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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Adventures In Painting - the Quest Quadrunner

I tend to write really long posts, and this is no exception. But, if you're new to rockets, I hope you find something of value here to use in your rocket builds. If you're more experienced, I hope you at least find this amusing.

I'm finishing up one of my three latest builds - the Quadrunner, by Quest Aerospace. This is a cool-looking rocket, and has a cluster of four motors - that's a lot of thrust, fire, noise and smoke compared to what I've launched in the past.

Check out what the kit looks like:

I'd been eyeballing this rocket for some time. I loved the idea of clusters, even before I'd tried them out. My first was the Quest Magnum Sport Loader, which was a lot of fun. Even with only two motors, you get a much more dramatic launch than with a single motor.

Here's the video of the Magnum launching on two Estes C motors - slo-mo at the end:

The Quadrunner is a little pricier than the standard basic kits I'd built in the past - about $45. I got it for less, thanks to an Amazon gift card. But I was nervous about building this rocket. It's more complicated than a Skill Level 1 rocket, and I really wanted to make it look good. It has, of course, the cluster of four motors, which means that putting together the motor mount is more complex, and needs to be more precise than a single-motor mount. You need all four motors to be perfectly aligned.

The finished mount

This means measuring and gluing precisely, and then rolling the whole thing across the table to make sure it's not wobbly.

There's another part that goes in the middle - it holds the bolt in place you see in the above photograph, which will hold the motors in - this is in place of the usual hooks most beginner's models use.

This meant threading the bolt through two diamond shaped bits, attaching a Kevlar shock cord to the top, then pulling the whole thing through the middle of the four motor tubes - then gluing that in place.

This little bit keeps the motor retention system in place, and is key to assuring your parachute ejects. And it's a tight fit.

It requires some precise sanding to make it fit, and without a precise fit, the ejection gasses from the motors will escape out the back of the rocket. The nose cone won't come off, the parachute won't eject, and the whole thing will come crashing down to earth and destroy all your work.

Then there are the fins. They're two-part fins which you have to glue together to make one piece - something I was nervous about, as I was pretty sure it would be easy to get them crooked.

How on EARTH was I going to get these perfectly straight? And how was I to round the leading edges, with that
joint right there?? This was an area I really could have screwed up this build and made a shabby-looking rocket.

The joint here needed to be pretty close to perfect.

I thought of several options, since in the directions you're meant to lay them flat on waxed paper to glue them together flat. I thought no way that's gonna come out straight - it's gonna dry crooked! I finally emailed Chris Michielssen of the Model Rocket Building blog, to ask what he'd do if he built this rocket. I hadn't found any references to the build on his blog, but I knew that if anybody knew what to do, it would be him.

He gave me some advice - turns out he had built this rocket. He said that gluing them flat on waxed paper was a good idea, and then putting a book on top would keep things straight. I sanded a rounded edge on part of the fins, then beveled the trailing edges, and put them together.

So far, so good.

I took all the steps I've learned so far to make the rocket look (and fly) great - carefully filled in those spiral grooves in the paper body tube, filled and sanded and filled and sanded and filled and sanded the fins, attached the fins to the body as carefully and precisely as I could...

Ted Macklin's guillotine fin jig...

Totally worth the money I paid for it.

Hi, Ted!

I got the fins down perfectly, with no gaps between them and the body tube. Made the best glue fillets I'd ever done - I mean, guys, this thing was looking really solid. Carefully filled the seam in the nose cone with some plastic putty, then refilled when I saw a little divot in the putty. Got the thing as smooth as possible.

Was I going to let this little flaw in the nose cone go unfilled? Oh, no I was not!

Up to this point, I was really proud of what I'd done. I had to take it slow, but I hadn't made any major errors I was bothered with.

Then, I had to wait to do the painting. Or, so I thought.

It's been cold here in Bloomington, and my understanding was that you should never spray paint if it's below 50 degrees Fahrenheit. So I waited.

Then, on a warmer day, I primed the rocket. I used a new primer to me - Rustoleum Sandable Primer. I always use primer that's "sandable," but had never used "Sandable Primer."

Actually, this was my first mistake. It turned out fine, and I really liked the primer. But I had intended to use Rustoleum Filler Primer. This stuff has a "build" to it, and apparently it can fill in flaws in your work. I believe it's used to fill in gouges in a car's paint job. I had a few spots I wanted filled on the fins, and a few weeks ago when shopping, I had been looking at both the Sandable and the Filler Primers. I thought I got the Filler stuff. I realized only after priming the rocket that I'd gone home with the Sandable stuff.

