Sunday, November 25, 2018

Build Series - The SBR Diablo - Part 1: Unboxing

Scott Binder of SBR sent me his latest high power rocket kit to check out: The Diablo. This is a simple, easy to build, short and fat (4 inches in diameter!) high power kit, as easy to build as a common model rocket - just bigger. Because it's a draggy rocket, it's good for small fields. With its light weight, it can fly on anything from an F motor all the way up to an I or maybe some J motors. And its small size makes it easy to transport to a launch even if you have a compact car.

I'm working on a video build series of this rocket to show how simple it is to put together. Here's the first installment, shot on my phone (sorry about the sound quality) - Unboxing:

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Thursday, November 15, 2018

Built from Scratch - A Tale of Two Berthas - Part 1

I recently finished building two Big Bertha rockets - from scratch, rather than from kits. As a guide, I used the original plans, published by Estes in Model Rocket News in 1963, when Bertha was a free plan, before it was sold as a kit (the Big Bertha is now the longest continually produced model rocket kit in history).

I meant to build just one scratch Bertha, but ended up building two of them, almost by accident. I do this a lot.

I started this project for a few reasons. My original Big Bertha kit was one of my earliest builds, detailed a few years ago on this blog. I mention the Bertha a lot here, not because it's always been my favorite model rocket (it hasn't), but because it's so iconic, and simple, and with its large parts, it makes a good demo rocket.

But last season, I flew my now beat up Bertha for my final flight of the fall on a C6-3 motor. Our Launch Control Officer, Kenn, commented "There's no better combination of rocket and motor, folks!" and I thought, you know what, he's right. That is a great rocket!

The Bertha is such a pleasure to see fly. I always brought it to a launch as a kind of afterthought, thinking I should take something that doesn't fly too high, just in case. But each time I flew it, it was just... fun! It goes up relatively high, but you can still keep your eye on it, and it floats gently back down on its 18 inch parachute, and... Well, I guess I just have a soft spot for the Bertha. So, I wanted another one.

That was reason 1. Reason 2 is that, while it does fly really well on C motors, I've always wanted to put a D12 in it, but I built the Bertha kit before I knew you could upgrade these things. The Berthas I've seen fly on D12-5 black powder motors are really fun. With its large fins, the Bertha is plenty stable, and can handle extra weight at the back. So, I decided to build a Bertha with a 24mm diameter motor mount, for those Estes D12 motors.

Reason number 3 is that I always wanted to build one of the plans from the old Estes Model Rocket News. Back in the 1960s, many of these early newsletters featured rockets either designed by Vern Estes himself, or by readers of the Model Rocket News, who'd send in their own designs, which used Estes stock parts. In those days, it was pretty common for model rocketeers to have a fleet which was at least in part built from scratch, rather than from kits.

I figured the Bertha would be a good place to start. No odd, out-of-production tube sizes or nose cones to buy. Just a BT-60 tube, some 1/8 inch fin stock, and a stubby elliptical nose cone. Speaking of which...

Reason 4: I had always wanted to try turning my own nose cone from a block of balsa.

 The plan was to see how hard this could be, using a hand drill as a kind of lathe. From what I'd read in both the Model Rocket News and the Handbook of Model Rocketry, this was supposedly not only doable, but not all that difficult. I wasn't sure that was true, but I wanted to try my hand at it.

It's been a while since I've done this kind of Rocket N00b stuff where I try to figure out how something is supposed to be done and then publish it here. I thought this would be a good place to start, and it would save me having to buy a cone. I wanted to keep this Bertha cheap, if I could.

Spoiler alert - it turned out pretty nice!
So, despite my promise to myself to finish building everything I'd been working on for a year and a half before starting any new rocket builds, I began working on the Bertha. Then I ended up making a second one. More on that when we get to it.

The next few posts will detail my scratch build of the two Berthas.

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Thursday, August 30, 2018

Zero Ns in 2018

Hello, stranger. How've you been?

For the past few year over on The Rocketry Forum, there has been a thread each year where people can log the Newton-seconds they've burned flying rockets that year, called the Newton Tally Thread (here's this year's). I can never participate in this thread, because I always quickly lose track. I'm not one of those rocketeers who diligently record the details of every flight, though I should be.

This year, however, it's been easy. I have flown exactly 0.0 Ns to date in 2018. What with the birth of Little N00b and our current lack of a car, I haven't been able to attend any launches at all. And I'm really itching to get out there and fly.

Mrs. N00b and I have plans to get a decent vehicle, and we've got plans to go to at least one launch this year, even if we have to rent a car. The last annual FlisKits launch, run by CMASS, celebrating Jim Flis' 16 years in business, will be on September 15 in Amesbury, MA. It's going to be a big one.

