rail button placement and fixation

[jkovac]

Thanks to everyone here for all the help so far in building my first HPR rocket.

For rail button attachment, the kit directions (Madcow Twitch) say to install one button 1 1/2" from the aft end , and the other one at the CP, making sure it's behind the forward centering ring. That would put both buttons nearest the aft end of the rocket. Other info I've found online says to put the forward button much further toward the nose, just below where the nose cone shoulder ends. That makes more sense to me. What do people usually do?

The other question I have is, are the buttons supposed to spin? It seems they should, but the kit directions say to use epoxy to fix the screws in the holes, and it appears that it might be hard to do that without some epoxy getting on the backside of the button and fixing it to the body tube. Maybe just keep spinning the button after attachment until the screw hole epoxy dries?

 

[tim cubbedge]

I usually put the aft button about 1/2 to 1" from aft end of air frame. I attach the forward button right around the CG of rocket.
They should spin so you don't end up with a flat spots from wear. I have encountered flat spots for that very reason. I try to make my a little loose but not always the case. I ususally use a drop or 2 of CA to secure the screw but I also use well nuts. That depends on rocket diameter.

 

[RKeller]

I like to put the aft button 1/2" to 1" from the back, then the front button is close to the CG. with the buttons too far apart the front button will be off the rail quickly and will not provide guidance. too close together and they will have a lot of loading on them, have more drag on the rail and wear quickly. most of mine end up around 6 calibers apart if that helps. obviously a long skinny rocket will have more and a short fat rocket will have less.

 

[fyrwrxz]

+1 for what Tim and Riley says. The CG location has always made the most sense for me. Most buttons I mount with T-nut, but they are below (or on) centering rings. Well nuts are great for blind holes. I like mine to spin with the theory it reduces friction but you have to realize also where you put the button dictates where it intrudes (if it does) in the payload (chute) compartment. You don't want anything too high up in the body tube that allows your recovery train to hang up. I'm also a big fan of ACME rail guides-your mileage may vary. Check the new printed guides from Wildman, too. Might as well mention fly-away guides and towers are also options.

 

[JDcluster]

I place the lower button near the top of the fins and the upper button near the CG.
There has been a long winded debate over this, many times in the past.

I don't like placing the aft most button near the bottom for one reason:
If it were to hang up (for any unknown reason) on the top of the rail, it will affect the trajectory more than a button place higher up the airframe.


JD

 

[Barkley]

I was told by a very good source to keep the buttons low on the rocket, significantly below the CG. The reason being that you want both buttons in the rail until the rocket is going fast enough to ensure stable flight. The closer the top button is to the nose, the quicker it leaves the rail, the quicker it becomes useless for it's intended purpose.

 

[jkovac]

Thanks all for the replies. Putting the buttons lower as suggested.

 

[Bat-mite]

I was told by a very good source to keep the buttons low on the rocket, significantly below the CG. The reason being that you want both buttons in the rail until the rocket is going fast enough to ensure stable flight. The closer the top button is to the nose, the quicker it leaves the rail, the quicker it becomes useless for it's intended purpose.

You have to find the sweet spot. If both buttons are too far aft, then you have a very short radius to determine what "straight up" is. You can get a millimeter or more of leaning if the buttons aren't spread out, and the closer together they are, the more this is magnified toward the nose.

If you are concerned about having more time on the rail, use a longer rail.

 

[Barkley]

Bat-mite, I hear what you're saying. When constructing my L2 bird I was told by several at my club to put my forward button near the top MM centering ring, which was very close to my CG. However, at the launch a well respected gentleman who will remain nameless - but who has several 100k+ flights - gave me the information I've passed along.

 

[Bat-mite]

I don't think there's any right answer. Lower buttons allow for more time on the rail to stay straight as thrust builds up. This probably compensates for the situation in my illustration. And it's not uncommon to find two L3 TAPs disagreeing on how to do a given thing.

I have no 100k+ flights to brag about, but it still seems to me that if you put one button about half-way up and use a longer rail, you get the best of both worlds.

Of course, if the launch site doesn't have longer rails, then my theory is useless. :grin:

 

[watermelonman]

I put the aft button as far back as possible, figured out recently how to modify some backing hardware to fit it behind the aft centering ring despite my tricky dimensions. I like to think that far back is good for the drag equation.

For the forward button, I usually get as far aft as I can while still being able to easily replace it. It is not scientific but feels about right.

I am not concerned with maximizing time on rail. I like to fly fast and high, so any club rail I find is more than enough to hold my birds until stable.

 

[DRAGON64]

I just received a rocket in the mail yesterday, which is already built, and in my opinion, the rail guides are installed in the wrong locations...



