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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Monday, May 21, 2018

Messing Around with Glue - Strength, Flexibility, Et Cetera

After my little experiment to find out the water weight of white and yellow glue - weight which would be lost as the rocket dried - I got curious about some other properties of adhesives used in model rocketry, and wondered if I could test them out.

I started thinking about that epoxy - Bob Smith Industries hobby epoxy - which I and many other first-time epoxy users turn to when building larger projects. BSI epoxy (for short) is easy to find, available in many hobby shops and online vendors. It's simple to use for an epoxy, as you mix the resin and hardener together in equal batches by volume. You can buy a small amount of it relatively cheaply. It's sometimes re-branded with other names - Aerospace Specialty Products, or Great Planes, for example. You can find fast cure and longer cure versions.

I've used BSI epoxy for a number of applications.

When building my Estes Leviathan, which I used for my high power Level 1 certification flight, I used it because I wanted to add strength by adding internal fillets - reinforcing fillets of adhesive inside the rocket, much as you would have at the root of a fin on a model rocket, but at the base of the long tab which runs through the body of the rocket and attaches to the motor mount tube (this is known as through-the-wall construction).

Internal fillets on the Estes Leviathan. The fins are through-the-wall, meaning a tab goes through the body tube and is attached to the motor mount tube. I built this to fly on a high thrust H motor, so I reinforced it with internal epoxy fillets. Once all the epoxy had cured, I put on the aft centering ring and glued it in place. The whole rocket is very strong.

With my Quest Big Dog, I used BSI epoxy to install the motor mount, because the centering rings were a little too small. The outer diameter of the rings was smaller then the inner diameter of the body tube, leaving a slight gap. Since epoxy is nice and thick, I used that to fill the gap.

And on a few occasions, I've used BSI epoxy to make the fin fillets on low or mid power rocket builds.

In these instances, the epoxy was mixed with a filler material called microballoons, a powdery substance which is actually made of microscopic spheres or bubbles of glass.

Fillers like these make the epoxy stiffer, so you can form a fillet shape which won't sag, making a nice, round fillet when the epoxy cures. It also makes the epoxy much lighter in weight, and easy to sand if you need to.

While all the rockets I'd built with epoxy fillets turned out very pretty, a few of  them had a major drawback - after a flight or two, there were long cracks all down the fillets of at least one side of the rocket.

It seemed that the epoxy fillets were rather brittle, and only light pressure on the fins caused them to crack.

So I wondered if I could figure out a way to investigate a few more properties of the glues I use. Specifically, could I examine the brittleness or flexibility of the adhesives? What about the strength?

Now, I'm going to state here again, this is not a terribly scientific test. Frankly, I don't have the equipment to definitively answer some of these questions. Or, at least, I don't have much data. But though playing around with these glues, I was able to satisfy my own curiosity about the flexibility or brittleness and the adhesive strength of my Elmer's white glue, Titebond II yellow glue, and Bob Smith 30 minute epoxy, both on its own and mixed with microballoon filler.

First, I should say that there are many, many different brands and types of epoxy, with different properties, strengths, uses, etc. This is obviously not an exhaustive test. But BSI epoxy is a pretty common hobbyists adhesive. It's great for certain applications, but I'd like to address those rocketeers who feel they need to reach for the epoxy for all their builds, large and small.

We've established that epoxy is heavy in the previous post. Using it on an Estes Alpha or Big Bertha will work, but it's more weight than you want to add. Furthermore, it's not necessarily "better." If you're building a low power model rocket made of paper tubes and balsa fins, white or yellow glue is usually the adhesive you want.

Some rocketeers want to make their rockets "bulletproof," so they figure epoxy is the glue they want. Well, first of all, model rockets shouldn't be indestructible. Destructibility is a safety feature!

Also, epoxy may not actually make for a stronger bond.

Okay, let's take a look at what I did. Remember, this is grain of salt time. If you have doubts about my methods or findings, that's fine. I encourage you to test things out for yourself - it's easy!