The FlisKits launch was my first club flying experience three years ago. There's cake and food, and hundreds of flights in one day. If I only get to attend launch one this year, this has to be it.

So, I haven't flown anything, and if you follow this blog, you know that I haven't written much in the past few months, but that doesn't mean rocketry has completely been on the back burner for me. In addition to doing The Rocketry Show podcast, I've been building.

It's gone much more slowly for me recently, since I don't get as much time to myself. If Little N00b needs something, the rocket has to wait. But I've been chipping away at it, and have finished a number of builds I'd like to share.

A lot of my progress has already been posted on Instagram (you can follow me @rocketn00b), and some on Twitter, and a little bit on my Facebook page.

Since I don't always have the time lately to do little experiments or research a topic to write up something for this blog, I'm going to start a few build series. I've got a few rockets I'm pretty proud of - and some which I learned I should have done differently. I have a good backlog of photos, so there's plenty of potential material there.

Look for the first one soon.

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Sunday, June 17, 2018

Sheer Joy

Watch this:

Joe Barnard is right to have such a reaction to such a successful flight. Amazing, incredible, awesome work.

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Friday, June 15, 2018

Applying Self-Adhesive Decals - or - Sticking Stickers Where You Want them Stuck

Many model rocket kits come with decals to add visual interest to a model - and if it's a scale kit, to add some realism.

Decals come in two varieties: waterslide and self-adhesive or self-stick.

Many builders, myself included, prefer waterslide decals. You soak them in water for a minute or two, and they slide right off their backing. You slide them in place on the rocket, and if you've made sure to dab a little water on the model first, you can then slide the decals around until they're in just the right spot. Then you dab lightly with a paper towel or cotton ball to remove any excess water and air bubbles, and when the decals set, they're pretty much permanent.

Waterslide decals are a little tricky to get used to at first, but once you do, they're much lighter and thinner, and they look like they're actually a part of the model's paint, rather than something just stuck there afterwards.

Most significant, they are re-positionable if you do them right, so you don't have to get them in exactly the right spot straight off the backing. With a little water, you have time to work with them before they're stuck.

Self-stick decals are really just stickers, and many rocketeers hate them. They're not only thicker and heavier than waterslides, if you don't do them right, they're stuck, wherever you happen to touch them to the model. So if you don't get them on straight the first time, you may never get them straight. It can be very frustrating.

The Estes Silver Arrow - an easy to assemble kit, and my very first build.

I got the sticker on crooked, and tried to re-do it, but I just tore the darn thing.
My very first rocket - the Estes Silver Arrow, from the launch set I bought when I started out - had stickers. Man, I hated that rocket! I got the silver decal on crooked, then tried to straighten it out, only to have it rip itself and the body tube. It was so ugly! I was happy to give it away to the son of a friend of mine, who broke it a day later.

I had a bit better luck with my first Estes Crossfire ISX, a Skill Level 1 kit that also came with stickers. But I still wasn't happy. I had bubbles under the decals, raised edges, and if you look closely, you can see that things are crooked.

Some imperfectly placed self-stick decals, with overhangs, bubbles, etc. And my feet.

Not only that, I found the process of putting them on so stressful, as I was trying to be so careful not to mess up, that it took me a couple hours and I needed a break after each decal.

I hated self-adhesive decals so much that for a while, if a model came with them, I'd either not buy that model, or I would decide to do a completely different paint job.

But since then, I've learned the trick to doing it right. You can get those self-adhesive decals on your rockets where you want them to go the very first time, and if you mess up, you can take them off and re-position them. It's a lot less stressful than just trying to jab a sticker into the right place on your rocket.

There are three newer rockets in my current active fleet with self-stick decals, and despite my continued preference for waterslides, these three are among my nicest builds. They are:

The Quest Superbird...

...a newer Estes Crossfire ISX, which I modified, adding a drag-reducing tail cone...

(I really should post the build I did of that rocket, because I like how it turned out.)

...and the Estes U.S. Army Patriot, seen at the top of the post.

All of these might be challenging, and two of these have decal wraps, meaning the sticker goes all the way around the model - and is supposed to meet itself on the other side! That's tricky enough to apply if you aren't also worried the decal will be stuck once you place it on there.

My method is similar to the method used when applying vinyl decals - such as those sold by With one or two little tweaks, the same method can be applied to self-adhesive decals.

