What weight kevlar is suitable for my rocket?

[Bill S]

Had a question. I normally use 100# kevlar on my low powered rockets as the shockcord (plus elastic attached to that). Now I went and built an Estes Executioner, which ended up being about 10 oz, and I was concerned about the 100# kevlar not being able to take the stress of the heavier rocket, so I used 200# kevlar.

Ended up with a situation today wherein the rocket was stuck 45' in a tree, and we were using the club's long pole to get it down. We simply couldn't break the kevlar and at least recover the main body, leaving the parachute and nosecone in the tree. That I could have lived with. But since we couldn't break the 200# kevlar cord, eventually we tugged enough to break the rocket free, but in the process the main body tube got somehow zippered (don't think it was due to flight damage). Everyone was astonished that we couldn't break the rocket free of the parachute/nosecone.

So my question is this: is 200# kevlar excessive for mid-powered rockets. I'm only using E and F motors now, don't plan to get level 1 certification in the forseeable future. Is 100# adequate for these rockets?

 

[teepot]

The kevlar strength is only part of the equation. Can your attachment point to the rocket take 100 lbs? Can the elastic take 100lbs of stress? Can the attachment point to the nose cone withstand 100 lbs of force. If the answers are no then 100Lb kevlar is fine. I ty to balance all of the forces by using a long enough shock cord and matching up each of the components strengths. I don't want 2000 lb kevlar matched with an eye nut rated to 500 lbs.

 

[heada]

100# is plenty good for any LPR. Use 5ft to 10ft of it from the main BT, then whatever Estes provided from the kevlar to the nosecone. Attach parachute to the nosecone. That way the kevlar is near the ejection charge where it's needed, the elastic is on the nosecone and parachute where it's needed and you can cause the elastic to fail if needed and it's easily replaced.

 

[Bill S]

The kevlar strength is only part of the equation. Can your attachment point to the rocket take 100 lbs? Can the elastic take 100lbs of stress? Can the attachment point to the nose cone withstand 100 lbs of force. If the answers are no then 100Lb kevlar is fine. I ty to balance all of the forces by using a long enough shock cord and matching up each of the components strengths. I don't want 2000 lb kevlar matched with an eye nut rated to 500 lbs.

Okay. I have no idea as to how to figure out what stresses are present, let alone what kevlar strength would be required to mititgate them.

 

[Handeman]

I think you are falling into the trap of TRF Overthinking it. If the Kevlar is not too big, doesn't break at ejection, and doesn't take up too much space and allows the recovery to work correctly, use it.

Getting caught is a tree is an anomaly you don't have to design for. If you want to go the TRF Overthink route, than design for the worst case scenario and figure how much force the rocket parts can take when you hang from the long pole pulling it from the tree. Don't forget to calculate the tear forces on the tube, nose cone, parachute, and elastic cord. You might also want to include the burnt motor weight in there and time of day and temperature somehow.

 

[Bill S]

I just decided to downgrade to 100# kevlar and go with that.

 

[rharshberger]

I think you are falling into the trap of TRF Overthinking it. If the Kevlar is not too big, doesn't break at ejection, and doesn't take up too much space and allows the recovery to work correctly, use it.

Getting caught is a tree is an anomaly you don't have to design for. If you want to go the TRF Overthink route, than design for the worst case scenario and figure how much force the rocket parts can take when you hang from the long pole pulling it from the tree. Don't forget to calculate the tear forces on the tube, nose cone, parachute, and elastic cord. You might also want to include the burnt motor weight in there and time of day and temperature somehow.

Or...just run a couple of sims and see what the max gee's will be then just figure needed strengths from that. For HPR's I usually use either 50 or 100G's, for LPR it should be WAY less probably not more than 10G's or so. But that might be TRF Overthinking....

 

[BABAR]

I think you are falling into the trap of TRF Overthinking it. If the Kevlar is not too big, doesn't break at ejection, and doesn't take up too much space and allows the recovery to work correctly, use it.

Certainly true, but seems like ideally you’d like to know it’ll work BEFORE you launch the rocket.


Reminds me of the story of how to know if how long to cook a potato in your particular microwave.
The answer was to pick two identical potatoes.

Put the first one in for 30 minutes and watch it.

Measure the time it takes for it to explode.

Take it out, clean the microwave, and put the second microwave potato in for two minutes less.

 

[Bill S]

Or...just run a couple of sims and see what the max gee's will be then just figure needed strengths from that. For HPR's I usually use either 50 or 100G's, for LPR it should be WAY less probably not more than 10G's or so. But that might be TRF Overthinking....

