For low power, how important IS elastic shock cord?

Seems like having elastic as part or all of a shock cord for low power rockets sets up a weak point. Elastic compared to Kevlar is more likely to burn if you don’t get the wadding or baffle right, is weaker than Kevlar, and also more bulky and likely to get stuck. Also I guess you don’t get the snap back dents on the nose cone if you use Kevlar.

On the plus side for elastic, I guess as it stretches it slows the release of kinetic energy so there is less of a “jerk” when the nose cone reaches the end of the tether, and presumably the same when the chute opens.

Sooooo, all other things being equal, would an extra foot of Kevlar be just as good as a segment of elastic and easier to pack?

I am already missing der MicroMeister. He’d have an answer to this with 8 color pics.....

There are two things you can do with a non-elastic cord. 1) "Z"-fold the cord and wrap the "corners" in a bit of tape. When the cord tries to straighten, the bits of tape get ripped, dissipating energy. 2) make a "bridge" with a short piece of elastic cord. Gather a loop in the non-elastic cord, and tie a piece of elastic across it. The elastic is shorter than the non, so it will stretch and absorb energy. This is the old Centauri "bungie cord" method. Wish I still had access to all the old photos I've uploaded. a lot easier to explain.

...well searches still show up the files...

This is how the tape works:

BABAR said:

Seems like having elastic as part or all of a shock cord for low power rockets sets up a weak point. Elastic compared to Kevlar is more likely to burn if you don’t get the wadding or baffle right, is weaker than Kevlar, and also more bulky and likely to get stuck. Also I guess you don’t get the snap back dents on the nose cone if you use Kevlar.

On the plus side for elastic, I guess as it stretches it slows the release of kinetic energy so there is less of a “jerk” when the nose cone reaches the end of the tether, and presumably the same when the chute opens.

Sooooo, all other things being equal, would an extra foot of Kevlar be just as good as a segment of elastic and easier to pack?

I am already missing der MicroMeister. He’d have an answer to this with 8 color pics.....

Click to expand...

So just to emphasize the importance of the "shock cord" its the only thing that ties your recovery system to your rocket.. and actually the shock cord is actually less vunerbal to breaking if singed... giving ever LPR ricketier knows that the strings that connect the parachute are more important... Also the melting point of Kevlar is above 500 °C (930 °F). Rubber starts to change property of elasticity at 200 o F.



Kevlar 36" Shock Cord 1.7MM 400LB Non-Flammable Kevlar Made In USA - Free Ship
cost $ 4.50



Model Rocket Fire Retardant Shock Cord 25 Feet of 3/8"
cost $5


So to answer your question off the bat, no dont buy Kevlar. Just stick to the Rubber shock cord .... LPR rokcetry does not usally need to have over complicated stuff , unless it befinfts you , via you shooting the rocket a lot... Just to standpoint dont buy it ... small rocket parts wear down after years of proper use ( including putting proper wadding ) .

I've made dozens of low-power launches on the same rockets without replacing the elastic shock cord. Honestly, there's absolutely no reason to replace it. Inspect it when repacking it, making sure it isn't getting brittle, and fly on. If you want to replace it when it's worn out, use what you want, but I don't see any issue with the elastic.

I will argue that it is a bit bulky on small-bodied airframe, but I've had no issues it.

Edit: And if you're forgetting to put wadding in the rocket, your chute won't survive anyway. Again, "upgrading" the equipment shouldn't be an excuse to not be careful installing recovery gear properly.

If I am building for 13 or 18mm motors, I do not worry about the elacticnshock cords. By the time they dry rot, it is usually time to retire the rocket. 24 and above, I like kevlar or nylon.

I’d guess the elastic allows for a more energetic ejection charge. This helps by allowing a single motor to effectively work with rockets having a wide range of internal volumes.

Oh, I guess the question is really whether or not you need it to be elastic. To answer that question, no, it does not have to be elastic. If you're using a non-elastic shock cord then you need to increase the length of it so that it doesn't tear out of the mount on ejection. I think the rule of thumb is 3 times the length of the rocket body. That gives the nosecone time to dissipate all its forward momentum, lessening the shock when it hits the end of the shock cord length.

Tyler P said:

Oh, I guess the question is really whether or not you need it to be elastic. To answer that question, no, it does not have to be elastic. If you're using a non-elastic shock cord then you need to increase the length of it so that it doesn't tear out of the mount on ejection. I think the rule of thumb is 3 times the length of the rocket body. That gives the nosecone time to dissipate all its forward momentum, lessening the shock when it hits the end of the shock cord length.

Click to expand...

Absolutely, no real reason to replace the elastic in most estes kits, but also nonspecific reason to use elastic over nylon. Kevlar can tend to shred tubes if you do not build appropriately.

On lpr kits that come with elastic, maybe this is overkill, but I like to use the kit elastic at the nose cone end and attach that to a length of Kevlar that is attached to the body. The part of the Kevlar at the top of the body tube gets wrapped with some tape or a cardstock/glue sandwich to reduce zipper risk. You might say that is overbuilding, but you get a longer heat resistant cord at the body end with some elastic shock absorption at the nose cone end. If I feel like there is room in the tube and margin for the extra weight I attach the Kevlar line to a homemade baffle near the middle of the tube where you are more likely to grab hold of the rocket for additional support as well.

The width and the stretching of elastic also helps prevent a "zipper" in slightly heavier rockets. Elastic can be used all the way through Level 3 if you understand how to handle it. It should be replaced every so often. Kevlar is much more durable and resistant. And even in longer lengths, will much more easily zipper your tubes and can still fray and break if you CA your tuber ends. Elastic could as well, but the stretching would take some of that energy. In light weight, Kevlar would last as long as you hold the kit together. So there are pro's and con's to both. Price, durability, reaction with other components, etc... There never really is a "better than" in the hobby. Only something may be "better" for your style and technique. We use both depending on the situation.