OK, here's the method. You will need:
  • Your finished, painted rocket
  • Your decal sheets
  • A pair of scissors
  • Some masking tape
  • A spray bottle filled with a mixture of water and a few drops of dish soap

First, you will notice that the decals are printed on a plastic sheet with a backing on them. No surprise there - just like any stickers you have ever seen. You need just the stickers, not the extra plastic it's printed on. The first step is to remove all the surrounding sticker plastic while leaving the decal itself stuck to the backing. You may need to hold the decal down as you peel the excess off.

Above are the decals from the U.S. Army Patriot. I've removed the excess from the bottom one. You can see that all that's left is the decal itself - there's no extra black around it. This allows you to accurately scan the decal sheet, so that you can either make your own waterslide decals if you want, or clone the rocket and print your own decals at a later date, or share the decal scan with someone who needs it. It's also necessary for this method.

Next, cut the individual decals out with scissors - still leaving them on the backing.

Take your first decal and trim the backing as closely as you can to the printed decal, taking care not to cut into the sticker itself.

Now you will place the decal where you want it. You're not going to stick it on just yet, so again don't remove the backing.

If you're doing a wrap, make sure to wrap it nice and tight around the rocket, and make sure the two ends meet and that the wrap is nice and straight.

Then you will take a piece of the tape and carefully tape one edge of your self-stick decal in place. Again, the backing is still on the sticker at this point. That tape will act as a hinge, so that you can let go of the rocket and sticker, and it will stay in the same place.

(You can see the tape in place in the photo above - that's just because it's tough to take pictures while doing this, so I had to take that shot out of sequence, after I'd already secured the decal in place.)

Now that you've got your tape hinge in place, you can set the rocket down if you need to. The decal will go right back where you want it in a minute.

Get your spray bottle with the soap and water mixture ready and close at hand.

Now peel the backing off your decal, holding it away from the rocket. Spritz the decal and rocket with a light mist of the water and soap mixture.

Keeping the decal taught, carefully lay it down on the rocket, working from the tape end. You can squeeze out extra water and air bubbles with your fingers as you go.

If you are doing a wrap, make sure to remove the tape hinge before you get to the end of the decal. Peel the tape off in the opposite direction from which you're laying the decal - so that the tape doesn't pull the sticker off!

Oops - my feet are showing again!
Now you can press the decal the rest of the way down. If need be, squeegee the rest of the excess water and air bubbles out with fingers or a poster squeegee.

Sometimes I use a poster squeegee, seen here.

The decal should be perfectly in place, but if you've made a mistake, and the decal is crooked or the ends of your wrap don't match up, you can easily pull the decal back up, thanks to the soap and water!

Take your time getting the placement right before you put your tape hinge down, and you shouldn't have much of a problem.

This method will help you get your stickers on straight, where you want them, and eliminate bubbles.

It takes some care, but this method is much faster and easier to get the results you want than merely trying to get a sticker perfectly in place the first time. There's no way I would have made this rocket look this nice if I didn't use the method described here.

This can be used for pretty much any self-stick decal. You just have to figure out where the best place is for the hinge to go. It might be on an end, or it might be on a longer side.

Take your time, and you'll get those self-adhesive decals where you want them. Your rockets can look the way you envision them!

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Thursday, June 14, 2018

Magical Weight-Reducing Balsa Filler (???)

I recently wrote about a test of various balsa grain fillers and sealers and the weight they add.

The clear winner of that test was the Brodak butyrate dope sanding sealer, which added 0.3 grams of mass to a piece of 3 inch by 3 inch balsa, sealed on both sides. In other words, the sanding sealer added only 0.033 grams per square inch or 0.005 grams per square centimeter of fin area. That's pretty light!

Well, it's one thing to test out something in the abstract - on square pieces of balsa. I was curious how much I would add to an actual rocket I was building.

I'm trying to finish all the rockets I started last year before starting anything new. It's been tough! I've got so much on the build pile, and I'm itching to build everything. But experience has taught me that I should limit myself to one or two projects at a time, focus entirely on them, and I will actually be more productive - and enjoy building much more.

I started to build the Estes Hi Flier XL. Sometimes, when I start a kit, I'll cut some spare fins, and practice shaping them into airfoil or streamlined shapes, so I feel confident when I move on to the kit fins. Sometimes, if those fins turn out well, I go ahead and make a clone of the whole rocket - if I have the parts, which I often do. Usually, the clone ends up finished long before the original kit.

This is what happened with the Hi Flier XL. I built a clone, added a payload section, since it's long enough to require two body tubes, and painted it like my original little Hi Flier. It looks great, but it feels a little heavy, and doesn't fly as high as I'd thought it would.

I filled the fins on the clone with Elmer's Carpenter's Wood Filler. I also think my paint job was a bit heavy. And there's the payload section which included a 1/4 inch thick basswood bulkhead, screw eye, and a small dab of epoxy.

