Mid Power - Building the Quest Big Dog (Part 2)

Click here for Part 1

Back to building the Big Dog, from Quest Aerospace...

After assembling the motor mount, I next decided to mark fin lines and a launch lug line - though I'll be using mini or micro rail buttons for this rocket.

If you're new to rockets, here's what I mean. Launch lugs were the standard for all hobby rockets until not too long ago. But in the last - I don't know for sure - decade or so, a lot of rocketeers have switched over to rail buttons. Instead of a thin, wobbly launch rod, which can suffer from wire whip if it gets too long for its diameter, mid and high power rocketeers often use something called t-slot rail to keep a rocket upright on the launch pad.

One of the most common rails used is a square aluminum extrusion, used to build all kinds of modular industrial things, with a cross-section that looks like this:

Instead of launch lugs, two (or sometimes more for really big rockets) small buttons are attached to the rocket, and they slide down into the slots in the rail. The result is a really sturdy launch platform.

Launch button (bottom) fitting into a rail slot.
Image from CoastRocketry.com

Sorry to digress. Back to the subject at hand.

I cut out the paper fin guide from the instructions, I carefully lined it up around the tube and taped it in place. There was a slight gap between the fourth fin line and the edge of the paper guide, which I figured was intentional. I marked each fin line with a little pencil dot.

As always, I decided which end I wanted to be the forward and which end the aft, and marked them so as not to
forget. I don't remember how I chose on this rocket, but it may have had to do with how the centering rings fit.

Estes fin guides have an additional line, the launch lug line, exactly between two of the fin lines. The Quest guide didn't have that. In order to get the launch lug line exactly spaced between two fins, I folded the guide over and matched up to of the fin lines, then put a pencil mark on the fold.

Then I attempted to match up the fin lines with the little pencil dots on the airframe, and that's when I noticed a flaw in the kit - the dots didn't line up with the lines. I can only have noticed this because I must have laid the guide down between the first and fourth fins. I double-checked the lines with the other dot, and they all matched up. Something was off here!

A few months ago, I bought a digital caliper, because it was inexpensive, and because I figured I'd use it for... well, something. In any case, I like having precise measuring tools.

I got out the caliper and measured the diameter of the body tube. The box claimed the airframe was 1.92 inches. The caliper told a different story - 1.97 inches!

I knew this thing would come in handy!

I opened the free RockSim file for the Big Dog, available from Apogee Components. I don't have RockSim, but OpenRocket will open those files. I saw that, according the the sim file, the rocket should have an outer diameter of 1.97 inches. That's not much of a difference, but if the paper fin guide was made with a smaller diameter in mind, you'd get fins that are ever so slightly unevenly placed - all too close together, except for the first and fourth fin, which would be slightly too far apart. You might not even notice the difference unless you looked closely, but I look closely. It would have bothered me.

So, I used OpenRocket to print off a new fin guide - with an additional advantage that this one would already have a launch lug line marked.

I taped this guide to the rocket, and everything matched up correctly, so I marked all the lines.

Finally, I grabbed a length of aluminum angle and extended all the lines.

Normally, I extend the launch lug line all the way down the body tube, so I can sight along it and verify that I have the lug on straight. I keep the fin lines rather short. But, due to probably a little sloppiness on my part, I've gotten a few fins on not quite perfectly straight when using the Guillotine fin jig lately. The Guillotine is a great tool, but like all tools, you must use it with care.

Just to be sure I'm getting the fins on true, I extended the fin lines most of the way down the body tube. That way, I can check the fins are on the same vertical line on both ends of the fin jig before gluing them down.

You can see that before I did all this, I marked the tube spirals with a pencil so I could see the lines clearly when I filled them in.

The spiral groove on this rocket is really tight, and I had trouble in spots keeping the pencil tip in them. In spots, I couldn't even feel the groove with my fingernail. It's possible I could get away with just priming the rocket with a good coat of filler primer, but I don't want to get to the end of the build and see grooves. So I filled them in.

I didn't take any pictures of this process, but I just blogged about how I do it.

I filled the grooves, let them dry, then lightly sanded.