Never mind - it worked great. A few minor flaws that perhaps could have been fixed with the Filler stuff, but after some rigorous sanding, it still looked pretty good.

Warmer than it looks - time to prime

Primed and sanded, ready for painting

Round about this time, I got a suggestion on The Rocketry Forum that you can, in fact, paint when it's cold outside. You warm the paint can with some warm water, run outside with the warm paint and rocket, do a light coat of paint, and bring everything inside to dry and warm up for a few minutes between coats.

I had a launch scheduled for Saturday (cancelled, in the end, due to rain), and I thought I'd have time to finish this rocket by then. I always like having something new to launch each time, because I don't have any other rocketeers to fly with - I launch with friends who are only interested because I try to make it fun for them. Basically, every launch I do is a kind of "demonstration launch," and I'm secretly trying to get one of my friends interested enough to start building and launching, so I have someone to talk rockets with.

I wiped the rocket down with rubbing alcohol to rid it of any dust, and did my undercoats of white. The rocket looked great. No problems due to cold weather - a nice, smooth, shiny finish.

It's hard to light a white rocket indoors, but you get the idea.

After letting the white cure for two days, I was ready to paint the red top coat.

This is where the trouble started...

Now, I was going to include this paint job in a post for my fellow rocket n00bs on the fact that you need to plan certain parts of your build, particularly painting, to avoid a messy job, and to keep from being frustrated. In the process, I learned that I have a lot to learn about certain materials with rocket building...

OK, so the Quadrunner comes with a paper mask for painting.

You're supposed to cut this out and apply it to the rocket, with the round half at the top, 1 5/8" from the top of the body tube, then extend the side lines of the mask down to the base of the rocket with masking tape. Then you mask off that whole section with tape and paper, spray the rocket red, and when you're done, you should have a beautiful half-red, half-white rocket with a nice round part near the top of the white side.

This presented a challenge. How could I be sure to get the mask centered and straight? If I applied it the correct distance from the top, how could I be sure that the part of the round top I started with was even the true center of that bit?

Well, I thought about it a bit, and came up with what I think was a pretty good plan, and I thought this is perfect blog material, too! What a smart guy I am!

First, I measured the bottom of the mask, finding the center of that line. Then, using a right angle, I drew from that point up the center of the mask, through the top.

To check my work, I did a little geometry to find the center of the semicircular top - and you can see was off by just a little bit. I made sure the line was straight up the center from the bottom to the top, and carefully cut out the mask.

I cut up the sides with my hobby knife and a metal ruler. I tried to rig up a tool to do the top round cut precisely, using my high school compass and my hobby knife. I think I found this idea in an old Estes or Centuri rocket design publication I found online.

This didn't work as planned, so I just carefully cut the top out with scissors. Not to worry - the black and gold stickers would probably cover any minor imperfections in the shape.

Then I found the centerline between two fins by measuring the distance between the leading edges of the fins and making a mark with a pencil. Then I drew a line all the way from there to the top of the body tube, and on that line, I put a mark 1 5/8" from the top.

Now I had a line on the rocket, and a line on the body tube. If I could get those lines to match up with the top of the mask at that little 1 5/8" mark, I knew I'd have the mask centered and placed correctly, and we'd be good to go.

See? This should work great!

I still think this is a good idea, despite what came next...

To apply the mask, temporary spray adhesive is called for. I have some Elmer's Craft Spray Adhesive, which is either temporary or permanent, depending on how you apply it.

I applied the adhesive to the back of the mask, waited three to five minutes for it to be less tacky, then applied it to the rocket. Success! I got it on perfectly!!

Then I built a beautiful, elaborate mask. It fully covered everything, yet would be easy to remove when I was done painting. I was feeling pretty good up to this point. Why was I so worried about this rocket? Now all I had to do was paint - this would be a cinch!

Doesn't look like everything's covered, because the edges are Scotch tape. But this was
a masking masterpiece, guys. Perfectly aligned edges, full coverage... Great work.

Then came The Ordeal.

To seal the edges, I did a quick pass of the white paint first. Then, to my horror, the edges lifted away from the rocket and began to curl up. Gahh!!! Stop it! Stop it!!

There was no way around it - I'd have to stop my work, remove all the masking, and start again the next day.

I carefully peeled everything off, and laid the paper mask flat. It was a little wrinkled, but the edges were mostly fine, so when it was dry, I laid a book on top of it to flatten it out and left it overnight.

