Calculating Shock Cord Size and Lengths

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gary7

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After an exhaustive search on line, including this forum, I am unable to find any formulas for calculating shock cord sizes, lengths, etc.
What I have found are opinions about cord lengths. A "general rule of thumb" stating 3-5 x the total rocket length is followed by many of you because "it works for me" or "this is the way I have always done it" but you cannot say otherwise why. I would like to see something proving why a cord should be a certain material, size, length. Some sort of physics/engineering tests would be helpful. We use such data for calculating shear pins and ejection charges for instance but again, I cannot find any such data for shock cord lengths.

Opinions are fine. But I want concrete evidence as to why, not just someone's opinion "just because".

Does anyone have any such study or evidence to share with the rest of us?
 
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"It works for me" is real world experience and that basically is the scientific data you are looking for.
 
^^What he said...Generally it just comes down to experience..Judging the weight of the rocket, the speeds at deployment and your own comfort level when deciding whether the braided cord, 1/2", 9/16", 3/4" or 1" nylon straps is appropriate..
 
You can find many resources for this problem, but not with "rocket" key words. The problem you wish to solve has many variables. I suggest breaking it in to two pieces; one being the ability of the cord to withstand tensile forces and the other being significant length to prevent component collisions and chute problems. The first problem is basically a mass on a spring problem. Hold one mass at rest and optimize cord characteristics for a given set of mass and velocity variables that match your case study. The length is way more complex and of course combining the two studies creates another differential equation to solve. The last study is basically a CFD with multiple dependent bodies. So yes, there is a way to solve it mathematically. However, there is also a bunch of people that have collectively tried just about every combination within reason, they have essentially used Newtons method to experimentally optimize the shock cords used in the hobby for both utility and cost. If you wish to improve on the current widely excepted method of shock cord use, I would add something to absorb the "shock". For instance a consumable that will undergo plastic deformation in the deployment process.
 
I have a script written in Matlab that calculates the energy of the rocket components at separation, and uses this information along with the properties of the recovery harness to estimate the the tension in the harness. I've been meaning to convert it to excel but haven't got around to it yet.

I haven't seen any web pages or forum posts devoted to this kind of thing, so like Random Flying Object said, you're sort of on your own. I have the advantage of an engineering background which made things a tad easier. If there's one thing I learned, it's that having a harness with some stretch will significantly reduce the load placed on the attachment points etc. Most of my rockets use TK with a section made up of Tubular Nylon. The TK is stronger/fireproof and uses less volume for a given load rating, while the Tubular Nylon provides some stretch.

Ethan
 
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I have a script written in Matlab that calculates the energy of the rocket components at separation, and uses this information along with the properties of the recovery harness to estimate the the tension in the harness. I've been meaning to convert it to excel but haven't got around to it yet.

I haven't seen any web pages or forum posts devoted to this kind of thing, so like Random Flying Object said, you're sort of on your own. If there's one thing I learned, it's that having a harness with some stretch will significantly reduce the load placed on the attachment points etc. Most of my rockets use TK with a section made up of Tubular Nylon. The TK is stronger/fireproof and uses less volume for a given load rating, while the Tubular Nylon provides some stretch.

Ethan

Did you code it by hand or use simulink?
 
After an exhaustive search on line, including this forum, I am unable to find any formulas for calculating shock cord sizes, lengths, etc.
What I have found are opinions about cord lengths. A "general rule of thumb" stating 3-5 x the total rocket length is followed by many of you because "it works for me" or "this is the way I have always done it" but you cannot say otherwise why. I would like to see something proving why a cord should be a certain material, size, length. Some sort of physics/engineering tests would be helpful. We use such data for calculating shear pins and ejection charges for instance but again, I cannot find any such data for shock cord lengths.

Opinions are fine. But I want concrete evidence as to why, not just someone's opinion "just because".

Does anyone have any such study or evidence to share with the rest of us?

Hi Gary,

The following is my opinion, with a little bit of evidence mixed in. I typically use roughly 5x the length of the rocket, or 25' for a typical high power rocket. Nothing magical here, but the reason is that such lengths allow the sections to slow down a bit before pulling the recovery harness taught. I tape the lines with masking tape after z-folding them as well to help absorb the deployment shock.

