Why 4X the Length of Rocket for Shock Cord

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Chunker78

Active Member
Joined
Dec 20, 2014
Messages
39
Reaction score
0
I plan to use 1/4 in Kevlar for a shock cord on my LOC IV for recovery. the rocket is 48" tall. That would make my shock cord 16'. I've even heard 5X the length of the rocket. Why so long a shock cord?
 
Nylon and even less so Kevlar have limited shock absorbing ability, the long shock cords allow the ejection forces to dissipate before the shock cord reaches the maximum extension (which puts shock loads on the mount points if it reachs max extension). All my 1/4" Kevlar shock cords are 25' long, and I can Z-fold them with masking tape wraps every 5 folds. As the shock cord extends the tape breaks creating a decelerating action, the length really takes up very little space and is just insurance. I also size my ejection charges to extend the nose cone/payload bays apart by about 3/4 the length of the shock cord when ground testing.
 
Nylon and even less so Kevlar have limited shock absorbing ability, the long shock cords allow the ejection forces to dissipate before the shock cord reaches the maximum extension (which puts shock loads on the mount points if it reachs max extension). All my 1/4" Kevlar shock cords are 25' long, and I can Z-fold them with masking tape wraps every 5 folds. As the shock cord extends the tape breaks creating a decelerating action, the length really takes up very little space and is just insurance. I also size my ejection charges to extend the nose cone/payload bays apart by about 3/4 the length of the shock cord when ground testing.

+1...
Everything. ...
I also size my charges to about 3/4 the length of the harness. ...
Remember, a harness should not be taking a shock load..
If it is there's something fundamentally wrong with your set up..
Usually too energetic of an ejection charge,,
Or too short of a harness....

The harnesses long length is to allow the energy of separation to dicipate before reaching full extention and transferring the shock to the attachment points....

Teddy
 
+1...
Everything. ...
I also size my charges to about 3/4 the length of the harness. ...
Remember, a harness should not be taking a shock load..
If it is there's something fundamentally wrong with your set up..
Usually too energetic of an ejection charge,,
Or too short of a harness....

The harnesses long length is to allow the energy of separation to dictate before reaching full extension and transferring the shock to the attachment points....

Teddy

+1 on all of that.

On that rocket with that cord, you shouldn't have any issues. If you start using much longer cords, 10x or more, then you run into the issue where the chute opens and hangs still in the air without weight until the rocket falls and hits the end of the cord. Besides causing zippers, that can also rip out anchor points and cause separations. Those sort of things usually happen with DD flights, but it is possible with motor ejection.
 
longer shock cords usually puts one section(typically the fin can) closer to the ground in the event of a tree landing :).
Rex
 
longer shock cords usually puts one section(typically the fin can) closer to the ground in the event of a tree landing :).
Rex

That was a joke I really got a good chuckle off of..
I think it was from Matt...
When someone from the midwest asks someone from the northeast
how long should my harness be the answer is just a little longer then the nearest tree is tall.....lol.....

Teddy
 
those of us who fly the 'Bong' learn to use a longer shock cord :).
Rex
 
How about that longer shock cords just look cooler coming down. We all want to look like the high power big boys rather than the short Estes rubbers found on the whoosh poppers. I have seen hugely long shock cords on LPR rockets. If 3X body length is cool then 4X must be even cooler and so on. Crazy lengths to look cool.

OMG is that grown man crocheting his awesome kevlar shock cord! Turn in your man card now. Is that old dude actually using underwear elastic?

Anti zipper and heat resistance are key and always good for a nice round robin discussion.

Shock cord length is also subject to rocket type, material used and recovery space. There are some odd rocs I have seen with very short shock cords. Whats up with that? Dude you know the shock cord rules are at least 3X body length. But my rocket is like the discount medical clinic down the alley; no recovery room. Lots of tiny plastic chutes coming out of pods, poor boy dual deploy, nose cones coming down on their own chutes, a real mess. All makes for some really good laughs from the higher level guys, before, during and after recovery.

I have always wondered what the rule is for the Mean Machine and other super rocs?
 
Last edited:
Good comments in this thread. I'll just add my two cents.

Until we change the terminology there is going to be misunderstanding on just what the part does that connects all the pieces of the rocket together as it is recovered.