I figured I hadn't used enough of the spray adhesive. So the next day, as soon as I got home from work, I started again. This time, I made sure to get plenty of adhesive on all the edges, and reapplied the mask. Then I built up the whole brown-paper-and-tape masking apparatus and started again.

Here's where I grew impatient. I skipped the white sealing coat (not best practices, but not what led to disaster), and went straight for the red. And what a red! Rust-Oleum 2X Apple Red - bright, shiny, smooth... This rocket was looking beautiful.

I waited for about ten minutes, then began removing the mask. I pulled off the tape and the paper, and the fins were a brilliant white. The line was clean and neat. I pulled off everything but the paper mask... and then the mask wouldn't come off!

I used fingernails and lots of swearing, but the thing wouldn't budge. I even tried to gently, carefully slide the tip of my hobby knife underneath the mask to loosen it. Nothing!!

Look at this monstrosity!

This obscenity!!

This abomination!!!

 
After getting only tiny pieces of this mask off and gouging some of the white paint in the process, I took to The Rocketry Forum for help.

I went from happy to pretty desperate very quickly. What should I use? Water? Goo Gone? Razor blades?? After several more attempts to just coax the thing off, I'd scratched through in a few spots to primer, and in a couple tiny places down to the white body tube, and I worried I'd even gone through the glassine coating to the fibers below.

A lot of people suggested water (until I mentioned breaking through the tube - then it became too risky), WD-40, white vinegar. Problem with all of these is that the paint had made the paper pretty impenetrable.

Finally, in frustration, I got out a sheet of 220-grit wet/dry sandpaper and went to town on this rocket. Probably a terrible idea, I thought, but at this point, I don't care if the finish is smooth and beautiful - I just want that red paper OFF OF THERE!

Now, I don't have this next part documented with pictures, because I was acting in desperation. I wetted the sandpaper, and scrubbed like hell. Then, the sandpaper started breaking through the paint. Then through the paper mask. Then, I could see the white paper underneath!

I had three thoughts: 1) This is a terrible idea, and this paint job is ruined, but I don't care. 2) Wait... Is this working? I can't believe this is working!! 3) Holy crap! I can't believe this isn't causing more damage to the surface than it is!!

After scrubbing and scrubbing with the sandpaper, I finally got the paper mask off!! There was still some residue on the white part of the rocket, but the cursed red paper was gone!

Flash! Aaaaaah! He saved every one of us!

OK, so how to get rid of the sticky residue? I mean, maybe the rocket wouldn't be perfect, but there must be some way to get rid of that stuff!

This is the point when Our Hero, inventor of the guillotine fin jig, rocketeer extraordinaire, stand up guy, and my Internet buddy Ted Macklin, posted those three magic words: Ronsonol lighter fluid.

According to Ted:

As long as this paint is sufficiently dry Ronsonol (which is actually naptha) will not dissolve the paint. It will soften and loosen most adhesives, especially those that are relatively fresh. A little Ronsonol on a cotton ball should work, but test first. 

Ronsonol? I think I have some of that!

So I searched in the depths of the darkest closet in my apartment, and found the Ronsonol. I dabbed some on a cotton ball, and began wiping down the rocket. Dudes, this stuff is AWESOME. Took the residue off so easily, and I could feel the smooth finish I'd started with below.

At this point, I was so happy, I did the Dance of Joy.

The rocket is on its way to greatness. I have some chips to repair, and I have to figure out how to safely mask off the red paint so I can retouch the white. But the ordeal is over. The Seal Team Six of rocket painting came through, and we are safely back on home ground - a little scarred, a little bruised, but safe for painting once again.

A Few Things I Learned From This

If you're a relative rocket n00b like me, here are a few things you might learn from my mistakes:

1. Paper masks = bad. Much better is a low-tack substance called Frisket Film. Also suggested possibilities would be vinyl drawer lining (though I'm not sure how low-tack that stuff is) and some kind of vinyl sign stuff. Still not sure exactly what this is called or where to get it.

2. If you're going to use a paper mask, make sure you use temporary, low-tack adhesive, not this "temporary/permanent" stuff.

3. Maybe make a photocopy of paper parts you need if you might accidentally destroy them while making a terrible mistake? I'm not sure how I'm going to cleanly cut a mask for repairing the white part, now that the paper mask is dead.

4. Ronsonol lighter fluid safely takes off adhesive residue.

5. Don't be impatient. If I'd found out about the lighter fluid solution before attacking my rocket with a hobby knife, I would have no repairs to do. The stickers would be on it right now, and after nearly a month and a half building, the rocket would be done and ready to fly.