Fixing a possible shock chord mounting mistake

[user 39157]

Hello everyone, new member to the forums here.

I'm going through some of my old projects that never got complete in order to try and finish them up. (crazy concept, right?) I came across a LOC park flyer Iris and cringed at the way I used to install shock chords. I somehow interpreted the instructions in a way that made me epoxy the nylon string to the motor mount, and then loop the actual shock chord through that. So my question is, is this flyable? I did it on another rocket that survived one flight, but I'm not sure how long it'll last. If it's not flyable, how would you go about fixing it?

 

[waltr]

Seems to be flyable from I can see in the photos.
The question is "is the cord secure to the MMT?" If so then it is good.

A way I have fixed (or added a new) cord mount is build a baffle to attach the cord to and glue into the tube a few inches above the MMT.
Another is simply use a coupler and epoxy. Get the new cord between the coupler and body tube.

 

[user 39157]

Seems to be flyable from I can see in the photos.
The question is "is the cord secure to the MMT?" If so then it is good.

A way I have fixed (or added a new) cord mount is build a baffle to attach the cord to and glue into the tube a few inches above the MMT.
Another is simply use a coupler and epoxy. Get the new cord between the coupler and body tube.

I guess a better question would have been "can that nylon string take the stress of an ejection charge". I'll keep your tips about possible fixes in mind, thanks!

 

[waltr]

Is it Nylon 'string' glued to the motor mount? It looked like kevlar in the photo. the photos are too blurry to be sure.
Nylon melts pretty easy from ejection charge HOT gases so would not recommend it as the part on the MMT.

It is not the ejection charge the recovery harness needs to take the stress of...
It is the MASS and Velocity of the two rocket parts.

With a long enough cord between booster and nose there can be little or no stress except weight of rocket under the chute. Then adding taped bundles or braid the cord can reduces the stress.

Since it is LOC Park flier guess 1.6inch BT and total weight around 300-400 gram.
What material and strength rating is the "string" attached to MMT?

 

[user 39157]

I took a minute to find LOC's nomenclature online, you are correct it is kevlar not nylon. I got the "shock cord mount" (thin kevlar string" mixed up with the "tubular nylon shock cord" (the actual shock chord). ~Three inches of the cord are epoxied onto the motor mount on each end of the cord.

Is this done to prevent the ejection charge melting the shock cord then? Having the shock cord epoxied directly to the motor mount seems simpler with less points of failure.

 

[Scott_650]

I took a minute to find LOC's nomenclature online, you are correct it is kevlar not nylon. I got the "shock cord mount" (thin kevlar string" mixed up with the "tubular nylon shock cord" (the actual shock chord). ~Three inches of the cord are epoxied onto the motor mount on each end of the cord.

Is this done to prevent the ejection charge melting the shock cord then? Having the shock cord epoxied directly to the motor mount seems simpler with less points of failure.

Yes, the Kevlar loop is much more heat resistant than the nylon strap.

 

[waltr]

Kevlar---> good.
It is good to fly.
Do remember to examine that cord to the MMT when preping for flight. Kevlar is heat Resistive but does degrade.

 

[mh9162013]

I'm no rocket expert, but as long as what's attached to the motor mount is Kevlar of sufficient strength, I see no issue here.