I'm using Rocksim. Does it show said data anywhere? Once I know max G's, how does that translate into lbs of stress on the kevlar? Not a math major here, sorry.

 

[rharshberger]

Take your gee's x weight of component/rocket to find how strong the kevlar would need to be to resist breaking. As for where Rocksim has the max G's, not sure I use Open Rocket.

 

[tsmith1315]

Never used Rocksim, but you'd want Max G's due to deployment, not acceleration under vertical flight. Open Rocket displays it on a graph, IIRC.

I guess it uses the parachute parameters you specify to figure that, I haven't looked that far into it yet.

 

[Philip Tiberius D.]

I always worry more about using too fine of Kevlar string - I’ve got a 1000’ of 100 lb for my LP stuff and worry it will cut through a cardboard tube like a knife even with CA around the nose cone area.

 

[Bill S]

Philip, I haven't had that happen yet, and the vast majority of my rockets use about 2x the body length of kevlar. Only a couple are CA reinforced around the end of the tube, and while a couple rockets have come close to a zipper (slight mark on the end of the tube where the kevlar was pressing), after about 200 launches, not one zipper in flight. I'm using 100lb kevlar.

 

[Michael L]

I'm using Rocksim. Does it show said data anywhere? Once I know max G's, how does that translate into lbs of stress on the kevlar? Not a math major here, sorry.

I don't have Rocksim on this computer but when you do the launch simulation, where the rocket graphic flies up, etc, there's a detail tab that you can click, lower right corner if I remember right, and it'll show tons of info in a column on the right side.

 

[heada]

Here is the non-science part of rocket science....

You want the kevlar just the right length. Too short and the ejection charge puts too much stress on the attachment point and it'll rip out or zipper the tube. Too long and the falling body will pickup too much speed before the kevlar gets tight and it will zipper the tube. How long is just right? Depends on lots of factors and really up to you and how you build and what model and kind of kevlar and, and, and...

 

[BABAR]

Or...just run a couple of sims and see what the max gee's will be then just figure needed strengths from that. For HPR's I usually use either 50 or 100G's, for LPR it should be WAY less probably not more than 10G's or so. But that might be TRF Overthinking....

Okay, I’ll bite.

is there a direct relationship between potential nose cone ejection forces and max G acceleration? Seems like more relevant factors would be maximum potential velocity in case of premature or late ejection (the premature case may be more than simulator predicts, as if rocket weathercocked and went sideways under thrust it may be going faster than a vertical boost, the latter could potentially be terminal velocity if the rocket has a VERY late deployment, goes down ballistic but deploys before hitting the ground.). Other obvious factors would be mass of the rocket body and motor and mass of the nose cone, force of the ejection charge, length and strength and degree of stretch of any elastic used....)

your probably right, but I can’t wrap my brain around the max G factoring into the equation.

 

[rharshberger]

Okay, I’ll bite.

is there a direct relationship between potential nose cone ejection forces and max G acceleration? Seems like more relevant factors would be maximum potential velocity in case of premature or late ejection (the premature case may be more than simulator predicts, as if rocket weathercocked and went sideways under thrust it may be going faster than a vertical boost, the latter could potentially be terminal velocity if the rocket has a VERY late deployment, goes down ballistic but deploys before hitting the ground.). Other obvious factors would be mass of the rocket body and motor and mass of the nose cone, force of the ejection charge, length and strength and degree of stretch of any elastic used....)

your probably right, but I can’t wrap my brain around the max G factoring into the equation.

I just use max gee's as an assumption based on something going sideways aka early or late ejection, horizontal ejection (landshark or heavily weathercocked, etc). You are correct that there are many other variables that can be accounted for as well, shock loading being on of them aka nosecone slams to end of kevlar upon ejection, but that usually more complicated than I feel like being....

 

[Philip Tiberius D.]

Philip, I haven't had that happen yet, and the vast majority of my rockets use about 2x the body length of kevlar. Only a couple are CA reinforced around the end of the tube, and while a couple rockets have come close to a zipper (slight mark on the end of the tube where the kevlar was pressing), after about 200 launches, not one zipper in flight. I'm using 100lb kevlar.

I go somewhere between 2x - 3x body length as well and in full disclosure it hasn’t happened to me either…. Watch next launch, I’ll somehow zipper a fiberglass tube.

