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Shock Cord length and size

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ColumbiaNX01

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Hey folks agian. I am building a 80 inches long, 4 inches diameter blue tube rocket. What is the best length of tubular nylon I should use for this rocket and the width of the tubular nylon? By the way it will fly on a K motor. It is not finished yet but the final weight may between 4 and 6 lbs.
 

cjl

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I would use something like 20-25 feet of 9/16" TN.
 

ColumbiaNX01

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could I use like 40-50 foot long or is that overkill? Whats the advantage to a long shock cord?
 

cjl

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You could, but it's overkill. Will it be single or dual deploy?
 

ScrapDaddy

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could I use like 40-50 foot long or is that overkill? Whats the advantage to a long shock cord?
I think Kevin mentioned on another thread that if it's too long it will "jerk" when it gets tithe end of the line.
 

Pantherjon

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5 or 6 pounds, 80 inches long..Dual deploy..I would recommend 10-15 feet for the drogue and 20-25 for the main. 1/2" tubular nylon would be plenty..
 

cjl

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5 or 6 pounds, 80 inches long..Dual deploy..I would recommend 10-15 feet for the drogue and 20-25 for the main. 1/2" tubular nylon would be plenty..
That's interesting - I would tend to go the other way. 20-30 feet for the drogue, and 10-15 feet for the main. On DD rockets, I almost always use less on the main than I do on the drogue.
 

Pantherjon

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That's interesting - I would tend to go the other way. 20-30 feet for the drogue, and 10-15 feet for the main. On DD rockets, I almost always use less on the main than I do on the drogue.
Those were the number I was given when I first set up my first DD rocket..Seems to work for me..If it ain't broke...;)

Now, on my L3, I am going maybe a little extreme with 50' sections of 9/16" TN for both drogue and main...But, it is a 12' 35 pound rocket...
 

cjl

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Those were the number I was given when I first set up my first DD rocket..Seems to work for me..If it ain't broke...;)

Now, on my L3, I am going maybe a little extreme with 50' sections of 9/16" TN for both drogue and main...But, it is a 12' 35 pound rocket...
50'?

Wow - that's a lot. I know the feeling though - I used 40' for my L3 drogue, and 25' for the main.
 

kandsrockets

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15 feet for the main and 25 feet for the drogue. You want more in the drogue area so the parts are further away from each other so there is less chance of the main getting tangled in to the booster.
 

troj

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If you insist on ungodly lengths of harness, make sure you pack the parachute such that it cannot inflate before things are stretched out.

How often have you seen a canopy inflate and stop, while the rocket is still in free fall underneath? Then, what happens when it hits the end of the harness? Major loading on everything -- I've seen several rockets break the harness and/or something else, and no longer have a recovery system.

We use under 20 feet for rockets that weigh hundreds of pounds.

-Kevin
 

WillMarchant

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http://www.tulsarocketry.org/rot.htm is a very interesting collection of "rules of thumb."

Their recommendation is that you select harness that is 50 times as strong as the static weight of the rocket. So, for a 10 pound rocket, you'd use 500 pound harness.

For harness length they recommend 5 times the rocket length. For an 80" rocket they would recommend about 33 feet of harness.
 

troj

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Their recommendation is that you select harness that is 50 times as strong as the static weight of the rocket. So, for a 10 pound rocket, you'd use 500 pound harness.

For harness length they recommend 5 times the rocket length. For an 80" rocket they would recommend about 33 feet of harness.
So, based on that, our 350 pound Redstone should've had a harness rated at 17,500 pounds, and over 100 feet long.

Instead, we had a webbing rated at about 5,000 pounds (but the attachment points weren't rated near that), and well under 20 feet along.

Result?

Perfect recovery.

As the rockets get larger, the need to understand how to reduce loads becomes more critical, and excessively long and over-built recovery components become, at best, a band-aid, and at worst, a cause for some of the failures and recovery damage that we see.

-Kevin
 

WillMarchant

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So, based on that, our 350 pound Redstone should've had a harness rated at 17,500 pounds, and over 100 feet long.

Instead, we had a webbing rated at about 5,000 pounds (but the attachment points weren't rated near that), and well under 20 feet along.

Result?

Perfect recovery.

As the rockets get larger, the need to understand how to reduce loads becomes more critical, and excessively long and over-built recovery components become, at best, a band-aid, and at worst, a cause for some of the failures and recovery damage that we see.

-Kevin
Kevin, as you are well aware, rules of thumb are rough guidelines and not intended to replace analysis in critical applications. Andrew is talking about a fairly standard "low end" HPR rocket for which "general guidelines" are perfectly applicable. Your example is definitely *not* flying in that regime...
 

troj

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Kevin, as you are well aware, rules of thumb are rough guidelines and not intended to replace analysis in critical applications. Andrew is talking about a fairly standard "low end" HPR rocket for which "general guidelines" are perfectly applicable. Your example is definitely *not* flying in that regime...
Right, but my point is that I totally disagree with the "rules of thumb". They're bad on the low end, and as rockets get larger, they get progressively worse.

