High Power Min Diameter Motor Retention

[jqavins]

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.

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.

 

[Williamhimka]

First, what diameter is this?
Motor retention doesn’t have to be that complicated if you’re using electronic deployment. Let me know what diameter it is and I’ll shoot you some ideas.

William

 

[jqavins]

It's vapor for some unknown project in the future maybe. Just an idea to throw out there for the community's consideration and comment.

 

[Williamhimka]

It's vapor for some unknown project in the future maybe. Just an idea to throw out there for the community's consideration and comment.

Diameter?

 

[jqavins]

Any diameter in which context you wish to consider it. I drew the figure with a 3.9" body tube, i.e. for 98 mm, and with the nuts approximately #10. But that doesn't matter.

The notion is the structure and it's use to hold a retaining bracket, not the scale. If it seems more appropriate at some size and less at another, that's just the sort of comment that would be welcome. The whole thing is just for conversation, not for any particular application.

 

[G_T]

You'll probably be weakening your fin attachment if you do it that way.

 

[rocketgeek101]

I'm by no means an expert, but this seems like it would seriously compromise the strength of the fin/body joint, which on a min-diameter rocket (especially a large one) is not something you'd want to do...

 

[Williamhimka]

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

 

[tsmith1315]

If you're using fins with a long root chord, the length of a coupling nut removed from the aft end of the fin root may not be a deal breaker. Especially if you're not out to break records.

An interesting idea.

 

[Off Grid Gecko]

I'm by no means an expert, but this seems like it would seriously compromise the strength of the fin/body joint, which on a min-diameter rocket (especially a large one) is not something you'd want to do...

Was thinking the same but a glass layer was mentioned, I assume for tip-to-tip securing. Seems like it could work fine you just have to do some trimming on a metal gasket to hold the motor in place. Or simply cut a slot in the back of the fin for the coupling nut and have the rest pasted securely to the body.
I haven't done any HPR min-diameters yet but my idea has always been to use a threaded assembly and flare the bottom of the tube to match the diameter.

 

[H_Rocket]

My only thought is that this would not be minimum diameter when set up as flanges or "ears" would have to protrude for the screws that attach the retainer plate. Of course you could design a plate that matches the aft profile of the fins, fillets, etc.

Interesting idea though.

 

[jqavins]

If you wanna take the risk for that large of an airframe (98mm) there’s always the option to friction fit your motor...

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.

Please, William, there's no harness system under consideration, because there's no rocket under consideration. Shootin' the breeze here.

Yes, as I said in Post #1, I'm aware of the AeroPack system. And of course I know about friction fitting. I'm not looking for alternatives, just for discussion of the possible merits and difficulties with this proposed system.

You'll probably be weakening your fin attachment if you do it that way.

I'm by no means an expert, but this seems like it would seriously compromise the strength of the fin/body joint, which on a min-diameter rocket (especially a large one) is not something you'd want to do...

Hmm. I wouldn't have thought so, but I'm certainly no expert either.

If you're using fins with a long root chord, the length of a coupling nut removed from the aft end of the fin root may not be a deal breaker. Especially if you're not out to break records.

An interesting idea.

I didn't draw a view from the side, but I guess I should have. In my head it looks like this: one fin is face on with the fillet removed (jagged lines mean the fin profile is not shown), and one fin is edge on.

The coupling nut is epoxied to the tube. The fin root is notched so it is epoxied to the tube and the the nut, i.e. it has glue along it's entire length. Then the nut is fully buried in the fillets. If there's a reason that this makes the joint weaker - and there may well be - it is beyond me. Please explain.

My only thought is that this would not be minimum diameter when set up as flanges or "ears" would have to protrude for the screws that attach the retainer plate. Of course you could design a plate that matches the aft profile of the fins, fillets, etc.

My thinking exactly. Both parts. In that order.

When I wrote "a sheet metal retaining bracket or whatever else one chooses to use", I was thinking one might fabricate a "boat tail with ears" whose front face does the retaining, which has a large enough axial hole for the nozzle and exhaust plume, and is attached by long bolts. But that's not germane to the basic concept.

Lest I become defensive of my idea, I shall now just sit back and read what others say unless asked a question.

 

[Williamhimka]

No worries.
Just a misunderstanding on my end.
I hope i was a help in some way.
William

 

[rocketgeek101]

No worries.
Just a misunderstanding on my end.
I hope i was a help in some way.
William

I think I may have had the same misunderstanding.

From the OP's first post, I thought the coupling nut was the same length as the fin root. As long as most of the fin root is directly in contact with the airframe tube, I don't see why this idea wouldn't work. You wouldn't be able to get as nice of a taper on the rear of the fin fillet though, so it might still be a problem for ultra high performance flights, but again I'm no expert.

 

[Neutronium95]

I would probably just embed a chunk of threaded rod into one of the fin roots. It'll present the same frontal area increase when the hardware is installed and should be easier and stronger.

 

[mtnmanak]

In the spirit of having fun with a design, this would be fun to try.

However, I would probably not put the nuts under the fins. I would just put then between the fins. You aren't going to gain a lot of drag with a couple coupler nuts on the sides, so there doesn't seem to be a good reason to compromise your fin integrity. Try it out as an experiment with the nuts between the fins, you can always try them under the fins in a future design.

If you go with a between-the-fin installation, JB Weld them on and then put a layer of glass over them. Don't worry about trying to keep the JB Weld/epoxy out of the threads, just re-tap them when you are done. If you use some JB weld to taper off the leading edge of the nuts, you would end up with a couple of really tiny fins and they would be quite aerodynamic.

