Yep.
Zippered Tube: Time to Lengthen Shock Cord
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mh9162013
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Not if it's glued properly.
The problem with kevlar is it's stiff and doesn't stretch so when the nose cone reaches the end of the cord it can pull hard on the mount. A piece of long rubber or elastic will probably have the nose cone slowed down before it reaches the end, but I've also had elastic torn when it reached full elongation. This is another consideration- kevlar is probably stronger than elastic.
I never had a cord mount fail before because I used rubber or elastic. When I switched to stronger kevlar I wanted a stronger mount so I did some experiments. Some types of mounts, even fiberglass cloth, could easily unzip and not provide much strength. I would recommend doing like I did- do some mockups of your proposed mount and test them. Some of the methods that I was sure would work really well, didn't.
https://www.rocketryforum.com/threads/shock-cord-mount-studies.167751/
Also my experience. The rubber or elastic shock cord breaks in the middle after being singed and stretched one too many times. Never had a shock cord mount fail. Never had a shock cord pull out of the mount.
For what it’s worth, I dislike rubber shock cords. They get old and brittle and break. I prefer elastic to rubber shock cords.
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GlenP
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For typical LPR kits, I like to use 3x body length of Kevlar mounted to the body, then about 1x body length of elastic to the nose, or whatever came with the the kit. The elastic can be checked and replaced if it looks damaged.
That’s a good compromise! The strength and light weight and small size of Kevlar but with a little stretchy elastic on the end, just in case.
High-tech device for applying glue inside a body tube.
So you just add CA and no paper, card stock or masking tape to patch the "rip". I never tried the cardstock mount with kevler so I can not answer that question.
Yep. Just added a lot of CA to the rip (from inside the tube) with some masking tape on the outside to hold the tube in the proper shape. Added some more CA to the top of the tube to try to prevent zippering in the future. That is when I overdid it with the CA glue. The CA glue ran down the inside of the tube and glued the shock cord to the inside of the tube. Doh! Lesson learned: A little CA goes a long way. No need to overdo it.
On the issue of the kevlar shock cord mounted to inside of the tube, here is what I am trying: I created the G. Harry Stine “shock lock” type shock cord mount made out of card stock as found in the Handbook of Model Rocketry. Instead of three big holes, I made two rows of smaller holes (4 holes in each row). I then “laced” the kevlar string into the holes - sort of like lacing up a shoe. I then glued that whole thing in the body tube as usual using wood glue (Titebond III). I wish I had taken a picture before putting it into the tube but I did not. Anyway, I hope the “lacing” spreads out the force on the mount and keeps the kevlar from tearing out. We shall see.
MidOH
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Switch to super thin BSI CA glue. It soaks in fast, and rarely enlarges the tube much. Would most likely have prevented your zipper. And it also prevents your nose come from flareing the tube out.
Hold tube level and use the nozzle to spread the glue. Don't squeeze.
My shock cords get epoxied to a coupler. Usually one of the couplers for my baffles.
Hold tube level and use the nozzle to spread the glue. Don't squeeze.
My shock cords get epoxied to a coupler. Usually one of the couplers for my baffles.
I am now doing this on the Maniac shock cord replacement.
Brock, when he is too tired to work on rockets but does it anyway.
Hobby knife “extension” for working deep in a body tube: top part of an Estes 1/4” launch rod. And duct tape.
OK, Kevlar thread people. Here goes.
GlenP
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I think that is a pretty typical method I have seen used around for quite a while, not anything that I invented personally, but glad to have made the suggestion that brought it to your attention so you could put it to good use here!
The Maniac’s body tube is 24 inches long. I now have 72 inches of kevlar shock cord and 24 inches elastic shock cord on the end of that. A total of 96 inches of shock cord.
That’s long enough, right?
That’s long enough, right?
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EDIT: replied to old part of thread.
waltr
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IF, ejection happens near apogee AND the flight is near vertical AND the Nose cone is not very heavy, this will be fine. That is the rocket has very little velocity.
When ejection is very later, rocket has increased speed downward or has weather cocked badly and has high horizontal velocity then there is a chance of huge forces upon ejection and risk of a zipper and/or recovery failure..
GlenP
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The longer shock cord mostly helps protect from nose cone slap back, which is a common thing when using only the stock kit elastic cord. Pretty common to see a little arc dent in balsa nose cones, like a little grin or smile, from when the nose cone has slapped the top of the body tube at ejection due to that bungee effect, in the typical LPR kit with a balsa nose and a too short elastic shock cord.
GlenP
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would a Jolly Logic Chute Release help a little with that, so that the chute does not immediately deploy at ejection, but the rocket gets a chance to slow down to a more manageable terminal velocity before the chute gets unfurled totally?
One thing to watch out for: The knot(s) used to join the Kevlar and elastic. I had one come undone after a couple of flights.
And then spent the rest of the afternoon reading knot threads on this forum..... Kevlar knots seem to sort of expand when not under tension and undo themselves. But with the help of the nice folks here, I now have secure Kevlar knots.
