Gluing shock cord to MMT

[Handeman]

While building one of my latest rockets, I glued the rear CR in place on the MMT. Used the fins to position the upper CR and glued that in place. Then I made a really boneheaded error. I filed slots and glued the shock cord to the wrong end.

I pulled the shock cord straight out from the MMT and gave it a good jerk and it ripped right out of the Rocketpoxy. It was surprisingly easy. Not that it could ever get pulled in that direction once it as assembled, but still.



When I went to reinstall on the other end, I decided to use laminating epoxy instead. What I found was when I brushed some onto the side of the cord that had been glued down to the Rocketpoxy, you could tell it was being absorbed into the Kevlar.

I haven't tried to rip the cord off the laminating epoxy, but I suspect it would be harder to get off.

I believe the laminating epoxy is a better choice for this application that Rocketpoxy or other similar epoxies.

Anyone else have any experience with this?

 

[JohnCoker]

That's very interesting. It's possible the Rocketpoxy didn't penetrate well and the slickness of the Kevlar kept it from forming even a good surface bond. But it certainly doesn't give one confidence in that joint!

I have made the same sort of joint several times where space is tight and hardware won't fit. This reminds me of the attachment point on a Mac Performance kit, which I thought was a clever idea:

 

[Handeman]

Now that you showed the pic, I do remember seeing that method before. The only thing that concerned me as how much thinner the CR was where the slot was cut. I think if I were to use that method, I would just use a slot for the cord and some epoxy to tack the metal pin in place on the bottom face of the CR.

As for the Rocketpoxy joint, I do think it's weaker than the one with the laminating epoxy, but since it would only be pulled parallel to the MMT and not straight away, I don't think that joint would ever fail in use.

 

[dhbarr]

For some of the tool-making epoxies they recommend you stipple the surfaces with a layer of laminating epoxy before using the filled stuff. I bet a similar approach would be useful here, i.e. wick up the kevlar before bedding.

 

[Steve Shannon]

I don’t use Rocketpoxy, but whenever I make and use thickened epoxy I brush the area or parts first with unthickened epoxy first in order to facilitate penetration.
I’ve epoxied shock cords to MMT sides for years. I don’t think any of them would pull free like you experienced.

 

[MClark]

When I glue a shock cord I wash it with acetone or similar and put a wrap of fiberglass over it.

M

 

[dr wogz]

It also depends on the force acting on the shock cord. Typically, they will shear: needing to pull the whole glue joint off in one shot.

You likely pulled the cord, at a 90° to the tube, so all your force is at that one point where it makes the 90° bend. Much easier to pull off.

I will always work the weave between my fingers to loosen it up, to make sure the epoxy can penetrate into the weave.

 

[crossfire]

I have used JB and RP for shock cord to MMT never seen that before. Never had a cord pull out.

 

[kbRocket]

Unweave and frizz out a couple inches of the Kevlar. This increases the area of the bond and makes it easier to get the epoxy to coat the materials. I prefer laminating epoxy for this application but you can do it with Rocketpoxy.

 

[Handeman]

It also depends on the force acting on the shock cord. Typically, they will shear: needing to pull the whole glue joint off in one shot.

You likely pulled the cord, at a 90° to the tube, so all your force is at that one point where it makes the 90° bend. Much easier to pull off.

I will always work the weave between my fingers to loosen it up, to make sure the epoxy can penetrate into the weave.

That's exactly what I did. I was just surprised at how easy it was to pull it up like that. I have no doubt it would never have pulled off once it was installed in the rocket where all forces would be parallel to the tube surface.

 

[Theory]

Interesting to be sure.

I typically only use rocketpoxy in situations where I absolutely do not want the epoxy to move and “penetration” is not required.

I used T88 on my last build and “worked it” into the Kevlar, then held it in place with masking tape. Confident this will hold

 

[markjos]

In the belt-and-suspenders mode, with adequate space, you can also put a knot in the cord below the notched centering ring.

Mark