I decided the easiest way for me to get an idea of how brittle filled epoxy is would be for me to simply take a hardened piece of it, and - by hand - see how much it could bend or flex before breaking, as well as how hard it would be to snap it in two.

Included here are screen captures of the video I recorded, and you can see the whole video at the end of the post.

Since I'd already looked at the water weight of white and wood glue, as well as the fact that epoxy loses no weight as it cures, I decided to continue comparing these adhesives.

What I needed were dried, hardened sticks or batons of glue. These were easy enough to make.

I laid out a piece of aluminum angle so that it was flat, with its open side upwards. I put a folded piece of waxed paper in the angle. I mixed up a batch each of plain Bob Smith 30 minute epoxy, plus a batch of the same epoxy filled with microballoons until they reached the consistency I like to use for fillets - thickened to a stiffness such that if you shaped it with a craft stick, it would not lose that shape. (I don't tend to measure it when I do this. I go by feel.)

I then poured a line - or fillet - of each epoxy into the waxed paper. For the yellow glue and white glue, I simply poured a line into the waxed paper. Then I let everything dry or cure.

Because white and wood glue are mostly water, they shrink as they dry. I was left with a very thin bit of glue after one application. So, while my epoxy batons were ready once they had cured, I had do make the white and yellow glue sticks in several applications, pouring a new fillet on top of the first once it had dried. This took several days for everything to dry thoroughly and for me to have a thick enough glue baton to actually handle.

So, you can see that each baton was different. I didn't measure an amount of epoxy for the two epoxy fillets, so they were uneven and of different thicknesses. The white and yellow glues were built up in layers. I would not be comparing these fillets to one another, so much as simply seeing what each one would be like on its own.

When all batons were ready, I peeled them from the waxed paper.

First, holding each between my thumbs and first fingers, I tried to see how easily each one would flex. Then I tried snapping the baton in two to see how easily it would break.

Here's part of what I mean by not having much "data" - I am doing this by hand, so I can't apply any numbers to what happened. But you can see the video for yourself, and I can report on what I found.

The first example was the Elmer's white glue baton.

Here was my first surprise. Of the three adhesive samples (that is, not including the epoxy with microballoons), this was the least flexible. It would flex a little in one direction, but not the other. It took a bit of effort, but the glue baton snapped without distorting much at all from its original shape.

That's not to say that I would call it brittle, though. While it was the easiest of the three adhesive batons to snap in two, it did require some force on my part, and though you may not be able to tell from the video, it actually cut my finger in two places because of the sharp edges on the glue fillet!

So, Elmer's white glue adds the least amount of weight, as we saw from the previous blog post. And it's the least flexible.

Now, I don't know whether that lack of flexibility is an advantage or a disadvantage, or perhaps even neither. While you could argue it might add rigidity to a rocket's fins, making the rocket stronger, you could also say that being inflexible might increase the risk of a fin breaking off during a hard landing. Then again, you use such a thin layer of glue when building a model rocket, perhaps it's just too thin for flexibility to be a factor.

Next was the Titebond II yellow glue.

Here was another surprise. Whereas the white glue was almost totally inflexible, the yellow glue was so flexible I couldn't break it! I could bend it back and forth like a Twizzler's, but the glue baton stayed intact.

It's not until the end of the video where I manage to break the Titebond II baton - by twisting it around and around until it basically tears apart.

So, the yellow glue is the second lightest in weight of these adhesives, and is ridiculously flexible. Again, maybe an advantage, maybe not.

Next was the pure, unfilled epoxy fillet. This, too, surprised me.

While I expected it to be quite brittle, it was in fact somewhat flexible. More so than the white glue baton, in any case.

It was also harder to break than the white glue - but of course, it was a thicker stick, so take that as you will.

The Bob Smith Industries unfilled epoxy adds the most weight of all three adhesives, and is somewhat flexible.

