North coast rocketry "cluster duck" build question...

Hello,
I'm working my way through North Coast's Cluster Duck (PDF). The kit come with about 2 meters of aramid cord and some elastic tape. The instructions say to connect the cord and elastic in between the body and nose cone.

The elastic seems to be pretty soft and i'm not understanding the point of it. There have been a couple of other things in the instructions that seem dopy (like adding CA glued to a wire rope ferrule) So i'm starting to questions the instructions. I'm thinking of leaving the elastic out.

The elastic is attached between the aramid and the nose cone. It offers more shock absorption when the chute deploys than just the aramid alone. The
CA on the ferrule is just insurance in case the crimp on the ferrule gives way and the wire tries to slip out. The instructions seem to be standard with other kit instructions.

I flew a Cluster Duck clone a few months ago. I believe it was an E-9 and 6 C6 booster motors. All seven motors ignited, nice straight flight . Parachute deployed at apogee, and a nice LONG ten minute walk for recovery. You may want to consider adding a Jolly Logic chute release or adding dual deployment. Some dings and minor damage at landing: we launched at an unused aircraft runway, so it landed and got dragged on the concrete. It's a fun rocket, good luck on your build and enjoy!

I built a Cluster Duck a few years ago and I too was trying to figure that out, and it made since once I laid it all out, but then I just took some longer Aramid from my supply and just made a single overly long shock cord from it, I didn't want to risk any tangles having two side by side shock cords.

The idea though is what kiririn stated above, to add some extra shock absorption. All of the North Coast kits have been this way for some time. I know some people that have used the Aramid then tied it off to the Elastic, then tied the elastic to the nose cone. The elastic will work this way for several flights provided it is protected from your exhaust gases.

Cool, I'm just finishing up a Cluster Duck. I questioned the dual shock cord as well. To actually have any damping effect, I'm going to shorten the elastic by several inches, otherwise the two cords are almost the same length and any damping would be minimal. I actually had a bigger question with the placement of the 18mm motor blocks. When built as directed, it leaves almost half the motor hanging out the back. I contacted Matt Steele and he explained that this was a simple fix to the issue of having the tube over pressurize when the booster motor burns through. Having the extra inch is supposed to help the tube from rupturing.

My solution was to position the outboard motor blocks so that the inserted motors exactly match up with the 24mm core, and then cut off the outboard motor tubes just above the motor blocks. When glued between the fins, the tubes overlap the main airframe by a millimeter or two, so there is no visible change. Now, instead of trying to vent out of a relatively small hole a few millimeters from the inside of the airframe, it has all the space between the fins to vent the booster blow back. The rear centering ring was given a good layer of epoxy to help with the scorching.

It's great to hear from people that have actually successfully launched this beast and not suffered any damage. Maybe I'm just overthinking this.

Another way of doing this is the old school "bungie cord" method.



If the elastic breaks, you still have attachment with the kevlar.

sooner.boomer gives good advice. Bungees have been around for decades. Just add an additional length of aramid, about 1.5 times more than the elastic, and attach the elastic to each end. As the elastic extends to take up the ejection and parachute opening shock, the parallel aramid acts as a safety strap in the case of the elastic breaking.

Chas

thanks all,
The plan is to launch this weekend.