Question about rail button spacing

[qquake2k]

I'm working on a 3" scratch build. The body tube will be 34" long. I normally attach the rail buttons to blocks on the centering rings. But I'm planning on making the motor tube 19" long, with a centering ring at each end, so it will act like a stuffer tube. That means the rail buttons would be approximately 17-3/4" apart. Is there any problem with the rail buttons being so far apart?

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[Steve Shannon]

No.

 

[K'Tesh]

So long as the CG isn't too far away from the forward rail button, I'd guess it'd be ok.

 

[rms]

The rail guidance is only valid while the top button is engaged in the rail. By having the buttons far apart you are effectively shortening the rail. So long as your rail exit velocity is ok when the top button leaves the rail, you will be fine This of course is assuming only two buttons.

 

[Nathan]

So long as the CG isn't too far away from the forward rail button, I'd guess it'd be ok.

This subject was discussed in another thread a while back and my understanding is that CG location does not need to be taken into consideration when determining rail button location.

 

[KennB]

Could you add a third centering ring to move the upper button further aft?

 

[mccordmw]

I only follow two rules for positioning:

1. front lug/button is as close to the CG as possible.
2. after lug/button is as close to the bottom of the rocket as possible.

Distance between the two should not be a problem.

 

[dhbarr]

I only follow two rules for positioning:

1. front lug/button is as close to the CG as possible.
2. after lug/button is as close to the bottom of the rocket as possible.

Distance between the two should not be a problem.

I've been playing with this in my mind vs CP ( incl. body lift & AOA ), and this -feels- right as far as minimizing the force rail buttons have to apply to the rail ( and thus less tipoff / railwhip )

Has anyone done an actual study?

 

[qquake2k]

Could you add a third centering ring to move the upper button further aft?

That's what I've decided to do. It may not be necessary, but I'll feel better about it.

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[Bat-mite]

Like everything in life and rocketry, there is give and take, and no one right answer. The higher the top button, the closer to straight up the rocket will be on the rail. That's a plus. But the minus, as rms wrote, is that as soon as the top button is free of the rail, the rail is no longer guiding the rocket. So a high top button, while helping to straighten the rocket on the pad, also shortens the rail. As you can see in your sims, the shorter the rail, the greater the likelihood of a crooked launch.

If anyone out there has a scientifically valid reason for putting your top button at the CG, I'd like to hear it. Thanks.

 

[soopirV]

Like everything in life and rocketry, there is give and take, and no one right answer. The higher the top button, the closer to straight up the rocket will be on the rail. That's a plus. But the minus, as rms wrote, is that as soon as the top button is free of the rail, the rail is no longer guiding the rocket. So a high top button, while helping to straighten the rocket on the pad, also shortens the rail. As you can see in your sims, the shorter the rail, the greater the likelihood of a crooked launch.

If anyone out there has a scientifically valid reason for putting your top button at the CG, I'd like to hear it. Thanks.

It's far from scientific, but my understanding of the CG recommendation is simply that the CG is where the rocket will want to pivot, so if your buttons are far above and below, there's a greater chance for binding as the rocket tries to rotate around the CG. Right at (or near the CG) will minimize or eliminate any of that "levering", ensuring no binding.

 

[Bat-mite]

It's far from scientific, but my understanding of the CG recommendation is simply that the CG is where the rocket will want to pivot, so if your buttons are far above and below, there's a greater chance for binding as the rocket tries to rotate around the CG. Right at (or near the CG) will minimize or eliminate any of that "levering", ensuring no binding.

Absolutely no offense intended, and you may well be right, but I sort of think this is one of those things that sounds logical, but may be lacking in substance. Personally, I think the best solution is to put your buttons far apart, and use a longer rail. But since a longer rail is not always an option, that's probably not realistic. I've put my buttons in a wide variety of positions, and I have never had a launch failure. I guess we need a physicist!

 

[caveduck]

Not lacking in substance, but it's a little complicated and a fair amount of the discussion around the net is oversimplified or wrong. There are at least four sources of binding torque. 1) Crosswind on the rocket on the pad creates a force that is effectively applied at the center of pressure for 90 degree angle of attack (CP90). This is *way* forward of the conventional low-alpha CP and for all intents and purposes is at the center of area of the rocket (think of the old "cardboard cutout" method). If both rail guides are behind CP90 (often the case), you have a binding torque with a moment arm equal to the distance from CP90 back to the forward guide. 2) If the rocket is not vertical on the pad there may be an additional torque from gravity, with moment arm equal to the distance from the CG back to the forward guide. If the forward guide is about at the CG (and CP90 is usually not too far away), then both of those torques will be zero since the net forces will be applied right at a rail guide location. 3) There is a third torque arising from the fact that the center of thrust is offset by ~1/2 caliber from the rail guides, but there is not much we can do about that. 4) Lastly there is one more torque that arises if the rail guides are misaligned with (not parallel to) the center of thrust, which can be eliminated by proper construction. All of these are more pronounced when the guides are closer together.