So, now that I'm building the kit, I'm weighing it at each stage, just to see where the mass is coming from.

On the kit, I filled the grain with Brodak sanding sealer. I weighed all three fins together, and they were 33.1 grams.

Then I used two coats of sealer, followed by a good sanding. Then I did a third coat and sanding, and a final fourth coat.

Then I weighed the fins. They came in at 34.1 grams before I sanded off the final coat of sealer.

So, with no sanding, the whole set gained 1 gram - and these are large fins! That was really encouraging.

I sanded rigorously, until the fins were glass smooth, then wiped off all sanding dust, and weighed the fins again.

33 grams even.

What?? The fins lost a tenth of a gram after sealing and sanding! Is this possible? Did the sealer allow me to sand off a bit of extra weight while maintaining a smooth finish? Is my scale acting up?

Instead of adding a tiny amount of weight, my fins lost weight! At the very least, they didn't gain weight.

I have a history of owning things that don't work very well - cars, appliances, etc. So when I see a result like this I tend to be skeptical that I got the right answer. I tend to think there's something up with my equipment or with the way I'm using it.

So, maybe the fins lost weight after filling. Maybe not. I guess this means I'll have to weigh future builds carefully to be sure.

Why am I so obsessed with what my rockets weigh, and where the mass comes from? Two reasons. First, I think readers of the blog will find it interesting, and I hope some will find it useful. The second is that when I started out, my rockets were all lighter! As my paint jobs have gotten prettier, my rockets have gained weight, and their altitude has probably suffered.

My Big Bertha - not smooth and shiny, but very lightweight!

I used to paint very light coats, moving the spray can quickly, until I got full color coverage. Doing this, though, I rarely got a nice, shiny gloss coat on a rocket. To do that, I needed to paint a little heavier, so that the atomized droplets of paint could run together to form a nice shiny shell of paint. It's a tricky thing to get - you don't want to go so heavy with the paint that it runs and sags!

My Estes Goblin. It looks nice, but that shiny coat of paint is on the heavy side.

I've got the wet coat pretty well down, but the rockets have gotten heavier. I want to see if I can have pretty rockets that also don't weigh a ton. So I need to be rigorous with my testing. Maybe I can save weight another way. Or maybe I need to change my painting method.

In any case, it does fit with the original mission of this blog - I learn about this stuff, then I share it with you - and any other Rocket N00bs out there can benefit.

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Thursday, May 31, 2018

Sanding Block Pro Tip

A while back, I showed a way to remove self-adhesive sandpaper from an aluminum sanding block, like the Great Planes Easy-Touch Hand Sander*, seen above.

Self-adhesive sandpaper, such as the stuff I buy from Klingspoor, has such a powerful adhesive on it that you can't simply peel it back off the aluminum.

My solution was to hold the sander under a stream of warm water and slowly but firmly peel the sandpaper off.

Any remaining adhesive is cleaned off with a cotton swab and an adhesive solvent such as lighter fluid.

While this certainly works, someone suggested to me a much easier, faster way, and it doesn't require running water.

All you need is cheap masking tape.

The Great Planes hand sander is just a hair over 2 inches wide. I got the cheapest masking tape I could find over 2 inches - a 3-inch wide roll for about $5. (That's a Boston hardware store price. In most parts of the U.S. you should be able to find it cheaper.)

Just adhere a strip of the tape directly to your sanding block.

Trim the tape down to the edges of the sander.

Now you can attach the self-adhesive sandpaper as you would normally, and trim that down as well.

Despite being a lot less sticky than the sandpaper itself, the masking tape will hold up to pretty heavy use. Once you're ready to change paper, you can just peel it off the sanding block with a fingernail.

Now you can quickly and cleanly change paper without any running water, and with no need to use a solvent to clean up any remaining adhesive residue.

You should use one piece of tape wide enough to cover the whole sander. If you try to put two narrower pieces side by side, the ridge will show through on the sandpaper side, and your sanding surface won't be perfectly flat.

Even if you don't own a Great Planes Hand Sander, the masking tape method should work with even a simple wood block sander. Whereas the sandpaper's adhesive would take the surface off a wood block, cheap masking tape should peel off easily. Having the sandpaper adhered flatly against the sanding surface is much better and flatter than simply wrapping a piece of sandpaper around a wooden block, as it gives you more control.

