Technique for Rail Button attachment to Existing Rocket

[DMcCauley]

Anyone have any good tips on how to SECURELY attach rail buttons (1010 type) to an existing rocket design - in particular one where you have no access to internal portion of rocket (i.e. fin can)

For new designs, i always use a wood block or some other backing piece with 1/4-20 threaded inserts which are solid as a rocket.

However, i'm wary to simply "epoxy" some rail buttons on the rocket, especially since it will be seeing warp-9 loads. (Rocket has previously flown many times on Warp-9 loads using 3/8" lugs up to I1299s, but I hate lugs, especially when it comes to rod whip)

One idea I had was to simply drill a 1/4" hole in the airframe and cut out a small space in the fin can foam, then inject some Aeropoxy into that space and attach the rail button with a screw embedded in the aeropoxy. Or something similar.

Any help appreciated!

Thanks
Dan

 

[fox_racing_guy]

Just drill into your top and bottom centering rings, fill the hole with CA or epoxy and screw them down. I have rail buttons on 30+ rockets like this and have yet to run into any kind of problem.

 

[DMcCauley]

Just drill into your top and bottom centering rings, fill the hole with CA or epoxy and screw them down. I have rail buttons on 30+ rockets like this and have yet to run into any kind of problem.

The centering rings are only about 1/8" thick fiberglass. Its a 3" diameter rocket.

 

[SSenesy]

Dan;

Alternatively, you could use the PML or Giant Leap rail guides. There's enough surface area there that they can mount without screws using just epoxy. I've been unhappy with the amount of 'rail rash' though...

 

[ben_ullman]

Dan;

Alternatively, you could use the PML or Giant Leap rail guides. There's enough surface area there that they can mount without screws using just epoxy. I've been unhappy with the amount of 'rail rash' though...

make sure and use urathane because some clubs wont allow aluminum because they tear up the rails.

Ben

 

[jderimig]

Anyone have any good tips on how to SECURELY attach rail buttons (1010 type) to an existing rocket design - in particular one where you have no access to internal portion of rocket (i.e. fin can)
Dan

Use a well nut.

https://www.emhart.com/products/pop/wellnut.asp

You can get them at McMaster-Carr (or any well stocked local hardware store).

They will SECURELY hold your rail button. I use them all the time.

 

[Handeman]

Another alternative is to drill the 1/4" hole and remove some of the foam. File a few small grooves in the inside ID of the hole and then fill to the surface with JBWeld. The grooves in the hole ID will help the JBWeld hold and unlike other epoxies, it is machineable. You can drill and tap holes in it. I would recommend course threads, but if it taps really well, then fine thread might be better.

 

[H_Rocket]

If it's a fiberglass rocket, a #6 button head sheet metal screw into a pilot hole is more than enough. Some will scoff, however how much load in the direction that would pull the lug off gets applied?

I've also used Molly fasteners for 1/8" material thickness on glassed cardboard.

 

[als57]

As has been said before a well nut works fine in existing rockets.

You mention 1/4-20 threads. Most buttons use #6 or #8 thread screws. PML plastic rail guides use #6 screws.

On G10 or Quantum tube I drill an under size hole and thread it using a 6-32 or 8-32 tap. Works great and never had a button come off.


Al

 

[troj]

Drill a hole in the airframe, put a little epoxy in through the hole to one side, install rail button, roll rocket so that the hole is down and the epoxy runs back to the screw. Done.

-Kevin

 

[DMcCauley]

Thanks for all the tips everyone.

Looks like I'll be going with the well fastener for this application. The rocket is a 3" PML Amraam which has been retrofitted with dual deployment and has many flights under its belt.

I plan on flying this bird with a J1999 Warp-9. So it needs a good solid rail button connection.

 

[bobkrech]

I second well nuts. They take less than a minute to install and no back side access is required.

1.) drill the hole.

2.) insert the well nut.

3.) screw on the rail button.

4.) you're done.

from https://www.mcmaster.com

Screwdriver-Installed Rubber-Insulated Rivet Nuts

​

Also known as well nuts and well-nut threaded inserts, these flanged neoprene bushings have a brass insert and are great for fastening metal to plastic, damping vibration, and sealing out moisture. They can be installed easily with a screwdriver—no special tools needed. To use, place bushing in a hole and insert a machine screw (not included). Tightening the screw causes the bushing to expand, securing the nut.

