Shock Cord length & diameter

[Rocketmaniac]

When one gets into the full "L" and baby "M" motors strength must be added to rocket design by 10-fold. Those of you who have flown this size rocket, what size shock cord did you use? How long? In my "I", "J" and "K" rockets I have used 5/8" shock cord, but lots of it and I have not had a problem. I have a large roll of 1" nylon that looks like L3 kind of stuff, but I haven't measured how much I have. (well, I purchased 300', but don't know how long each lenght is)

 

[Johnnie]

I will be using 1" tubular nylon I purchased from an Industrial lighting outfit. They use it to build safety harnesses for workers that install any kind of event lighting. Good stuff and comes in designer colors. I believe this is also used for mountain climber rigging.

I had loops sewn into them at a local uhpolstery shop.

 

[quickburst]

I suggest designing your recovery system to meet a 1200 lb load. The diameter or width of a shock cord is not as important as it's tensile strength. There are many products on the market that hae been used with great success. For example, some tubular nylon is rated at 1500 lbs. and it is only 9/16 inches wide. This material is much stronger than its flat braid counterpart which is 1" wide.

Tubular nylon can and has been used for many L-3 projects. The TN will need to be protected from ejection charge heat and inspected regularly. Another choice is the Kevlar harnesses. More expensive, stronger and more heat resistance.

I use the TN and have had great luck with it. Sewn loops are a wise move. Most flyers do not have enough "Knot Know How" to do the job properly. Including me.

QuickBurst

 

[Rocketmaniac]

Johnnierkt & QuickBurst,

What about length? any recommendations? (for both apogee and main)

 

[Johnnie]

I have 20ft for the drogue, and 30ft for the main, and I used 5 more feet to make up various loops for double tie-offs. Honestly, I feel I should go longer, but we'll see when I do a dress rehearsal.

 

[jrogers]

50' Drouge, 20' main in my L3

 

[BrnAgainRoc]

ive been told to use 3x's the length of my rocket for your main and 1 1/2x's for the drogue chutes.

 

[DPatell]

on big rockets I use 40' drogue, 30' main. I prefer a longer drogue cord so that if I hang it in a tree, it's accessable on the ground.

 

[rstaff3]

Dan, I've seen the arguements for 3x the rocket length....no use 4x..5x...for years. But I never remember seeing your arguement of length being related to recovery from trees. I'm sure it's been discussed, just don't remember seeing it. Very good slant on the problem!

 

[BrnAgainRoc]

well everyone ive talked to and joined in launches pretty much have always went 3x the length even on Estes rockets.Iam not much of a recovery info guy just know what has worked with me the past and present. As for it allowing to recover from large trees i dont know but i suppose its very possible.

 

[gregzo]

Here's another rule of thumb I have heard.....

If you still have room in your payload bay - you do not have enough shock cord.....

Now this can obviously be taken to an extreme - but the point is - for a couple of extra bucks - it can't hurt to have the extra length provided it fits comfortably in the rocket.
Rocketrage 1" (4,000 lb) Cobra 3D shock.
40 foot is $24
60 foot is $28

Quickburst is correct about focusing on the strength and not the size. General rule of thumb on strength I have heard is 50:1.

Good Luck

 

[Rocketmaniac]

Originally posted by Gregzo
Here's another rule of thumb I have heard.....

If you still have room in your payload bay - you do not have enough shock cord.....

Now this can obviously be taken to an extreme - but the point is - for a couple of extra bucks - it can't hurt to have the extra length provided it fits comfortably in the rocket.
Rocketrage 1" (4,000 lb) Cobra 3D shock.
40 foot is $24
60 foot is $28

Quickburst is correct about focusing on the strength and not the size. General rule of thumb on strength I have heard is 50:1.

Good Luck

Most of the guys who I fly with use a lot of shock cord. In my mid-size rockets I use about 75' for the apogge and 40' for the main.

The roll of 1' stuff I have says it rated at 4200 lb. Later today I will un-roll it and see how long each section is.

Thanks for everyone's opinion........

 

[Smokin' Rockets]

If you can fit it 40-50' for the drogue is good. 20-30' for the main..

Tubular nylon is much stronger than tubular kevlar.

The wider the better to help prevent zippers. 1" is usually fine.

GL....Bill

 

Originally posted by Badboy1982
well everyone ive talked to and joined in launches pretty much have always went 3x the length even on Estes rockets.Iam not much of a recovery info guy just know what has worked with me the past and present. As for it allowing to recover from large trees i dont know but i suppose its very possible.

Speaking strictly of high-power rockets, if you have the extra room, longer does not hurt anything. The old 3X rule is baloney if you’re talking about shorter rockets.

3x is great if you have an 8ft rocket, but not so good if you have a 3ft rocket.

I learned this lesson the hard way. I went with 3X on a 3ft rocket, and the nosecone would snapback and smack a bodytube or fin every time upon ejection. This caused damage to the fins, the finish on the BT, and would knock a big chunk of paint off the NC.I put in a 25ft harness and had no problems after that.

