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.

Follow me on Twitter.

Like my Facebook page for blog updates and extra stuff.

Have a question you'd like to see addressed on this blog? Email me at iamtherocketn00b@gmail.com.

Saturday, May 12, 2018

Back On The Rocketry Show Podcast


Tonight, I recorded a nice, casual episode of The Rocketry Show podcast, and it's nice to be back on. We were hoping to talk to someone about the TARC finals results (the top spot was taken by Creekview High School of Canton, GA), but they've been pretty busy all day, and by the end probably needed to go to sleep!

Gheem managed to find me a new machine - a podcast-dedicated computer in the form of a used, refurbished MacBook Air, seen above. It cost the show a mere $50 and seems to have solved the problems with my sound and connection to the studio, so I shouldn't have such bad sound quality issues in future episodes.

It was good to have a nice chat with people about rocketry.

Follow me on Twitter.

Like my Facebook page for blog updates and extra stuff.

Have a question you'd like to see addressed on this blog? Email me at iamtherocketn00b@gmail.com.

Thursday, May 10, 2018

Accurate


I really need to stop buying kits and build something...

Follow me on Twitter.

Like my Facebook page for blog updates and extra stuff.

Have a question you'd like to see addressed on this blog? Email me at iamtherocketn00b@gmail.com.

Wednesday, May 9, 2018

Glue and Water Weight


In some of my more recent posts, I've done a number of informal experiments testing out various materials. In thinking about what order to present them in, I've decided to start with this one, because it's rather simple, and I may want to refer to it when I write up the others.

If you scroll to the bottom of the post, where it says "Labels," you will see the tag "testing things out." Clicking on that will take you to all the other little tests I've written up. 

Often, on the various online model rocketry forums, people will ask "What's the best glue?" We've received emails on the subject at The Rocketry Show podcast. The question of which glue is "best" is a tricky one. First, it depends on what you're using it for. Secondly, there's a lot of opinion out there, and while it must be true that certain facts exist somewhere, it can be hard to sort them from the mass of opinion masquerading as fact. Even the information that looks completely factual might be incorrect, so how do you really know? A great way to start a huge debate online is to ask "What's the best glue?"

So, I'm not going to dare to do that. I have my preferences and opinions like every rocketeer. But rather than present my ideas as the ultimate truth, I thought I'd share a little of my own testing certain properties of glues out, so people can take that information and do with it what they like.

* * *

Everything you add to a model rocket changes the weight and the center of gravity, which in turn affects how the rocket flies. This includes not only components - engine hooks or motor retainers, shock chord mounts, etc. - but also things like primer, paint, and even the glues you use to put the whole thing together.

Around the time I was trying to figure out how much weight I added to a rocket by painting it, I got a little curious about weight and glue. White glue and yellow glue - the two most commonly used adhesives for low power model rockets - are pretty dense. A full bottle feels nice and heavy. But, of course, both glues have a lot of water in them. And you really don't need to use much yellow glue or white glue at all to get a good bond. Less is usually better with white and yellow glue.

So, assuming you're adding weight when you glue on fins and launch lugs, and then make fillets of glue at the roots of the fins (and the launch lug, if you choose to fillet it), how much of that weight will be lost as the glue dries? What percentage of the weight of the glue is just water - which will evaporate - and what percentage is actually left behind?

I decided to do a little test. As with the Weight of Paint experiment, this isn't really exhaustive or scientific. I tested one batch of one formula of one brand each of white and wood glue - the two I had lying around my Rocket Room. I didn't do multiple batches. I only tested the glues I had on hand - not all formulas and all brands. I just did enough of a test to satisfy my curiosity, and hopefully shed a little light on this subject for others.


White glue is usually made of a compound called polyvinyl acetate, or PVA. Yellow glue, often called "wood glue" or "carpenter's glue," is often made of aliphatic resin. Then again, both white and wood glues, either PVA or aliphatic resins, may be used on wood, and can be called wood glue. The yellow stuff is more commonly used in carpentry, and comes in various formulas, so often when people say "wood glue," including myself, they mean yellow.