*As of this writing, it's difficult to find a Great Planes hand sander. Great Planes was owned by Hobbico, and as a result, was part of the whole Hobbico bankruptcy. We in model rocketry got really lucky that Estes was purchased by Estes Industries, LLC, run by people who actually really care about the hobby itself. Hobbico's other holdings have apparently been purchased by Hobbico rival Horizon, and so far, the future of all its holdings is uncertain. The Easy Touch Hand Sander has been on backorder for some time. I hope it returns to market some time soon. It's such a simple, elegant tool - my favorite in the tool box. I own three! I'll try to keep track of when the Great Planes sanders come back and post about it here. I recommend you get one. They're inexpensive and a great little tool.

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Tuesday, May 29, 2018

Balsa Grain Fillers and Added Weight

Most model rockets you build will have fins made of balsa wood. Because balsa is so porous and has such a loose grain, you have to give the wood some kind of treatment if you want smooth fins. You have to fill or cover the wood grain with something.

Anything you put onto a model rocket while you are building adds some amount of weight. Adding weight changes a rocket's flight - usually lessening the possible altitudes a rocket can achieve on a given motor.

I decided to investigate how much weight common balsa grain filling techniques might add to a model rocket build and share it here. I hope any weight-conscious builders might find it useful, or at least, interesting.

* * *

I decided to do this for a couple reasons. First, a while back here on the blog, I investigated how much weight I was adding to my rockets simply by painting them. The results were surprising, as the rocket I built for that series turned out to be nearly 25% paint, if you counted only the weight.

The added mass from the paint would certainly have depressed the altitude this rocket could achieve!

Second, I recently built an Estes Photon Probe, and because I had the parts, made a booster for it, so that I'd have a great, two-stage rocket. Lots of people have Photon Probes, but I'd have one that was just a little different!

Because the balsa fins provided with the kit were rather soft and warped, I used a technique I don't often use to fill in the balsa grain, and flatten the wood. I papered the fins, by gluing on skins of simple copy paper. Out of curiosity, I weighed one set of the large, main fins before and after papering.

By papering the fins, I added 3.3 grams to one set of fins alone. This rocket had four main fins, four booster fins, and 8 tiny "dorsal" fins.

The added weight of the paper skins, plus the optional payload section, plus a Jolly Logic Altimeter Two, plus however much the paint job weighed, plus not one but two motors - one in the booster, one in the main body of the rocket - ended up being too much for the motors I had selected, and resulted in one rather disappointing flight.

* * *

There are many ways rocketeers fill in or cover balsa fin grain. I decided to compare five of the most popular.
  • Carpenter's Wood Filler (CWF) - sometimes called Elmer's Fill-n-Finish by long-timers
  • Sanding sealer - specifically, a butyrate dope product used in model aircraft building
  • Two different methods of papering fins - one with glue and copy paper, one with self-adhsesive labels from Avery
  • A slurry of wood glue, thinned with water, then thickened again with a filler

I'll be talking about fin stock here, but three of these methods - CWF, sanding sealer, and glue slurry - can also be used to fill the grain on balsa nose cones and transition parts.

For this test, I selected a nice plank of 3/32 inch thick "Very Hard Balsa" from SIG Manufacturing.

The balsa stock was three inches wide, so I cut five square sections three inches long.

The result was a 9 square inch (or just over 58 square centimeters) test piece for each method.

What I would be attempting to determine is how much weight each filling method adds per square inch (or square centimeter). Results may vary slightly, but should give you a ballpark idea of the kind of weight penalty your rocket may have to pay depending on how you fill in your balsa grain.

After a light sanding with 400 grit paper and a sanding block, each piece was carefully weighed on a metric scale accurate to 0.1 gram. I checked the calibration of the scale with a 100 gram weight before each use to ensure accuracy.

It was important to weigh each piece before and after treatment, because the density of balsa varies greatly. I wasn't interested in how much my test pieces would end up weighing - I was only interested in measuring how much weight the filler treatment added.

Below, you'll see the results of each treatment, and a little pro and con list for each method. I should state that these pros and cons are based on my own experience, and some techniques which I like, others don't, while some others may prefer are not my cup of tea. As always try things out and do what works best for you while building. As long as you're doing things safely and you are happy with the results, don't let other rocketeers tell you you're doing it wrong!

Elmer's Carpenter's Wood Filler

This water-based putty is the first thing I ever used to fill in wood grain. It's one of the cheapest methods available. What you are looking for is Elmer's brand, and it comes with the orange lid (you can also find it in a tube now). Any other carpenter's putty or filler is not ideal, as they are rougher in texture.

The SIG balsa was really nice and uniform. Nearly every piece weighed in at exactly 2.9 grams before treatment.

CWF must be thinned with water, then brushed onto the wood with a paint brush. You allow it to dry thoroughly, then sand it smooth with a sanding block and 400 grit fine sand paper. A really good ratio of CWF to water for thinning is Chris Michielssen's 2.5:1 formula from the Model Rocket Building blog. It thins enough so that the filler is brushable, but not so much it takes forever to dry.

