Rail Guide vs Button

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Harbinole

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Hi all,

First post! Sorry if this is posted elsewhere, but nothing jumped out when I searched.

Considering going with rail guides on my next 4” high powered build.

How durable are rail guides?

Something about them being epoxied from the outside as opposed to screwed in (like rail buttons) makes me uneasy. Most likely because I’ve never tried it.

Thoughts?



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Are you asking about launch lugs (tubes) vs. rail buttons?

A lug bonded to the airframe with epoxy could be stronger than a rail button that is just screwed in to the fiberglass tubing (vs. into a backing block or hardware bonded to the inside wall). Lots of people do that and it works fine. So if your question is really "lugs vs. buttons" I think the thing to consider is the rigidity of the rod vs. the rail.

A 6 or 8 foot aluminum rail for buttons is much more rigid than a large launch rod. I think that is the main reason why most people use buttons on high power rockets.

Apologize in advance if I misunderstood your question.
 
Are you asking about launch lugs (tubes) vs. rail buttons?

A lug bonded to the airframe with epoxy could be stronger than a rail button that is just screwed in to the fiberglass tubing (vs. into a backing block or hardware bonded to the inside wall). Lots of people do that and it works fine. So if your question is really "lugs vs. buttons" I think the thing to consider is the rigidity of the rod vs. the rail.

A 6 or 8 foot aluminum rail for buttons is much more rigid than a large launch rod. I think that is the main reason why most people use buttons on high power rockets.

Apologize in advance if I misunderstood your question.

Thanks for reply. Should have been more specific. I’m referring to these:

https://www.apogeerockets.com/Buil...for-4in-Tube?zenid=40vfe1e60mqf7eplu0p6lht3i6


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I believe that the OP was referring to a rail guide (think stretched rail button). I have used the pml rail guides on a couple of rockets...tolerances are rather tight and it takes 2 or 3 flights to loosen up a bit. it can be bothersome to load the rocket on the rail until they loosen up.
Rex
 
Thanks for reply. Should have been more specific. I’m referring to these:

https://www.apogeerockets.com/Buil...for-4in-Tube?zenid=40vfe1e60mqf7eplu0p6lht3i6


The correct name is conformal rail guide. There are printed plastic and aluminum versions depending on source. They tend to be long to afford more contact area for the epoxy to airframe bond. They are a bit more critical for alignment than rail buttons as they have a longer "contract patch" to the launch rail. Likely a but more resistance on the rail than buttons, but that would be difficult to measure without experiment.
 
Got it...

My comment about bonded lugs still applies. Those rail guides have a lot of surface area for the bond. I'd guess they are quite a bit stronger than a button screw in fiberglass without a backing blockif you prepare the surfaces properly and are using good epoxy.
 
I've never tried rail guides, and have no desire to. It seems to me that alignment could be an issue. I think rail buttons are more forgiving. If you get one slightly offline from the other, there's still little chance that they'll bind. And if you screw the rail buttons into something solid, there's little chance they could strip off. I epoxy blocks to the centering rings to give more solid mounting points.

https://www.rocketryforum.com/showthread.php?142491-Make-Tube-Fly-With-Fire!&p=1728724#post1728724

https://www.rocketryforum.com/showt...t-Thump-In-My-Down-quot&p=1687812#post1687812

https://www.rocketryforum.com/showt...t-Thump-In-My-Down-quot&p=1688039#post1688039
 
I also have never used rail guides but have seen a few folk who have and seems like they have had repeated issues with them falling off the rocket.
 

I have them on two PML kits - no problems with either.
You trade off broader surface contact area for epoxy to bind to the airframe, for risk of potential misalignment and additional friction with the rail rod.
I would be hesitant to rely on them with paper air-frames.

Personally, I think that a rail button with a flange is the best of all worlds:
https://www.apogeerockets.com/Build...ton_Standard?zenid=40vfe1e60mqf7eplu0p6lht3i6

a
 
I used rail guides from Apogee on my Level 1 rocket and have had no trouble. As I recall, I prepped the surface of both the BT and the rail guides with 80 grit and used rocket poxy to secure them. I too was worried about the friction between the guides and the rail, but a lightsanding of the rail guides took off stray bits of epoxy and smoothed out the edges of the rail guides just fine. Would rail buttons have worked? Sure, but with a cardboard BT I'm hesitate to make more holes than absolutely necessary.

That was my first high power build and I'm far from an expert builder. But, IMHO if you can attach launch lugs in a straight line, then rail guides aren't that different. I've launched this rocket at least 5 times on small to midrange H motors and the rail guides are solid. I'll be using the same rocket on a mid-sized J for my L2 in March.


