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Fafnir was one of my more beloved rockets. It was a 3"/38mm beastie made from a mailing tube, with plywood bat wings. It was a workhorse of my fleet since I could fly it on either a medium to full G on a Class 1 field or up to low I's on a high power field (more thrust isn't great--the body tube has a few wrinkles from over-enthusiastic flights). The most commonly-flown motor was probably a G125 Red Lightning to a thousand feet or so--a lovely flight to show TARC teams what's possible in rocketry. Alas, a hard landing on frozen turf near the end of last year broke one of the fins off just above the fillet. The root cause was that the chute didn't fully eject because I didn't add enough BP to augment the 29mm ejection charge, and the rocket has a large body tube to pressurize.
Fafnir in happier days
So I definitely needed to rebuild. I really like having a 3" rocket in my Class 1 capable fleet, so being under 1500g liftoff weight was imperative. I stuck with a 38mm motor mount to allow for high power flights later on. As with the original version, the fins are traced from an image of a fruit bat, then scaled to a more reasonable aspect ratio.
I actually started the build way back over winter break, when I had a dad-daughter bonding moment laying up the body tube. She's in a plastics and composites engineering program and does fiberglass boat repair for pay and fun, so we made a good team. The first layer was a carbon fiber braided sleeve that had been gathering dust forever, followed by a couple of layers of medium-weight fiberglass to give room for finishing and a little more strength. I didn't love the sleeve, but that's probably partly because of unfamiliarity. After trimming up the ends, it came out right at 24" long, a little shorter than the original. That's OK, storage will be easier. I sanded the tube smooth and then added another coat of epoxy to give a nicer surface. I'm quite pleased with how the CF sleeve shows through the glass on top.
Next up was rounding/airfoiling the fins. That took a fair amount of work with one of the 3/4" Dremel sanding drums. I wore a respirator because the last time I did this I came down with pneumonia about a week after doing a lot of sanding without respiratory protection. The doc didn't think it was a cause, but it was sure coincidental. No pneumonia yet, so it seems to have worked. The motor mount is a little longer than the original and has three centering rings so that I minimize the volume the ejection charge has to pressurize. I'm guessing this will fly mainly on 29mm motors. It's a lot easier to prep those flights if I don't have to add BP to the ejection charge, so a little extra weight is worth it. I cut the fin slots with a Dremel cutoff wheel, and sanded where the fillets will land.
Here's the stack of parts, vise-grips for scale. The nose cone isn't pictured--I'll re-use the same nose cone as the original. Construction of the motor mount and the rest is fairly normal. The bulkheads are glued to the MMT with 5-minute epoxy5-minute. I even peeled back the glassine in the areas where the glue will hit. The only slightly unconventional thing for me is that the aft centering ring won't sit on the aft side of the fin tabs. I wanted to keep the weight out of the back end, so the tabs are relatively short. Since the aft centering ring is also my motor retention and I've had trouble with very long screws in the past, so the bulkhead will be right at the back of the airframe.
As of now, I'm expecting the all-up weight to be around 900-950 grams, though I may add a little nose weight if I have room in the Class 1 weight budget so that I can fly 3-4 grain CTI I motors. The main thing is to stay below the transonic range--these fins aren't built for Mach. If OpenRocket is to be believed, I'll be seeing altitudes of around 1200 feet on a G126, 1800 feet on a G125, and up over 3000 feet on an I170. I suspect it won't go that high, but that's a discussion for flight testing.
Fafnir in happier days
So I definitely needed to rebuild. I really like having a 3" rocket in my Class 1 capable fleet, so being under 1500g liftoff weight was imperative. I stuck with a 38mm motor mount to allow for high power flights later on. As with the original version, the fins are traced from an image of a fruit bat, then scaled to a more reasonable aspect ratio.
I actually started the build way back over winter break, when I had a dad-daughter bonding moment laying up the body tube. She's in a plastics and composites engineering program and does fiberglass boat repair for pay and fun, so we made a good team. The first layer was a carbon fiber braided sleeve that had been gathering dust forever, followed by a couple of layers of medium-weight fiberglass to give room for finishing and a little more strength. I didn't love the sleeve, but that's probably partly because of unfamiliarity. After trimming up the ends, it came out right at 24" long, a little shorter than the original. That's OK, storage will be easier. I sanded the tube smooth and then added another coat of epoxy to give a nicer surface. I'm quite pleased with how the CF sleeve shows through the glass on top.
Next up was rounding/airfoiling the fins. That took a fair amount of work with one of the 3/4" Dremel sanding drums. I wore a respirator because the last time I did this I came down with pneumonia about a week after doing a lot of sanding without respiratory protection. The doc didn't think it was a cause, but it was sure coincidental. No pneumonia yet, so it seems to have worked. The motor mount is a little longer than the original and has three centering rings so that I minimize the volume the ejection charge has to pressurize. I'm guessing this will fly mainly on 29mm motors. It's a lot easier to prep those flights if I don't have to add BP to the ejection charge, so a little extra weight is worth it. I cut the fin slots with a Dremel cutoff wheel, and sanded where the fillets will land.
Here's the stack of parts, vise-grips for scale. The nose cone isn't pictured--I'll re-use the same nose cone as the original. Construction of the motor mount and the rest is fairly normal. The bulkheads are glued to the MMT with 5-minute epoxy5-minute. I even peeled back the glassine in the areas where the glue will hit. The only slightly unconventional thing for me is that the aft centering ring won't sit on the aft side of the fin tabs. I wanted to keep the weight out of the back end, so the tabs are relatively short. Since the aft centering ring is also my motor retention and I've had trouble with very long screws in the past, so the bulkhead will be right at the back of the airframe.
As of now, I'm expecting the all-up weight to be around 900-950 grams, though I may add a little nose weight if I have room in the Class 1 weight budget so that I can fly 3-4 grain CTI I motors. The main thing is to stay below the transonic range--these fins aren't built for Mach. If OpenRocket is to be believed, I'll be seeing altitudes of around 1200 feet on a G126, 1800 feet on a G125, and up over 3000 feet on an I170. I suspect it won't go that high, but that's a discussion for flight testing.