Rail Button Placement on HPR

[Rocketmaniac]

I'll be using some 1500 series rail buttons on my HPR cluster. In reviewing railbuttons.com's article on rail button placement, it is recommended to use 3 buttons. The first 2 placed very near the rear (aft end) of the rocket. The 3rd button should be up closer to the CG (w/motor) plus or minus a caliber or two.

On this rocket, the CG is 36" from the aft end of the rocket. This is 9" from the end of the lower body tube. The 3rd centering ring is 20.5 from the aft end. This is more than 2 calibers from the CG.

Should I put the 3rd button on my electronics bay? This would be 1.6 calibers from the CG, but the elctronics bay can rotate.

 

[rstaff3]

The use of the third button depends a lot on the guide length, speed of the rocket wehn the top button leaves, etc. In general, I think I'd just use the two bottom buttons and use a nut and washer on the inside of the tube on at least the upper button.

 

[lalligood]

3 buttons is overkill on all applications except for *extremely* long rockets (read: major length to diameter ratio) like the FlisKits Richter Recker.

With 2 buttons, place one pretty much as far aft as possible. Second button should be reasonably close to the fully loaded CG point (closer to CG the better as the rocket increases in weight). The only reason for the second button being in the ballpark of the CG is so that it doesn't place so much stress on the aft button while the rocket is sitting on the pad.

HTH,

 

[Rocketmaniac]

Originally posted by lalligood
Second button should be reasonably close to the fully loaded CG point (closer to CG the better as the rocket increases in weight).

The CG is only 9" away from my electronics bay. Should I put it threre (e-bay) or attempt to mount button #2 with some kind of t-nut or something like that? I am just afraid to have the main chute get hung-up on the rail button mount.

 

[rstaff3]

If you support the buttons on the inside you shouldn't need them that close to the CG.

 

[Rocketjunkie]

Put one button about 6" from the rear of the rocket. Put a second about 18" above the first. If you want support higher, put a third on the payload section.

We do not have any 1515 size rails, all use the standard size buttons. The club rails are Blacksky. Although the HP Blacksky rails look bigger, they use the same buttons.

 

[DynaSoar]

Originally posted by Rocketmaniac
I'll be using some 1500 series rail buttons on my HPR cluster. In reviewing railbuttons.com's article on rail button placement, it is recommended to use 3 buttons. The first 2 placed very near the rear (aft end) of the rocket. The 3rd button should be up closer to the CG (w/motor) plus or minus a caliber or two.

On this rocket, the CG is 36" from the aft end of the rocket. This is 9" from the end of the lower body tube. The 3rd centering ring is 20.5 from the aft end. This is more than 2 calibers from the CG.

Should I put the 3rd button on my electronics bay? This would be 1.6 calibers from the CG, but the elctronics bay can rotate.

Putting one on a section that can rotate is no problem. You just have to line them up when you take it to the rail.

If you're afraid the screw will interfere, use a shorter screw. Or put a washer on top the bottun so the screw doesn't go it as far.

As railbuttons says you don't even really need them. As long as it's going into the body tube wall, it can be glued there, and give more grip than an otherwise adequate surface mount. You could also glue a cap os some sort over the end of the screw so that there's a smooth, wide convex coevring on it. Everything should slide by something like that just fine.

The lower they all are, the longer they'll guide the rocket. With 3 buttons, the need for one to be at the CG are greatly reduced. I'd place one just aft of CG, one as far back as possible, and the middle one between half and 2/3 of the way between them towards the aft.

Use lug placement as your guide. If you were using a lug, how long would it be and where would the ends be? There's two button placements.

 

[Todd Knight]

Our club saw a few problems creep up with rail buuton location a few years ago when rails started to become the norm. When the buttons were placed at the very aft end of the rockets we were getting a funny kick coming off the rails. We fly in Ohio, so most of our rockets are heavier to stay under the waivers we were living with at the time. Since then we have been mounting the buttons a caliber or so above the rear of the rocket and seems to have cured the problems.

Personally, I only use 3 buttons on the largest rockets. I have mounted one button on a payload bay before and not had any issues, just make it is lined up correctly. As has been stated before, on the 1515 buttons use the T-nut that is included, it is definitely much stronger. If the forward button is in a parachute bay you can put some epoxy or clay over the nut on the inside to eliminate any hang up points.

 

[JDcluster]

Using 3 is absurd,
He just wants to sell more buttons.
I've never used more than 2 on any of my rockets which includes everything from H to M & the M rocket is almost 12 ft tall.

I usually place 1 just above the fins & the upper most 1 on or just past the CG if possible.



JD

 

[Ryan S.]

I agree, there is no reason you need 3 rail buttons, the rocket should never be hanging by the railbuttons. You load it onto the rail always supporting it, support it on the way up, and then rest it on the pad when it is raised. the rail buttons only need to guide the rocket up

Using 3 is also tough because lining 3 points up in a straight line is much harder than two so you could rip buttons off

 

[DynaSoar]

Originally posted by Ryan S.
I agree, there is no reason you need 3 rail buttons, the rocket should never be hanging by the railbuttons. You load it onto the rail always supporting it, support it on the way up, and then rest it on the pad when it is raised. the rail buttons only need to guide the rocket up

Using 3 is also tough because lining 3 points up in a straight line is much harder than two so you could rip buttons off

Nobody said anything about them supporting weight.

A long rocket's CG leave the rail way before the tail and before a shorter rocket's. It's got a lot more surface area to catch any wind, and a lot more leverage. I'd rather pay another 50 cents and make sure my rocket keeps going straight.

It's not difficult at all the get 3 buttons in line. You draw the line, you drill. You mess up drilling, you fill it and drill elsewhere.

 

[Rocketmaniac]

I ended up putting 3 buttons on my rocket. First one was on the aft centering ring. Second one was on the second centering ring, about 11" from the aft end. And the third button I put 32" from the aft end. That is a little less than 1 caliber from the CG. It was in the apogee deployment section. I used a T-nut and put some epoxy around the edge to smooth it out.

I had no problems with the placement...... Rocket flew great....

 

[rstaff3]

Glad it went well! Er...and where are the pics?

 

[Rocketmaniac]

Originally posted by rstaff3
Glad it went well! Er...and where are the pics?

Camera battery died, Rocketjunkie got a video of the launch.......