Mid Power: Building the Quest Big Dog (Part 7)

Click here for Part 1

 The Big Dog kit comes with a launch lug for a 3/16 inch diameter launch rod, but I scavenged that for my previous Estes Cosmic Explorer build in which I adapted that rocket to hold an E-sized motor (24mm instead of 18mm). But I wasn't worried about not having the right launch lug for the Big Dog, because I decided a while ago that, for this rocket, I would use launch rail buttons.

Most rocket kits, even high power kits, come with a traditional launch lug. Larger high power kits can have a huge launch lug, and require a really thick, long launch rod. But a lot of high power rocketeers have abandoned the lug-and-rod system in favor of launch rails - long metal extrusions with a t-shaped slot running its length.

A launch rail from Apogee Components

A launch "button" is a small piece of plastic material shaped like a little spool. The thin middle part fits into the slot on the rail. With two (or more, for bigger rockets) buttons attached to the rocket, you slide the whole thing onto the rail, and that keeps the rocket pointed straight up at launch.

Rail button sliding into a launch rail. From CDI Model Rocketry

Launch rails are sturdier than rods, and less likely to "whip" back and forth due to wind and the weight of a rocket and the force of its taking off. And people say that launch buttons produce less aerodynamic drag than lugs. I don't know if that's been confirmed scientifically, but a small button is less obtrusive than a huge lug. So it may reduce drag, and in any case, it looks better.

Some rail buttons are even airfoil-shaped.

Airfoil-shaped rail buttons from GiantLeapRocketry.com

The standard rail size for high power rockets is called 1010. The 1010 rail is 1 inch wide. Some really heavy rockets use a rail size called 1515 - 1.5 inch wide.

But, like a lot of things in rocketry, the t-slot rail was never intended for use on rockets. It's an industrial product you can use to make all kinds of stuff - furniture, kiosks, machining equipment... Whatever. It's like an industrial Erector Set. Rocketeers adopted the rail and came up with the rail button. So, there are other sizes of t-slot extrusion. They come in metric as well as English sizes.

For a while, the 1010 and 1515 rails were the only option if you wanted to use buttons. Hypothetically, you could put a 1010 rail button on any rocket, but for smaller model rockets, they're a bit too large to be practical. A small launch lug looks better, and produces less drag.

However, there is a company which now sells smaller sized rail buttons for use with lighter rockets, and teeny-tiny ones for use with small low power model rockets. Rail-buttons.com sells a mini button, which fits the metric sized t-slot rail, and even a micro button. With the micro button, you can make a launch rail system using one of a couple of smaller t-slot rails now available. The Makerbeam and the OpenBeam, both of which are used to make smaller, lightweight industrial stuff (a lot of people use them to make frames for 3D printers), can now be used to launch rockets.

A 1010 button would work fine on the Big Dog. But I don't currently have a 1010 rail, and a button of that size is larger than I need. In my opinion, as long as it works, smaller is better.

Mini button on the lower left; 1010 button on the upper right

I do have a metric rail and some mini buttons. I used those when I built my Estes Partizon (as was recommended by Randy of rail-buttons.com).

But even those looked a little larger than necessary for this rocket. The Big Dog is pretty light, so I have decided to go with the really tiny micro buttons, and launch the rocket from my Makerbeam rail, which is 1500mm long - about five feet.

The micro button - even smaller than I'd imagined

The first step is to drill holes for the buttons to screw into. I'd marked my rail button line back in Part 2 of this build series, and it was time to decide where exactly to place the buttons on that line. While the kit comes with one launch lug, you always need a minimum of two buttons. The kit instructions say to place rear end of the lug 10 inches from the bottom of the body tube, so that's where I put my forward button. But where to place the other one?

Well, as long as you have two buttons, you can hypothetically place them pretty close together. With a rocket kit with, say, a 3-inch lug, you could place the buttons 3 inches apart, exactly where the ends of the lug would go. But, I don't have a drill press, just a hand drill. What if I get the buttons slightly out of line with one another? If the buttons were close together, that would mean that the rocket would be pointed at an angle to the launch rod, rather than straight up. The further apart the buttons, the less extreme that angle would be.