When using wood filler on fins, once they are dry, they will look something like this:

Because it is water based, CWF can cause fins to warp, as the cells in the wood absorb moisture. Doing both sides of the fin at the same time should help prevent this. I like to let my fins dry on a cooling rack, seen in the picture at the top of this post.

Even though your fins shouldn't warp if you are coating both sides of a fin, it can sometimes still happen. Check them occasionally over the first 20-30 minutes, and if it looks like they're curling, put them between some pieces of waxed paper and press them overnight between some flat, heavy books.

After drying overnight, I sanded the piece as smooth as I could. You should leave a very thin skin of filler on the balsa - don't sand all the way down to the wood, or you will expose the grain and have to reapply the filler. It's easiest to use CWF before gluing fins on, because it's easier to sand them smooth if you don't have a rocket body tube in the way.

The finished piece looks like this.

It may be hard to see with the lighting in the photograph, but the wood grain is faintly visible, yet there is a thin skin of filler left on. If you sand all the filler off in a few places, you will have to retouch the fin and re-sand.

The final weight:

The CWF added 1.1 gram of weight. That equals 0.122 gram per square inch, or 0.0189 gram per square centimeter.

This is a technique where you may find some variability in results. Sometimes I accidentally sand CWF too far, until some of it is completely sanded away. If the wood filler is sanded off in spots, your balsa grain will show through and you will need a second coat.

So, when I use CWF, I sand until I'm convinced it's smooth, and then I stop. In fact, when I first sanded this test piece, it came in at 4.2 grams, an increase of 1.3 grams of total mass - 1.44... grams per square inch or 0.022 gram per square centimeter.

I decided to see if I could sand further and still have a smooth surface. So I did, and ended up with the lighter result. Could I have sanded even more off? Perhaps, but I know I'd probably have exposed some wood grain if I pushed it too far.

  • Very inexpensive
  • Easy to clean with water
  • Non-toxic
  • Available at most hardware stores or online
  • Can be used to fill in deep gouges as well
  • Doesn't seal balsa, so if you get some on a root edge of the fin, you can sand it off, and it won't prevent you from getting a good glue bond
  • Creates a lot of very fine dust, which really goes everywhere (though I have a solution for this, in an upcoming blog post)
  • Can warp fins, as it is water-based
  • I often get little pock marks in the surface after I think I've sanded it smooth. I'm pretty sure this is due to air bubbles getting trapped inside when I'm stirring in water. I've never heard anyone else complain about this, though, so perhaps it's just me
  • Doesn't do anything to harden or strengthen soft balsa 

Sanding Sealer

Sanding sealer is a longtime rocketeer's standby. In the old days, this was the standard grain filling material. While there are basic lacquer sanding sealers used in woodworking to get a smooth finish on projects, what rocketeers often use is a type of sealer called butyrate dope or aircraft dope. In the early days of aviation, aircraft dope was applied to the canvas skins of airplanes. It would cause the canvas to shrink and tighten against the wooden spars of the aircraft, giving the plane its structure and rigidity.

Today, RC aircraft builders still use it to shrink tissue paper skins onto their models. Rocket builders can use it to get a smooth finish on balsa, and because it shrinks, it can tighten the balsa and make it slightly harder. Because of this, though, you need to brush sanding sealer evenly on both sides of the fin, or the shrinking will cause the fin to curl and warp.

Sanding sealer actually seals the pores in the wood, so when you sand it smooth, you are able to sand the whole piece to a near glass-smooth texture. Any little imperfections can later be filled in with a good primer before painting the rocket. You brush the sanding sealer on, allow it to dry, then sand it smooth. It usually requires a few coats - between two and five. Rocketeers traditionally apply sanding sealer after fins are glued on, though I usually do it before. You do need to be careful doing that, though, because it will seal the wood, making a glue bond harder to achieve, if you get any on the root edge of the fin.

The sanding sealer piece in my test again started out at 2.9 grams before treatment. I used a total of four coats, finally determining I was happy with how smooth the piece was. I did two coats before sanding, then applied a third, then sanded, then a fourth, and sanded again. The finished piece looks almost like the before picture, but you can see it's smoother and a little ashen-white from sanding dust.

The final weight of the piece:

3.2 grams. Total weight added, 0.3 grams. This is a gain of 0.03 grams per square inch, or 0.005 grams per square centimeter.

Some people use carpenter's sanding sealer - such as Deft or Minwax. It's much cheaper, but you buy it in larger quantities. I've heard it does work, but that it takes more coats and more sanding to get a smooth finish. Since it's not aircraft dope, I do not know if it shrinks.