The guides are too far apart, and the aft guide is too far forward. I am debating removing the guides, and installing buttons in locations I would consider to be more beneficial to the over-all launch profile of the rocket; The first button near the aft, and the second at or near the cg. MY rule of thumb in some cases, the forward button ends up 15 to 18 inches in front the aft button. Of course that is full dependent on the size and length of the rocket

 

[bob jablonski]

I usually add a extra centering ring at the top of the motor tube then put the rail button into the top and bottom rings which adds a bunch of strength to the button mount.

 

[KenECoyote]

I usually add a extra centering ring at the top of the motor tube then put the rail button into the top and bottom rings which adds a bunch of strength to the button mount.

That's a very clever idea!

 

[Bat-mite]

I still prefer Teddy's method -- epoxy mixed with carbon fiber bubble over a pem nut. Just make sure you put Vaseline on the screw first, so you don't get epoxy inside the threaded hole.

 

[Jmhepworth]

I like to put the aft button a half inch or so from the aft end of the rocket and the forward button 1 caliber forward of the center of pressure. That makes it a good guide for a final stability check. Putting the forward button at the center of gravity is too indefinite for me, since the CG changes depending on the weight over the motor.

Joe

 

[RKeller]

I just received a rocket in the mail yesterday, which is already built, and in my opinion, the rail guides are installed in the wrong locations...

View attachment 271632

The guides are too far apart, and the aft guide is too far forward. I am debating removing the guides, and installing buttons in locations I would consider to be more beneficial to the over-all launch profile of the rocket; The first button near the aft, and the second at or near the cg. MY rule of thumb in some cases, the forward button ends up 15 to 18 inches in front the aft button. Of course that is full dependent on the size and length of the rocket

While those aren't where i would put them, they'll work just fine. What is the payback for all the effort to move them?

 

[KenECoyote]

I think some confusion is due to old learned experience on where launch lugs go; however lugs (like guides) are more stabilizing individually than buttons. In the above case, the previous owner may have replaced lugs with guides in the same positions there were previously.

However I would think that rail guides aren't so much a problem to have farther apart since they are more stabilizing individually than rail buttons, so the aft rail guide can still help to keep the rocket moving straight upwards (like a launch lug).

I've been split about placing the rail button on the CP vs. closer together for more time on the rail, but now I'm thinking how about a rail guide aft and a rail button on the CP?

 

[DRAGON64]

While those aren't where i would put them, they'll work just fine. What is the payback for all the effort to move them?

My peace of mind should I decide to remove them. I mounted the booster on a rail and gave a few tugs, but there was no movement of the guides. I can only assume the original owner built the rocket with JB Weld...

 

[Zebedee]

You could leave them and mount some more between a different set of fins. Saves risking damaging the tube getting them off. Not as pretty though.

 

[tbonerocketeer]

Orange crush flew on a p6000 at airfest, used 2 large rail buttons, one 1" from the bottom, and the other about 36" from the bottom. The rocket was 190lb on the pad and is 16 ft in length.

 

[watermelonman]

My peace of mind should I decide to remove them. I mounted the booster on a rail and gave a few tugs, but there was no movement of the guides. I can only assume the original owner built the rocket with JB Weld...

Dreamed and Bondo solves most problems.

 

[watermelonman]

Dremel, Dremel.

 

[jeff2space]

View attachment 271560

I'm sorry, I have a problem with your diagram on the left. First, as shown the top rail button is not centered on rail. That is why rocket is tilted. Second, if both rail bottoms were in rail and your rocket is at a different angle than the rail, then your buttons are offset to rocket's centerline. Why would placing buttons 2 inches apart or 20 inches apart change the angle on the rail as long as they are both on centerline of rocket's cross section.

 

[UhClem]

Why would placing buttons 2 inches apart or 20 inches apart change the angle on the rail if they are on the same line?

There is always going to be a little play in the fit between the buttons and the rail. If there were none, there would be a lot of friction.

Assuming 50 thousands of play, that would result in 1.43 degrees of slop with a two inch spacing and .14 degrees with 20 inches. Some might see that 1.43 degrees as excessive. Estimate the amount of play you have and how much angular error you can tolerate and you will know your minimum distance.

 

[jeff2space]

There is always going to be a little play in the fit between the buttons and the rail. If there were none, there would be a lot of friction.

Assuming 50 thousands of play, that would result in 1.43 degrees of slop with a two inch spacing and .14 degrees with 20 inches. Some might see that 1.43 degrees as excessive. Estimate the amount of play you have and how much angular error you can tolerate and you will know your minimum distance.

Good explanation, thanks.

 

[Bat-mite]

Sorry for the drawing. It was simply to illustrate the point. It was not to scale, and not plotted on a graph.