Fafnir, a 3" Dragon-Themed Rocket

[boatgeek]

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.

 

[boatgeek]

A quick discussion of the name. Around the time I was building the original, my son was deeply into dragons, so that's why I built a dragon-themed rocket. He did the illustration on the original. I didn't want to do something obvious (Smaug!), so I went to a slightly deeper cut. Fafnir is a dragon from Norse/Germanic lore. Fafnir was a man who became a dragon after killing his father to steal his father's hoard of gold. He was later slain by Siguro (Siegfried for you Wagner fans). The slaying of Fafnir appears on a number of Viking Age runestones (carved roughly 900-1000 CE) in Scandinavia and beyond.

The original was flat black and gloss red, which made for a pretty nice look. the braided sleeve is a good approximation of scales, plus it's a crime to paint carbon fiber, so the body tube will just be clear coated. I'm not entirely sure where the fins and nose cone will land. I do have another copy of the illustration, but (a) it's old enough that I'm not sure the vinyl will still stick and (b) the dragon is more Tolkein than Old Norse, so I don't think I'll use it. I am thinking of having the name in Norse runes, possibly in gold. If I do that, the fins and nose will likely also be gold.

 

[jqavins]

The only slightly unconventional thing for me is that the aft centering ring won't sit on the aft side of the fin tabs.

Since there are three centering rings, wouldn't the tabs be snug between the forward and middle ones? Sure, if there were only two rings then it would be unconventional to keep the tabs short of the aft one, but it this case it doesn't seem odd at all.

I'm not entirely sure where the fins and nose cone will land. I do have another copy of the illustration, but (a) it's old enough that I'm not sure the vinyl will still stick and (b) the dragon is more Tolkein than Old Norse, so I don't think I'll use it. I am thinking of having the name in Norse runes, possibly in gold. If I do that, the fins and nose will likely also be gold.

Could you commission a new drawing from your son? Make it even more of a family project?

No pneumonia yet, so it seems to have worked.

The lion repellant that I spray on my lawn is working too.

 

[boatgeek]

Since there are three centering rings, wouldn't the tabs be snug between the forward and middle ones? Sure, if there were only two rings then it would be unconventional to keep the tabs short of the aft one, but it this case it doesn't seem odd at all.

The tabs are between the middle and aft rings. The tabs are securely glued to the middle ring, motor mount, and body tube, and "loose" on the aft end in the ~3/4" wide annular space between the motor mount and body tube. The forward ring is ~6" forward of the middle ring.

Could you commission a new drawing from your son? Make it even more of a family project?

I suppose I could, though the options for printing vinyl are a little more limited with Stickershock out of commission for the foreseeable.

The lion repellant that I spray on my lawn is working too.

Good news! Given that I did get pneumonia once and I suspect that you've never had lions, I think I have a slightly better causal link. That said, even if the dust didn't cause pneumonia the last time, it's still not that great for a person. I'll stand by the statement that the minor discomfort of the respirator is worth it.

 

[boatgeek]

Wow, it's been a while. Fafnir is basically flight-worthy, though I will need to clear coat before flight to prevent damage to the paint. Here's what happened along the way:

Glued in the fins and motor mount.
Filleted the fins (System 3 GelMagic with black dye), sprinkled a little gold glitter over the fillet surfaces
Shot a little clear over the glitter to protect it from masking tape, primed the fins. This picture also shows the yellow cast of the older epoxy that we used for the body tubes. It fits really nicely with the gold theme.


Filled and sanded the fins, primed the nose cone, sprayed them all gold


Spent far, far too much time researching runes. Short description: Germanic-to-Scandinavian runic alphabets are generally called futharks after the first letters: F, U, TH (aka thorn, the "Y" in "Ye Olde..." in English), A, R, and K. I went out to look at available runic fonts, and came up with a few that I kind of liked. The top and bottom ones are an older form, and the middle one is newer. But none of them really struck me as awesome. If I'm going to have to explain what it means at launches, I want it to be awesome.


At this point, my beloved wife said that since we were only looking at 5 characters, she would help me draw my own font. This is pretty close to the final, though some development is ongoing. Both F's will be like the middle one, intended to be more like trees than a poorly-made menorah.


So all that remains is to buy and cut the vinyl, scuff sand the clear coat, fill a small holiday in the gold paint on the nose cone, and clear coat the whole shooting match. It may be ready for flight in two weeks or so.