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Commonwealth Rocketry, LLC.

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Canton, Michigan 48188
734-722-Rocket (7625)
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These 1.5 inch counter sink buttons "save your rail"!

No more wear and tear on the rail, the counter sink ELIMINATES rail gouges.

You supply your own 1/4-20 pan head screws.

Rail Button Installation:
The Basics

I've had a lot of questions about installation, so I thought I'd cover my experiences here. There is no simple answer to "Where should I place myrail buttons, and how many should I use?" Each rocket is uniquein geometry, construction, mass, and lift-off profile, so one simple set ofinstructions is not going to cover all cases. Once you decide on how manyand where they go then the "how" of installation is fairly simple.

First off, how many buttons do you need and where do you place them? Inany case you will always need at least two buttons. It's typically agreed that one buttonshould be placed as far to the rear of the rocket as possible. As to the forwardbutton, there are all sorts of theories floating about revolving around the CP and CG. But keep this one simple idea in mind: once there is only one button onthe rail you essentially lose most or all of your guidance due to the geometry of the buttons.

When your rocket is on the pad, in a standard two-button configuration, the distance from the upper most button to the top of the rail is your effective guidance length. With lighter rockets I usually aim for the two centering rings that aresandwiching the fin tabs. That may seem awfully close together, but I've never had a rocket hang on the rail (yet). The only issue is whether or not the weight of the rest of the rocket has enough leverage to rip your uppermost button out. The greater your launch angle from vertical the more of a problem this can be unless your rockets are fairly light.

However, I highly recommend what has become a standard three-button configurationon most of my larger rockets. Two of the buttons are placed a few inches apart at the very rear and then a third button up closer to the CG (w/motor) plus or minus a caliberor two. This allows positive guidance for the entire length of the rail while the upperbutton helps support the weight of the rocket while it's sitting on the pad.

In either the two-button or three-button configuration, if you do put a button forward of your most forward centering ring then make sure the button won't interfere with 'chute deployment or piston movement since the screw extends into the interior. For example, in a three-button configuration it is sometimes convenient to place the uppermost buttonthrough the electronics bay airframe in a standard mid-body dual-deployment configuration.

So, now you know where you are going to place your buttons. Next you need todrill pilot holes in your airframe to accept the mounting screws. I've found thatdepending on the airframe material and type of drill I'm using that I needanywhere from a 1/8" to 5/32" bit for the Series 1000 buttons. Do some testsfirst on a scrap of similar airframe material. You do not want the screw to just push through without engaging the threads. You should be able to screw and unscrew the button just as if a nut were in place. If you're using cardboard then you might want to wick some thin CA into the edge of the hole and let it dry to harden the edge.

If you're installing these during construction and it's convenient then youcan simply use a nut and lock-washer on the inside and you're done.However, since I've retro-fitted most of mine I prefer to do the installation after construction. Ideally I try and drill into one or both of my centering rings, but I've found it's not critical if you can't or if you miss.

Rough up the airframe with sandpaper just around the hole and remove any andall paint. Rough up the bottom surface of the button as well. If you've managed to drill into a centering ring, insert a drop or two of epoxy into the hole so it can be wetting the side surfaces. Take the assembled button and smear some epoxy onthe exposed threads. Also smear some epoxy on the bottom surface of the button. It doesn't take much!

Now, screw the button into the airframe and check for alignment. If you'veused the correct amount of epoxy you should see a small fillet form around thelower lip as you snug down the button. (Do not over-tighten, as you could deform,compress, or crack the button.) As long as the epoxy doesn't extend above the top of the bottom washer you should be okay.

I've flown rockets with buttons installed using this technique (epoxy-only) on everythingfrom D12 to K550 and have never had a button ripped off the airframe. I'm not going to say it's the only way or even the best way. Use some common sense andfeel free to experiment. I'd love to hear any new ideas and feel free to contactme with any questions.
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