High Power Min Diameter Motor Retention

[UPscaler]

The best minimum diameter retention is a nice friction fit! If you're really worried, throw a little aluminum tape around the closure/airframe joint.

I like the ingenuity, though.

Braden

 

[jqavins]

That standoff had to be offset to clear the nut holding the rail button. Come to think of it, I'm not sure how you would use rail buttons on a min. dia. design. I suppose you would have to go with guides instead.

Of go with FARGs.

 

[AllDigital]

I use two techniques for high power minimum diameter retention (on 98mm and 75mm). For both methods I use a piece of coupler glued high up inside the fin can for the forward motor stop. I size it for the longest possible motor I'll fly and then use coupler spacers for shorter motors.

1) Method one is to use a 3/8" or 1/2" aluminum ring that has the same OD as the motor case. I leave a 3/8" gap of airframe at the nozzle end of the fin can (MM) and insert the ring. I then have 6-8 small PEM screws that hold in the ring through the airframe, along with the lower launch lug threaded through the ring.

2) Method two is to run a threaded rod through the eye bolt at the top of the motor and secure it to the airframe. I use two threaded metal inserts from launch lugs and I turn them upside down, so the flush part is on the outside of the airframe. I then run threaded rod through the eyebolt and the airframe and secure it with the launch lug threads. This method works great, but you have to drill your holes perfect to align to the top of your eye bolt, so there is no play.

 

[JackC]

If you wanna take the risk for that large of an airframe (98mm) there’s always the option to friction fit your motor. Weave tape around the motor in an “inclined plane”manner as if it was a screw. So don’t just put tape vertically on it. Then test fit it. You just want enough tape on the motor to have it quite snug in the motor tube/airframe etc.

What’s your harness system? Is there a “Y” harness setup? Or is your harness attached to a forged eye bolt that can be threaded into the motor?

Aero pack has a minimum dia retainer system that epoxy’s right to the inside of the airframe up to as large as 98mm.

Slim lines retainers are unfortunately not around anymore. I do believe Ken Allen from Performance hobbies has a few slim line min diameter retainers available. Not too sure about sizes tho. Give him an email or call if you like that idea.

His website is here :
https://performancehobbies.com/

William

Deleted.

 

[Viper3]

For large rockets with electronic recovery . Epoxy a bulk head with a small hole in the center in the body tube above the longest motor you plan to use. Measure the distance between the motor's forward bulk head and the top of the epoxyed bulk head add a couple of inches to the length. Cut a threaded rod equal to the previously calculated length. Screw the threaded rod into the forward bulk head of the motor. Insert the motor into the airframe and the top of the threaded rod will pass through the hole in the epoxyed bulk head. Reach into the top of the air frame and add a washer, lock nut, and bolt .
For motors < = to 54mm I would just use tape friction fit.

 

[jqavins]

Screw the threaded rod into the forward bulk head of the motor...

CTI cases don't have threaded forward bulkheads. And this isn't a comment on the strengths or weaknesses of the method proposed in post #1 as was requested.

 

[Sooner Boomer]

Between the fins probably won't work because that's where the launch lugs/rail buttons/guides go, I used something like this in my L2 rocket with washers under the heads of the screws, and I have to modify one of the washers to clear the rail buttons.

View attachment 528423
View attachment 528424

I flew this rocket at Argonia Aug. 7th. It was the first time I've used altimeter eject. Don't know if that had anything to do with it, but when I pulled the motor casing out of the rocket, I found that two of the standoffs were completely loose in the body. I guess it was only the third one that kept the motor in. They weren't loose when I installed the motor and screwed the screws in. I had originally used JB Weld (I think). I'm going to drill out the holes and file grooves in the standoffs before I glue them back in. In fact, I'll probably use *new* standoffs. I wish I had some phosphoric acid. That's what I used to prep aluminum surfaces when bonding strain gauges to it. I'll just have to make sure all surfaces are clean and grease-free.

 

[jqavins]

https://homebrewing.org/products/phosphoric-acid-10-8-oz

 

[Steve Shannon]

I've come up with a notion that might be of use. Yes, I know there are multiple ways, including AeroPack's interior anchor system; this is another idea. Comments please.

The notion is to attach nuts to the outside of the tube, then cover them up with the fins and fin fillets. These would be anchor points for a sheet metal retaining bracket or whatever else one chooses to use. There's a lot of epoxy involved in their installation, so work with a well greased bolt inserted until it's done. I'd go with a coupling nut and long bolt to ensure lots of thread engagement. And LocTite.
View attachment 527856

1. Glass the bare tube (brown, optional)
2. Attach a long coupling nut (grey)
3. Apply epoxy filler (turquois)
4. Attach fin (green)
5. Glass from fin to tube across nut (optional)
6. Apply full size epoxy fillet (pink)
7. Apply final glass layer

The turquoise and pink sections may not need to be applied separately; a lot of you have experience at applying epoxy fillets that I don't.

I see no reason that wouldn’t work. You might want to do something so that whatever screws into the long coupling nut cannot vibrate loose, either a jam nut or some thread locker. Would you do more than one? Is there a long screw that has something at the end to grasp the aft end of the rocket motor or does this extend to the end of the rocket?

 

[jqavins]

There's a nut buried in each fin root, so typically either three or four screws. I'd use LockTight. The screws hold a sheet metal plate, like a three or four lobed version of these:

 

[Steve Shannon]

There's a nut buried in each fin root, so typically either three or four screws. I'd use LockTight. The screws hold a sheet metal plate, like a three or four lobed version of these:
View attachment 531648

Looks good to me. It’s in the aerodynamic shadow of the fin, so it might not be too bad aerodynamically.

 

[prfesser]

I flew this rocket at Argonia Aug. 7th. It was the first time I've used altimeter eject. Don't know if that had anything to do with it, but when I pulled the motor casing out of the rocket, I found that two of the standoffs were completely loose in the body. I guess it was only the third one that kept the motor in. They weren't loose when I installed the motor and screwed the screws in. I had originally used JB Weld (I think). I'm going to drill out the holes and file grooves in the standoffs before I glue them back in. In fact, I'll probably use *new* standoffs. I wish I had some phosphoric acid. That's what I used to prep aluminum surfaces when bonding strain gauges to it. I'll just have to make sure all surfaces are clean and grease-free.

I don't know how well it will work, but some rust removers are based on phosphoric acid.

 

[C.O.B.H.C.]

Slim Line motor retainer. This is a competitor 3 I modified into a 75mm minimum diameter.