Rail Button Locations

[bytesiz]

Quick question... The forward rail button should be located at the CG and the aft button most anywhere aft of the CG. Sound about about right?

 

[JDcluster]

I mount the aft button near the CP & the forward button above the cg. I try to keep them about 8-18" apart depending on the size of the rocket.

Some people like mounting the aft button on the very bottom of the rocket. I feel this is the wrong approach; if it binds & gets hung up it will affect the launch angle more then if it were mounted above the fins.



JD

 

[MarkII]

Once the upper rail button is clear of the rail, the rocket no longer has rail guidance, right? (The actual guide distance is the distance from the upper button to the top of the rail.) One button alone cannot keep the rocket traveling parallel to the rail. Although rail buttons are usually sold in pairs, I have read that using three is more effective: one up on the airframe, and two near the aft end, separated by a short distance. I haven't tried this arrangement yet, but it kind of makes sense. After the upper button clears, the rocket still has the two near the aft end to provide guidance. The aft buttons continue to be functional until almost all of the rocket has cleared the rail, increasing the effective guide distance. You get to use much more of the rail with this arrangement.

 

[rstaff3]

This is a subject that seems to come up a few times a year and usually gets a wide range of opinions. I usually put my lower button as low as I can. The upper goes as close to the CP as possible but I opt for abutting a centering ring over that criteria. I have mounted both near the middle of a rocket around the CP/CG. So far, the only button problem I've had is when I mounted them in a foam pool noodle. :y: The rocket flew fine but the buttons were both MIA.

The statement about the guidance from the rail disappearing when the upper button leaves the rail is true, but if the rocket is stable at that point then you should be OK. Luckily, rails are usually nice and long.

 

[JDcluster]

If it's not up to speed when leaving the rail you'll still be in trouble no matter where you put the buttons......

By placing them at the very bottom & it binds the rocket can vector further from vertical than if it were above the fins.




JD

Once the upper rail button is clear of the rail, the rocket no longer has rail guidance, right? (The actual guide distance is the distance from the upper button to the top of the rail.) One button alone cannot keep the rocket traveling parallel to the rail. Although rail buttons are usually sold in pairs, I have read that using three is more effective: one up on the airframe, and two near the aft end, separated by a short distance. I haven't tried this arrangement yet, but it kind of makes sense. After the upper button clears, the rocket still has the two near the aft end to provide guidance. The aft buttons continue to be functional until almost all of the rocket has cleared the rail, increasing the effective guide distance. You get to use much more of the rail with this arrangement.

 

[MarkII]

If it's not up to speed when leaving the rail you'll still be in trouble no matter where you put the buttons......

By placing them at the very bottom & it binds the rocket can vector further from vertical than if it were above the fins.




JD

I wasn't talking about putting both of the lower buttons right at the aft end of the rocket. On a 5 foot long rocket, for instance, the lowest button would be just above the aft end, the middle button would be at approximately the CP location, and the forward button would be at or just forward of the CG location. If you are at a launch and find out that the the only pad that you can use for your 5 foot rocket has a 6 foot rail, wouldn't you feel a bit more comfortable if you knew that your rail buttons could take advantage of nearly all of that length for guidance? Your statement about the rocket being in trouble if it is not up to speed by the time it leaves the rail is true of any rocket, whether it uses a rail or not, right down to micro-sized rockets. The statement does add support to the idea of installing a third button, though.

BTW, I didn't invent this idea. I have encountered it in a few places on the web in the past few years, one of which was Matt Stum's railbuttons.com site.

Of course you want to have the rocket be up to speed when the forward button clears the top of the rail; adding the third button isn't meant to render it useless or redundant. The third button is meant to keep the aft button from becoming useless once the forward button is clear. It is an option that can be installed for a little extra insurance. I mentioned it as a suggestion, not a commandment. Consider it or dismiss it; I don't care. I don't have any stake in how you or anyone else builds their rocket.

