Rail Button location . . .

[eastvolt]

I know the general rule of thumb which is posted on many of a website is to place one or two railbuttons as far back on the booster section as possible, and another placed within one or two calibers behind the CG point.

However, looking at a wide arrange of L3 projects, i've seen the upper rail button placed both one to two calibers above and some below the CG point.

What are your thoughts on this placement around the CG?

 

[Chicagonative17]

I put them about 1 caliper above CG and one at the very bottom of rocket.

 

[jderimig]

It doesn't matter as long as the top one is far enough back to stay on the rail long enough to establish stability and both are securely attached to the airframe. A good place for the rear one is a mmt centering ring.

 

[SpartaChris]

I know the general rule of thumb which is posted on many of a website is to place one or two railbuttons as far back on the booster section as possible, and another placed within one or two calibers behind the CG point.

However, looking at a wide arrange of L3 projects, i've seen the upper rail button placed both one to two calibers above and some below the CG point.

What are your thoughts on this placement around the CG?

There is NO formal rule about rail button placement. None. The whole "Place the button around the CG" thing was made up, probably because of lack of a better answer at the time. Truth is that the buttons just need to be spaced far enough apart to support the rocket on the rail without it binding. That's it, that's all there is. I have never followed the CG rule and have never had a problem with any of my flights getting off the rail. I've always kept it simple and just installed the buttons into the forward most and aft most centering rings, or somewhere in their vicinity.

 

[uncle_vanya]

There is NO formal rule about rail button placement. None. The whole "Place the button around the CG" thing was made up, probably because of lack of a better answer at the time. Truth is that the buttons just need to be spaced far enough apart to support the rocket on the rail without it binding. That's it, that's all there is. I have never followed the CG rule and have never had a problem with any of my flights getting off the rail. I've always kept it simple and just installed the buttons into the forward most and aft most centering rings, or somewhere in their vicinity.

The only thing that anyone has said in my circle is that if the forward button isn't near the CG then when it comes off the rail if the rocket is not yet stable the rocket could pivot around the CG - but this seems to be a risk to me even if the button is near the CG.

 

[Larry_Pollack]

The only thing that anyone has said in my circle is that if the forward button isn't near the CG then when it comes off the rail if the rocket is not yet stable the rocket could pivot around the CG - but this seems to be a risk to me even if the button is near the CG.

Better to make sure that the rocket will be stable well before that.

As a couple people previously said, buttons should be placed so that the rocket is supported, they dont bind, and they are securly attached.

IMO those are the only thngs we should be making sure of.

 

[SpartaChris]

The only thing that anyone has said in my circle is that if the forward button isn't near the CG then when it comes off the rail if the rocket is not yet stable the rocket could pivot around the CG - but this seems to be a risk to me even if the button is near the CG.

Exactly. Rail button placement has nothing to do with stability once the rocket leaves the rail. The buttons are simply a guide to aid the rocket in vertical trajectory and nothing more.

Likewise, if the rocket isn't travelling fast enough for the fins to take over and guide it straight up, then you're under powered or using too small of a rail. And rail button placement isn't going to help that either.

 

[eastvolt]

Cool. Good information.

Thanks!

 

[brianc]

https://www.railbuttons.com/rb/installation.asp

 

[cjl]

If a rail button is placed near the Cg, there will be less force on it when the rocket is on the rail than if it is much farther back. This doesn't matter much other than with really big rockets, but it is a factor...

 

[ben]

I think each rocket has its own circumstances for rail buttons.

Ben

 

[SpartaChris]

If a rail button is placed near the Cg, there will be less force on it when the rocket is on the rail than if it is much farther back. This doesn't matter much other than with really big rockets, but it is a factor...

Less force on what exactly? Less force on the button because it is placed near the CG? I mean, sure, but only because the button is physically placed higher up on the rocket, so it leaves the rail first whereas the one below it is on the rail longer.