Just because it's an old "rule of thumb" doesn’t mean it's right for every situation.

 

[edwardw]

Originally posted by DPatell
on big rockets I use 40' drogue, 30' main. I prefer a longer drogue cord so that if I hang it in a tree, it's accessable on the ground.

Trees??? What trees?

Edward

 

[wwattles]

Originally posted by edwardw
Trees??? What trees?

Edward

Well, since Edward sent us off down this bunny-trail first, I'll blurt out what I was already thinking...

If we base our harness length upon the height of the nearest trees, my shock cords would be 30 inches long, since the tallest thing near my HPR site is about 30 inches tall, and those are few and far between!

As for what I'm using on my L2 shot, it's a 20' piece of TN. (Single deploy at apogee)

WW

 

[rstaff3]

Just as a data point, I've never used much over 3x the rocket length on any rocket F-J. The only time I've had a cone bounce into my fins is on my NCR Archer, when all it had was the stock elastic. One rocket length of Kevlar leader, and it doesn't have that problem either. I'd contend that if your cone impacts the fins with 9' of cord on a 3' rocket, then the deployment probably was too fast and the rocket passed the cone as the latter slowed.

Over about 15lbs, I'd go for the mega-lengths to absorb the shear force of deployment and to keep the parts separated.

Just offering my experience, doen't mean I will be right 100%.

 

[Johnnie]

Hey Edward,

Is that the Island of Japan? What motor did ya use?

As for trees, The longer the cord, the more tree tops it can drape across...not a good judge for cord to me.

I like a cord long enough for the two halves to travel and slow before reaching the length of the tether. I do not like the damage caused from snap backs.

 

[edwardw]

That is probably the only major geographic feature of our launch area. A small old creek bed that has dried up and become a ditch. It is probably 100 yards wide by 1/2 mile long. Other than that we have cactus *everywhere* and gopher holes. The motor was a Pro38 I170.

Edward

 

[quickburst]

Originally posted by Rocketmaniac
Johnnierkt & QuickBurst,

What about length? any recommendations? (for both apogee and main)

I use the 3 X rule, like most here have stated. That is three times the rockets length, as a minimum.

QuickBurst

 

[tinteffects]

I used 2 25' lengths of 1/2 tubular kevlar for my L-3 cert. Kosdon M1845, rocket 110# Sky-Angle 'chute

 

[rstaff3]

BTW, I never had much problem with my Rocketman Freedom to Fly rocket. The cord barely extends past the end of the tube. The one problem was that the shroud lines are longer than the exposed part of the shock strap, and you can't always cound on the payload section pulling the chute all the way out

(I now use an extention cord! no not the electrical type)

 

[Shawn K McDavid]

What tinsel strength shock cord is necessary for a seventy pound rocket with a 12 inch diameter?

 

[Buckeye]

Tinsel is for Christmas trees, not shock cords.

 

[Shawn K McDavid]

Sorry I was using voice recognition and didn’t notice the error. Any way do you know the what strength would be needed?

 

[kalsow]

I usually design for a 20G shock load. 20 x 70lb =1400 lb. More conservative people design for 50G loads. 50 x 70lb = 3500lb. With this size of rocket extensive ground testing is needed!

How did you work your way up to such a big rocket without already having some rule of thumb for shock loads??

 

[Shawn K McDavid]

My L2 rocket was a 7.5 ' version of the super DX3 withy 3Lb of concrete in the nose to keep it under 3,000 Ft.
I over built the recovery, added a piston ejection, and electronics. This all works well. I have since built several other rockets. Then I got the wild idea to build a 12’ x 12” rocket using sonotube with a chassis, and filling the booster section with expanding foam around the motor mount tube. First I built a launch trailer with a 16’ tower, and electric wench for lifting. I am using a 75mm Motor tube, and a L2200G motor for first flight. Rocksim predicts 2918’ altitude with 83FPS at departure. If all goes well I plan to use a M1350 for my L3.

 

[Onebadhawk]

Did someone just say use 1400 lb material for a 70 lb rocket ??
My 1/4" Tubular Kevlar is stronger then that and it's for 3" rockets..
My 7/16" Tubular Kevlar is 5000 lb material and when doing custom work I will go up to 100 lbs on the pad..
At 70 lbs you're almost 3/4 of the way to the end of where I would use my 5000 lb material..
Of course, length is always far more important than a weight rating.
It's far more important that there is enough length of the boosters harness to allow the two separating
parts slow on their own before slamming to the end of a short harness..

Again,
It's not a shock cord..
It's a harness..
The distinction is that in a good set up the harness should never be subjected to a "shock" load..

Teddy

 

[Shawn K McDavid]

I am new to these very large rockets, so all help is
appreciated

 

[GaryT]

For this rocket I'd recommend 1/2" Tubular Kevlar, 40' for the booster 30' for the main. Thing is your only going 2900' so by the time the 40' unfurls you'll be popping the main and for a bird this big I'd pop the main at 1000' Min, a main that big and 30' of SC will take some time to open.