 

[ClayD]

I mount mine aft of the cg. Thrust from the motor pushing on the cg, means that in trying to keep the rocket going "up" you want to keep the trust from having a moment arm on the cg. If they are above the cg, or the aft button is too far forward, you could see more "work" on the button itself. The more work you see at the button, the more rod whip you are likely to see(maybe- a 6 pound rocket on 6' 10-10 rail should be nearly imposible to whip.) CP doesnt come to play till about half the way up the rod when aerodynamic stability occurs. -"idealy"- you are already pointed in the desired direction, and the buttons are irrelevant. Rod whip at this point could send the rocket in an undesired direction. Binding, after the cp takes control, should never! happen. That would be a no-no on the flyers part, of an unstable rocket.(trying to change direction on the rail.) Both the rod and the rockets direction should have been established to be in the same, and continue without effort on the rockets or the rods part.

I would not worry about button placement at all unless you are flying research(hard to start-unproven motors), Low-velocity rockets, heavy rockets, even airstarts where the motors might ignite in the first 12' feet.

All my rail buttons go into the centering rings. I drill in with a 1/8" hole, and just screw the 8-32 treads in. No epoxy, nothing... (on a g10 airframe with birch centering rings.) with paper tubes, I may put a brass wood insert in. I use the conformal rail lugs to, and they work great on 4" and smaller.

The further aft, the less chance of rod whip!

 

[bobkrech]

For most rockets the CG and CP are not deciding factors. You need both buttons to be engaged in the rail for guidance, so you want the button near the aft end of the rocket ot provide maximum guidance. The lower button should be a few inches forward of the aft end of the rocket, and the forward button about 3 +/- 1 airframe diameters ahead of the aft button, to provide the maximum guidance length.

Bob

 

[cbrarick]

The only rocket I have 3 buttons on is my Ultimate Wildman. The only reason for the third button is that when the rocket is raised on hydraulic towers it wants to pop off, it's so long! Once it's up, the third button doesn't do anything much! The bad news is that that third button adds significant complexity trying to align up all three.

 

[Pantherjon]

I do mine the simple way...Lower rail button into the aft centering ring and forward button into the forward centering ring..All of my HP rockets are set up this way and travel up and off the rail no problemo!Including the monster Ultimate Wildman.

 

[AndyC]

Like Jon, I just try to hit the top & bottom rings, unless they are really close together (rare). Sometimes I'll glue a ~1/2" square of plywood to the inside of the tube just on top of the rings if they are thinner than 3/8", since my aim with the drill never seems to be as good as I expect. In 2 rockets I use a third button: these are long rockets - usually in the ebay coupler where it can be twisted to align as it goes on the rail. Its useful to support the rocket while loading and for those somewhat more breezy days.

 

[Buckeye]

I also like 3. Two in the centering rings and one in the avbay. A long rocket with heavy avbay on an angled rail can put torque on the bottom 2 buttons. The 3rd button fixes that.

The lower button should be a few inches forward of the aft end of the rocket, and the forward button about 3 +/- 1 airframe diameters ahead of the aft button, to provide the maximum guidance length.

What is the significance of 3 +/- 1 airframe diameter for guidance?

 

[ClayD]

I also like 3. Two in the centering rings and one in the avbay. A long rocket with heavy avbay on an angled rail can put torque on the bottom 2 buttons. The 3rd button fixes that.



What is the significance of 3 +/- 1 airframe diameter for guidance?

anything that has a 60% variance has no significance.

 

[Buckeye]

Exactly my point. 3 +/- 1 airframe diameter spacing for buttons sounds rather meaningless.

 

[bobkrech]

I also like 3. Two in the centering rings and one in the avbay. A long rocket with heavy avbay on an angled rail can put torque on the bottom 2 buttons. The 3rd button fixes that.

What is the significance of 3 +/- 1 airframe diameter for guidance?

Many aerodynamic and mechanical properties depend on the length to diameter ratio of an object. For example, the stability of a rocket is measured in calibers, or diameters, between the CG and CP of the rocket. Most designer favor stability margins around 2 with variations ranging for as low as 1 to has as high as 4. Rockets with stability margins less than 1 are not very stable and rockets with stability margins that are very high are over-stable.

The buttons should be as low as practical so the rocket is guided by the rail for the longest possible distance. You can express the location and separation between the buttons in caibers as well, thus on a 2" rocket a separation distance of 6" is 3 calibers where as on a 4" rocket 12" is 3 calibers. Certainly on rockets of 6" diameter or smaller, the torque load measured in foot-pounds is readily carried by the airframe alone and a well nut inserted into a drilled hole is more than sufficient to secure a rail button to the airframe.

