13mm (mini) Screw on motor retention

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R3verb

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Why should the MPR and HPR guys have all the fun?? One of the things I LOVE about my MPR and HPR rockets right now is the screw-on motor retention. There's something very satisfying about screwing that motor in there nice and tight. At the moment, I'm taking a short break from building HPR stuff to get my park flyer squadron back up to speed which includes rebuilding an Estes Mini Honest John that met an unfortunate end when it's chute didn't deploy. One of the things I HATED about that kit was the motor retention strategy was just "tape it up 'till it's tight". To me, that doesn't feel nearly good enough so I started looking for a 13mm screw on motor retainer. Turns out that doesn't exist (at least not that I could find).

So that brings me here. I broke out the laptop, fired up Fusion 360 and here's what I came up with:

Screenshot 2024-06-11 at 8.36.22 AM.png

It's essentially just a clone of the Estes one for 29mm that I have but that fits the 13mm motor tube. So far I've printed about 4 iterations getting it dialed in but I'm printing the "flight ready" version out of PETG right now and It'll get installed on the new MHJ today. I'll give it a few flights to see how it does and if it survives I'll post the files!

Here's a few shots of the iterations:
IMG_5637.JPG

IMG_5638.JPG
 
Does the OD fit inside the OD of BT-20?

I probably wouldn't bother with this personally, just because 13mm powered rockets are so weight-sensitive, both in overall weight and its location. The additional mass of a screw-on retainer on the aftmost part of the rocket is a double whammy.

I'm in the tape retention camp for the little guys. It's cheap, it's light, it's easy, it works.
 
Does the OD fit inside the OD of BT-20?

I probably wouldn't bother with this personally, just because 13mm powered rockets are so weight-sensitive, both in overall weight and its location. The additional mass of a screw-on retainer on the aftmost part of the rocket is a double whammy.

I'm in the tape retention camp for the little guys. It's cheap, it's light, it's easy, it works.
Nope, it does not. BT-20 = 18mm ID, The outermost diameter of the retention ring is 21.952mm:

Screenshot 2024-06-11 at 11.26.43 AM.png

Now, if you're talking about just the internal screw part, that should fit at 16.85mm:

Screenshot 2024-06-11 at 11.28.34 AM.png

As for weight and balance, I thought about that as well. With the current print setting in PETG, this comes in at a scant 3 grams:

Screenshot 2024-06-11 at 11.30.10 AM.png

I took the time to model the whole rocket and we're still at a stability factor over 1 so should be fine even with pushing the motor back a bit:

Screenshot 2024-06-11 at 11.31.58 AM.png

Yes, I totally understand this is overkill but it's great practice in simulations and 3D design so why not. If anybody wants any of the files (rocket or fusion) let me know and I can send them to you. I want to fly it a few times before I publish them for the masses just to make sure it holds up to repeated use but for anybody here I'm happy to share since I assume you all know what you're doing ;).
 
There are few options for screw-on engine retainers for mini engines, so thanks for sharing OP. The only other screw-on option I'm aware of (that's off-the-shelf) is taking one from the Estes Bandito.
 
Interesting. I had never seen a good off the shelf option (I assume nobody cares at this scale). One thing I want to try, especially with the BT-50 tube that the MHJ uses is a screw on boat tail. Weight might be a concern but it's possible with some smart design I can keep it about the same with low infill and thin outer walls. I'd love to try and shove the Estes Altimeter in there and see if it makes any difference lol.
 
Does the OD fit inside the OD of BT-20?

I probably wouldn't bother with this personally, just because 13mm powered rockets are so weight-sensitive, both in overall weight and its location. The additional mass of a screw-on retainer on the aftmost part of the rocket is a double whammy.

I'm in the tape retention camp for the little guys. It's cheap, it's light, it's easy, it works.

Back in 1974 I read "In Defense of Masking Tape" in the NAR Model Rocketeer...

Dr. Kosden used it on an O motor at BlackRock back in his day.
 