  • Lightweight
  • Creates less dust than CWF
  • Hardens balsa
  • Easy to use
  • Dries quickly - I was able to apply and sand all four coats in under two hours
  • Much more expensive!
  • Puts out noxious fumes - you really need to have adequate ventilation when using it
  • Hard to find - I ordered mine from
  • Seals the wood, so if you get it on the root edge of a fin, you may have trouble getting a good glue bond.

Papering Fins with Glue and Copy Paper

 Papering fins is a technique for both concealing balsa grain and adding strength to the fins, by essentially gluing paper directly onto the fin surfaces. This creates a smooth surface (the paper) for painting, and the paper and glue add stiffness and strength to the fin itself.

There are four main techniques for fin papering - 1) using glue and paper, 2) using a glue stick and paper, 3) using spray adhesive and paper, and 4) using self-stick Avery labels. I'm going to examine two here techniques here.

Probably the oldest technique for papering involves cutting two pieces of paper, slightly larger than the fin itself, from ordinary office printer or copy paper. Glue - either white glue or "yellow" wood glue - is spread on the surface of the fin. Then most of the glue is wiped off with a finger. This leaves only a very thin layer of glue on the fin. The paper is pressed down to one side of the fin and burnished down with any object which can act as a kind of squeegee to squeeze out any excess glue from between the paper and the fin (I use the body of a Sharpie marker). The fin is then flipped over, the process is repeated on the other side of the fin, and the whole thing is left pressed under flat, heavy books until the glue is dry. After that, usually a bead of thin CA - cyanoacrylate or hobby grade super glue - is run along the edge of the paper and fin, sealing the edge down and making any overhanging bit of paper nice and stiff. The overhanging paper is then sanded off and you're left with a really solid, strong piece, which, if done correctly, should not show any balsa grain.

If you're having trouble picturing this process, watch this video from Apogee Components.

My test piece was, again, 2.9 grams before the treatment. When it was dry, I ran a bead of CA around the entire edge. Normally, you don't apply CA to the root edge, as it may seal the balsa and impede a good glue bond between the fin and the body tube. But I wanted to make sure I got a good clean edge all the way around, and a little extra CA on one edge shouldn't really make a weight difference.

The end result looked like this.

I expected this to be the heaviest application, but the result surprised even me.

This test piece gained 2.1 grams, weighing in at 5 grams after treatment! That's a 72% increase in weight. Put another way, 42% of the weight of this final piece is just the paper and glue!

So papering as I did with 20 pound copy paper and Titebond II wood glue resulted in an increase in weight of 0.233 grams per square inch of fin area, or 0.036 grams per square centimeter. While that might not sound like much, you must bear in mind that most rockets have 3-4 fins, and some of them have fins with a lot of area - certainly something to consider when building.

  • Inexpensive
  • Many people find it easier than sealers or fillers
  • Much quicker
  • Strengthens fins
  • Can straighten warped fins 
  • Requires almost no sanding - except for fin edges

  • The CA can glue your fingers together and cause chemical burns - use with caution!
  • Can be trickier to get the results you want if you shape your fins into airfoils
  • Much heavier
  • Fin edges remain unsealed

Papering Fins with Self-Adhesive (Avery) Labels

Some builders like papering fins, but prefer using self-adhesive Avery labels (the brand doesn't matter, but Avery seems to be the Kleenex of self-stick office supply labels). It's faster, cleaner, and easier. There's no messy glue to apply, no need to squeegee out excess glue once the paper has been applied, and no drying time. You simply press the label onto the fin, burnish it down, trim the edges, and seal them down with a bead of CA.

Once the CA is cured, you sand off the excess from the edges. As you can see from my finished piece, I cheated a little bit here.

I didn't have a label large enough to do my test piece with one label, so I had to lay them edge to edge. It would make for a sloppy looking finish, but since I was only testing the weight, it would be good enough. When papering fins, though, make sure you use a piece large enough for the whole fin - two pieces would look just terrible, and could compromise the added strength from the paper skin.

My test piece started out at 3.0 grams. Once the paper skins were on, the fin stock weighed 4.3 grams, a gain of 1.3 grams total mass. That's 1.44... per square inch or 0.022 gram per square centimeter. That's only slightly heavier than my final result with the CWF.

Using Avery labels is probably the fastest, easiest method of filling balsa grain, but it does have at least one drawback. Because the adhesive on mailing labels isn't wet - as it would be when papering fins with wood glue - it doesn't penetrate the balsa. It just adheres to the surface of the wood, like a piece of tape. As a result, the paper skins can come loose. You may find an edge lifting up after a hard landing. I've found bubbles lifting in the middle of the fin, causing a raised bump in the paint.