Bob

 

[Buckeye]

Bob:

I understand the definition of calibers and how it is applied to stability, since it is based on cp/cg relationships. However, I don't see how 33% variation in non-dimensional L/D defines "maximum guidance length" without knowing the other factors. Is there some data from which this rule of thumb is derived? Furthermore, in your first post, you combined "a few inches" and "airframe diameters" to place the buttons, which is inconsistent and confusing.

I am not concerned about pulling the buttons out of the airframe. Rather, the torque on the bottom 2 buttons (if too close together) can bind them in the rail and put the rocket off-line (not parallel to the rail). There is enough slop between button and rail for this to happen. That's why I like 3 buttons in this scenario, with the cg factoring into the 3rd button placement to properly carry the load when the rail is angled.

 

[berlinetta]

I mounted my rail buttons on my Big Nuke and Big Nuke 3E into small blocks of wood with t-nuts epoxied into them. I mounted the bottom button onto the aft-most centering ring, and the top button on the second to top-most ring, as I do not want to risk my recovery system getting snagged. You can see how I did my Big Nuke 3E buttons below. I don't remember the distance between the two on the Big Nuke 3E, but they are 16 3/4" apart on the Big Nuke. I am a little concerned as the Big Nuke Nuke is 7.93 feet tall and weighs 9 pounds 7 ounces empty. Will the buttons hold the Big Nuke on the rail? I am sure the Big Nuke 3E will be fine as it is also 7.93 feet tall (no final empty weight yet). Below is a picture of the Big Nuke 3E's fin unit before it was mounted into the booster tube. you can see the rail button mounts on the centering rings. The holes line up perfectly.

 

[ClayD]

I mounted my rail buttons on my Big Nuke and Big Nuke 3E into small blocks of wood with t-nuts epoxied into them. I mounted the bottom button onto the aft-most centering ring, and the top button on the second to top-most ring, as I do not want to risk my recovery system getting snagged. You can see how I did my Big Nuke 3E buttons below. I don't remember the distance between the two on the Big Nuke 3E, but they are 16 3/4" apart on the Big Nuke. I am a little concerned as the Big Nuke Nuke is 7.93 feet tall and weighs 9 pounds 7 ounces empty. Will the buttons hold the Big Nuke on the rail? I am sure the Big Nuke 3E will be fine as it is also 7.93 feet tall (no final empty weight yet). Below is a picture of the Big Nuke 3E's fin unit before it was mounted into the booster tube. you can see the rail button mounts on the centering rings. The holes line up perfectly.

Looks great, you should have no problems. I have never seen recovery "Snagg" on buttons, but I guess anything is possible...

 

[bobkrech]

Bob:

I understand the definition of calibers and how it is applied to stability, since it is based on cp/cg relationships. However, I don't see how 33% variation in non-dimensional L/D defines "maximum guidance length" without knowing the other factors. Is there some data from which this rule of thumb is derived? Furthermore, in your first post, you combined "a few inches" and "airframe diameters" to place the buttons, which is inconsistent and confusing.

I am not concerned about pulling the buttons out of the airframe. Rather, the torque on the bottom 2 buttons (if too close together) can bind them in the rail and put the rocket off-line (not parallel to the rail). There is enough slop between button and rail for this to happen. That's why I like 3 buttons in this scenario, with the cg factoring into the 3rd button placement to properly carry the load when the rail is angled.

I'll try to make it really simple.

1.) You only need 2 rail buttons to keep your rocket on the rail. You may add a third further forward to stabilize a long rocket or to support the rocket when you raise the rail, but this third button does little to aid the rail guidance, and can get in the way of recovery system deployment.

2.) You want to mount the lower button a few inches above the aft end of the rocket so it won't catch the launch clips.

3.) The separation between the lower lug to the upper lug should be be not less than 2 and doesn't need to be more than 4 calibers apart.

4.) If it's closer than 2 calibers, you're allowing the rocket to twist excessively and putting more torque on the lugs than necessary.

5.) If it's more 4 calibers, you're throwing away guidance length.

6.) These suggestions are not rules. You are free to do whatever you want. These are common sense suggestions based on the CG/CP relationships in most rockets, to minimize clip entanglement, and maximize guidance length.

