Minimum spacing for two rail buttons on a rocket

[Astrofox]

Recently I found myself in the possession of a LOC 163 kit as a practice rocket for tip-to-tip fiberglassing, and I started thinking about where/how I would attach the rail buttons as it is intended to be a minimum diameter kit. I got to thinking, maybe I could put two rail buttons about 3" apart from each other towards the top of the booster section, as I personally believe it would not only allow for plenty of space to exist for larger motors but would allow me to remove the zipperless booster from the bit of airframe that holds the parachute so that it can be cleaned/modified for future flights. Is 3" separation between rail buttons on such a rocket a sufficient distance, or should I look into other rail buttons or other means of attachment?

Thanks in advance,
Astrofox

 

[blackjack2564]

Just cut/grind screws short so they only go into airframe not inside of to. Then put them where they belong....one at the rear and other near the cg.

Or glue them on surface mount.

 

[o1d_dude]

“Railbuttons? What railbuttons? Don’t need no stinking railbuttons!”



I bought a flyaway guide from Additive Aerospace for my LaserLOC 163.

Not cheap ( bought the 12” long one ) but extremely well thought out and built.

Pretty standard for speed junkies and Mach monkeys.

 

[XrayLizard]

I wonder where the idea of one one being at the very bottom came from. I *guess* its ok since people do it.

 

[o1d_dude]

I wonder where the idea of one one being at the very bottom came from. I *guess* its ok since people do it.

Guessing the idea is to get maximum stability on the rail during the boost and acceleration.

Dunno.

It’s one of those rules of thumb we always hear about.

 

[XrayLizard]

yes Sir

 

[UhClem]

I wonder where the idea of one one being at the very bottom came from. I *guess* its ok since people do it.

Once the you are down to one button it isn't doing you any good at all. The further that last button is from the CP the more adverse yaw it will generate so the bottom of the rocket is the worst place for it.

The minimum distance is determined by how much play there is between buttons and rail. Combine that with button distance and you get the angular variability.

 

[JDcluster]

About 2-3 calibers seems to work.
I never place the buttons at the aft end. My first one goes above the fins, the second is north of the cg.

 

[Kelly]

I wonder where the idea of one one being at the very bottom came from. I *guess* its ok since people do it.

Just thinking out loud here, feedback welcome...

You want your buttons as low as possible, since the longer they stay on the rail the faster and more stable your rocket is when the buttons leave the rail. So, for any given spacing, that means you want to put the lower button all the way at the aft end.

The second button, you again want as rearward as possible for the same reason, but of course if it's too close to the first, the rocket can wobble in the slot, or can generate enough torque to shear off a button. So put the second button as rearward as possible to avoid these issues - maybe a couple calibers above the first?

I'm don't know why Cg or Cp would figure in to this. Neither has any impact on how the rocket moves, while both buttons are in the slot.

 

[jqavins]

Also just thinking out loud. That sounds like it makes sense, mostly. When the rocket is being put onto the rail and prepped, and is not upright - let's say it's horizontal for simplicity - the pull of gravity acts through the CG. If both buttons are below that then there will be a torque that the buttons have to resist, which can wind up putting lots of force on the higher of the two, trying to pull it out. So one at the CG is a good idea, or two that bracket it.

All this seems to make a good case for three buttons. One as low as practical, one some thing like "maybe a couple of calibers above the first?" for going straight during launch, and one at or above the CG for pre launch operations.

 

[Kelly]

If both buttons are below that then there will be a torque that the buttons have to resist, which can wind up putting lots of force on the higher of the two, trying to pull it out. So one at the CG is a good idea, or two that bracket it.

I can see where that could be a concern. My rockets, though, are light enough that I support them while putting them on the rail, and/or the buttons are secure enough that they can support the weight of the rocket. So I just worry about maximizing stability.