Anchoring the Kevlar in an MD LPR rocket, and other motor mount issues

[neil_w]

Is the "Kevlar protection" issue related solely to distance from the top of the motor (source of the ejection charge)? If I have a long motor mount tube that extends several inches above the top of the motor, does that provide the Kevlar the equivalent protection of several inches of heat shrink?

 

[jrap330]

If you've ever had to replace a tri-fold in an old rocket you'll never use one again. With kevlar, it's almost excusable but with kevlar there are other methods that are both better and easier. Tri-fold with rubber shock-cord is an especially bad combination, because the rubber cords will dry out and fail after a few years. If I have one complaint about Estes it's that they're still using tri-fold with rubber shock cord in their kits.

you really do have to replace it, just install another. Too many Rocketeers are afraid the recovery will get "hook up" I believe that is just over cautiousness.

 

[kuririn]

Is the "Kevlar protection" issue related solely to distance from the top of the motor (source of the ejection charge)? If I have a long motor mount tube that extends several inches above the top of the motor, does that provide the Kevlar the equivalent protection of several inches of heat shrink?

Intuitively I would say yes.
I had a kevlar burn through on a maiden flight over the weekend. but that was a thin 100# thread.
The same thread has lasted numerous flights on MX rockets.
And I suppose airframe volume has something to do with it also. A thread 5" away from the heat source in a BT-50 tube will probably experience hotter ejection gas than a thread 5" away in a BT-60 tube.

 

[cwbullet]

Intuitively I would say yes.
I had a kevlar burn through on a maiden flight over the weekend. but that was a thin 100# thread.
The same thread has lasted numerous flights on MX rockets.
And I suppose airframe volume has something to do with it also. A thread 5" away from the heat source in a BT-50 tube will probably experience hotter ejection gas than a thread 5" away in a BT-60 tube.

did you use wadding?

 

[kuririn]

did you use wadding?

Yeppers.
The wadding was stuck at the top of the tube when I recovered the rocket, but the laundry ejected.
Don't know if wadding would have helped in this case as the burn through was at the motor mount.

 

[cwbullet]

Yeppers.
The wadding was stuck at the top of the tube when I recovered the rocket, but the laundry ejected.
Don't know if wadding would have helped in this case as the burn through was at the motor mount.

It is a bummer when the shock cord separates from the rocket. I tend to user a thicker cord for the anchor.

 

[BEC]

+1
New Way kits include a piece of heat shrink tubing to protect the kevlar near the motor mount.

I’m also doing that to protect against the lip-of-the-body-tube wear-through. I’ve had only one fail since I started doing so. It failed just past the heat shrink on the 46th flight (according to my logbook). I redid the tri-fold mount as described in a prior post and made the heat shrink longer. That same model‘s next flight is number 90. There’s little wear in the Kevlar at the moment.

 

[jqavins]

I know, I know, late to the party again.

Consider a 24mm minimum diameter cardboard rocket. OK it's not really minimum diameter, it's more like a rocket with a full-length MD stuffer tube, but that's near enough the same thing, as far as this question goes.

Is it? Is it really? If this is not truly an MD rocket then can't you attach the kevlar to the outside of the stuffer tube? That would make things a whole lot simpler.

I have never seen such wide thrust rings before. Where did you get those?

Heavy walled thrust rings/couplers can be made up to 1.75 inches long (for BT-5), 2.75 inches (for BT-20), or 3.75 inches (for BT-50) from stock that's readily available at the end of every launch day. There's plenty of thickness on those to file or cut in a groove that will accommodate a cord as thick as 1/16", and up to nearly 3/32 with some care.