So back in April, I put up a gofundme, and after some really awesome friends and relatives threw in some cash, I picked up a Madcow DX3 XL for my birthday. The intention is to get my L3 at Airfest this year. Because of my internship this summer, a couple of NASA projects, and some medical issues with my shoulder, I haven’t been able to start the build until the last two weeks. It’s given me lots and lots of time to think about it though. Construction began with the most detailed Open Rocket model that I could muster. (This was done back in may) I did several versions of the file for different features and weight considerations. I’ve built enough rockets to know that sometimes the best laid plans don’t always lead to success, there have to be options to deviate towards if something goes wrong, or a part has to be built heavier than expected to fix a mistake. For a bird like this, where all I’m after is safety and consistency (not trying to break any records), I’m primarily concerned with ensuring that everything is built extremely well, and any weight discrepancies can be corrected with ballast. However, I’m not throwing caution to the wind on weight! Assembly began by measuring each component of the fin can and dry fitting everything together. The fin slots needed sanding, as usual, but otherwise the slots were nice and straight. The centering rings needed sanding as well. I labeled the fins and slots to match them up for best fit, and then I made markings on the MMT for the positions of the centering rings. After I had diagrammed the fit, I set everything down for a couple of days and came back to do it again. I am a deep believer in this tactic. I have found MANY of my own errors by embracing the fact that a little bit of distance can breed an incredible amount of clarity and perspective. This time it paid off big time. When I had marked the position of the middle centering ring I had misread the ruler several times. For some reason that day ¾ meant ¼ . It was enough of an error that If I hadn’t caught it when I did, and had installed the MMT, I would never have been able to install the fins and the motor retainer without seriously compromising the design (and possibly my L3). I rubbed the old marks away with acetone, and re-marked to the proper measurements. With the markings on the MMT fixed, I placed a clamp to support the middle centering ring and epoxied it into place using West Systems 105 resin with 206 Hardener. The upper centering ring was notched in four places to allow ½” Kevlar shock cord to be attached. Two notches held the ends of a long sling, and two notches were used for smaller loops that act as attachment points for the future (after the permanent sling wears out). After the notches were cut, the Upper ring was secured in place with the WS 105/206 mixture from before. After curing, fillets were applied to the centering rings using WS 105/206 mixed with WS 404 filler to a “catsup” consistency. This was followed fully saturating the shock cord roots with WS105/206, and then four 4”x6” sheets of fiberglass being laid up over them. Next up, installing the MMT, Building the rear centering ring, and tacking in the fins.