Mid Power: Building the Quest Big Dog (Part 8) - Fins, Fillets, and Fillers

Click here for Part 1

Once I get to the part of building a rocket where I attach the fins, I feel like things are really taking shape. The rocket starts to actually look like a rocket. Most of the hard stuff is over, and I can relax a little.

That's not to say that I can take less care - you have to be precise when you attach your fins, or things will go on crooked, and the rocket will never be all it can be.

As usual, I used the Guillotine Fin Jig.

Sometimes, I'll post a picture on The Rocketry Forum or the NAR Facebook group, and people will ask me about the jig - "Is it worth the price?"

I would say yes. I've found it so helpful - I don't fret about attaching fins the way I did when I first started. And I use it for more than that. You can use the Guillotine to hold the rocket while you work on it - which I did when I drilled the holes for launch rail buttons. I take the guide arms off so I don't catch them on something - a fin, or a sleeve, or whatever - and it's like having an extra set of hands.

You set the fin jig by using two fins, or two pieces of the same thickness material as the fins you're using. You can then remove them from the jig, and attach the fins to the rocket. Here's a fun video creator Ted Macklin sent me demonstrating the Guillotine prototype. You can see how easy it is:

I prefer to leave the front piece in place while I work, and even stick a second piece in the middle, just to make sure I haven't accidentally moved anything - by, say, squeezing the arms together.

Sometimes, I'll find I have everything aligned properly, and I find I have a mysterious tight spot. The fins fit between the arms everywhere except where I want to put them, and I can't slide the fin in place. What has happened is that I have accidentally gotten some glue on the aluminum guide arms when I built the last rocket.

If this happens, there's an easy way to fix this. I take some steel wool or some very fine sandpaper - nothing coarser than 400 grit - and gently rub off the glue. Aluminum is soft, so you don't want to use anything too harsh on the guide arms, or you'll take off material. But once you have any leftover glue removed, you should have no problem fitting the fins in place.

It would be nice if the Big Dog had through-the-wall or TTW fin construction - where the fins have tabs that go through a slot in the airframe and attach directly to the motor mount tube. On higher power rockets, this makes the fin connection much stronger. The Big Dog can take large E, F and G motors. You could even put a small H motor in there - which would be a high power motor, and you'd need a certification to do that. But I imagine that would be a bad idea - the Big Dog fins simply get glued to the outside of the airframe, like many smaller, low power rockets.

Because of this, I want to make sure the fins are on really securely. I don't want them to rip off during flight - an event called a shred or shredding. But I also want to make sure they're tight and sound when the rocket lands. These are some big fins!

I'll start by using wood glue, as I usually do with all my smaller models. And the best practice is to use a double glue joint. Put the glue on the fin root, attach the fin to the rocket, then remove the fin again and allow the glue to dry.

Double glue joint - the fin is in the foreground.

Then, you'll run glue along the root of the fin again, and this time, when you attach the fin to the rocket, it will lock in place, creating a much stronger bond.

I used to attach one fin, move to the next, then the next, and the next, attaching fins around the rocket. And sometimes I'd find they'd be out of line with one another. Your pencil line will be thinner than the fin itself, so it's easy to get things slightly misaligned when you do it that way. Now, I do one fin, then turn the rocket 180 degrees, and do the opposite fin. That way, I can check the alignment as I go, and if my fin lines are marked correctly, everything will be correctly aligned.

Unfortunately, the shadows in this picture and the fact that my camera fisheyes a little bit close up distort the image and make this look a little crooked - but I double and triple checked the alignment of these fins.

Once I got all the fins on and the glue had set overnight, it was time to do the fin fillets.

I've noticed lately that on all of my previous rockets, you can't see the fillets. I know they're there, because I put them there. But I used wood glue, and I made them small. Wood glue shrinks as it dries, so unless you look very closely at the rocket, you wouldn't see them at all. They do add strength, but there's more to fillets than simply reinforcing the bond between the fins and the airframe.