Off-center rail buttons are less of a problem when they're far apart.

Many rocketeers like to place one button far aft, and one about the center of gravity. Or the center of pressure. Or wherever - there's a lot of discussion. I placed my aft button about 2 1/4 inch from the back - enough so that I wouldn't interfere with the centering rings. I chose the spot where the rail button line crossed an inner seam - just to have a nice mark to use.

I wish I could tell you what size drill bit I used, but I threw the packaging away. It might be 1/16 of an inch, but I think it's a little smaller. In any case, it's smaller than the diameter of the screw.

I drilled the holes for both buttons, then wicked a little thin CA (cyanoacrylate - hobby super glue) into the hole, and wiped away the excess.

The CA makes the hole harder, so you'll be able to thread the button screw in once it's dry. It will also allow you to sand off the burrs that come up from the body tube from the drill. I threaded the screws into the holes and then removed them. As it happens, I got the holes lined up pretty well, so I don't think there will be any issue with aligning the rocket on the launch rail.

I'll leave the buttons off until the very end of building the rocket, after it's painted. I could add them now, but leaving them off saves me from having to mask the buttons off while painting - and potentially leaving a bald spot in the paint job from masking tape.

Next up, I'll put on the fins and do some nice fin fillets - with epoxy, instead of my usual wood glue. This will add strength and give them a nice shape.

Click here for Part 8

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Building the Big Bertha - Part 6 (For N00bs)

[Click here for Part 1]

We're getting closer to finishing our Skill Level 1 rocket!

Once the fins are on, attach the launch lug. That's the short tube that looks like a piece of drinking straw.

First, I like to scuff it up a bit, so the glue holds better. I do this on one side with rougher grit sandpaper. Then I do a double glue joint, putting a bit of glue onto the roughened surface of the launch lug and attaching it to the rocket for just a moment, as perfectly straight as I can along the launch lug line we've already marked. I remove it, and allow the glue to dry a little, then apply more glue and attach it to the rocket.

Scuff one side of the launch lug

    

Apply a thin layer of glue the entire length of the launch lug

Place it on the rocket where you have marked the launch
lug line, getting some of the glue on the rocket

Allow the glue to dry for several minutes. These reverse-
action tweezers aren't necessary, but they are a nice little tool!

Add a bit more glue and attach the launch
lug to the rocket - make sure it's straight!

You must make sure it is straight - the launch lug will slide down over a metal rod called the launch rod (when we actually take the rocket out and launch it). The rod keeps the rocket flying straight for the three feet or so it takes to gain enough speed for the fins to keep it stable in flight. If the launch lug is askew at all, the rocket can grab hold of the rod, and either not take off, or fly in a weird direction, or even try to take the rod with it! Site down the length of the body tube to make sure the launch lug is straight and true. You can even (carefully) slide a launch rod into the lug and verify that it follows the line you marked on the tube - then you'll know it's straight. Just (carefully) remove the rod so it doesn't weigh down on the glue and push the lug out of alignment as it dries.

Site down the length of the rocket to verify the launch lug alignment. Be sure to adjust it
while the glue is still pliable if it's crooked. Work fast - a double glue joint sets quickly!

Let the glue on the launch lug dry before moving on.

The fins are on, so let's reinforce the joints. We're going to make fillets. A fillet is a rounded joint in a corner, often found when two pieces are welded together at a right angle. In model rocketry, this makes the joint where the fin connects to the rocket stronger, and it also reduces interference drag, making the rocket more aerodynamic.

For our rocket, we'll use more wood glue.

The simplest way to do this is to apply a bead of glue at the base of the fin at the leading edge, then take your finger and run it along the base of the fin, dragging the glue with it. You should get smooth, even line of glue connecting the fin to the body all the way down the root of the fin.