  • Fast
  • Easy
  • Adds some strength
  • Pretty lightweight, as far as papering goes

  • Expensive
  • Leaves edges unfilled
  • Still need to be careful with that CA
  • Can come loose

Glue Slurry

This one is less commonly used, but I've used it myself, and you do see it recommended by certain rocketeers on the various online rocket building forums. The technique is simple. You take ordinary yellow wood glue, thin it a bit with some water, then re-thicken it with a filler. The commonly used filler in this case is flour.

It's possible to use straight up glue, but most rocketeers who use this technique do use water and a thickener. The purpose for thinning and re-thickening is that it makes the glue more sandable. The glue not only fills in and seals any balsa grain, it also hardens the balsa.

How much strength you add to the balsa depends on your ratio of glue to filler. More glue makes a harder (but probably heavier) fin, while more filler makes it much easier to sand. Your results may vary slightly, depending on the ratio you use. Still, as we saw in a previous post, wood glue loses about half of its mass as it dries, so this shouldn't add too much weight.

When I use a glue slurry on my fins, I don't use flour. It works, but I find that when it mixes with water, the flour hardens into a kind of cement, and it forms such a hard shell I have trouble sanding it. So I'd recommend trying it with another filler - talcum powder. I use talcum-based baby powder for this.

Feel free to play around with ratios to find your ideal mix. But if you want a place to start, here's my recipe, which is pretty easy to sand.

  • Pour some wood glue into a mixing cup
  • Add the same amount (by weight) of warm water*
  • Stir to combine
  • Shake talcum into the cup and stir until lumps are mostly gone
  • Keep adding talcum until the mixture is about the same viscosity as the original glue

*If you don't have a digital scale, it will probably work just fine if you go by volume - or just eyeball it.

You can then brush the mixture onto the fins and allow the them to dry completely. When brushed on and dry, this recipe looks like this:

Slurries with more glue and less filler will look more yellow. It will make for a harder - but heavier and harder to sand - balsa fin.

Then you can sand them smooth. As you sand, the talcum will be released.

The finished piece looks like this.

It is a little harder to sand this stuff than CWF or sanding sealer, but you should only need one coat, and there's not much chance you'll accidentally sand too much. If you use, say, a 220 grit sandpaper to knock off any large bumps and then smooth the fin with a 400 grit paper, it will take you a long time to accidentally sand all the way through the filler. You will know you are done when the fin is nice and smooth, and you can just see the wood grain through the translucent skin of the glue. It looks a bit the same as the perfectly filled fin with CWF. You'll just see the grain, but you won't be able to feel it.

My finished fin weight in at 3.9 grams, having gained 1 gram exactly. That's 0.111... grams per square inch, or 0.017 grams per square centimeter.

For evidence of the strength a glue slurry can add to fins, here's a rocket of mine - the Ceres B Booster (design from Make: Rockets: Down-to-Earth Rocket Science by Mike Westerfield). I built this for strength, and finished the fins with a glue slurry.

You can see some slight paint scuffing on the fins. That's all the damage they've sustained after multiple flights, a couple of hard landings, and one time being dragged by the parachute through the dirt for about 150 feet - a wild ride which would have badly damaged the fins of most of the other rockets in my fleet. At the very least, the corners of these fins should be gone - yet they remain intact.

I don't often use a glue slurry on my fins, but when I do, I never regret it.

  • Cheap
  • Non-toxic
  • Adds a lot of strength to balsa while adding less weight than paper
  • Can be cleaned up with water

  • Tough to sand
  • Messy - needs to be cleaned up before glue dries

Quick Summary

From lightest to heaviest, here are the methods I've tested, along with my results:

  • Sanding Sealer - 0.033g/square inch, 0.005g/square centimeter
  • Glue Slurry - 0.111g/square inch, 0.017g/square centimeter
  • Carpenter's Wood Filler - 0.122g/square inch, 0.0189g/square centimeter
  • Papering with Avery Labels - 0.144g/square inch, 0.022g/square centimeter
  • Papering with Glue and Copy Paper - 0.233g/square inch, 0.036g/square centimeter

So you can see, each technique has pros and cons, and each will affect the weight - and therefore, the performance - of your rockets differently. There are other methods you can use as well - thin CA, sanding sealer mixed with talcum, papering fins using a glue stick, etc. These are five common methods that I have tried.

I've used each of these techniques on my builds, and they've mostly turned out quite well. My two personal favorites are Elmer's Carpenter's Wood Filler and Brodak sanding sealer. Try different techniques and pick your favorite one!

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