Fillets also make the rocket look better. Nice, smooth round fillets are a source of pride to a rocketeer. And a good, smooth transition from the fin to the airframe can reduce aerodynamic drag, by reducing turbulence at the joint.

But fillets can go too far.

As was recently pointed out to me by an aerospace engineer (online - we don't have a lot of aerospace engineers where I live), there's a tradeoff. Because in The Handbook of Model Rocketry it is asserted that good fillets = less drag, a lot of people go kind of overboard on the fillet radius, making fillets that are far larger than necessary. They look cool, but all that extra surface area creates more friction drag, which can negate the reduction of turbulent drag you're after. Basically, this guy says you should use a fillet with a radius no larger than necessary to strengthen the joint between the fin and the airframe.

But for my purposes, and for a lot of rocketeers, I think a good idea is to choose a radius that looks right, and is proportional to the size of the rocket itself. Unless you're into competition rocketry, and have access to a wind tunnel, a nice smooth fillet that's not too large or too small will improve the look and performance of the rocket enough to be satisfactory.

A convention among some rocketeers is to go with a fillet radius that's as close as possible to 4% of the root chord of the fin - that's the length from the front to the back of the fin where it meets the rocket. I think it makes a little more sense to base the fillet radius that has to do with the diameter of the body tube. As it happens, I lucked out and got a good balance of both.

You can create decent fillets with just your finger, and a lot of people do (though you definitely need to wear skin protection if you're working with epoxy). But to create an even, smooth fillet with a consistent radius, you might use a simple tool. I looked in my tool box and found some 5/16 inch dowel rods. Half of that thickness will be the fillet radius, and it looked pretty good for this rocket.

Here you see a 5/16 inch dowel and a soft lead pencil.

In order to get a straight, clean fillet, I needed to tape off the areas where I did not want epoxy to go. To do that, I needed to mark the rocket where the edges of the fillets would go. For that, I used a soft lead pencil. Run the lead of the pencil around one edge of the dowel rod...

It was hard to do this and take a picture at the same time, but you get the idea.

Then run the leaded dowel along the fins and body tube...

This will leave a line on the fin and the airframe, showing you where the edges of your tape should go.

I taped off the fins and the airframe, plus the forward edge of the fins down to a point, and also masked the airframe in front of the fins. All this is to catch excess epoxy. Then I ran some tape around the motor retainer to protect it from drips.

Epoxy is great for fin fillets for larger rockets. Longer cure-time epoxies are better, because they're stronger. They're also self-leveling, which means that the epoxy will smooth out as it sits, before it cures. This is a good thing, but it does mean that you can lose a little of the roundness of your fin fillets. Epoxy can also be hard to sand, which you might need to do if you want to touch up your fillets.

Because of this, some people add fillers to epoxy. Fillers thicken the epoxy and give it different properties. In rocketry, one property people want is sandability. Microballoons are good fillers if you want to make your fillets weigh less and if you want to be able to sand them later.

Microballoons from TopFlite

These spheres are microscopic, and are so tiny they look like a fine powder. A full bottle feels like it has nothing in it. They are so fine, you need to wear some protection when you work with them. I wore a dust mask, some gloves and some safety glasses. When I first took the cap off the bottle, some of the microballoons drifted upward, like smoke - that's how fine these things are.

I used some more 30-minute Bob Smith Industries hobby shop epoxy.

Fillers make epoxy start to stiffen up faster than they would otherwise, giving you less time to work. Because of that, you want to have everything in place before you start. Some things in rocketry are like cooking - you're much less likely to mess up and search for things in a panic if you apply the concept of mise en place - everything in place.

Safety glasses, gloves, dust mask, tiny cup for mixing epoxy, tiny cup of rubbing alcohol, epoxy and filler, dowel rod, two craft sticks for mixing epoxy and applying it to the rocket. Also on hand but not in the picture: paper towels, folded and ready to go, and a squeeze bottle of rubbing alcohol.

Also, because I had to work fast, there are no pictures of the fillet-making process. But here's how it goes down.

