Rail button spacing and rail length Qs

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BeerorKid

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IMAG0229 by beerorkid, on Flickr

Picked up some rail buttons with the well nuts from Dog House Rocketry and even though I have yet to launch with them it is obvious that they are smoother than the PML guides I picked up and had a problem with out on the pad.

I know that most often rail buttons are meant for MPR and above, but I want to use them so much and have worked them in on my LPR and built a nice launch pad from a Rockwell Jaw stand.

1. Seems in MPR and HPR the rail buttons are screwed into the engine rings and that sets their placement. In LPR that really is not possible. With the well nuts on the ones I bought I can place them anywhere and I would guess that spreading them out over the body of the rocket would be beneficial. Is that true?

2. my current rail is 40 inches of usable rail, but I picked up a 72 inch rail and can extend my current setup by two feet. If I am using rail buttons would there be any disadvantage of me using the longer rail? I am guessing there might be a bit more drag, but I line the rail with graphite powder just in case. Also since I am mostly LPR I thought maybe there would be some possible negatives.
 
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Keep them both positioned in the lower section otherwise the rocket will be supported by only one lug for too long while at the top of the rod and could tilt one way or the other.
Basically the rail is only effective until the upper lug is free of the rail.
 
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As burnout said, once you're down to one button, the rocket is free to pivot on the rear button.

Too close together, and there's a lot of torque on them. Too far apart, and the upper button may clear too soon.

Longer rails are beneficial for heavier rockets, or flights where there's a lower thrust:weight ratio, and you need more distance for the rocket to build up sufficient velocity.

-Kevin
 
Figured velocity was key. Keeping the buttons a bit close and maybe under 10 inches apart makes sense. Was wondering what to do with the wifes Giraffe build. It had a PML guide on it, but I ground it all the way down so I could do buttons. Will place them about 8-10 inches apart. Mostly so they look cool.
 
I'm doing a scratch build and using rail buttons for the first time. The lower button will be attached with a wood screw into a block of wood glued to the lower CR. The upper button will be a couple of inches above the motor mount so I will be using a well nut. The upper button will be about 26 inches from the top of the 2.3" diameter body tube. My question is; what is the technique for getting the well nut in position and pushed into the hole from the inside? Also, I have bent the flange on the well nut to match the curvature of the tube so I need to be able to position it in the correct orientation. The only thing I can think of is to tape the well nut to a long dowel, stick it into the tube and position it under the hole, then start the screw until it grabs and pull the well nut up into the hole.
 
I'm doing a scratch build and using rail buttons for the first time. The lower button will be attached with a wood screw into a block of wood glued to the lower CR. The upper button will be a couple of inches above the motor mount so I will be using a well nut. The upper button will be about 26 inches from the top of the 2.3" diameter body tube. My question is; what is the technique for getting the well nut in position and pushed into the hole from the inside? Also, I have bent the flange on the well nut to match the curvature of the tube so I need to be able to position it in the correct orientation. The only thing I can think of is to tape the well nut to a long dowel, stick it into the tube and position it under the hole, then start the screw until it grabs and pull the well nut up into the hole.

If you are using well nuts like this:

well-nuts.jpg

Then I think you might have them flipped the wrong way. You drill the 5/16th hole and insert them from the outside of the BT. When you screw down the bolt the rubber that is now inserted into the BT will squish and create a washer when it is tightened. Let it sit for a few days and back it off so it can spin a bit.
 
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My rail buttons aren't like that. I got these from Apogee Components.

rail_button_flanged_main.jpg
 
I'm doing a scratch build and using rail buttons for the first time. The lower button will be attached with a wood screw into a block of wood glued to the lower CR. The upper button will be a couple of inches above the motor mount so I will be using a well nut. The upper button will be about 26 inches from the top of the 2.3" diameter body tube. My question is; what is the technique for getting the well nut in position and pushed into the hole from the inside? Also, I have bent the flange on the well nut to match the curvature of the tube so I need to be able to position it in the correct orientation. The only thing I can think of is to tape the well nut to a long dowel, stick it into the tube and position it under the hole, then start the screw until it grabs and pull the well nut up into the hole.

