Monday, April 13, 2015

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
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. 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

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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