Mix the epoxy according to directions. Then add in the microballoons and stir until you have a good consistency. With a craft stick, apply epoxy to the area between the fin and the airframe. Use the dowel rod to shape the fillet by running the end of it all along the joint between the fin and the airframe. On the first pass, you'll take off a lot of excess epoxy. You can clean up wet epoxy with rubbing alcohol, so you'll take a paper towel soaked in alcohol and wipe the excess off the dowel. Then you'll go back and do a second pass over the fillet with the dowel, and wipe off excess.

Basically, you want to wipe off the excess until you stop seeing the epoxy which gathers on the tape trying to creep down into the joint. Then, you can make the fillet smoother by dipping the dowel rod in the little cup of alcohol, shaking off the excess, and running the dowel over the fillet a couple of times.

Once you're satisfied with your fillet, carefully peel off all the tape, making sure not to get any excess epoxy on anything. Finally, check your rocket - if you got any drips of epoxy anywhere - on a fin, on the airframe, or especially on the motor retainer, grab a paper towel, soak it in alcohol, and wipe off the stray epoxy. Make sure to check the fins on the opposite side of the rocket to make sure epoxy didn't drip down and fall on them - once the epoxy hardens, you're going to have a heck of a time getting it off there with some rigorous sanding.

When you're done, you'll have smooth, well-rounded fillets.

I let the epoxy cure for a few hours before moving on to the next batch. You want to make sure that the fillets are cured and firm, and that the epoxy isn't sticky. Overnight is best, but a few hours should do just fine.

Now, because this was my first time using microballoons, I wanted to make sure I was using the right amount, and I was curious how strong the fillets would be with fillers. I had a scrap of body tube to which I'd glued a couple scrap fins. I had papered these fins when I was experimenting with that process. These ones turned out pretty well, and I'd wanted to see what they'd look like when they were primed - to see what a finished product with papered fins might look like.

So, I took some of each batch of epoxy from my fillets and slathered a rough fillet on the scrap fins. I did this with each batch. As you can see from the fillet on the left, the first batch was sandable, but not terribly easily. I'd have to sand more than I wanted to. More sanding means more chance to scuff up an airframe or fin. The fillets on the right were much closer to ideal - sandable, but strong.

Once the fillets were cured, I tried to snap off the fins, to see how strong the epoxy and filler was. I twisted and bent, completely distorting the body tube, but the fins would not break off! This gave me confidence - the fins on the Big Dog weren't likely to snap off on landing, at least, not at the root.

So, the first set of fillets wouldn't be that sandable, but because I'd taken extra care to shape them and smooth them with alcohol, it wouldn't be a huge deal. The main thing would be to lightly sand off the ridges between the fillet and the airframe and fin. You don't want to see a sharp edge - a nice, smooth transition is what you want. If you do it with care, there won't be much to sand.

It was tricky to see how much filler to use. The Top Flite instructions advise 3:1 epoxy to filler. But that's hard to eyeball, especially if you're trying not to get your face too close. Even with eye protection, there's no guarantee a current of air wouldn't waft some microballoons up and around my safety glasses, and call me crazy, but I don't want microscopic bits of glass in my eye.

Consistency and color are what I went by. For some fillers, you want to get the consistency of peanut butter. I felt that would have been a bit too far for this epoxy, as the thicker it got, the less time I had to work - down to minutes. So I got a consistency that was nicely thickened, such that when I lifted some epoxy out of the cup, I got a slow ooze in a single strand from the mixing stick.

As for color, the first batch had the appearance of watery skim milk - a little too translucent. The mixture I finally settled on had more the appearance of 2% milk.

Here are some pictures of the first and second batch of fillets for comparison.

Rather translucent - strong, but not very sandable

Much closer to an ideal epoxy to filler ratio

It's hard to photograph semi-translucent fillets and get a good idea of what they look like, so here's a preview of the nearly-finished rocket. You can see how nicely rounded the fillets are.

We're nearing the home stretch of the Big Dog build. Next, we'll turn our attention to the nose cone. After that, all that's left is to prime, paint, add decals, and attach the rail buttons (which you can see above).

Click here for Part 9

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