The glue I use most for building rockets is Titebond II yellow carpenter's glue.

Why Titebond II? I have no idea. I went shopping for glue one day, saw some Titebond (original, in the red bottle), and some Titebond II, and figured well, if Titebond is good, Titebond II has to be better, right?

I'm sure there's no validity to that thinking, but I do like the nice blue label.

Anyway, I decided to find out how much of the mass Titebond II was water weight, which would be lost as the glue dried.

Once I'd done that, since I had a little Elmer's Glue All lying around, I figured I should try the same test with that, and compare the two.

I knew that Elmer's was a PVA-based glue, and assumed that the Titebond was an aliphatic resin. Not because I'm a glue expert - I'd just read somewhere that yellow glue is made of aliphatic resin, and assumed it applied across the board.

It turns out that, while the original Titebond is aliphatic resin, Titebond II is another PVA-based glue. And Titebond III is something else altogether. This is not to mention other brands of yellow carpenter's glue - Elmer's, Gorilla Glue, etc.

Despite the fact that both the Elmer's and the Titebond were PVA glues, they would turn out to be quite different.

Yellow Glue


I wanted to find what percentage of the glue's weight was water. The test would involve carefully measuring out exactly 10 grams of glue, allowing it to dry out completely, and then weighing the dried hardened glue again.

I have a stack of these little plastic sample cups which had been discarded from a Starbucks coffee shop, and you may have seen me use them for mixing epoxy on this blog. They're perfect.
 

First, I weighed the cup. Exactly 2.8 grams. Once I had the cup base weight measured, I tared the digital scale back to zero, and carefully poured exactly 10 grams of glue into the cup.



With that done, I double checked the combined mass of the glue and the cup - exactly 12.8 grams.


I always have to double check with this scale. It's so accurate, it can easily slip out of calibration and be off by a tenth of a gram. I always check the tare with a 100 gram weight, so I can be sure of my results.

A thin layer of wood glue dries very quickly. But a large mass in a cup like this takes a long time to dry fully - I mean many weeks. To prevent the glue collecting dust, I covered the cup gently with a bit of cotton cloth, stuck the glue on a shelf, and checked it from time to time.


Two months later, I was satisfied that the glue was as dry as it would get, and decided it was time to check the results.

I carefully tared my scale and weighed the cup of glue. The combined weight of cup and glue was exactly 8 grams.


Subtracting the 2.8 grams of the cup's mass, that meant the glue had gone from 10 grams to 5.2 grams. It would seem that in the Titebond II yellow carpenter's glue, the mass was 48% water weight, which would be lost as the project dried.

That's a significant loss in weight. Since glue is generally used sparingly, the weight it adds to your rocket is pretty small (though I admit I thought the final dried glue would weigh even less).

I was all set to write this up here on The Rocket N00b blog, when I realized I really should compare my favorite glue with another commonly-used adhesive, Elmer's Glue All. If I wanted this information to be of interest to more people than just myself, I needed some context.

White Glue

Again, I weighed a sample cup, and again, it came to exactly 2.8 grams (I wanted to make sure in case there were some variance).

I poured in exactly 10 grams of Elmer's Glue-All, weighed the final combined cup, and it was exactly 12.8 grams.



Interestingly, the Elmer's dried much faster than the Titebond II. It only took 2-3 weeks for me to be satisfied the glue was completely dry all the way through.

Once it had dried, the combined cup and white glue weighed exactly 7 grams.


Minus the 2.8 grams of the cup, the glue went from 10 grams to 4.2 grams! So, the Elmer's Glue All is 58% water weight!

For lightweight building, it would seem that, at least between these two choices, the white glue is the winner by a small margin, although either one looks to be pretty darn good. Both white and yellow glue are cheap and nontoxic, they form a nice strong bond between wood and paper (such as balsa fins and body tubes), and any mess can be cleaned up with warm water before the glue dries. Since both are nontoxic and easy to work with, you can control how much you apply - using fingers to wipe off excess - so you can use a small amount, and once the glue dries, it will weigh about half as much as when you put it on.