Thread, Thickness range, Drill Size, P/N, Price and Quantity per pack
6-32 0.015"-0.156" 5/16" 93495A120 $12.76 for 25
8-32 0.015"-0.156" 5/16" 93495A130 $14.50 for 25
10-32 0.015"-0.192" 3/8" 93495A180 $6.28 for 10
1/4"-20 0.031"-0.187" 1/2" 93495A321 $8.86 for 10


Bob

 

[H_Rocket]

I plan on flying this bird with a J1999 Warp-9. So it needs a good solid rail button connection.

My 4" Endeavor ate itself at LDRS on the J1999. Motor assembled well so I don't think there were any problems there. On ignition flight was nominal for about 150', then there was a bright flash at the aft end. The motor blew the rear closure, tore the 3 point PML retainer off, and spit the liner and half burned grains. The case was driven into the lower recovery bay and made a mess of the aft bulkhead for the avionics. Of course the case is ruined as the aft end is seriously bell shaped.

The rocket coasted to what looked like maybe 1,000' turned over and was a serious bunker buster. Only about 3" was sticking out of the ground.

I discussed this at length with Gary and the other AT people. I'm sending pictures to them and I got the impression that they will make me whole with respect, at least to the motor. I'm not one to ask for the rocket back in warranty as I accept the risk there, especially with a brute like the J1999.

I'm not trying to discourage you, just wanted to share my experience. I will try this motor again, just after some thinking.

Hey I pushed the envelope - The envelope pushed back!



Why the concern with the rail button mounting? With almost 600# of thrust instantly on, it ain't gonna be on the rail all that long!:jaw:

 

[bobkrech]

Regarding the original question, the peak thrust of a I1299 is 420 pounds. Only a small fraction of this will be transferred to the rail buttons and the launch rail as a torque load. Assuming the rail is stiffly mounted, the fraction should be not more than the rocket radius/moment arm and could be less. If the rocket radius is 2" and the moment arm is 12", the maximum fraction of the thrust transferred should be 2/12=16.7% or 70 pounds distributed over 2 rail buttons if my thought process is not warped.

That's not a very high load.

Bob

 

[peter_stanley]

Drill a hole in the airframe, put a little epoxy in through the hole to one side, install rail button, roll rocket so that the hole is down and the epoxy runs back to the screw. Done.

-Kevin

This is what I've always done and never had a problem.

Peter

 

[jderimig]

Regarding the original question, the peak thrust of a I1299 is 420 pounds. Only a small fraction of this will be transferred to the rail buttons and the launch rail as a torque load. Assuming the rail is stiffly mounted, the fraction should be not more than the rocket radius/moment arm and could be less. If the rocket radius is 2" and the moment arm is 12", the maximum fraction of the thrust transferred should be 2/12=16.7% or 70 pounds distributed over 2 rail buttons if my thought process is not warped.

That's not a very high load.

Bob

The above only if the rocket gets stuck on the rail.

Otherwise the rail buttons will see the inertial loads of acceleration, ma, where m is the mass of the rail button (very small) and the frictional load of the button and rail.

You'll have the same button frictional loads whether the motor has 50, 500, or 5000# of thrust. (I think)

Edit: In my experience the largest loads the buttons will see is when you are putting the rocket on the rail.

--jd

 

[DMcCauley]

The above only if the rocket gets stuck on the rail.

Otherwise the rail buttons will see the inertial loads of acceleration, ma, where m is the mass of the rail button (very small) and the frictional load of the button and rail.

You'll have the same button frictional loads whether the motor has 50, 500, or 5000# of thrust. (I think)

Edit: In my experience the largest loads the buttons will see is when you are putting the rocket on the rail.

--jd

Good info to know.
Thanks!

 

[RocketMonkey]

Use a well nut.

https://www.emhart.com/products/pop/wellnut.asp

You can get them at McMaster-Carr (or any well stocked local hardware store).

They will SECURELY hold your rail button. I use them all the time.

Yea what he said

 

[als57]

The above only if the rocket gets stuck on the rail.

You'll have the same button frictional loads whether the motor has 50, 500, or 5000# of thrust. (I think)

Edit: In my experience the largest loads the buttons will see is when you are putting the rocket on the rail.

--jd

I'd expect the frictional load to increase as a product of velocity on the rail. So higher impulse should create a higher frictional load. Due to the small contact area the force may effectivly "plateau" at some point. A robust attachment should render the issue moot.

Al