My article on Replaceable Kevlar Shock Cords was published in the Apogee Peak Of Flight newsletter yesterday:
You can download issue 338 HEREThis is a simple new way to install a Kevlar line in motor mounts in BT-50 mainframe and larger models.
I've had a few Kevlar lines burn through after repeated exposures to ejection charges. Replacing a Kevlar line installed using standard construction is nearly impossible. You can't access the inside of the motor mount.
With this new method you can check the condition of the line between flights and tie on a new line if needed.
I've been using this Kevlar mount on many of the new builds and flight testing with great success. Sometimes a solution is simple.
Until today, I still preferred the Estes tri-fold attachment because the parachute wadding also protects the shock chord. As of now, I'll be using the loop around the motor mount. This is the kind of solution, so elegant in its simplicity and leaving us all thinking "Why didn't I think of that?" which is the mark of genius.
ReplyDeleteThanks Joe!
ReplyDeleteThe tri-fold mount works but can block the ejection of a parachute in a smaller diameter model.
This is one of those problems I've thought about for a while, then a few months back the answer came. It is too simple and does the job.
I hope to follow up with an article on replaceable Kevlar in minimum diameter models.
I had come to an inferior solution due to a cord burn through on my Der Red Max. I used a very long drill bit and created the channel on an already built rocket, then used a skewer to 'thread' the new kevlar cord through the holes. Planning for this, and using a q-tip or thin straw for a conduit is a fabulous mod.
ReplyDeleteThanks!
Hi Kristian,
ReplyDeleteBe sure you use the smallest diameter guide straw. You don't want any of the ejection charge going out the back.
At first I tried a 1/8" launch lug for the guide tunnel but thought the ejection charge would leak.
I'm trying this on the rocket I'm building now.
ReplyDeleteOne thing I've seen suggested (just now) is to keep the Kevlar well away from the ejection charge by running it through the OUTER part of the motor mount centering ring(s) rather then alongside the motor tube. I would think doing this might also reduce chances of heat damage to the guide straw, though I don't know how much likelihood there is of such a problem. On the other hand, seems to me running the Kevlar up the outside poses more risk of its tearing into the centering ring.
Maybe a compromise would be to go from the inner part of the aft centering ring to the outer part of the forward one? Of course then you'd have to install the guide straw at an angle...
Any thoughts?
This replaceable Kevlar loop is a new idea, any improvements and suggestions are welcome.
DeleteI don't see why you couldn't set the guide straw to the outside of the centering ring.
You bring up a good point about possibly tearing the outside of the ring. Rings are much stronger now then they were in the 1970's. With the Kevlar looped around the ring the "pull" is spread out.
The purpose of pulling out and checking the line is to be able to replace it if needed.
I haven't noticed the top of the straw melting yet - time and many more launches will tell.
Running the straw at an angle might make it harder to pull out or feed a replacement through.