As for calculating the load at deployment, which dictates what harness material I'll use, I use the worst case scenario as follows. Force = mass * acceleration. I use the total mass of the rocket (could probably use just the mass of the nose or airframe, whichever is heavier, but I'm conservative). For acceleration, looking at past accelerometer data from flights, it seems that deployments may inflict 30G's of force; so I use this number. As an example, lets use a 20lb rocket. F= 20lb (rocket) * 30 (acceleration at ejection) = 600lbs. I know I need a recovery harness with a break strength of at least 600 lbs. For reference, 9/16" tubular nylon's reported break strength is reported to be ~2200lbs and 1" tubular nylon's break strength is ~4000lbs.

A few notes: nylon will stretch a before it fails where kevlar will not. Kevlar may fail with only a few good tugs. Burn marks in either will weaken the material, but nylon is much more affected by heat than kevlar is. Knots supposedly weaken the harness material significantly, yet I can't recall ever seeing a harness fail at the knot.

My standards, for what they're worth, are 9/16" tubular nylon on most high power rockets. 1" tubular nylon on really big high power rockets. I've used 2" on a 11.5" rocket, but that was overkill. I like 1" flat Kevlar in high performance applications where space is a premium. I don't like 5/8" tubular kevlar: strong like ox but inflexible and takes up a lot of room! I like the apogee harness to be longer than the main harness, as there is generally more force involved at apogee. Nomex cord protectors are great when placed near ejection charges. I don't trust stitched ends; many use them, I just don't trust 'em (sorry); yes I've seen them fail on rare occasions (as I mentioned before: never seen a harness fail at the knot). Last, and one that a lot of people won't like: I absolutely use (prefer!) underwear elastic on light mid power (and even a few high power) rockets. Put in a long length, and it will save cardboard airframes from zippers that a product with less stretch would have provided.

-Eric-
 
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There are too many variable factors involved in sizing for there to be a concise formula.

Are you using nylon? Elastic? Tubular Kevlar? Flat Kevlar?

What width? What thickness?

How energetic are your charges? Barely enough to get separation? "Blow it out or blow it up?"

How are you containing your charges? Wells? Surgical tubing? Glove fingertips?

How tightly does your chute fit in the airframe?

How many shear pins are you using? What size? What material?

These are just a few of the factors that determine how much shock cord is needed. But there's good news! You don't need to find the right length down to the millimeter. You just need enough to get your chute out with room to open.

When people tell you "3 to 5 times the rocket length works for me," you are getting an amalgamation of results of experiments done by many different people under almost every combination of the factors above.

Of course you will want to ground-test your deployments. You may discover in ground-testing that you did not use enough cord. In that case, buy something longer. And vice versa if it is too long.

I don't know where you are buying your cords, but Teddy at One Bad Hawk is an excellent resource for knowing how to size harnesses to any rocket. And he makes great stuff that lasts longer than most rockets do.
 
After an exhaustive search on line, including this forum, I am unable to find any formulas for calculating shock cord sizes, lengths, etc.
What I have found are opinions about cord lengths. A "general rule of thumb" stating 3-5 x the total rocket length is followed by many of you because "it works for me" or "this is the way I have always done it" but you cannot say otherwise why. I would like to see something proving why a cord should be a certain material, size, length. Some sort of physics/engineering tests would be helpful. We use such data for calculating shear pins and ejection charges for instance but again, I cannot find any such data for shock cord lengths.

Opinions are fine. But I want concrete evidence as to why, not just someone's opinion "just because".

Does anyone have any such study or evidence to share with the rest of us?

Gary, what you are looking for doesn't exist, or in my opinion, it shouldn't exist. A rocket is a total system and should be designed that way to meet desired goals. The whole process of rocket design is about compromise as you prioritize each aspect of design to get as close to the design goals as you can. It is a system and you can't separate the up part from the down part and still optimize both. Do you want low and slow with big thundering motors, a rock solid sport flier, a mach buster, or peak altitude or something else?