Next, I installed the motor mount and began airfoiling the fins. I'll talk about that in the next build post on this rocket.

Click here for Part 3.

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Building the Big Bertha - Part 5 (For N00bs)

[Click here for Part 1]

In Part 4, the first thing we did was to reinforce the ends of the body tube with a bit of thin CA - cyanoacrylate, or super glue. Turns out, this is a really good idea.

A couple days ago, I took a few rockets out to try out my new homemade launch controller.

I took my newly-completed Big Bertha with me. Since it was a little windy, and I didn't want to lose my beautiful, new rocket to a big gust of wind, I put in the weakest motor I had - an A8-3 (We'll talk about motors later, but I'll just say, the A is the least powerful of the standard sized motors). This was a mistake, because it turns out this motor is far too weak for a rocket this heavy (always read the instructions!).

The controller works great. But the rocket lifted off the launch rod, went about 50 feet maximum into the air, then the motor ceased its burn and the rocket took a nose dive straight at the Earth. The ejection charge blew the nose cone off about 6 feet from the ground. The parachute didn't even open, and the rocket body embedded itself into the ground.

The parachute is still folded...

This could have been the end of the Big Bertha. But I pulled it out of the ground. Smoke still oozed out of the top of the tube. Apart from a bit of mud, grass and clover leaves, there was no damage!

So that Rocket Pro Tip was well worth it!

OK, back to our build. Now it's time to add the fins to the rocket body.

The first thing we need to do is determine where the fins will go, and mark the tube, so that we get them on straight. Kits come with a fin marking guide for this purpose.

You'll cut this out and wrap it around the body tube, somewhere near the base. Make sure the lines on the ends line up perfectly - that's how you know you have the fin marking guide on straight.

You can see here that my fin marking guide has a bit
of a gap in it. Some Estes kits are a bit sloppy on a few
details. But the fin lines did line up perfectly.

The lines marked FL are fin lines - put a pencil (NEVER a marker or pen) mark on the body tube where they indicate. The LL line is for the launch lug - mark this as well. In the above photo, I had it off-center a bit while manipulating the camera. Often, the launch lug and the engine hook are lined up.

Next, what you'll need to do is draw a perfectly straight line up the body tube to show exactly where to place the fins. Well, how do you do that?

The traditional way is to place the body tube in a door jamb and use that as a straight rule to mark a line up the tube. People have been doing that since the birth of model rocketry in the 1960's, and it works. Many kits will explicitly include that in the instructions. If that's all you've got, it's fine. But it has drawbacks.

Some door jambs are not as straight as they should be - especially in an older house that has settled, or been repainted many times. And they may have chips in them. And you'll get pencil lines on your door jambs.

Using a door jamb to mark a fin line -
notice the nick in the jamb?

 There's a better option.

You can make yourself a really cool, cheap tool, with aluminum angle. This is a piece of aluminum, sold at hardware stores, and it's used for... I dunno. Construction or something. Doors or windows. I have no idea. I only know it's used for rockets.

It comes in 3-foot lengths, and it costs less than two bucks. Get yourself a piece of 1/2 inch aluminum angle, the kind with equal-length sides, and cut off an 8- to 12-inch piece with a hack saw. Here's mine:

I even put 1-inch marks on the side with a Sharpie so I could use it as a crude measuring tool. I wasn't sure that would come in handy, but it has. Not as precise as a ruler, but it's nice to have some times.

See how the angle sits exactly along the tube?

Line the angle (or the door jamb, or the Estes Tube Marking Guide) up with the fin marks you made, and draw a line up the body tube, for several inches. Then do the launch lug line. I like to mark the launch lug line with an LL so I don't accidentally glue a fin to the wrong place.

The yellow thing is an Estes Tube Marking Guide part.

See? Perfect lines!

Now it's time to glue on the fins. This gave me a real panic the first time I did it. I knew I would have to do this freehand, and I was pretty sure I would screw it up.

There are options for assuring that you get perfectly straight fins. But if this is your first rocket, try it freehand. It's not as hard as you think - you'll just have to hold the fins in place until the glue sets.