 

[jrap330]

Okay, I’ll bite.

is there a direct relationship between potential nose cone ejection forces and max G acceleration? Seems like more relevant factors would be maximum potential velocity in case of premature or late ejection (the premature case may be more than simulator predicts, as if rocket weathercocked and went sideways under thrust it may be going faster than a vertical boost, the latter could potentially be terminal velocity if the rocket has a VERY late deployment, goes down ballistic but deploys before hitting the ground.). Other obvious factors would be mass of the rocket body and motor and mass of the nose cone, force of the ejection charge, length and strength and degree of stretch of any elastic used....)

your probably right, but I can’t wrap my brain around the max G factoring into the equation.

Most important...the energy of the ejection charge, dissipated by the volume of the rocket. As eluted to, if the Kevlar length is just right, "just like a rocket at Apogee" there will be little forces as the "moving" nose cone with parachute decelerates to almost zero mph.

 

[DaHabes]

I always worry more about using too fine of Kevlar string - I’ve got a 1000’ of 100 lb for my LP stuff and worry it will cut through a cardboard tube like a knife even with CA around the nose cone area.

In addition to using CA on the body tube, I use multiple layers of shrink tubing on the kevlar where it meets the body tube. If you build up a few layers it becomes too thick to cut the tube unless it is really a catastrophic deployment.

 

[Philip Tiberius D.]

In addition to using CA on the body tube, I use multiple layers of shrink tubing on the kevlar where it meets the body tube. If you build up a few layers it becomes too thick to cut the tube unless it is really a catastrophic deployment.

I’ve seen others comment on a similar method and have used zip-ties to produce a similar effect…. Sounds like Monty Python dialogue.

 

[afadeev]

You want the kevlar just the right length. Too short and the ejection charge puts too much stress on the attachment point and it'll rip out or zipper the tube. Too long and the falling body will pickup too much speed before the kevlar gets tight and it will zipper the tube. How long is just right? Depends on lots of factors and really up to you and how you build and what model and kind of kevlar and, and, and...

I don't think the shock-cord can be too long.
Worst case scenario, the force of the ejection charge does not fully eject the shock-cord, but then the nose-cone (NC) will still put chute out and it will open, and pull the rest of the string out of airframe. If NC never gets ejected, the length of the shock-cord is irrelevant.
Too short of a shock cord can be a problem causing nose cone to rebound (or tear the harness), and bounce right back towards the inflating chute. This can, potentially, entangle nose cone in the chute itself, and cause it to not inflate properly.
3+ airframe lengths (for shock cord size) has often been sited as a good rule of thumb. I err on the longer side.

In theory, zippering is a function of airframe accelerating tangentially to the fully extended and tightly strung shock cord. This is WAY more likely to be correlated with ejection event missing apogee by a huge margin, than anything to do with the length of the shock cord. And event then, longer shock cord would be safer, since it will be less likely to be fully extended and thus less tightly strung out.

I wrote "in theory", because after years of flying, I am yet to experience a single zippering event. I've had my share of early/late ejections, shock-cord failures, and chutes failures (torn shroud lines, burned canopies, chutes missing altogether - don't ask). But zero first-hand zippering experiences.


a

 

[Bill S]

I don't have Rocksim on this computer but when you do the launch simulation, where the rocket graphic flies up, etc, there's a detail tab that you can click, lower right corner if I remember right, and it'll show tons of info in a column on the right side.

I found that tab, etc, and indeed it does give alot of information, but I don't know which info is useful to me.

 

[les]

The ejection charge goes off - the nose cone and chute get ejected (hopefully). Good old F=MA will determine their velocities relative to each other.

The rocket body, cone, and chute will, right after separation, be slowing down due to the drag on those different components. They will decelerate at different rates due to different drag forces. Least for the cone, most for the chute as it opens.

If the cord is too short, the velocities imparted by the force of the ejection will cause a significant shock force once the cord is fully stretched out. Again, these velocities are determined by the ejection force, the drag (how much did the parts slow down relative to each other), and the length of the cord (which really relates to time for the drag to decelerate the parts - the longer the cord, the more time for drag to slow things down and more time for drag to slow the parts down). The velocities once you reach the end of the cord will determine the shock force. You also indicated you use elastic combined with the kevlar. The force to stretch the elastic further slows down the parts and reduces the shock load.

So now if you can estimate the actual relative velocities of the cone/chute vs the body tube once you reach the end of your cord, you can determine the shock force measured in G's. Take the weight of the components, multiply by this G force, and that will determine the required breaking strength of the cord. Like a 1.5 pound rocket with 20G shock would require a cord rated for 30 pounds. Don't forget to factor that cords loose a significant portion of their rating where there is a knot.

Or - just use 2~3X the body length as a quick rule of thumb and assume a load less that 50G..........

