A couple of years back, my Big Daddy 4x18 had a partial failure of the Kevlar harness to which the shock cord was attached. The harness consisted of a piece of 100# twisted Kevlar attached at each end to opposite sides of the engine mount; on that particular flight, one side broke free. Fortunately, the rocket recovered without damage.



It sat around, unrepaired, until last month. I couldn't figure out a good way to attach a replacement harness to the engine mount... it's like building a ship in a bottle in the dark.

I was reading about various replaceable Kevlar shock cord mount ideas when I remembered the Big Daddy, and I started thinking, how can I do replaceable Kevlar in that rocket?

Then I remembered it has only one centering ring, on the bottom. The centering ring is made of photographic mat board, pretty flimsy but plenty good enough for the necessary gas seal. (The TTW fin tabs support the engine tubes.) That's when I thought of running the Kevlar out the engine hook holes.

But that flimsy centering ring would never hold the screws. I needed to beef it up without adding too much weight.

So I took a piece of 1/16" birch plywood to some friends of mine who have a laser cutter, and came home with a reinforcement ring. I attached it with yellow glue, then applied a thin fillet of epoxy to add a bit of extra strength.

Here's the result:


(Click for a bigger version.)

Now I just need some warm weather to fly it in...

__________________
NAR # 115523
Once upon a better day... SAM #0076
My site: http://rocketry.gonnerman.org

Cool. Nice repair. But why do I see 3 Kevlar cords?

Joe

Actually, there are four. Two lengths of 100# Kevlar braid, with loops at each end, forming a double harness.

I really, really don't want this one getting away...

__________________
NAR # 115523
Once upon a better day... SAM #0076
My site: http://rocketry.gonnerman.org