HMMM... sounds interesting.

Do you have any space aft of the fins? What if you had a band clamp (similar to a hose clamp but without the big screw fitting) holding a thin threaded plate for the rail button? Then you could cover with glass as part of the tip to tip and then feather it in.

The plate would give the threads some bite, and the stresses would be passed around the base of the BT. If you don't have any space aft of the fins than you could put the band and plate in place before the fins are attached.

It's kind of like glassing in an Acme rail guide with a band clamp.

Rather than puting the screw in from the outside, how 'bout puting the screw in from the inside and have the nut on the outside. Maybe a stove bolt or machine screw thats counter sunk into the tube just enough so the motor case doesn't touch it. I assume you are using the large buttons, so something like a 1/4 x 24 should do it and provide enough strength to hold the button firm. I have a tool that's used to create the tapers for stove bolts and other screws that have the tapered heads. With a 98mm tube there should be plenty of room to work.

Ken.

Oh, and I'm not sure if I'm insulted or complimented by the GEEK comment.

I was going to say that after all the tip to tip is done, drill it for a pem-nut, install the Pem nut with a bit of epoxy. Install the button and screw, then grind off the bit projecting through the inside of the airframe. With the right Pem Nut and enough epoxy I bet it'll be fine. How heavy will the bird be?

Warren

Oh, and I'm not sure if I'm insulted or complimented by the GEEK comment.

Ha! that is a compliment. The UprOar was flown by the "League of Extraordinary Geeks". Trust me, Ed-- you'd know it if I insulted you 🙂

I was going to say that after all the tip to tip is done, drill it for a pem-nut, install the Pem nut with a bit of epoxy. Install the button and screw, then grind off the bit projecting through the inside of the airframe. With the right Pem Nut and enough epoxy I bet it'll be fine. How heavy will the bird be?

Warren

Warren, that was actually one of my thoughts -- grinding down a PEM. The rocket will weigh perhaps 12# dry, twice that loaded.

Ken, your comment above is intriguing... so is the hose clamp. I could even wrap the motor with a piece of tape, and put the hose clamp on there, so the majority of the drag woudl be below the airframe, though that would dirty up the aft end of the rocket.

I've also thought about buying another aft ring and drilling and tapping a hole into the aft closure itself, though that would be "modified".

Keep the great ideas coming! I'll share my initial idea in a day or two... I'm curious if anyone else comes up with a similar idea? THANKS EVERYONE! This is helpful...

Depending on how you're retaining your motor casing... you could make a plate (with a hole in the center to allow the nozzle through but small enough to retain the motor) and weld either a 1/4x20 nut or bolt to it to screw down the button. Just use 4-6 screws to bolt the plate to the aft; this would take care of the rail button and motor retention. But the bolts to hold the plate would have to be bonded to the outside of the airframe. More drag, even with cool-looking covers. Of course, that's going to put that button awfully low, the one above will have to be relatively close to it. I've never seen a bird turn on the last button left in the rail... but I heard it can happen. Though you should be quite stable by the time that last button is riding up the rail alone. 😉

This is an enigma. Without really knowing the design, how the motor is retained, where the fins are positioned relative to the airframe, etc., it's an interesting challenge. But then, I guess that was the purpose of posing the question in the first place. Upon reading the initial question, I assumed the problem was only how to attach the button to the airframe securely, but I think there is more to it. How do you attach the button or something similar to the airframe while keeping the drag and turbulance induced by the button to a minimum.

John, can we have more specifics about the rear end of the rocket?

Here's another thought. Position the end of the motor about 1/2" into the tube - or - at least leave enough airframe exposed for a screw. Add the rail button to the bottom of the tube aft of the end of the motor as you normally would with a screw and a nut, but only after the motor is loaded and retained.

Along similar lines, depending on the weight of the bird...

How about heliarc welding an Acme rail guide to the outside of a slimline retainer? The acme has less frontal area than a traditional button. If you're partial to the inherently non-binding nature of the regular button, get one machined up out of aluminum with a flange and get it welded up. Should be sturdy enough. For the top, just a standard button or an Acme with countersunk screws in the flange for good measure.

Just my $0.02.
-Ken

Depending on how you're retaining your motor casing... you could make a plate (with a hole in the center to allow the nozzle through but small enough to retain the motor) and weld either a 1/4x20 nut or bolt to it to screw down the button. Just use 4-6 screws to bolt the plate to the aft;

Nope, no room... 4" motor in a 4" airframe. The only thing protruding is the aft closure. The nozzle is well up inside that. There is no "aft" to put the 4-6 screws you refer into. No centering ring, no fin tabs, just a 98mm aft closure hanging out the back end. I'll retain the motor from above....

JW

PS What is a "fin tab" ? 🙂

Duck tape, lots of duck tape... do the PEM thing, that should hold it fine. I'm glad you started this thread, John. Tim Thomas and I will be faced with this problem next year...

Upon reading the initial question, I assumed the problem was only how to attach the button to the airframe securely, but I think there is more to it. How do you attach the button or something similar to the airframe while keeping the drag and turbulance induced by the button to a minimum.

John, can we have more specifics about the rear end of the rocket?

This is similar to all my rockets, with one exception -- this one is 98mm. The fins are surface mounted. The airframe is 4" diameter, and the motor is 98mm. I'm not at all worried about drag or turbulence... I'm worried about the button falling off at an inopportune time.

There are no centering rings to tap into, since this is minimum diameter. Think of this as a giant Estes rocket that uses the airframe as a motor tube. I need to figure out a way to sink a 10/24 screw into the wall of the airframe, and all the anchorage I have to work with is the thickness of the wall of the tube, period... I'd guess 1/16" or so?

Hope that helps. This is a toughie.

JW

Here's another thought. Position the end of the motor about 1/2" into the tube - or - at least leave enough airframe exposed for a screw.

It won't fit -- the motor won't go into the tube, because the aft closure will preclude that. Minimum diameter rocket....