 

[Buckeye]

Or...just run a couple of sims and see what the max gee's will be then just figure needed strengths from that. For HPR's I usually use either 50 or 100G's, for LPR it should be WAY less probably not more than 10G's or so. But that might be TRF Overthinking....

The max acceleration of the flight trajectory has very little to do with the working load on the recovery harness. You want the shock load during recovery events. Flight simulators do not predict this. A different calculation or accelerometer measurement like this one is helpful. Though, I don't know how accurate these spikes really are, since this accelerometer has a 70G limit.

 

[Michael L]

I found that tab, etc, and indeed it does give alot of information, but I don't know which info is useful to me.

Man... I hear you... there's a lot of info on that sidebar. I use the manual slider, watch the little rocket and when it seems to be at apogee, based on velocity = 0 or really close (sometimes it's hard to get the mouse to stop), that's where I get the altitude. I also look at velocity once it's on the chute to gauge parachute size (or drogue but I'm not at the dual deploy stage quite yet).

 

[afadeev]

The max acceleration of the flight trajectory has very little to do with the working load on the recovery harness. You want the shock load during recovery events. Flight simulators do not predict this.

Totally agree with the first sentence.
One augmentation to the second sentence - there will be two shocks of interest to the shock cord's integrity:

1. During ejection charge firing.
2. During chute inflation. Both are a PITA to nail down.

For the first, estimating force of the ejection charge is fairly straight forward. Translating it into shock cord's g-forces is anything but. If you have the total BP charge's size and the volume of the airframe that will be pressurized before nose-cone (NC) is ejected, you can write a own formula to get the resulting pressure. Or use the following link:
http://www.rockethead.net/black_powder_calculator.htm
For example, for my 2.6" TLP Matra Super flying on Estes E12 motor (my notes say the BP weight is 0.75g on 24mm Estes motors), I get theoretical 13 psi of pressurization during ejection. For a smaller 1.2" airframe of Apogee Aspire, I get 3.3 psi.
But that assumes the entire payload bay is empty (21" x 2.6" in Matra), which it wont be. In practice, the volume will contain a Nomex blanket, the shock cord, and the chute itself. So the actual ejection pressure should be higher, in theory. By exactly how much depends on the volume the chute and other laundry items when those are compressed.
Even if we estimate all of the above, the resulting PSI will still be meaningless, since it is the maximum theoretical pressure that will never be achieved. The NC will start dislodging at some pressure before the theoretical maximum (else NC would barely nudge, and never separate from the airframe). So the actual speed with which the NC will be dislodged will depend on the NC to airframe friction and the PSI's required to overcome it. Then there is the ejection pressure dissipation as NC shoulder exists the airframe barrel, as well as the air pressure acting on the NC, which varies with speed. Someone is welcome to build the model to estimate all that. Alternatively, we just ground test on the first variable (NC to airframe friction) HP airframes, ignore the rest, and call it done.

Similar, but different, ambiguities exist with chute inflation g-forces. How you pack your chute matters (z-folded? rolled? shroud lines in our out? lines taped or not? etc.), what type of a chute you are deploying matters, how it gets ejected or pulled out during airframe separation matters. And the most important variable is the speed with which airframe is traveling during chute ejection and chute inflation.

Net-net: if it works, it works.
You can collect your own data over time, or start with something others have identified works for them, and improve as desired.

HTH,
a

 

[BABAR]

Regarding: “I don't think the shock-cord can be too long.” In post 22.

rocket internal space permitting, this may be true. I can only speak for low power, but some of Estes designs are exceptions that prove the rule that Estes makes great rockets. Specifically sometimes there is very little space available, and trying to stuff too much shock cord in there may make for a failed deployment (for low power, the rule, “if you can’t blow it out with your breath, something is too tight” applies.). Longer cords also need to be packed with a bit more care, they tangle easily, and a knotted long cord may have a very SHORT effective length.
Stronger (and thicker) cords are accordingly tougher to pack.

so I might say “put in as long a cord as you can easily pack.”

Here’s a nice trick for getting a lot of cord into a small space by Tim Van Milligan. It is a bit time consuming but it works quite well

Just jump to 3 minute in video.

 

[BABAR]

Also nice low power or mid power cheap easy anti zipper device

credit @BlaineS

https://www.rocketryforum.com/threads/blaines-kevlar-anti-zipper-using-foam-ear-plug.154996/

 

[BABAR]

Most of my shock cord failures have burn throughs of Kevlar thread. It’s heat resistant but not infallible.

here a trick for easy cord inspection and replacement
Credit @hcmbanjo
https://www.apogeerockets.com/education/downloads/Newsletter338.pdf