-Kevin
 

ColumbiaNX01

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People you guys need to stop arguing. I am going to most likley use 2 sections of 30 foot 9/16" tubular nylone for the recovery. If anyone disagrees tell me what you would use? I appriciate everyones input.
 

H_Rocket

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I would use about 10' in the booster/apogee section and about 20' in the main section. Basic 9/16" TN will be more than enough for the shock loads you are considering.

No math, just experience.

If you go any longer on the mains, I would seriously consider a d-bag.
 

sylvie369

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I'd use pieces no longer than about 20' on either side. The recommendation by H_Rocket sounds about right to me.

The main advantage to longer shock cords is that it increases the odds that you'll be able to reach part of the rocket when the parachute catches in the tallest tree in the area. But if you don't fly at Bong, that might not be important to you.

When you use longer shock cords you make it harder and harder to pack the recovery stuff, and in a 4" diameter rocket that's going to be an issue for you. Try stuffing 30' of TN into your space, and then try 15'. I'll bet that the difference is obvious. When your ejection charge blows the nose cone off, which is more likely to follow it out? Which is more likely to jam? (okay, well, that was too obvious).

I think the shorter cord will give a significantly more reliable deployment. And 20' isn't THAT short. Stretch it out and take a look.
 

Burner

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I think any of the above suggestions will work fine in the rocket you're building, don't stress. If it were me I would use 1/2" tn, 20'-25' for drouge and 15'-20' for the main.
 

troj

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People you guys need to stop arguing. I am going to most likley use 2 sections of 30 foot 9/16" tubular nylone for the recovery. If anyone disagrees tell me what you would use? I appriciate everyones input.
9/16" is fine; I'd not make them any longer than 20 feet, each.

-Kevin
 

cjl

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If you insist on ungodly lengths of harness, make sure you pack the parachute such that it cannot inflate before things are stretched out.

How often have you seen a canopy inflate and stop, while the rocket is still in free fall underneath? Then, what happens when it hits the end of the harness? Major loading on everything -- I've seen several rockets break the harness and/or something else, and no longer have a recovery system.

We use under 20 feet for rockets that weigh hundreds of pounds.

-Kevin
This I'll agree with, if at all possible. A d-bag is a good idea if you can, as it will allow the recovery system to stretch out during deployment and reduce the chance of a tangle or hard opening.
 

Handeman

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This I'll agree with, if at all possible. A d-bag is a good idea if you can, as it will allow the recovery system to stretch out during deployment and reduce the chance of a tangle or hard opening.
I agree with cjl and kevin. Long harnesses cause more stress then short ones. They need to be sized to the weight and deployment methods of the rocket.

I have a 92", 4" dia. 8.5 lb dual deploy rocket. I use 18' of 9/16 TN for the drogue and 15' of 9/16 TN for the main. I also ALWAYS use a drogue chute. The chute keeps the upper part even or above the fin can on the way down so when the main deploys, the harnesses are almost stretched out already and there is minimal shock on the harnesses when the main opens.

I have seen very few failed recovery harnesses on dual deploy rockets, but the ones I have seen where all very long harnesses and the fin can/motor broke away when if fell to the end of the harness after the main had opened and stopped. The last one I saw do that was a L3 attempt and the U-bolt tore out of a 3/4" CR on the fin can when it fell to the end of the 100 ft. harness attached to an upper half and main chute that was almost stopped.
 

cjl

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100 foot?

Wow - I've seen some long harnesses, but that takes the cake.
 

troj

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100 foot?

Wow - I've seen some long harnesses, but that takes the cake.
I've seen it. I've seen numerous in the 50+ foot range, as well.

-Kevin
 

BobCox

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Another reason not to use an excessively long harness... doing so increases the chance of getting snagged on a tree or a power line. This article has more detail. Basically, a longer harness makes for a bigger target that can get snagged.
 

Pantherjon

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100 foot?

Wow - I've seen some long harnesses, but that takes the cake.
There is a member of our club (Danny Dempsey) who has a rocket where he has 300' of cord in it and multiple parachutes:dark:..The fin can is almost on the ground before everything fully deploys.:roll: It's a light rocket and falling slowly that allows him to be that wild on his cord length...
 

Jeff

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The general consensus seems to be that shorter cords are better because they limit the stress on the harness system. Does anyone have any negative experiences with shorter cords - namely tangled chutes and components smacking into each other during descent?
 

cjl

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Well, the advantage of long cords is that it lessens the shock of parts hitting the end of the cord just after the deployment charge goes off. This is why I tend to use a shorter cord on the main, and a longer cord on the drogue. It's definitely somewhat of a balance, and a lot of people do go too far on the excessively long cord side.
 

JDcluster

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Most of my rockets have about 18-25ft between the booster section & payload then another 12-15 (at most) connected to the main chute. I use to use wider sections of TN, but lately I've been going the other way; A 1" leader strap about 6-8 ft long attached to 1/2" x 15 ft. TN.

For bigger rockets, say 5 inch & up; I'll go 30-35ft then another 15-20 on top.



JD
 
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