So there really is physics justification for putting the forward guide near the CG and CP90. There's wiggle room in the engineering decision though - it can work to have the guides further aft, but then you have to be more careful about crosswind and tilt. For a hilarious instance of what can go wrong, see the famous "pad hopper" video on FB.

 

[FredA]

The rail guidance is only valid while the top button is engaged in the rail.

Not quite correct.
Rail buttons ONLY provide guidance when TWO buttons are engaged.
NOTHING WRONG with using three buttons.

Putting one higher up - especially if you can get one above the Cg & Cp - will help with torque on the buttons while sitting on the rail in a cross-wind.

Keeping the aft two buttons reasonably close to each other maximizes the amount of guidance from the rail.

 

[mccordmw]

I can't find the pad hopper video. Anyone have a link? Pleeeease? :grin:

 

[mccordmw]

The rail guidance is only valid while the top button is engaged in the rail.

Not quite correct.
Rail buttons ONLY provide guidance when TWO buttons are engaged.
NOTHING WRONG with using three buttons.

Putting one higher up - especially if you can get one above the Cg & Cp - will help with torque on the buttons while sitting on the rail in a cross-wind.

Keeping the aft two buttons reasonably close to each other maximizes the amount of guidance from the rail.

While having 3 buttons won't cause an issue. It does mean you have to be extra diligent in making sure all three are close to being in line. Otherwise, wouldn't you expect binding issues?

 

[qquake2k]

While having 3 buttons won't cause an issue. It does mean you have to be extra diligent in making sure all three are close to being in line. Otherwise, wouldn't you expect binding issues?

That's what I was thinking.

 

[soopirV]

Absolutely no offense intended, and you may well be right, but I sort of think this is one of those things that sounds logical, but may be lacking in substance. Personally, I think the best solution is to put your buttons far apart, and use a longer rail. But since a longer rail is not always an option, that's probably not realistic. I've put my buttons in a wide variety of positions, and I have never had a launch failure. I guess we need a physicist!

Mark Canepa's book is on Google, and has this to say on the matter- unfortunately, it's about lugs...



Further on, about lugs is offered the following, but nothing on relationship to CG...the search continues!

 

[Bat-mite]

Not lacking in substance, but it's a little complicated and a fair amount of the discussion around the net is oversimplified or wrong. There are at least four sources of binding torque. 1) Crosswind on the rocket on the pad creates a force that is effectively applied at the center of pressure for 90 degree angle of attack (CP90). This is *way* forward of the conventional low-alpha CP and for all intents and purposes is at the center of area of the rocket (think of the old "cardboard cutout" method). If both rail guides are behind CP90 (often the case), you have a binding torque with a moment arm equal to the distance from CP90 back to the forward guide. 2) If the rocket is not vertical on the pad there may be an additional torque from gravity, with moment arm equal to the distance from the CG back to the forward guide. If the forward guide is about at the CG (and CP90 is usually not too far away), then both of those torques will be zero since the net forces will be applied right at a rail guide location. 3) There is a third torque arising from the fact that the center of thrust is offset by ~1/2 caliber from the rail guides, but there is not much we can do about that. 4) Lastly there is one more torque that arises if the rail guides are misaligned with (not parallel to) the center of thrust, which can be eliminated by proper construction. All of these are more pronounced when the guides are closer together.

So there really is physics justification for putting the forward guide near the CG and CP90. There's wiggle room in the engineering decision though - it can work to have the guides further aft, but then you have to be more careful about crosswind and tilt. For a hilarious instance of what can go wrong, see the famous "pad hopper" video on FB.

Okay, but if it's that windy, you should be using a higher thrust motor. I guess what I'm saying is, if you are getting crosswind binding, you probably shouldn't be launching anyway.

 

[FredA]

While having 3 buttons won't cause an issue. It does mean you have to be extra diligent in making sure all three are close to being in line. Otherwise, wouldn't you expect binding issues?

Yes.....but so what....put three screws in a line.....not really that hard....
You already need to put two screws IN LINE with the axis of the rocket otherwise you'll make corkscrews coming off the flight.
Once you draw the line, drill a third hole.

Really - you can't drill three holes in a line????
Perhaps you need a "less crafty" hobby.....

Here is a hint - use a center punch.

 

[mccordmw]

While having 3 buttons won't cause an issue. It does mean you have to be extra diligent in making sure all three are close to being in line. Otherwise, wouldn't you expect binding issues?

Yes.....but so what....put three screws in a line.....not really that hard....
You already need to put two screws IN LINE with the axis of the rocket otherwise you'll make corkscrews coming off the flight.
Once you draw the line, drill a third hole.

Really - you can't drill three holes in a line????
Perhaps you need a "less crafty" hobby.....

Here is a hint - use a center punch.

I myself have no problem using angle irons, plumbs, spring-loaded center punches, etc to get nice, straight lines. However, there are builders here of all skill levels. This post is not in the HPR forum where people generally have a bit more experience. Wouldn't it be better use of this post to highlight the potential issues of 3 buttons (keeping it straight) as a reminder to less experienced builders vs demeaning others by recommending a less crafty hobby?