But again, I'll argue that the placement near the CG doesn't matter at all. On our Talon, we placed the buttons in the centering rings, well behind the CG. I did the same thing on my 8" Thor. Neither bird experienced any liftoff issues whatsoever.

 

[cjl]

Less force on what exactly? Less force on the button because it is placed near the CG? I mean, sure, but only because the button is physically placed higher up on the rocket, so it leaves the rail first whereas the one below it is on the rail longer.

But again, I'll argue that the placement near the CG doesn't matter at all. On our Talon, we placed the buttons in the centering rings, well behind the CG. I did the same thing on my 8" Thor. Neither bird experienced any liftoff issues whatsoever.

Less force on the button because of the reduced leverage effect when there is any twisting force on the rocket at all. Admittedly, this has nothing to do with the Cg, but it is true that the farther apart the rail buttons are, the less force is on them when any twisting force is applied to the rocket (not twisting along the rocket's axis, but along an axis parallel to the ground)

 

[Larry_Pollack]

Less force on the button because of the reduced leverage effect when there is any twisting force on the rocket at all. Admittedly, this has nothing to do with the Cg, but it is true that the farther apart the rail buttons are, the less force is on them when any twisting force is applied to the rocket (not twisting along the rocket's axis, but along an axis parallel to the ground)

This is a good point. It really goes to "will the rail button(s) stay on the rocket on the way up the rail, and what happens if it (they) dont"

Insetting them in the centering rings or installing a backing on the inside of the airframe gives you the best option for this.

What happens if it comes off the rocket, of course depends on when it comes off.

 

[jderimig]

Less force on the button because of the reduced leverage effect when there is any twisting force on the rocket at all. Admittedly, this has nothing to do with the Cg, but it is true that the farther apart the rail buttons are, the less force is on them when any twisting force is applied to the rocket (not twisting along the rocket's axis, but along an axis parallel to the ground)

If the twisting force is in danger of taking the rails buttons off heaven help you when the rocket clears the rail.

The largest force that rail buttons see is during the process of putting the rocket on the rail not during launching.

 

[Johnnie]

If the twisting force is in danger of taking the rails buttons off heaven help you when the rocket clears the rail.

The largest force that rail buttons see is during the process of putting the rocket on the rail not during launching.

...or waiting for the button to be pushed. If you fly in a stiff wind, waiting for launch could be fun if your buttons pop off. I had a set of ACME's pop once...grrr Rocket hit the dirt and scratched it all up. I hate that.

Lots of good info has been posted here, you'll just have to do what feels right for your peace of mind.

Johnnie

 

[bigone5500]

IMO if you place one at the aft CR and one at the top CR AND assuming the top CR is close enough to the CG, then you will be ok. My reason for saying this is that if the rocket is supported at or above the CG, then the weight of the upper part of the rocket will not 'pull' on the top rail button as much. Just a factor of leverage. I built my Expediter with a 24" MMT and put a button at 5.25" and at 16.25". It flew fine on an H242 and an I357.

 

[ben]

...or waiting for the button to be pushed. If you fly in a stiff wind, waiting for launch could be fun if your buttons pop off. I had a set of ACME's pop once...grrr Rocket hit the dirt and scratched it all up. I hate that.

Lots of good info has been posted here, you'll just have to do what feels right for your peace of mind.

Johnnie

or when a hydraulic launcher goes to vertical and has a small snap back. Saw a 10" Polecat Aerospace Nike Smoke fall right off the rail onto his dad!!! Onlny got his shoulder and nothing else thank god! Lesson learned, bolt, not glue buttons on!

Ben

 

[edwardw]

Most of my rockets get three railbuttons at these distances from the aft end, 2", 4" and 16". Has worked great for me for a long time. I've also used well nuts (expanding nuts) to secure them to the airframe and they work great. Even seen them used on very complex projects and they hold up. Best thing is if you break a railbutton you can just pull it out and put a new one in.

Edward