For low power rockets, most people use either wood glue - sometimes called "yellow glue," because it's yellowish - or "white glue" like Elmer's Glue All, so called because it's white. Some people use epoxy, but that's more for higher powered rockets, so for your first Skill Level 1 kit, just get some yellow or white glue. Do not use "school glue" or anything washable - it just won't hold as well.

Wood glue, white glue. I started with the Elmer's wood glue,
but I've really taken a liking to this Titebond stuff.

You can make the glue set better and in less time by creating a double glue joint. This works really well with wood glue, which is about all I use, because it creates a stronger bond - stronger than the material it's gluing, so the rocket itself will break before the glue does.

First thing to do is to put a small bit of glue on the root edge of the fin. Just dab a bit on the root edge, then spread it up the edge with a finger, so there's a thin layer of glue all along the edge. Then locate the place on the rocket where the base of the fin will sit - it's often, but not always, lined up with the base of the body tube (your kit's instructions will tell you where to put the fins - you may need to mark a spot on the body tube where they'll go). Put the fin exactly where you'll glue it down, pressing the glue to the body tube. Then remove the fin.

Yep. Remove it. We're going to let the glue on the fin edge and the body tube dry a bit. Then we'll re-glue it, and the "double glue joint" will set a lot faster, and be a lot stronger. Set the fin somewhere where the glued edge won't touch anything, and let the glue dry for a few minutes.

When I built my first rocket, the idea of getting the fins on straight was the part that made me hesitate. But I did pretty well with it. I did have to keep my hands on the rocket, and keep checking that I wasn't moving the fins, but I'm happy with the rocket.

But there are tools you can use. If you read Harry Stine's Handbook of Model Rocketry, you will find instructions for making a very simple fin jig, called a Kuhn fin jig. This is just a piece of wooden angle - used for doors or something - with a slot cut right in the center.

Works great - if you have the power tools to make it, and know how to get the slot cut exactly perpendicular to the angle, and get it the right thickness for your fins - none of which do I have or know how to do.

The Estes Tube Marking Guide has a Kuhn-style fin jig at the end, which is helpful - sometimes. I've used mine on a few rockets, but I discovered quickly that if the fins are too thick, they'll barely fit, and if you're building a smaller rocket with more than three fins, it won't work at all...

Building the Estes Mini Honest John. With the Estes Tube Marking Guide
fin jig, I can get opposite fins on, but not the
two side fins. Those, I had to do by hand.

Still, it has helped me get my fins on straight on some of my favorite rockets.

I recently invested in the coolest tool in my toolbox - the Guillotine Fin Jig, created by a guy named Ted Macklin, and sold by Apogee Components. Now, if you're just getting into rockets, and not sure how involved you're going to be, you might want to hold off on a lot of special tools. The Guillotine Fin Jig is not cheap - it's about $90. But it is so ingenious. It holds your rocket in place and holds your fins perfectly straight. So, if you're building your first rocket or so, do it by hand. But if you've decided that you really want to make more, better rockets, getting a fin jig like the Guillotine is a really nice investment.

It's also great for carrying stuff from one room to the other, if you tighten down all the nuts, because the body of it is basically a wooden box.

Back to gluing on fins! Once your glue has set for a few minutes, apply another thin layer of glue to the fin root edge, and carefully line the fin up with the fin line you marked on the body tube, and hold it - or jig it - in place. Let it set for a few minutes at least before you do anything. I leave mine in the fin jig for at least 20 minutes or so and get a snack or work on some other rocketry project.

 

There may be small gaps due to imperfections in the tube, or imperfect
sanding - this is fine. Press the fin to the tube tightly for a moment and
let the double glue joint do its work. We'll fill in any minor gaps later.

Perfectly aligned fin. This is why I love good tools. You need skill to make a nice
rocket, but a good tool can help you make a better rocket, with less of a struggle.

Repeat the process for all your fins.
























Now the fins are on, and your rocket is starting to actually look like a friggin' rocket!

Next time, we'll apply glue fillets to the fins to reinforce them (and make them more aerodynamic), attach the launch lug, and maybe even prime this sucker and start painting! Stay tuned...

[Click here for Part 6]