved most helpful as a zipper stop. attached to the overhand loop in the kevlar is at least 3 body lenths of 1/8" oval or round elastic. Larger models and LMR's get the same treatment but I increase the kevlar to 130 or higher anchor line and 3/16 or 1/4" flat elastic, still a minimum of 3 body lengths. I used to use only 6 feet of 1/4" elastic on the larger models but it was causing snap back damage to the bodies or nosecones. I haven't had a repeat of that problem since increasing to 3 time the body.
Straight 50lb kevlar for Micro-Maxx I find 4 to 5 times the body is really required to prevent seperations, I've started using 30" as a minimum length no matter what the model length on the really small and light models and at least 5 times for the "larger" 8 and 10" stuff. well except the extreme length experiment 103.5" Micro-superroc. it worked out to twice the "Main" 35.5"
Body lenght.

 

[Johnnie]

All,

I loose the elastic on any rocket I fly, as the snap back leaves too much to be repaired, when the nosecone snaps into the bodytube or fins. I use Pink nylon cord that you get at Wally World (Wal Mart) and a roll of that has lasted a lot of years now. I also use 3X the length of the tube.

For HPR, I use 1" tubular nylon, found at outdoor sporting goods stores, in 4" dia and up, and 9/16" tubular nylon for 3" and smaller. For the 7.5" project I built, I have 1" kevlar rope...

Johnnie Paul

The nylon cord at Wally World comes in a few different colors.

 

[iceage]

If you still have room in the rocket after you put the nosecone on, you do not have a long enough shockcord

Patrick

 

[als57]

Originally posted by iceage
If you still have room in the rocket after you put the nosecone on, you do not have a long enough shockcord

Patrick

Its amazing how much shock cord it in some of the level 3 project I've seen. One of the last ones had 150' of cord. It worked ; so I guess it wasn't too much.

Typically for HPR I use 3 to 5 times the body tube lenght when using tubular nylon or kevlar. On stubby rockets I use more like 5X. My 4" PE Fat Boy has about 15' of 9/16 tubular nylon. My PML Phobos has about 12' of the same stuff. My LOC Warloc has about 25' of 1" TN.

Al

 

[Rocketmaniac]

For my Mid-powered rockets(wart-hog, strong arm) I use about 10'-12' of 3/16" nylon shock cord. This comes out to about 3-4 times the lenght of the rocket.......

For my High powered ISP Caliper, I use 9/16" nylon cord. When I do dual deployment, I use about 50' for apogee and about 25' for the main......

 

[flashgc]

I always use as much as I can fit in the tube... so this sometimes relates to organization of the material. On my larger projects I use a 2" wide elastic that I bought surplus in a roll. Smaller projects might get the kind of elastic that can be bought at any sewing supply. In any case, it gets packed in an organized 'back and forth' package with 2 rubber bands (minimum) to keep that pack together during flight. It shapes up something like this:

0
| rubber band
|_/___________
_|_________|_|
|_|_________|_
_|_________|_|
|_|_________|_
_|_________|_|
| \
| rubber band
0

As one person noted in this thread, too much can cause entanglement, but this method of packing keeps that potential to a minimum. As long as you can avoid entanglement, there's no such thing as too much shock cord.

I try to keep the anchor inside the body tube and have the shock cord handle the exit to minimize possibility of zipper. As related in another post on another subject, saturating the BT edges with thin CA hardens the tube at that point and thus minimizes the chance of zipper.

 

[flashgc]

... The message parsing features of this board played hob with my illustration in the previous post. Hope you can make it out.

 

[Austin]

I am one of those guys that uses long shockcords. I have about 140' on my L3 birds...@60' on the top to middle section and 60' on the middle to bottom section. Most of my big rockets are around 12' tall, so 60' of cord is a 5 to 1 ratio. Note that the ENTIRE length of the rocket comes into play when determining shockcord length...something I think that hasn't yet been mentioned in this thread. Also, the extra 20' gives me some extra to tie my knots.

Carl

 

[trogdor]

This thread is heading in a direction that makes me want to propose a new NAR competition. Chute Altitude after Rocket Touchdown. CART.

 

[als57]

Originally posted by Trogdor
This thread is heading in a direction that makes me want to propose a new NAR competition. Chute Altitude after Rocket Touchdown. CART.

Well I've seen at least 150' on a couple of L3 projects for the CART.

Al

 

[jflis]

You could just call it "Parachute Duration", wherein the stopwatch *starts* when the rocket hits the ground and *stops* when the parachute hits the ground... LOL