Finally, I tried the fin fillet material - the Bob Smith epoxy filled with microballoons. Since these are made of glass, I put on some gloves to protect my skin in case there were any shattering. This turned out to be unnecessary.

Because the microsphere filler makes the epoxy less dense, this fillet was much lighter. And, unsurprisingly, it was very brittle. It barely flexed at all before breaking, and was quite easy to snap in two. Interestingly, the break was very clean with perfectly smooth and flat ends.

This is why microballoon filler is good for thickening and lightening epoxy for certain applications, but you wouldn't want to glue fins on with it. In boat building, microballoon filler apparently makes epoxy really good for non-structural fillets. I don't know much about boat building, but it makes sense. I wouldn't want this stuff to have to take much stress.

* * *

Next, I wanted to see if I could figure out a way to test the adhesive strength of the white glue, wood glue, and plain epoxy. In other words, I wanted to see, if you used one or the other glue to attach your fins to a model rocket, which fins might break off most easily? Which might be the strongest?

What I needed to do was to glue some fin stock onto a body tube with each type of glue, and then try to break the fin stock off. But just doing that alone wasn't going to show anything. I might get an idea myself of how strong one bond would be over another, but I would have no real numbers to attach to it, and it wouldn't be much use here on the blog for me to say "I tore off the fins, and X tore off easier than Y."

What I came up with wasn't perfect, but it did give me some idea of how the adhesives compare, and it gave me some numbers - kind of.

I made three "fins," 1/8 inch thick, with a three-inch chord (from the leading to trailing edge) and a 2-inch span (from the root to the tip). I took sections of BT-60 body tube and gave them a very light sanding. Body tubes are kind of slick, and to get a good grip, it's not a bad idea to take a bit of the shine off them for better adhesion - especially for epoxy.

I attached the fins to the body tubes and allowed them all to dry for several days. Glue was applied to the root edge only, and there were no fin fillets used. I would use my digital scale as a kind of gauge to measure roughly how much force I was applying as I pressed the tip of each fin to the scale at an angle, and slowly pressed until the fin snapped off.

I have read of wood glues that they are "stronger than the materials they are bonding," when used on wood and or paper. I've also heard that the paper body tube will fail before the glue does. I won't say that I found any proof to the contrary. But I did find this: while each fin broke off with a different amount of force, in each case, there was paper body tube still attached to the root of the fin. It did not seem that the adhesive had separated in the middle of the glue layer. So, perhaps some glues do not fail as easily as the paper body tubes, or perhaps the failure is partially the materials, partially the adhesive.

Anyway, on to the experiment.

I slipped a tube coupler into the body tube to give it extra support. Holding each piece to the scale at about a 25-30 degree angle, I slowly pressed until the fin separated from the tube.

I videotaped each example in the hopes of slowing down the video and seeing how much force, in grams, I was applying at the moment the fin snapped off.
Again, it's important to take this experiment with a grain of salt. First, of course, I was doing this by hand. It's impossible to say how well or poorly I was holding the fins at the same angle or pressing with the same motion each time. And a digital scale isn't really meant for this thing. As you'll see in the video, it takes a moment for the reading to catch up with what's happening.

I started with the white glue. I pressed the fin slowly against the scale, and when it began to tear, the scale read 210 grams. When it finally snapped, the scale reading had reached 460 grams. In this and other cases, after the fin broke off, the scale reading jumped up a couple hundred grams momentarily. Either the scale was catching up to the pressure I had applied a moment later, or maybe I pressed down on it with the body tube or something.

The Titebond yellow glue withstood a bit more force, with the fin breaking off at 778 grams.

Finally, the Bob Smith epoxy, which broke much more easily. Even if the white and yellow glues were close enough to take into account any discrepancy of the pressing motion on my part, the epoxy snapped off with very little pressure, with the tearing sound occurring at only 178 grams. In the video, it's much more apparent from the sound how easily it broke off.