Newer builders are hit with a mountain of things to think about when building more advanced rockets. It's easy to not think about the considerations of a 3rd button if they're used to eyeing it when building the simpler LPR models with lugs not too far apart.

We all had to learn some time.

 

[FredA]

Maybe - but this forum is constantly aghast at the notion of using three buttons because they can't get them aligned.
Sorry if I ruffle some feathers, but it is NOT rocket science to draw a line and get three holes on the line....really....

 

[Bat-mite]

Maybe - but this forum is constantly aghast at the notion of using three buttons because they can't get them aligned.
Sorry if I ruffle some feathers, but it is NOT rocket science to draw a line and get three holes on the line....really....

I have no trouble drawing the line. But when I go to drill, being a round tube, my hole sometimes ends up 1/32" or 1/16" off center. I have a drill press, too, but sometimes the size and length of the airframe are prohibitive. It is definitely not the line drawing that is the trouble.

 

[Exactimator]

Qualifying that I’m not a degreed engineer:

I recall learning at some point that three bearing systems (for instance a solid shaft with three bearings) was a big engineering no-no. Since it’s nearly impossible to get the three bearings lined up perfectly, what ends happening is two of the bearings turn the shaft into a lever, creating force against the third bearing. After a while either the shaft or bearings will fail because of the added force.

I see three rail buttons as a three bearing system. Admittedly, there’s enough slop in a 1010 rail slot to accommodate a minor misalignment. But if they’re out of line enough, two of the buttons will turn the rail into a lever on the third and there will eventually be some sort of failure.

 

[FredA]

Center punch...then drill.

Plus - tolerance is not that tight on rail buttons. You can be off a bit and they will still slide on.

Frankly, off-axis rail buttons are more of a problem as your thrust will not be aligned with the guidance - so you really want any and all holes in-line.
Plus - if you really blow the placement of one of the holes, don't use it and fall back to two buttons.

Really blowing this out of proportion.

 

[Exactimator]

Really blowing this out of proportion.

Meh. Something nerdy to contemplate on a slow Friday. Plus, this is the place that rolled an argument over adhesives for several pages.

 

[Trident]

Run a strip of wood between centering rings at their outer edge. This will give you an anchor strip along the body tube wall. If you are concerned about strip weight, put another along the opposite side. Then draw a line along the tube, make sure the strip is aligned, and put rail buttons where you want. Also try using a sharp awl to punch your hole drilling location. You can be very accurate. Think of it as shoving a needle into the spot -- easy to locate the exact spot.

 

[Buckeye]

The rail guidance is only valid while the top button is engaged in the rail.

Not quite correct.
Rail buttons ONLY provide guidance when TWO buttons are engaged.
NOTHING WRONG with using three buttons.

Putting one higher up - especially if you can get one above the Cg & Cp - will help with torque on the buttons while sitting on the rail in a cross-wind.

Keeping the aft two buttons reasonably close to each other maximizes the amount of guidance from the rail.

Yes.

3 buttons is not hard, especially if the top one is in the ebay coupler or payload. Just rotate the airframes until they line up. There is wiggle room.

This too: https://www.rocketryforum.com/showthread.php?138235-Loading-rocket-on-an-angled-rod-rail

 

[OverTheTop]

As someone also mentioned earlier, my method is the upper lug as near to the CG as practicable, and the lower lug as far aft as practicable. No hard and fast rules, as some designs can't be done this way.

I think that when launching, after the upper lug at the CG has disengaged, the lower lug still provides some guidance (just not quite 100% like when the both are engaged). The sideforce from any wind will be pushing at around the CG point (depends on your stability margin). This is trying to make the rocket translate sideways, but it is pinned at the rear. The rotational moment is applied to a lever that is effectively pivoting from one end. This makes the rotational inertia (second moment of area) quite high and provides just a bit more stability until the bird is off the rail.

If you want to feel what I am talking about, grab something like a piece of broom handle by the middle with one hand and twist it backwards and forwards. Then grab one end and try the same thing. It is about four times the force required for the same accelerations IIRC. That's one reason why you are taught to ski by pivoting your skis at your feet, and not around the tips of the skis.

 

[Incongruent]

What is the justification for having the upper point at the CG?

Other than being a fair distance from the other button and the feel goodness of having significance to the placement, I don't think anyone has explained why the CG should be the location of the forward button.

I've heard that it's because being the CG, the rocket would naturally pivot there, but I can't recall why that would be better. (Probably less stress on and less bouncing around of the forward.)


As for the rear button at the side center of pressure, (CP90) I understand the reasoning behind the placement; the rail button forces the rocket to pivot around it, so ideally the force pushes the rocket at the pivot point and it doesn't pivot. (as much)

A related question, though not about rail buttons:
If the CG was at CP90 (and front CP was behind the pair), would weathercocking be reduced? Weathercocking is a result of crosswinds, which should push at the side CP, so if the rocket pivots where the force is, rotatin should be lessened and the rocket should translate instead.


Corrections welcome.