Hans.
Thanks for the tip, Hans!
To connect the kevlar part of the shock cord to the elastic part of the shock cord, I use a “loop to loop” technique I learned on Chris Michielssen’s blog site:
http://modelrocketbuilding.blogspot.com/2016/06/a-better-shock-cord-knot-tips-part-1.html?m=1
https://modelrocketbuilding.blogspot.com/2016/06/a-better-shock-cord-knot-tips-part-2.html?m=1
For tying a loop knot that won’t slip in almost any type of line or cord, including Kevlar, I use the “Lefty Kreh” fishing knot. Hasn’t slipped or come undone yet:
https://www.101knots.com/non-slip-kreh-loop-knot.html
Here is the shock cord on the Maniac, showing Chris Michielssen’s loop to loop connection and two Lefty Kreh knots to make the loops:
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waltr
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Still need to have initial ejection near apogee or zero velocity. Then the recovery system needs to be strong enough to withstand opening the chute at 50-100fps velocity.
I saw in a previous thread that you've used the Lefty Kreh in chute shroud lines. But I'll be damned if I can get my fat fingers to maneuver those tiny strings into that knot. Evidence points to it being a good knot. I just can't get it tied.
Hans.
I am a Kreh knot fanatic, I admit.
The keys to tying it are:
1. Wrap the loop through something to hold the loop in place. The something should come apart unless you want the loop permanently on the something. I use one of those “bulldog” big paper clips.
2. Use a LOT of the end of the string or cord when trying the knot. You want enough “tag” end left over after you wrap it 5 times around the standing end to still be able to easily pass the tag end through the overhand knot and pull it tight.
Use lots of line for the tag end. More than you think you need. You can always cut off the excess tag end. Make sure you allow extra line for your application to account for the amount of line you are going to use for the Krek knot.
3. Tie the Kreh knot with it laid out flat on your work surface. That way, you can see the knot as you tie it.
4. When doing the wrapping (the hardest part), hold the overhand knot with your non-dominant hand to keep the loop and the overhand knot from moving. Then use your dominant hand to wrap the tag end around the standing end 5 times. If the tag end is too long to comfortably wrap, undo the knot and shorten the tag end. Then try again.
Practice helps for sure.
If all you want is a nice, secure loop in the end of a cord or string, and the loop should be free standing (that is, the loop does not pass through an object when you tie it), just use a Surgeon’s Loop. It is much easier to tie than a Kreh knot.
https://www.101knots.com/surgeons-loop.html
I'm trying this one for shroud lines:I am a Kreh knot fanatic, I admit.
The keys to tying it are:
1. Wrap the loop through something to hold the loop in place. The something should come apart unless you want the loop permanently on the something. I use one of those “bulldog” big paper clips.
2. Use a LOT of the end of the string or cord when trying the knot. You want enough “tag” end left over after you wrap it 5 times around the standing end to still be able to easily pass the tag end through the overhand knot and pull it tight.
Use lots of line for the tag end. More than you think you need. You can always cut off the excess tag end. Make sure you allow extra line for your application to account for the amount of line you are going to use for the Krek knot.
3. Tie the Kreh knot with it laid out flat on your work surface. That way, you can see the knot as you tie it.
4. When doing the wrapping (the hardest part), hold the overhand knot with your non-dominant hand to keep the loop and the overhand knot from moving. Then use your dominant hand to wrap the tag end around the standing end 5 times. If the tag end is too long to comfortably wrap, undo the knot and shorten the tag end. Then try again.
Practice helps for sure.
If all you want is a nice, secure loop in the end of a cord or string, and the loop should be free standing (that is, the loop does not pass through an object when you tie it), just use a Surgeon’s Loop. It is much easier to tie than a Kreh knot.
https://www.101knots.com/surgeons-loop.html
https://daveroot.neocities.org/knots/Knots_SingleLoops.html
Look at Alpine Loop, method #4.
For everything else, I'm using the Figure 8 knot, reverse threaded if needing to go around something. I've gotten quite good at it.
Hans.
I do a square knot between the kevlar and elastic. It's easy, compact, and I'm telling you it is incredibly solid.
Previously described here: https://www.rocketryforum.com/threads/shock-cord-material-for-lpr-mpr.165939/post-2253397
Previously described here: https://www.rocketryforum.com/threads/shock-cord-material-for-lpr-mpr.165939/post-2253397
OK, tried the Lefty Kreh for all 8 corners of a chute. 1st one took a half hour. But with persistence, finally got it down to 8 minutes each. So about an hour and a half to do the shroud lines on the one chute. The big breakthrough came when I clamped the initial overhand loop with a tiny smooth jaw alligator clip. (would that be an alligator without his dentures?) That held the loop as a unit keeping it from unraveling while I continued to grip, drop, regrip, twist, insert, and swear. My fingers and thumb are wide enough that I can't even tell if I've gripped the thread.
LOC and Top Flight make chutes with the lines already attached!
Hans. (off to have a few beers after that)