What About Epoxy?

Sometimes on the forums, a newcomer will upload a small, low-power model rocket build they've been working on, and in an attempt to do a really good job, they reach for epoxy as their main adhesive.

There are some instances where epoxy might be appropriate on a low or mid power rocket build. Attaching a plastic Estes screw-on motor retainer, for example, calls for a small dab of JB Weld steel-reinforced epoxy.


For low power model rocketry, though, use of epoxy should be sparing - if used at all. For most of the main parts of the build - attaching balsa fins and a launch lug to the paper body tube, for example, or installing the motor mount into the rocket - epoxy is overkill.

There's nothing wrong with epoxy. You certainly can use it for small model rockets if you choose. But here are a few reasons you might go with white or yellow glue instead.

First, epoxy is messy. It's a two-part adhesive made up of the epoxy resin itself and a hardener, which you have to mix together. It's toxic, so you shouldn't touch it with your skin. Because of it's toxicity and its thick, honey-like viscosity, it can be hard to get just the right amount onto the part you're trying to glue on (you can use a brush, but the brush will have to be discarded), so you'll probably use more than you need. And, going with the topic of this post, epoxy starts heavy and stays heavy.

Unlike wood glue or white glue, epoxy doesn't dry, it cures. The two parts are mixed together and a chemical reaction occurs in which the molecules in the epoxy and hardener join together to form longer, more complicated molecules, and the substance changes from a liquid to a solid. Because of that, epoxy, which is already pretty heavy, doesn't lose any mass, and you'll end up with a heavier rocket than you may want, without necessarily even benefiting from a better adhesive bond.

I think the reason some beginners reach for the epoxy for their smaller rockets is that they see high power rocketeers use epoxy on the online forums, and perhaps think "this is what the REAL rocketeers use, so it must be better!"

There are good reasons for high power rocketeers to use epoxy. Some high power rockets are made of fiberglass or carbon fiber, neither of which will stick to wood glues. And the forces encountered in high power rocketry can call for heavier duty adhesives, even on paper tubes and plywood fins (though there are times when a good wood glue will work just fine). But while some epoxies are up to the task, others are... well, they're fine for general use, but not necessarily better than wood or white glues. Higher performing epoxies are out there, but they come with a higher price tag. There are many different kinds of epoxies, and it's hard to make a general statement which applies to all of them.

Let's look at what is often a hobbyist's first epoxy, Bob Smith 30 minute. Epoxies have a set cure time, and they may be 5 minutes, 15 minutes, 30 or more - up to hours. Longer cure times generally make for stronger epoxies. The Bob Smith 30 minute is a pretty good one for a low price, and I've used it for a number of mid power builds.


Here, I've measured some out. Getting just about 10 grams was an accident. Because it's hard to work with, and you have to stir it, I was just trying to get some amount in the cup, but here you see I've nearly hit 10 grams, with the cup weighing 12.9g.

A few hours later, well after it had cured, the cup weighed 12.8 grams.


This means one of two things: either the epoxy did lose a small amount of mass during its curing process, or far more likely, I needed to re-tare my scale.

Epoxy certainly will work as an adhesive for low power model rocket building. But because it is toxic, you shouldn't handle epoxy without protective nitrile gloves. When mixing large batches, you may also need to use a respirator, to protect your lungs from toxic fumes. Since it's messy and harder to handle, you are likely to use more than you need, adding extra weight. And since epoxy cures rather than drying, none of that extra weight will be lost before you paint and fly the rocket.

There are some reasons you might want to use epoxy in low power builds, depending on what you're trying to do, and I'll talk about those in an upcoming post. I'll also attempt to test out a few properties of these adhesives, including adhesive strength and brittleness vs. flexibility. I found a few surprises.

Follow me on Twitter.

Like my Facebook page for blog updates and extra stuff.

Have a question you'd like to see addressed on this blog? Email me at iamtherocketn00b@gmail.com.