NAR 103770
L1: H143 9/17 - Loc Nuke Pro Max "Orange Peel"
 
I have used both without issue. I have also learned to cut the guides in half. One half goes at the CG and one near the bottom. This reduces any binding, makes them less prominent, etc.
 
If you use rail guides, don't get paint on them. They work much better without the slight amount of added width x depth. A few .001 make a difference.

I personally prefer buttons.
 
If you use rail guides, don't get paint on them. They work much better without the slight amount of added width x depth. A few .001 make a difference.

I personally prefer buttons.

Good point, I forget to mention that I used masking tape to keep them clean after I poxied them on.


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I've never tried rail guides, and have no desire to. It seems to me that alignment could be an issue. I think rail buttons are more forgiving. If you get one slightly offline from the other, there's still little chance that they'll bind. And if you screw the rail buttons into something solid, there's little chance they could strip off. I epoxy blocks to the centering rings to give more solid mounting points.

Ah, but on a minimum diameter rocket there's between little and nothing to screw into. I believe that's the only place I've seen conformal guides used is on min diameter rockets. Surface prep and adhesive choice is imperative.
 
Likely a but more resistance on the rail than buttons, but that would be difficult to measure without experiment.

It would depend on the coefficient of friction of the rail guide or button material against aluminum and the normal force acting by rocket on lip of that rail. When it first begins to slide just as it breaks the static friction you have combined frictional force a static and a kinetic friction coefficient multiplied by mass of rocket multiplied by acceleration of gravity then static and kinetic results added. Then just a kinetic frictional force as its moving which is lower than a static frictional force. If the launch rail was angled then you'd have components of the force to deal with. Perpendicular to the launch rail the component of force acting as normal force is m*g*cos theta for use in friction force calculations similar to finding friction with blocks on inclined planes. The static and kinetic frictional coefficients come with some materials tables. Zirrconium metals had lower coefficients than other metals so they are used in hip implants and joints that move a lot for example. I think there were academic papers on banana peels on floors at one time too.

From like a college physics or high school physics class. I think a spring force gauge and even time trials of the rocket sliding down the rail a marked distances with some more algebra would be enough of an experiment. Had a lab with blocks sliding down a ramp like 6 years ago details seems fuzzy.

But if you feel it dragging more it probably has a higher coefficient of friction without the testing or math. Then there is drag force from the rail button or conformal guide itself in flight which will relate to the cross sectional area of it but the differences will be very slight compared to no rail guide/button at all.
 
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An advantage of rail buttons is they can be mounted further apart which may decrease binding of the rocket on the railing. For rockets over several pounds you can screw and epoxy the area yet still allow the roller to roll freely. Rollers can be replaced as they wear versus buying a new conformal rail guide section.

My opinion is theoretically a rail conformal guide has less drag force in flight but only slightly compared to buttons.

I've seen MD rockets under two pounds have rail buttons epoxied directly on. Designed team comp rockets without buttons or conformal. I've had that glue button tactic fail on a 3.5 pound rocket non MD with rocketpoxy but that may have been a mix error. Screwing the button in and a five minute epoxy was sufficient on non MD.

If you want to set a competition record for personal or team goals it will be worthwhile to build a tri or quad rail tower and forego the rail button or conformal guide completely by very slightly reducing drag force. It may be 10x more expensive than a rail button and not as affordable is the downside. Either money on kit pad or raw creative effort to build one.
 
Thank you everyone for your replies. Lot to consider for sure. While it feels the rails can work well I get the feeling like there is just a bit more risk with them.

I’m building my first 2 stage and with all the variables at hand, I really don’t want to add unnecessary risk (as small as it may be).

I’ll hold off and try them out on a future build where I can get a bit more familiar and comfortable with them.


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How heavy of rockets has anyone flown on conformal rail guides? That's what I don't know about.
 
I screwed up the trig function in my earlier post when rail was angled then edited. It was mass*gravity acceleration* cosine theta for normal force on angled scenario and not sine. Sine would be the force parallel to rail if rail is angled if you treat it like inclined plane block friction problem. And the frictional formulas always used the normal force even in components.

Normally a regular force vertical component is sin and a horizontal is cosine but this case was different.
 
Needed force perpendicular to surface object is sliding on.
 
It would depend on the coefficient of friction of the rail guide or button material against aluminum and the normal force acting by rocket on lip of that rail. When it first begins to slide just as it breaks the static friction you have combined frictional force a static and a kinetic friction coefficient multiplied by mass of rocket multiplied by acceleration of gravity then static and kinetic results added. Then just a kinetic frictional force as its moving which is lower than a static frictional force. If the launch rail was angled then you'd have components of the force to deal with. Perpendicular to the launch rail the component of force acting as normal force is m*g*cos theta for use in friction force calculations similar to finding friction with blocks on inclined planes. The static and kinetic frictional coefficients come with some materials tables. Zirrconium metals had lower coefficients than other metals so they are used in hip implants and joints that move a lot for example. I think there were academic papers on banana peels on floors at one time too.