I use tape folded back on itself with the sticky side out, stuck to the angle aluminum that we use to draw lines on body tubes. I stick the well nut to the tape, position it on the hole, and push it through the hole. After attaching the rail button and screw, I put a few drops of epoxy into the hole and tighten it the rest of the way. Then I add more epoxy on the inside to cover up the well nut and keep the chute or shock cord from hanging up. It is easier than I worried it would be.

Joe
 
I'll try either the magnet or tape method. But I'm going to wait to install the rail buttons until after the rocket is finished and painted, probably about 2 weeks. Less stuff to have to sand around.
 
Less stuff to have to sand around.

So true. The buttons are the last thing I do.

just a friendly FYI, weld and well nuts are different. I just learned that a couple days ago and wanted to share. So sorry if I sounded all smarty pants or whatever. Since my builds just scratch MPR, I am fine with the well nuts and love the buttons, but it is difficult to keep them loose for the rail. A couple of mine are worn well from just a few flights being tightened down.
 
Just a few little points of clarification. The items shown in Post #6 by BeerorKid are well nuts. The items shown by Nathan in Post #7 though are either acetal (most likely) or else nylon rail buttons along with blind nuts for mounting them. The two types of fasteners have different methods of operation. Well nuts are inserted from the top side (the outside of the rocket) while blind nuts are inserted from the back side (inside the rocket). In Nathan's photo the black things are the rail buttons proper.

Friction is always your enemy when you use any type of launch guide. Rail buttons made out of acetal (Delrin) or nylon impart considerably less friction than the soft rubber of a well nut and consequently they don't need to be lubricated or be able to rotate. They can thus be firmly affixed to the rocket with epoxy. When done so they will last a very long time and will never require any maintenance.

Another method that can be used to attach rail buttons when there is no solid part inside the rocket to screw them into is to use speed nuts on the inside. These are very thin and do not need to be pushed through the tube wall; they sit entirely on the backside of the hole. The installation is completed by placing a drop of epoxy onto the combined speed nut/machine screw threads inside the rocket after the button has been installed. Speed nuts usually aren't quite as strong as the other fastener types but in this installation once the completed attachment is given a drop of epoxy it is quite strong enough to keep the rail buttons in place. Speed nuts also have a slightly curved profile so they will have more non-deforming contact with the inside wall of the tube without any modification. Rail buttons don't normally experience all that much stress anyway because they are only active for a fraction of a second and if they are properly installed they don't receive very much torque. In a low-power rocket the weight saved by using speed nuts instead of well nuts or even blind nuts can easily be enough to make a difference.

Finally in order to insure that any installation of multiple launch guides doesn't produce unnecessary friction and binding the builder must insure that all guides are installed perfectly in line with each other and also perfectly parallel to the thrust axis of the rocket. But you probably knew that already, right?
 
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Rail buttons don't normally experience all that much stress anyway because they are only active for a fraction of a second and if they are properly installed they don't receive very much torque.

For light rockets loaded onto a vertical rail, I agree. However, for heavy rockets loaded onto a horizontal rail it's another story. I suspect that it's not the launch that stresses rail buttons, but instead the loading, raising and resting forces.
 
For light rockets loaded onto a vertical rail, I agree. However, for heavy rockets loaded onto a horizontal rail it's another story. I suspect that it's not the launch that stresses rail buttons, but instead the loading, raising and resting forces.
Good points. I have no experience with more massive rockets like those that you refer to, but I sort of assumed that they received enough support before being raised to vertical to spare the launch guides from undergoing too much stress. In any event, speed nuts would be totally inadequate to back up the rail buttons on such rockets anyway, so that suggestion wouldn't apply to them. One of the members posting above was talking about using rail buttons on LPRs and that type of scenario was more of what I had in mind.
 
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