How far apart should rail buttons be?

Starfire73

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I have a LOC EZI-65, 4" diameter, 57" long.

I understand the bottom rail button should be just in front of the rear motor mount centering ring; however, where do I place the forward rail button? How many inches forward of the rear button should I locate it? What's the general wisdom on placement?

This is my first HPR and my first experience with buttons instead of launch lugs. This will be my level 1 cert rocket.

Thanks!
 

Titan II

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We frequently screw them into the rear and front CRs. I think your tube is 17". If your front CR is near the end, that will work well.

DS....the directions call for a launch lug, not buttons.
 
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Starfire73

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We frequently screw them into the rear and front CRs. I think your tube is 17". If your front CR is near the end, that will work well.

DS....the directions call for a launch lug, not buttons.
I asked LOC about how to install the buttons and they told me the same thing you did Titan II, but I thought I maybe misunderstood them. Do you actually try to screw the wood screw into the centering ring… like you actually try to hit the rings? So, you’re buttons will only be as far apart as your motor mount’s centering rings then?
 

Titan II

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I asked LOC about how to install the buttons and they told me the same thing you did Titan II, but I thought I maybe misunderstood them. Do you actually try to screw the wood screw into the centering ring… like you actually try to hit the rings? So, you’re buttons will only be as far apart as your motor mount’s centering rings then?

In this rocket, yes, if the rings are 1/4". It is nothing to hit the center of the rings.
 

David Schwantz

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If you are worried about them being to close you can always add a third button with a backing plate inside. I fly lots of 3 button rockets. With that being said, Buttons in both CRs will be just fine for this size and weight of rocket. And hitting the CR is easy. Just measure down, on the inside, to the top of your CR. Add half the thickness and mark on the outside of the tube. Drill pilot hole and screw button on.
 

crossfire

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I have a LOC EZI-65, 4" diameter, 57" long.

I understand the bottom rail button should be just in front of the rear motor mount centering ring; however, where do I place the forward rail button? How many inches forward of the rear button should I locate it? What's the general wisdom on placement?

This is my first HPR and my first experience with buttons instead of launch lugs. This will be my level 1 cert rocket.

Thanks!
Old rule of thumb put upper button close or just below the CG of the rocket.
 

gary7

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First of all, it would as usual be nice to have a more scientific reason for placement of rail buttons (or launch lugs or rail guides). I believe I have seen a paper on it before somewhere but could not find it for this thread.

For what it's worth, I have attached a screen shot of my upscaled Fliskits Tesla. See the details in the screen shot. Note also the CP, CG and rail button placement. The buttons were located out of necessity due to constructions techniques I used in the build. The rocket has flown fine without problems regarding the button placement.

 

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UhClem

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First of all, it would as usual be nice to have a more scientific reason for placement of rail buttons (or launch lugs or rail guides). I believe I have seen a paper on it before somewhere but could not find it for this thread.
 

llickteig1

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The simple fact is that rail button placement isn’t very sensitive. As long as one is near the bottom and the other is far enough above it to keep the rocket from turning before it gets to the end of the rail you’ll be fine.
This so true. I read the analysis paralysis regarding rail buttons and think to myself about how I have never seen even one of the tens of thousands of rockets I have seen fly fail due to poor choice of rail button placement.

Choose a convenient location on the outside of the rocket and go fly it.
 

Five

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Attach the button whenever it is convenient ( as you can see in the first picture). Just make sure the buttons don’t get caught on anything like the shock cord or chute. On my Super DX3 I had a screw sticking out which would have caused it to snag on something when the chute deployed so I just put some JB weld on it and smoothed it out.
 

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mtnmanak

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The simple fact is that rail button placement isn’t very sensitive. As long as one is near the bottom and the other is far enough above it to keep the rocket from turning before it gets to the end of the rail you’ll be fine.

This so true. I read the analysis paralysis regarding rail buttons and think to myself about how I have never seen even one of the tens of thousands of rockets I have seen fly fail due to poor choice of rail button placement.

Choose a convenient location on the outside of the rocket and go fly it.

These statements are both true. Far more important than the location of the buttons is the alignment and anchoring of the buttons. While I can't recall seeing any rockets go sideways because the buttons were too close or too far apart (although, that scenario has the potential to be concerning - you only have positive flight control when two or more buttons are in the rail), I have seen numerous times where buttons bind in the rail, even on high powered rockets. This seems to happen more with longer launch lugs (like the PML ones) that are misaligned, but I have seen it happen with buttons too. I have also seen at least two cases where one of the buttons came off in the rail (technically, one of them was again one of the PML launch lugs) and it caused the rocket to spin off in a crazy direction. Most likely, the rocketeer had poorly anchored the button/lug, then broke it off while loading the rocket in the rail, but it stuck well enough that they didn't notice it was broken until the motor ignited.

The one case where the distance between buttons could have a big impact is on rockets with really large fins launched in high wind conditions, especially if the rail is short and the velocity at the end of the rail is relatively low. If you find yourself in that situation, though, you probably have bigger problems on your hands than button placement... The most difficult decision in rocketry is knowing when to abort a launch.

Either way button placement is not rocket surgery.

So what’s the general consensus? Do most people attach the rail buttons before or after painting it?

Doesn't matter. I tend to install Rotaloc nuts in my high power rockets for buttons, so I can remove/change them, so I am not concerned about the sequence of painting. Whatever you do, don't paint the rail buttons. Painting the buttons also could contribute to them binding in the rail.
 
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NateB

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So what’s the general consensus? Do most people attach the rail buttons before or after painting it?

I drill the holes for buttons before painting and attach them after paint.

For what it's worth, I've been placing the top button at the CP as a way of marking the CP when asked.
 

Starfire73

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I drill the holes for buttons before painting and attach them after paint.

For what it's worth, I've been placing the top button at the CP as a way of marking the CP when asked.
That’s what I was planning. That way I don’t have to tape them off. I’ll just have some alcohol nearby when I expoy them in so I can clean up the already painted rocket.
 

FredA

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Remember - guidance ends when the penultimate button leaves the rail.
If you are using a 8-foot rail, with two buttons 4-feet apart, you get only 4-feet of guidance.
Make sure that's sufficient.

This is why I recommend three - two at the top and bottom of the fincan and one further up to help with wind-load while on the pad.
 

beeblebrox

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Think logically, before the rocket is stable from enough velocity, it could try to pivot, most likely around the CG. The rail buttons or launch lugs should then be separated and placed one above and one below the CG of the model. The above idea of a third one up higher is great for a long rocket. As for paint... I put a short piece of bamboo skewer into the threads of the button attachment piece. Remove after painting and install the button, unless you want the button the same color as the body. Paint certainly will not hurt the button, it will wear off a bit after a launch or two anyway.
 

Kelly

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it could try to pivot, most likely around the CG.

A rocket with two or more buttons on the rail will not pivot.
A rocket with one button on the rail will pivot about that button.
Only an unattached rocket will pivot about the CG.
 
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