From like a college physics or high school physics class. I think a spring force gauge and even time trials of the rocket sliding down the rail a marked distances with some more algebra would be enough of an experiment. Had a lab with blocks sliding down a ramp like 6 years ago details seems fuzzy.

But if you feel it dragging more it probably has a higher coefficient of friction without the testing or math. Then there is drag force from the rail button or conformal guide itself in flight which will relate to the cross sectional area of it but the differences will be very slight compared to no rail guide/button at all.

Not really. The normal force of the rail guide will be affected by gravity much less than by thrust. Thrust will cause the rocket to attempt to rotate about either the rocket’s center of gravity (when flying freely) or in this case about any counteracting forces, such as the friction from the guides. As the rocket tries to rotate about the aftmost railguide the forward end of the rocket will press harder into the outside of the guiderail. That normal force, times the dynamic coefficient of friction of aluminum on aluminum will determine the friction of the forward rail guide. The aft one will be pulling away from the rail, causing additional friction as the guide rubs against the inside of the rail. Gravity actually reduces the friction.


Steve Shannon
 
I've used aluminum Acme conformal rail guides on several rockets - off the top of my head, a 54mm fiberglass Wildman Jr., 3" paper airframe extended BSD 38 Special, and a 4" paper airframe modified BSD Horizon, ranging from 4 - 8 lbs on the pad. Properly prepped (cleaned, scuffed, etc.), epoxied with JB Weld, and aligned with a straightedge, they're just fine. Have I snapped some off when I wasn't careful loading on the rail? Yep, but rarely, and lesson learned.

Like most building techniques, they have their place, and work well in some cases, while rail buttons would be better in others.

Mark
 
Just to add to the shared experience / collective knowledge base on this topic: I've used Acme conformals without any problems on 2.2", 3" and 4" airframes with a range of motors H - K, the heaviest pad weight being just under 4 kg. I just use Devcon 30 min 'two ton' epoxy rather than JB weld. Scored in some cross hatching on the conformal mating surfaces with the tip of a needle file, scuffed the airframe, thoroughly degreased with isopropanol. I've used this on fibreboard, FG and CF wrapped airframes. With CF it's important that the aluminium doesn't come into contact with the actual carbon, but in the case of wraps the carbon is well below the surface sealed in by resin. To align: after making a pencil line I've used a wood ruler gently resting along the notches of both shoes. Built up masking tape on either side of the conformals will keep in place during cure. After the initial bond has cured I remove the tape and build up a small fillet around each shoe.
As per markjos's post, I can see that care/assistance is needed when sliding your vehicle onto the launch rail, to avoid any leverage that might twist the conformal off, although so far it hasn't happened to me (there, I've said it now..)
 
Not really. The normal force of the rail guide will be affected by gravity much less than by thrust. Thrust will cause the rocket to attempt to rotate about either the rocket’s center of gravity (when flying freely) or in this case about any counteracting forces, such as the friction from the guides. As the rocket tries to rotate about the aftmost railguide the forward end of the rocket will press harder into the outside of the guiderail. That normal force, times the dynamic coefficient of friction of aluminum on aluminum will determine the friction of the forward rail guide. The aft one will be pulling away from the rail, causing additional friction as the guide rubs against the inside of the rail. Gravity actually reduces the friction.


Steve Shannon
Thanks. I missed the torque acting on the rail guide as the thrust is off center from the rail and analyzed that one wrongly. Would there be a case were forward of the CG of rocket the tube could strike the rail creating a third frictional point near nose or top of Body tube due to offset thrust from rail position by rocket's center line of thrust and the rail guide height creating a moment?
 
In years past, it was feared that the ACME Conformal Rail Guides were galling the rails. I have used them on a number of rockets without the galling issue.
 
I could see a situation where a high enough thrust motor on a very long body tube with a short stubby sorta very aft mounted single conformal rail guide causing galling. The body tube itself would carry a torque onto the upper section of rail where body tube hits from the rail guide reaction force. Like a couple. Damn that's a high friction force with a lot of torque to gall aluminum.
 
The one bad thing I have experienced with the conformal rail guides is that if there is some wind and your rocket presents a good sized sail area (large fins, large body tube) it can press the guides against the side of the rail and increase drag considerably. This comes into play especially where the rails are not in the best of condition. I am primarily referring to the PML urethane guides.
 
I've used both, I prefer conformal rail guides mainly because they are so much easier to attach, just prep the surface and epoxy, no drilling, no adding blocks ect.
 
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