Instead of your finger, you could also use a craft stick (popsicle stick) or coffee stir stick, or even a narrow dowel rod.

I always get extra glue on the fins if I do it the simple way, which either I can't sand off, or don't have the patience to. So I mask off my fins with a little low-tack masking tape. I've been told by some real rocket craftsmen that this isn't necessary with glue fillets, but I find it makes mine a lot neater, so I always do it.

I start by masking two fillets - two joints between the fins and the body - at a time. You'll do two fillets in one go on each side of the rocket, and by side, here I mean the space between the fins. So, if you have a four-fin rocket, like the Big Bertha, you'll have four sides. Three fins, three sides. Etc. Each side will have two fillets each, one for each fin.

Here, let me just show you...

This is what I'm talking about. I've got a tiny bit of tape on the body tube at the leading edge of the fin, then one piece of tape running the length of each fin, and tape on the tube itself, with a little gap between the tape lines where the glue fillet will go. If you're building a rocket where the fins are not flush with the base of the body tube, I'd put a piece of tape there as well.

You'll want to lay the rocket on its side with the joints you're working on facing upwards, so you can work on the fillets, and so they'll dry evenly. Here's one of the many little things I love about rocketry - coming up with a creative solution to problems. There are big problems and little problems in building rockets, and they all require creative solutions.

Here's a few ways you can do this.

You could lay the rocket on its side, with the tube supported by a thick book.

Could be just fine. But you might not want to put any lateral stress on those fins before you reinforce the glue joint. And if you're building a three-finned rocket, this won't work.

The joints aren't facing upwards!

 You could lay a dowel rod on a table, place something heavy on it, and slide the rocket body over it.

You could place the rocket down on a table (or in this picture, a thick book) with the fins hanging over it, and lay a heavy blanket over the body of the rocket. This will hold the rocket down, but won't squash the body.

In my case, the problem is solved by the Guillotine Fin Jig. The long metal arms are for holding the fins when you attach them to the rocket. But once you're done, you can slide the rocket through the jig the other way to hold it gently but firmly in the horizontal position for working on it.

Whatever solution you come up with for this little problem, just make sure the rocket is secure, that it won't fall or get crushed, and that it can remain undisturbed in a horizontal position while you work and while the glue dries.

OK, enough of that digression - back to fillets!


Drop a healthy bit of glue in the gap between the tape lines. If you're building a small rocket, a bead may be enough. The Big Bertha is pretty big, and I find if I don't put enough glue, I can't get the fillet to run the whole length of the fins, so I put a bigger blob. Don't put too much, though. You are going to pull most of it off, and the more glue you use, the more of a chance you'll get a mess.

First, notice that I don't put the bead right at the fore of the fin.  With a craft stick or coffee stir stick (depends on the size of the rocket, and therefore, the radius of the fillet I'm making - bigger rocket, fatter stick), I push a little of the glue forward to the leading edge, then run the glue back down the length of the fin, smoothing it as I go. Do this quickly, so it spreads evenly, and if there are any bubbles when you're done, smooth them over with the stick.

 

Once the fillet is smooth and covers the entire seam between the fin and body tube, remove the tape. Don't wait long to do this - fresh glue will smooth out and make a nice smooth seam, but once the glue has started to set, it will leave a sharp ridge.

Remove all the tape, from the fins and also the little bits from the body tube.

When you remove the tape, you should have smooth, even fillets.

Let these set and dry a bit before moving on. I can't tell you how long - 20-30 minutes, maybe? I dunno. Go have a snack or something. When the glue looks dryish on the surface and the color has changed from off white to more yellowish, you're fine to move on. Turn the rocket to the next side and do those fillets. Repeat until you're all done.

Oh, and don't forget to put a fillet on each side of the launch lug! The launch lug takes some stress from the launch rod, so you want it to be secure!

We're nearly done building our first Skill Level 1 rocket! Next up, we'll prime and paint this sucker, then we're almost ready to fly!

Click here for Part 7