Bob

 

[Buckeye]

I am not seeking your permission or advice on buttons, and your smarty-pants attitude to "make it really simple" is not helping the discussion. I merely asked for the theory behind your statement to use 2 to 4 non-dimensional calibers to place rail buttons for maximum guidance length (dimensional) with no other knowledge about the rocket size, mass, or launch pad. Is this guideline based on data and scientific study, or is it anecdotal? I am guessing the latter, based on your responses to date, and that is fine. I just want to understand the background, since I have not seen a Barrowman-like investigation of rail buttons. I am a big boy with two engineering degrees. I can handle it.

For most rockets the CG and CP are not deciding factors.

These are common sense suggestions based on the CG/CP relationships in most rockets,

So which is it? Rail button spacing uses CP/CG or not?

 

[Skywriter]

I'll try to make it really simple.

1.) You only need 2 rail buttons to keep your rocket on the rail. You may add a third further forward to stabilize a long rocket or to support the rocket when you raise the rail, but this third button does little to aid the rail guidance, and can get in the way of recovery system deployment.

2.) You want to mount the lower button a few inches above the aft end of the rocket so it won't catch the launch clips.

3.) The separation between the lower lug to the upper lug should be be not less than 2 and doesn't need to be more than 4 calibers apart.

4.) If it's closer than 2 calibers, you're allowing the rocket to twist excessively and putting more torque on the lugs than necessary.

5.) If it's more 4 calibers, you're throwing away guidance length.

6.) These suggestions are not rules. You are free to do whatever you want. These are common sense suggestions based on the CG/CP relationships in most rockets, to minimize clip entanglement, and maximize guidance length.

Bob

Yeah, I know, this is an old post but what the heck. Time to add my two cents.
I've seen several rockets cock at just two buttons. When the first button comes off the rail the lower button does not provide any lateral stability; instead, the rocket is dependent on velocity to keep it stable. If you use rail guides instead, it solves this problem; however, I haven't had much luck getting rail guides to stay on heavy rockets when mounting them. Three buttons or a combination of buttons and rail guides would also solve both problems. However, if the buttons/rail guides are not PERFECTLY aligned they will cause binding to occur. Maybe there is a better way of mounting rail guides?

 

[Steve Shannon]

Yeah, I know, this is an old post but what the heck. Time to add my two cents.
I've seen several rockets cock at just two buttons. When the first button comes off the rail the lower button does not provide any lateral stability; instead, the rocket is dependent on velocity to keep it stable. If you use rail guides instead, it solves this problem; however, I haven't had much luck getting rail guides to stay on heavy rockets when mounting them. Three buttons or a combination of buttons and rail guides would also solve both problems. However, if the buttons/rail guides are not PERFECTLY aligned they will cause binding to occur. Maybe there is a better way of mounting rail guides?

Just mount two buttons where the bottom two would be when you mount three. [emoji846]

 

[Bat-mite]

If an inch here or there of rail button placement causes noticeably different flight characteristics, then you probably aren't using a motor with enough thrust.

 

[Nathan]

I'll try to make it really simple.

1.) You only need 2 rail buttons to keep your rocket on the rail. You may add a third further forward to stabilize a long rocket or to support the rocket when you raise the rail, but this third button does little to aid the rail guidance, and can get in the way of recovery system deployment.

2.) You want to mount the lower button a few inches above the aft end of the rocket so it won't catch the launch clips.

3.) The separation between the lower lug to the upper lug should be be not less than 2 and doesn't need to be more than 4 calibers apart.

4.) If it's closer than 2 calibers, you're allowing the rocket to twist excessively and putting more torque on the lugs than necessary.

5.) If it's more 4 calibers, you're throwing away guidance length.

6.) These suggestions are not rules. You are free to do whatever you want. These are common sense suggestions based on the CG/CP relationships in most rockets, to minimize clip entanglement, and maximize guidance length.

Bob

+1 What Bob said. ↑↑↑

CG is not a factor in deciding rail button location.

 

[Jmhepworth]

I sometimes put the forward button one body tube diameter forward of the center of pressure as a reference point for the farthest aft the CG can be. But more often than that I just put it where it looks right.