Props go to Rick Randall at New Way Model Rockets. He has incorporated this Kevlar protection cover in many of his LPR kits.
Kevlar lines start to break down and can fail at 10 to 15 launches.
Near the low end of the Kevlar, near the engine mount - Rick adds some heat shrink tubing over the Kevlar line. When you consider how hot a heat gun gets (to shrink the tube around a wire) it makes sense!
An black powder ejection charge is momentary, the heat gun take a few seconds to shrink around the Kevlar line.
Heat shrink tubing is not included in the School Rocket Kit. I found some on Ebay and am starting to use it in my builds.
For reference, here's a 1/16" diameter heat shrink tube slid over a 135 lb. LPR Kevlar line.
To fit it over the Kevlar knot under the engine hook,
I flared the open end a bit using a sharpened dowel. The flared end makes it easy to slide over the knot.
On the left,
The heat shrink is slipped down and over the knotted Kevlar line.
On the right,
A heat gun was used to shrink the cover tight over the Kevlar and knot. The hear gun was used on the LOW setting.
There is that square hole left on the other side. Some ejection gasses could escape out of the hole.
A small piece of scrap BT-20 was cut and glued over the hole.
I admit I too thought that was pretty nifty the first time I built a Newway kit. I assume the heat-shrink will break down over time as well, but it should extent the life of the Kevlar. Would be great to see some testing.
ReplyDeleteHi Neil,
DeleteThere would have to be an "autopsy" after a few flights to check the condition of the shrink wrap over the Kevlar.
Thanks for the post. I'm gonna use this idea on my Atlas 1/73 scale rockets. Often wondered about the Kevlar life.
ReplyDeleteHi Bill,
DeleteMy experience shows that Kevlar shock cords can degrade and break after a dozen launches. Kevlar is flame resistant, not flameproof.
When there are centering rings like this, I would use your old method of passing the Kevlar through a hole drilled in them and around the bottom end of the MMT, sealing the Kevlar where it goes through the lower hole with a dab of poster putty. The heat shrink is compatible with that method and the system remains easily removeable for inspection and replacement.
ReplyDeleteHi Anonymous,
DeleteYour description would certainly work, but I've found there isn't much space inside between the BT-20 and BT-50 tubes for a Kevlar tunnel tube. That's why I never sold a Heavy Duty Engine Mount to fit a BT-50 rocket body. The heat shrink tubing should work well in the BT-20 tube models. Once the heat shrink tube is shrunk down tight around the Kevlar, it would be difficult to remove the Kevlar out the back of the mount.