Button placement?

Hey guys!

Quick question here.

How do y'all decide to place buttons?

The first one I buttoned, I put where the lug was supposta be.
Second one I put nimbly bimbly in a straight line.

But, what I wanna know is, what's the "magic spot" for button placement?

Thanks!
Mitch

I usually put the lower one just above or into the bottom centering ring. The upper is usually around 2.5 calibers further up.

bottom one 1"-2" from rear upper one depends on how long rocket is. 54 mm rocket 80'' long Im not going to do 2.5 caliper maybe 10 caliper or more.

I usually put one in the aft centering ring. The other I try to put one caliper forward of the center of pressure as an aid to measuring stability. Generally, you want it far enough forward so it keeps the rocket straight, but not too far. Once the first button leaves the rail, the second one isn't doing anything.

Joe


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Jmhepworth said:

I usually put one in the aft centering ring. The other I try to put one caliper forward of the center of pressure as an aid to measuring stability. Generally, you want it far enough forward so it keeps the rocket straight, but not too far. Once the first button leaves the rail, the second one isn't doing anything.

Joe


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Click to expand...

+1
The second guide should be just below the CG.....

Teddy

Reading the first few responses, I am thinking I messed up on the nimbly bimbly one lol.

It's a basic Estes Tomahawk kit and I put the aft button about two inches above the plastic fin can and the forward button about 6 inches above that.

I didn't wanna drill into the plastic, but maybe I should and take the forward one off?

Also, I've never heard the caliper term in this form yet, what's that?
Circumference on a linear plane?

Picture for reference -

okay folks watch your spelling please. in this instance the term is 1 caliber = body tube diameter. I've heard that the aft guide should be roughly 1" from the aft end of the tube, and the forward guide about 1/2 between cg and cp. with buttons and a large rocket you want something sturdy to attach to and yet not interfere with the recovery system...centering rings and or baffles generally provide both. FWITW I've used lugs of 3" diameter rockets (mid power), for H impulse and up, rail guides (PML has some really nifty guides that can attach like a lug, but are for rails).
Rex

Not spelling mistake. Caliber is the internal measure of the diameter of a tube. Caliper is the tool used to measure the outside diameter of a tube. In the nursery business, a 3 inch caliper tree has a 3" diameter trunk. On a body tube, one caliper is the outside diameter of the tube. One caliber is a bit less than that, because the thickness of the tube is not included in the measurement. So both are correct.


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I have a little different take. I put one as close to the bottom of the rocket as practical. Then I get another in front of the fins, which in many rockets would be near the Cp. I want the rocket to be actively guided as long as possible, and to get going as fast as possible while guided.

On a couple larger rockets I actually have three rail buttons. Some folks think that is voodoo. I might agree if somebody was not careful in aligning the buttons. I have my own pad and rail so I make sure that they are aligned.


Mark Koelsch
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In rocketry parlance, a "caliber" is a reference to rocket diameter. A "caliper" is a tool to measure diameters, either inner or outer.

Anyway, to maximize active guidance, 2 buttons should be placed as far rearward as practical. Cp/CG relationships don't really mean anything for button spacing. If your rail is tilted, and your rocket is top heavy, then a button near the CG is helpful to carry some of the static load and reduce the moment placed on the aft buttons. I use a third button in this case.

Agreed, the term used in rocketry is "caliber" and is the outside diameter.

I also put one button as low as possible, usually screwed into the aft CR, or a block glued to the aft CR.

I put the upper button as high as possible without causing interference with recovery parts, pistons, etc. I have one rocket where the upper 1010 button is near the upper CR and I think it is too low. I've had several instances where it made hard turns into the wind on high thrust motors. Almost like rod whip, which I'm thinking the low placement of the upper button may be contributing to.

The guidance length that your rail provides is the distance from the top rail button on your rocket as it sits on the rail to the top of the rail.

Beyond, it pays to have the lowest rail button a close to the aft end of the rocket as practical, and the forward button at the CG of the rocket loaded with motor you are most likely to use.

The reason why you want to put the forward button at the CG is that the rocket will rotate around that point, and with the aft button located as close to the aft end of the rocket as reasonable possible. This provides the maximum torque to keep the rocket pointed in the right direction while on the rail.

Bob