Another rail button placement question (specific example)

[Wyatt]

Hi everyone,

Long time reader, first time poster, thank you in advance for your help!

I'm working on a 2.6" diameter scratch build, and wondering where to put the rail buttons (for a 1010 rail). I've read every previous thread on the subject that I could find, and come to the conclusion that there is no right answer regarding where to place them. I thought I'd post my current design idea, and see if anyone has experience to suggest that this might not work.

OpenRocket snapshot attached (electronic dual-deploy rocket, mass/CG are estimates), with the two rail buttons depicted on the aft section (they are about 6.5" apart). This placement represents the easiest option for me, as I can mount the buttons directly into the forward and aft centering rings (not depicted in that snapshot).

The question I have is: does anyone have practical experience to suggest that the buttons are too close to each other? Let me know if you need more details regarding the rocket design.

If the consensus is that the buttons are too close, I think I'll have to find a way to surface mount the forward button further up on the airframe, so as to not interfere with the drogue compartment? Also, since the airframe separates between the two aft-most sections, I'll need to find a way to ensure buttons remain longitudinally aligned (shear pins?).

Thank you!
-Wyatt

 

[heada]

I place one as far aft as possible, normally screwed into the aft centering ring. The other at the loaded CG.

 

[K'Tesh]

Like heada, I like to keep my forward button somewhere around CG, and the other as aft as practicable. My feelings are that if there's any kind of sudden wind gust, that this will reduce binding/friction on the rail. Then again, if you are facing wind gusts like that, you should be thinking about flying kites instead.

Oh, and... Welcome to TRF!

 

[Voyager1]

I agree with the advice you have received. If you purchase the rail buttons with the mounting flange you can easily install these securely without interfering with the drogue chute.
https://www.apogeerockets.com/Building_Supplies/Rail_Buttons/1in_1010_Rail_Button_Standard

 

[Kelly]

Like heada, I like to keep my forward button somewhere around CG, and the other as aft as practicable. My feelings are that if there's any kind of sudden wind gust, that this will reduce binding/friction on the rail.

I think if you do a free body diagram on a rocket, on a rail, you will see that the CG doesn't really enter into the equation, since the rocket is not free to rotate about it. If you have two buttons on the rail, then the total side force will be equal to the force exerted by the wind. The distribution of the this force - how much is borne by each button - will be determined by the placement of the buttons relative to the Cp.

• For instance, if both are equidistant from the Cp, both will be pressed against the rail with equal force.
• If one is at the Cp, and the other is - well, anywhere - then the entire force will be seen at the button on the Cp.
• If they are both below the Cp - as Wyatt shows - then the top button will be pressed against the rail (on the side opposite the wind) with a force GREATER than the force applied by the wind, and the lower button will be pressed against the rail on the windward side, to counterbalance the torque. (So, now you actually have greater friction, since you have both buttons being pushed in opposite directions. The sum of the absolute value of the forces (which determines friction) is now GREATER than the force applied by the wind, though the vector sum of these forces is still equal to the force applied by the wind.)

But, nowhere does Cg enter into this discussion.

So, to minimize sidewards force, your buttons should straddle the Cp, or have one button right on the Cp. (Note that this Cp is the Cp seen when the aerodynamic forces come directly from the side, which may be different than what the simulation shows. The simulation cares about the Cp when the rocket is heading straight into the wind, or at a minor angle.)

But to answer Wyatt's question, I think the placement is fine. Buttons should be as low as possible, but far enough apart to prevent excessive torque. This doesn't minimize rail rubbing, but as K'tesh said, if that's really the major concern you probably shouldn't be flying.

(Sorry for the ramble, I've just been doing some thinking about "ideal" button placement, and this discussion helped me.)