In the early days of model rocketry it was indeed a shock cord and used as such. I have had many experiences when a 10 or 12 inch rubber band shock cord has been stretched to nearly it's limit and then brought the pieces flying back together to snap off a fin, crush the body tube, or at least scrape off paint to show where the nose cone and rocket have collided. The first improvement was to go to elastic which made the cord stronger but did not prevent the above from happening. Sometimes, it seemed to me, it would be better if the cord broke. It would cause less damage. When shock cords absorb shock they redirect it. Watch Wile E. Coyote and his giant slingshot. It never works out well for him.

We should be using recovery harnesses not shock cords. If you think of them differently you will use them differently. Use strong material. Make them longer so they do not have to somehow dissipate the shock of the ejection charge but just hold the weight of the parts and the shock of the chute opening for a safe recovery.

I like my harnesses just long enough that one or two people will announce, "separation" before everything reaches it's limit. ;) Slow and graceful.
 
Good comments in this thread. I'll just add my two cents.

Until we change the terminology there is going to be misunderstanding on just what the part does that connects all the pieces of the rocket together as it is recovered.

In the early days of model rocketry it was indeed a shock cord and used as such. I have had many experiences when a 10 or 12 inch rubber band shock cord has been stretched to nearly it's limit and then brought the pieces flying back together to snap off a fin, crush the body tube, or at least scrape off paint to show where the nose cone and rocket have collided. The first improvement was to go to elastic which made the cord stronger but did not prevent the above from happening. Sometimes, it seemed to me, it would be better if the cord broke. It would cause less damage. When shock cords absorb shock they redirect it. Watch Wile E. Coyote and his giant slingshot. It never works out well for him.

We should be using recovery harnesses not shock cords. If you think of them differently you will use them differently. Use strong material. Make them longer so they do not have to somehow dissipate the shock of the ejection charge but just hold the weight of the parts and the shock of the chute opening for a safe recovery.

I like my harnesses just long enough that one or two people will announce, "separation" before everything reaches it's limit. ;) Slow and graceful.

Perfect Joe,, me too...
I completely avoid the term "shock cord" just for this reason...
I am forced to put the term on each page of my site so it will "return" on a search for "shock cords"...
I always say,,
If your harness is taking a shock load something is fundamentally wrong with your set up...
Usually too short of a harness or too energetic of an e charge....

Teddy
 
I always say,,
If your harness is taking a shock load something is fundamentally wrong with your set up...
Usually too short of a harness or too energetic of an e charge....

Teddy

I totally agree!!! I think most times it's too energetic of an e charge... especially with the apogee charge on DD.
 
I totally agree!!! I think most times it's too energetic of an e charge... especially with the apogee charge on DD.

100%....
How many times have you heard "blow it out or blow it up" ???
Typically,, that creates the exact same amount of problems that it solves,,,
or,, for each problem it solves, it creates one....

Ground test...
Ground test...
Ground test...

Then you won't be guessing at the amount of powder........

Teddy
 
Another way is with an elastic bungie cord across a non-elastic recovery harness. Let's see if a picture is still in my stuff...BUNGIE.JPG
 
Another way is with an elastic bungie cord across a non-elastic recovery harness. Let's see if a picture is still in my stuff...View attachment 273893

I agree that what you show is an alternate method of reducing the shock on the recovery system, but it really is a band aid. If you don't use too much powder to begin with, you don't need the extra elastic.

The way I see drogue deploy, you only need to separate the rocket enough for the drogue to get into the air stream. Even if the two halves only separate half the distance of the recovery harness, as long as the drogue gets into the air flow and it's big enough to keep the payload above the fin can, the harness will get stretched out and the drop part of the flight will work correctly.
 
I agree that what you show is an alternate method of reducing the shock on the recovery system, but it really is a band aid. If you don't use too much powder to begin with, you don't need the extra elastic.

Actually, if memory serves me correctly, this is the way Centauri designed their recovery systems back in the late 60's/early 70's. Along with the "twin-slit" method of attaching the cord to the body tube. This was back before Estes invented the tri-fold. This is for LPR, obviously; there was nothing else back then. Did Estes always use rubber bands?
 
Last edited:
Estes has used always rubber pretty much exclusively...I don't know of any exceptions. In HPR I like having a little elasticity in the system in non-FG motor-deploy rockets due to possible shock loads from early/late ejection. Pryo delay trains have kinda inaccurate timings; I've actually had 1/8" kevlar broken on a late deploy. But I don't make anywhere near the whole length of the cord elastic. For electronic DD a long-enough kevlar/nylon rig takes the cake, no absorption needed.
 
Back
Top