In an effort to be more thorough, I decided to run the test again, this time trying to shear the fins off at the root, rather than breaking them off by pressing on the tips. I used my razor saw miter box as a support and clamped the fins down.

The rig I came up with to try shearing the fin off at the root. The body tube is at the top of the picture in this screen capture.

This test turned out to be kind of useless. Shearing the fins off at the root was so hard it was nearly impossible. In each case, I exerted somewhere between 8-12 kilograms of force, pushing down on the body tube with my weight and resorting to twisting the piece in an effort to break the fin off. I squashed the tube coupler inside the body tube. I worried I might injure myself!

The yellow glue withstood the most force by far, exceeding 12 kilograms of pressure at one point.

But since I had to twist and press the piece, I don't feel that the shear test gave very reliable information.

Still, I found the fin tip break test to be at least somewhat enlightening.

So, here's what I found, just using the single example of each adhesive, and the method I used and the way I used the adhesive (more on this in a moment). I'm not saying it's completely reliable, but it is what it is.

  • The Titebond II yellow wood glue was the strongest bond, was reasonably light in weight, and also the most flexible.
  • The Elmer's Glue All white glue had a bond which was a close second to the Titebond, was the lightest in weight, and had the least flexibility.
  • The Bob Smith Industries 30 minute epoxy had the weakest bond, was the heaviest in weight, and had some flexibility.

Now, that's not to say that one glue is the "best" glue. (This kind of statement starts some bitter fights on online rocketry forums, and I won't participate in that. Besides, I don't mean this to be conclusive - it's just food for thought. Keep that in mind if you decide to leave a comment.)

Also remember that this question of weight and strength is partly aimed at beginning rocketeers who might be tempted to buy some expensive epoxy to make their Estes Bull Pup or Hi Flier XL "better" or "stronger," when actually the weight, messiness, and expense of epoxy may not give you better results. In fact, your model rockets will usually be fine no matter which adhesive you reach for.

I also don't want it to sound like I'm dismissing epoxy in general, or Bob Smith epoxy in particular. First, there are many different epoxies with many different properties. When building rockets with certain materials - fiberglass, carbon fiber, or a plastic-like material called "quantum tubing" used in kits by Public Missiles, Ltd. - epoxy is a must, as wood glue will not bond to these materials. And Bob Smith is fine stuff if you don't want to go out and buy the really fancy stuff - as I said before, I've used it in my builds before and I will use it again. If used properly and for the right purpose, it'll do a great job.

Also, the difference in strength between any of the two adhesives - especially white and yellow glues - is probably not enough to make a difference if you have a hard landing. If you're landing just hard enough to break something when using white glue and not yellow (which would be a really tiny margin, I suspect), then you probably just need to be more careful about recovery - switch to a parachute instead of a streamer, or pack your chutes so they come out more reliably.

So, why did I do all this? Well, in this hobby, it's possible to hear a lot of conflicting information about a lot of things. It can be hard to know what's correct, and what's just opinion. One of the many interesting things about this hobby is that you can test things out! You don't necessarily have to take someone's word for it.

Don't get me wrong - there are lots of really knowledgeable people out there who'll give great advice. And I don't mean this to be any kind of last word on glue. In fact, I don't even mean for you to take my word for it! If you see a flaw with how I've tried these little informal tests and can think of a better method - let us know! All I've done is the best I can with what I have to try and figure a few things out here.

The fact is, any of these adhesives will work for most builds, whichever you choose. Sometimes a rocket will land hard, and you'll think it definitely should have broken, and it comes out unscathed. At other times, a rocket with well-glued on fins will have a soft landing, yet somehow a fin will pop off (as happened with my beautiful Estes Photon Probe after a soft landing on grass). So take what I've done here however you want, and decide for yourself. As long as the rocket is safe to fly, it's fine - you are the rocketeer, and you make the decisions.

Alright, here's the video.

If you have trouble watching it on the blog, you can follow this link to it and see it directly on YouTube.

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