 

[Bat-mite]

As soon as the first button exits the rail, both gravity and aerodynamics start to become effective on the rocket. As others have said, you want the aft button as far back as you can put it. The forward button is the one everyone disagrees about. I look at it like this:

1. The farther apart the buttons are, the less tilt you get; however
2. The farther forward the forward button is, the sooner it exits the rail; therefore
3. Put them as close together as you can, as far aft as you can, without getting a ton of wiggle while it is on the rail.

Good luck to you! BTW, the reason that this is mostly insignificant is that the rocket rides the rail for a fraction of a second, and then CG and CP take over, and even if you found the magic, perfect rail button spots, the rocket still won't go straight unless it is perfectly stable throughout the flight.

 

[UhClem]

(Sorry for the ramble, I've just been doing some thinking about "ideal" button placement, and this discussion helped me.)

While your discussion of torques and the effect on rail friction is fine, it misses some things.

When loading the rocket onto the rail, the same torque problem arises if the buttons aren't located near the CG. Hopefully they are mounted well enough so this isn't a serious problem.

When leaving the rail, the aft button is useless (and is in fact worse than useless) once it is the only one. Two things happen. The first is that if the rail isn't perfectly vertical, the rocket mass will cause it to rotate. The second is that the wind forces at the CP will cause the rocket to rotate. The further that aft button is from the CP and CG the greater the moment arm resulting in more torque and the longer that torque acts on the rocket. If you don't care about this (and simulation programs certainly don't) then put the buttons where you want.

Simulation programs act as though the buttons use a simultaneous release system which is great if you have one. Otherwise they miss that aspect of launch dynamics.

In the case of this particular design, I have several rockets built along the same lines that have one button on the lower motor section (as high as possible) and one just below the electronics bay. (To keep it out of the way of the drogue parachute.)

 

[Kelly]

When loading the rocket onto the rail, the same torque problem arises

True. I always forget about 'loading' issues; my rockets are small enough I can just be careful enough to avoid this problem. The 'torque' I mentioned is more related to what John called 'wiggle': If the buttons are very close, a small wind force on the rocket can wiggle it significantly out of alignment, as well as posibly damaging the rocket.

When leaving the rail, the aft button is useless (and is in fact worse than useless) once it is the only one. Two things happen. The first is that if the rail isn't perfectly vertical, the rocket mass will cause it to rotate. The second is that the wind forces at the CP will cause the rocket to rotate. The further that aft button is from the CP and CG the greater the moment arm resulting in more torque and the longer that torque acts on the rocket. If you don't care about this (and simulation programs certainly don't) then put the buttons where you want.

I still don't understand how the Cg enters much into this. The wind forces will act on the Cp, and the rotation point will be the button. I suppose if you want to consider dynamic effects - for example how the rotational inertia affects the tendency to rotate - then Cg matters.
But the main point is that as soon as your first button leaves the rail, neither button is doing you any good, which is why I say put it as far back as reasonably possible.

 

[David Schwantz]

3 buttons. Done it many times.

 

[Voyager1]

I think if you’re worried about what the rocket will do when the forward button leaves the rail, then either the rail isn’t long enough and launch velocity isn’t sufficient for stable flight, or the wind is too strong for a safe launch, or all the above!

IMHO having a suitable spacing of the buttons will improve the stability as it accelerates up the rail. If they’re too close, and too far back from the CoG, then there’s the possibility of the rocket being less stable before any buttons leave the rail.

 

[Wyatt]

Thanks for all the replies! Based on the responses, I'm going to give the rocket a try with the rail buttons as shown in my initial post. If the rocket has any issues leaving the rail, I'll try moving the forward button further forward. I'll report back if that happens.

 

[Rocketclar]

Since I have a number of rockets that use piston ejection (both PML and scratch builds with FG pistons), I 'surface' mount my rail buttons or use rail guides since I don't want to interfere with the internal piston. The way I 'surface' mount them is using a small block of basswood epoxied to the AF and then epoxy a small screw with the rail button or rail guide. Granted more drag, but I'm not after altitude records. I've also used rail guides, both the polymer and Al Acme guides.