 

[tsmith1315]

The coupling nut is epoxied to the tube. The fin root is notched so it is epoxied to the tube and the the nut, i.e. it has glue along it's entire length. Then the nut is fully buried in the fillets. If there's a reason that this makes the joint weaker - and there may well be - it is beyond me. Please explain.

No argument here, I was trying to clarify the notch to those who seemed to share rocketgeek's interpretation. An idea likely to work it's way into something I do sooner or later!

 

[RalfB]

This is my Idea.

 

[fyrechaser]

Slim lines retainers are unfortunately not around anymore.

Slim Line retainers are available at Giant Leap Rocketry. I have a brand new one on my new 3" minimum diameter bird I just finished. I picked it up from them a couple of months ago.

https://giantleaprocketry.com/collections/giant-leap-motor-retainers

 

[BigMacDaddy]

I always prefer some type of retainer rather than just friction fit (even on my LPR stuff).

Following your logic of mounting in the fin root, I wonder if it is possible to do some type of integrated clip that would push little nubs into the tube to lock the engine in. This would assume that your engine was mounted a small way inside the tube. Something like this...



Just trying to go with the brainstorming spirit of things...

 

[mtnmanak]

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

An odd statement in that you link to Kenny's site and Kenny shows all Slimline retainers in stock and the manufacturer of Slimline retainers, Giant Leap, has never stopped making them, as far as I know. I have never had a problem getting the Slimlines from GL:

https://giantleaprocketry.com/collections/giant-leap-motor-retainers

 

[icyclops]

How to make something that is simple much more complex…..classic engineering. I really don’t mean to be harsh here, but come on guys…there are already enough proven ways to accomplish this without messing with the fin attachment and making it weaker…you want to chat about it….feel free. Enjoy!

 

[cwbullet]

I still think the best retention for high power minimum diameter is tape.

 

[Williamhimka]

Strange.
From what I’ve heard, they have been discontinued.
Sorry about that…

 

[Jmhepworth]

I still think the best retention for high power minimum diameter is tape.

I use aluminum tape.

 

[Sooner Boomer]

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)I've been using 1/8" ply, but may step up to 1/4" for this rocket since it will be surface mounted.
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 used standoffs that used #8 screws for my 54mm body/38mm motor L2 project. They fit perfectly between body and motor mount.
I plan on using something like what you describe for my 54mm minimum dia. I will probably use 1/4" fins rather than the 1/8" ply I've been using because the fins will be surface mounted (rather than TTW).
https://www.allelectronics.com/item/sp-318/1.5-aluminum-hex-standoff-f-f-8-32/1.htmlhttps://www.allelectronics.com/item/sp-321/1-x-0.25-dia-aluminum-hex-standoff-f-f-8-32/1.html

 

[Sooner Boomer]

Try it out as an experiment with the nuts between the fins, you can always try them under the fins in a future design.

If you go with a between-the-fin installation, JB Weld them on and then put a layer of glass over them. Don't worry about trying to keep the JB Weld/epoxy out of the threads, just re-tap them when you are done. If you use some JB weld to taper off the leading edge of the nuts, you would end up with a couple of really tiny fins and they would be quite aerodynamic.

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.

 

[Steve Shannon]

I would probably just embed a chunk of threaded rod into one of the fin roots. It'll present the same frontal area increase when the hardware is installed and should be easier and stronger.

I remember seeing bike spokes used years ago. The base of a fillet contributes least to the strength of the fin mount. That’s a good place to embed a slim threaded bike spoke. Some kind of retainer plate would still need to be fabricated.

But personally I would probably just use tape to friction fit a motor in a minimum diameter rocket. It’s boring but it has worked well as long as model rockets have been around and I’ve seen it work on everything up to and including 98mm motors.

 

[jqavins]

I've done a little more thinking since I last posted, and come up with an improvement.

I used standoffs that used #8 screws for my 54mm body/38mm motor L2 project. They fit perfectly between body and motor mount.
I plan on using something like what you describe for my 54mm minimum dia. I will probably use 1/4" fins rather than the 1/8" ply I've been using because the fins will be surface mounted (rather than TTW).
https://www.allelectronics.com/item/sp-318/1.5-aluminum-hex-standoff-f-f-8-32/1.htmlhttps://www.allelectronics.com/item/sp-321/1-x-0.25-dia-aluminum-hex-standoff-f-f-8-32/1.html

Bear in mind that the fillets as I drew them in the OP must cover the point-to-point (circumscribed) diameter of the nut. The description of a nut generally gives the dimension across the flats (inscribed). The point-to-point of these 1/4" diameter standoffs is probably 0.289 (nominal). Also, even the dimension across the flats might be giving more meat than we really need.

There are also round coupling nuts/standoffs that eliminate the greater point-to-point distance A little searching found this one; the price is not listed, but I'm sure a little more searching would find viable results. I found this by changing "coupling nut" in my search to "standoff", so thanks, that's a big help.

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

"Between the fins" doesn't have to mean "centered between the fins". I see that, indeed, your washers are not centered; they could have been only far enough from the fins to avoid the tabs. Of course, cutting down the washers the way you did gets them out of sight and out of the air stream, so one might want to do that in any case, if one is obsessive about such things. (Oh, what am I saying? No one here is obsessive.)

 

[Sooner Boomer]

"Between the fins" doesn't have to mean "centered between the fins". I see that, indeed, your washers are not centered; they could have been only far enough from the fins to avoid the tabs. Of course, cutting down the washers the way you did gets them out of sight and out of the air stream, so one might want to do that in any case, if one is obsessive about such things. (Oh, what am I saying? No one here is obsessive.)

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.