The longest part of your flight is the recovery. It's half the distance traveled but usually +10x of the time of flight. It is from the time the rocket comes apart until it touches the ground. That can include few events when using motor deploy or many using DD. As you design your recovery system, shock cord length is only part of the design and should be part of the whole recovery system design, not just a part of the recovery event (apogee deployment) design. The shock cord length is one of the variables that gets adjusted (compromise) depending on how you design the other parts of the system and the overall design goals of the rocket.

i.e. one of your design priorities is you want a sure, can't miss, always going to open apogee event so you use 5g of BP which meets the always open criteria, but to prevent damage to mounting points and shock cords, the length of the apogee shock cord has to be lengthened a lot to allow parts to slow down before they hit the end and then will impart less shock to the system. That shock load can be calculated BTW. Another options would be to drop the Bp amount down enough to ensure opening, but not push the parts to the end of the shock cord and let the drag of the drogue chute and gravity extend the apogee shock cord to get the right flight profile. In that case, you could shorten the apogee shock cord to save weight which may be a high priority criteria for your over all design.

Bottom line, there is no one formula for shock cord length because that is a design criteria that gets adjusted based on the overall design of the rocket.
 
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Hey guys.......do you realize you are beating a dead horse?

Thread is from 2013.

I would much prefer beating a dead horse. Beating a live horse is cruel and would make me feel bad.... sorry, couldn't resist. Jim, your contribution to this forum has been a tremendous help in my rocketry journey (oh, as a side note, I just got my RTx - your Quick Start Guide is great!)

This thread, despite it being dated, is timely for me as I am at the point of deciding on length and material for a current build. Rocket is a 3" diameter tube fin design using MAC Performance canvas phenolic tubing. Other details, dual deploy, under 7 lbs pad weight with largest 54mm motor I can fit in it. I am using MACs centering ring with pin and length of nylon as the connection for the booster. I'm debating specifically about what to attach to that for the apogee harness and also the main harness. I would love opinions/experience using 550 paracord in this scenario. I am using it in a 2.6 Madcow DX3 (cardboard, 29mm motor mount), but it weighs substantially less at under 2 lbs without motor. Considering the paracord for several reasons. Cheap, easy to pack, allows for some stretch under shock load. I am going to need protection over the MAC provided nylon attachment and would include this to the vulnerable areas of the paracord, both for heat and exposure to sharp edges of the canvas phenolic airframe. Thoughts anyone?
 
I would much prefer beating a dead horse. Beating a live horse is cruel and would make me feel bad.... sorry, couldn't resist. Jim, your contribution to this forum has been a tremendous help in my rocketry journey (oh, as a side note, I just got my RTx - your Quick Start Guide is great!)

This thread, despite it being dated, is timely for me as I am at the point of deciding on length and material for a current build. Rocket is a 3" diameter tube fin design using MAC Performance canvas phenolic tubing. Other details, dual deploy, under 7 lbs pad weight with largest 54mm motor I can fit in it. I am using MACs centering ring with pin and length of nylon as the connection for the booster. I'm debating specifically about what to attach to that for the apogee harness and also the main harness. I would love opinions/experience using 550 paracord in this scenario. I am using it in a 2.6 Madcow DX3 (cardboard, 29mm motor mount), but it weighs substantially less at under 2 lbs without motor. Considering the paracord for several reasons. Cheap, easy to pack, allows for some stretch under shock load. I am going to need protection over the MAC provided nylon attachment and would include this to the vulnerable areas of the paracord, both for heat and exposure to sharp edges of the canvas phenolic airframe. Thoughts anyone?

My only experience with paracord is while camping. When I flame-whip the ends, it does melt pretty easily. So, it needs protection for sure.

I am also building a 3" MAC. I like to use 100% Kevlar in the booster, because it can get heat from both the apogee charge and the motor backup charge. I have some Wildman 1/4" Kevlar that I will likely use. Calling it 1/4" is a bit generous, however, as it measures more like 3/16" at best. For the main, I use a short harness of Kevlar, then 9/16" tubular nylon.
 
Insert expected (and overdue) Mr. Ed joke.....



HERE.


Later!

--Coop
 
Digging up old threads again. Dang.

