I guess I didn’t have to deliberate over whether to build the Mustang or the Ventris. The Mustang is in primer and there’s almost three more weeks before URRF. Weather permitting, I can get both ready.
First decision: Shock cord. The Ventris comes with trifold and elastic, a setup John Boren has vigorously defended while denigrating Kevlar on the motor mount. Who knows, he could be right. He also says whatever shock cord system Estes provides, some people are going to complain and/or build it differently. On that he’s definitely right.
I dithered over a few options. I really like Chris Michielssen’s removable Kevlar method but until I can source 6″ long plastic stick cotton swabs, or some other source of much longer guide straws, that’s out for big rockets. In a fatter body tube I’d opt for a u-bolt or an eyebolt or even a loop of Kevlar on the front centering ring as an attachment point for a removable leader. But fitting that on the Ventris’s centering ring would be tricky, and reaching inside to attach and detach the leader would be close to impossible. I finally decided to go ahead with gluing a doubled Kevlar leader behind the front centering ring, sacrificing removability. So I started by notching a ring.
Then I put the motor mount together, and cut a piece of Kevlar which I tied in a loop, then put a slip knot in it. The slip knot goes around the motor mount tube and gets glued down in the fillet behind the first centering ring.
The centering rings were slightly more snug than I liked, but a little sanding took care of that. I used a dowel to apply tacky glue just past the fin slots before pushing the motor mount in. Tacky glue to avoid yellow glue’s grabbiness, but I think it was starting to grab too. I probably should have used epoxy, or else larger amounts of tacky glue. Once it was in place and I checked the position by dry fitting the fins, I applied yellow glue fillets, first to the rear ring, and then to the front — first covering the Kevlar with a straw to keep glue off it.
I tried to be careful with the centering ring positions but wasn’t quite careful enough: there was just a little forward-backward play on one of the fins. So I glued a cardstock shim to the end of the fin tab.
The Estes fin jig wasn’t designed for this. I used it anyway; I haven’t talked myself into buying a guillotine jig yet. I found I could peel off paper from the outside of a spent 24 mm casing to make a good fit inside a spent 29 mm. Then that fit over a 18 mm. And that fit over the 13 mm which fits in the jig. I made sure the jig was set up for four 1/8″ fins and went ahead with gluing.
If you’re more observant than I was you’ll notice I didn’t mention sanding the fins, mostly because I didn’t. Better late then never? I rounded the leading edges, but given the concave curve of the trailing edges I just softened those and the tip edges.
Fillets will be next, once the Lunar Eclipse Jr. comes off the rotisserie.