Building the Semroc Sprite (part 5)

So I did more sanding and priming and sanding, and I glued the nose cone on, and I filled some dents and the nose/body seam with Bondo spot putty and sanded some more, and I went to put the ring fin on after, as the instructions recommend, sanding the fins down to achieve a snug fit oops.

They didn’t need sanding.

The fit wasn’t snug. It wasn’t loose. It was entirely wrong. The fins’ height — body tube to tip — was too small. Way too.

How? Didn’t make sense. Someone else brought an assembled Semroc Sprite to the last club meeting and if they’d had the same problem they would have mentioned it. The fins are laser cut. The kits were bought within weeks of each other. If theirs were the right size mine had to be too. Unless I made them too small… yes, I did sand a little to remove the laser cutting tabs and a thin layer of primer I’d gotten on the tips, but I couldn’t have sanded off this much, could I?

How much was this much? I tried measuring but that was pretty much impossible on the assembled rocket, so I marked the fin tips on a piece of paper, scanned it along with a ruler for scale, imported it into a drawing program, and added a circle and some measurements:The black marks are the actual fin tip locations. The circle is the inner diameter of the BT-70 ring fin. It clears the fin tips by about 0.06 inches. The diameter mismatch is an eighth of an inch!

Which is impossible. Yet here we are.

I thought briefly about reducing the diameter of the ring, by cutting out a 0.125π-inch section and gluing the remainder, somehow, back together, but I didn’t like that thought. Instead I cut some slivers of balsa, 1/16 inch wide, from the pieces the fins were cut out of, and glued them onto the ends of the fins. Then I sanded those down a little to get a snug fit, finally, and glued the ring and the fin tips on.Because I primed and sanded the fins, they’re no longer the same thickness as the tips. There are also some small gaps to fill. Then there’ll be some work with an emory board to even things out and round the fin edges. Meanwhile I now see access to the fins and lower body tube is nowhere near as restricted by the ring as I was thinking it would be. Probably it would have been better to leave priming and sanding until after full assembly.

Well, this is only my first Sprite. I can make new mistakes on the second one.



2 thoughts on “Building the Semroc Sprite (part 5)

  1. Interesting – The kit fins should be supplied a bit wide (span) to allow sanding to the size of the ring. I always clean off some of the laser burnt edges usually so the edge is square. Laser lines aren’t a square cut but cut like a wedge, wider at the bottom where the beam is diffused by the material being cut. These fins are thick so the beam cut wedge would be even more pronounced. I’d let Randy at Semroc know about this. i doubt you could have sanded 1/16″ off both edges and ended up with a 1/8″ difference.

    • It’s weird. We had another club meeting last night and the other person brought theirs in; they definitely didn’t have the same problem. I don’t know how I could have lost that much height but I don’t see how it could be on Semroc’s end if the other kit was okay, so maybe I just was a lot more aggressive in my sanding than I thought. But why would I? Maybe I’m sleepsanding.

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