IMO it's a discussion forum and we can resume the discussion. I didn't read the original thread back at the time so I can read it now and maybe other people can too, and we can discuss it.

I've had the exact same question myself. If people say "5 times the rocket length" then I start thinking about my rockets that are similar in diameter and weight but somewhat different in length. Length doesn't seem to be a primary variable in this calculation so I figured rather than use actual length I would use what I thought would be an average length on the longer end of the scale for a rocket of similar diameter and weight.

The next part of the question would be what size or strength of cord. I'm mostly thinking about low power rockets (Estes B through D) and mid power rockets (G motors). I have used 3/8" and 1/2" wide elastic with success for rockets up to 2.6" diameter but 1/2" elastic failed in my 4" diameter level 1 cert. rocket (but not on the cert flight). I've now got some kevlar cord about 3/16" or 1/4" diameter to use, and I know to make it plenty long. But with Estes rockets I'm not sure. I've seen a few kits that include basically one strand of kevlar and it seems to work well but sure seems thin to me. I've been looking at slightly larger braided kevlar and that's what I'm thinking about trying just because it looks more appropriate.
 
IMO it's a discussion forum and we can resume the discussion. I didn't read the original thread back at the time so I can read it now and maybe other people can too, and we can discuss it.

Agreed . . . We still use Shock Cords and we still need to know how to choose the right components and make them the proper length !
 
IMO it's a discussion forum and we can resume the discussion. I didn't read the original thread back at the time so I can read it now and maybe other people can too, and we can discuss it.

I've had the exact same question myself. If people say "5 times the rocket length" then I start thinking about my rockets that are similar in diameter and weight but somewhat different in length. Length doesn't seem to be a primary variable in this calculation so I figured rather than use actual length I would use what I thought would be an average length on the longer end of the scale for a rocket of similar diameter and weight.

The next part of the question would be what size or strength of cord. I'm mostly thinking about low power rockets (Estes B through D) and mid power rockets (G motors). I have used 3/8" and 1/2" wide elastic with success for rockets up to 2.6" diameter but 1/2" elastic failed in my 4" diameter level 1 cert. rocket (but not on the cert flight). I've now got some kevlar cord about 3/16" or 1/4" diameter to use, and I know to make it plenty long. But with Estes rockets I'm not sure. I've seen a few kits that include basically one strand of kevlar and it seems to work well but sure seems thin to me. I've been looking at slightly larger braided kevlar and that's what I'm thinking about trying just because it looks more appropriate.

Without "pointing fingers" at any kit manufacturer, let me say that kit manufacturers stay in business by selling kits, obviously.
By that, I mean to say that a shock cord that is "good enough" for a few flights before it fails, ultimately "sells more kits".

Is that "intentional" on the part of the manufacturer ? In some cases, perhaps, but not in others.
"Intent" is very subjective.

Should we stop buying kits from manufacturers that use lower quality or lower strength shock cords ?
Absolutely not ! BUT, we should be wise enough to replace ANY component that is prone to premature failure !
 
a shock cord that is "good enough" for a few flights before it fails, ultimately "sells more kits".

Should we stop buying kits from manufacturers that use lower quality or lower strength shock cords ?

I don't know how this works out on average. A kit that is lost doesn't get enough flights to matter. A kit that crashes or catos doesn't get enough flights to matter. A chute that doesn't open fully doesn't stress the shot cord. So maybe there are fewer that last long enough to fail. Nevertheless we should certainly give feedback to the manufacturers that they should provide better materials.
 
I don't know how this works out on average. A kit that is lost doesn't get enough flights to matter. A kit that crashes or catos doesn't get enough flights to matter. A chute that doesn't open fully doesn't stress the shot cord. So maybe there are fewer that last long enough to fail. Nevertheless we should certainly give feedback to the manufacturers that they should provide better materials.

Better materials mean higher prices . . . Better materials = Fewer failures . . . Fewer failures = Fewer kits sold . . . Fewer kits sold = Higher prices.

Unstable rocket kits are a "liability" . . . Components that simply "wear out" mean flight damage, which means time to buy more kits.

BTW - Kits don't "cato", motors do . . . LOL !
 
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