This was actually one of the ideas I had. Use a bayonet style twist lock mechanism instead of the threaded mechanism. I may still try it just as a 3D modeling exercise but more practically I think it would be a better solution for less precise 3d printers. To get these very fine threads I'm having to print at VERY fine detail (.08mm layer height) which I know a lot of printers can't do. Most top out around .15 which won't cut it for this fine a thread.
 
Back in 1974 I read "In Defense of Masking Tape" in the NAR Model Rocketeer...

Dr. Kosden used it on an O motor at BlackRock back in his day.
hahaha that's insane. Look, I'm not saying it DOESN'T work, just that I have a hard time trusting it and especially here in Colorado where grass fires are a real risk, I like to be 100% certain my motor is staying in my rocket.
 
Does the OD fit inside the OD of BT-20?
Ok so I did some tinkering with sizes of everything (I love parametric design!) and it looks like if you did 0 knurling on the outside of the retainer ring, you could JUST get it under 18mm. If I build something with BT-20 tube in the future I'll give this a shot, or if anybody has a project they want to try it on, I'll send you a file you can print!
 
Nope, it does not. BT-20 = 18mm ID, The outermost diameter of the retention ring is 21.952mm:

View attachment 650022

Now, if you're talking about just the internal screw part, that should fit at 16.85mm:

View attachment 650023

As for weight and balance, I thought about that as well. With the current print setting in PETG, this comes in at a scant 3 grams:

View attachment 650024

I took the time to model the whole rocket and we're still at a stability factor over 1 so should be fine even with pushing the motor back a bit:

View attachment 650025

Yes, I totally understand this is overkill but it's great practice in simulations and 3D design so why not. If anybody wants any of the files (rocket or fusion) let me know and I can send them to you. I want to fly it a few times before I publish them for the masses just to make sure it holds up to repeated use but for anybody here I'm happy to share since I assume you all know what you're doing ;).
If you do want to reduce the outer diameter, how about a smooth outer ring instead of the serrated one you have created?
 
hahaha that's insane. Look, I'm not saying it DOESN'T work, just that I have a hard time trusting it and especially here in Colorado where grass fires are a real risk, I like to be 100% certain my motor is staying in my rocket.
Back in 1974 I read "In Defense of Masking Tape" in the NAR Model Rocketeer...

Dr. Kosden used it on an O motor at BlackRock back in his day.
Never used it on a O but used masking tape plenty of times on the old Eliss K motors, Friction fit, and a masking tape thrust ring. Never an issue. I even currently use it on my 54 mm Blackhawk with an Aerotech I65 single use.
 
I think I have 2 issues with this approach.

First, especially where I live, dry grass is an issue. Now, I'm not sure that a tiny spent A motor being ejected at 100' and falling into dry grass is going to cause a fire but I don't really want to take that risk. I'd so much rather KNOW that the motor is going to be in the tube. I've had motors eject from friction fit mini mounts in the past so I know this is not an unfounded concern.

My second worry is stretching. I really like to see how many times I can launch a rocket. I think half the fun of these little mini guys are the fact that most of them you can get like 15 flights out of, maybe more. If every time I'm launching the rocket I'm having to add tape to the motor to squeeze it into the motor tube, that likely will cause some motor tube stretching. While I'm sure it's minimal, I like the idea of a motor retention system that doesn't leave any permanent change to the rocket.

Not saying the friction fit doesn't work or that you shouldn't use it though! Like I said at the beginning of this thread, this is really just me liking the screw on retention system, thinking it's cool and wanting it on my mini rockets too!

For those of you who like to go deep into the regulations, what are the regulations around motor retention? Let's say hypothetically you did a friction fit motor and it did eject and start a fire. Are you held liable for that? Where would I even find that information?

Thanks for the discussion everyone!
 
If you do want to reduce the outer diameter, how about a smooth outer ring instead of the serrated one you have created?
Yup, this is what I was tinkering with! Check out this cross section inspection:

Screenshot 2024-06-12 at 10.52.14 AM.png

So the top number 19.95mm is from the edge of the knurling to the other edge of the knurling but the bottom number, 17.95mm is from the trough of those 2 knurlings. So if we got rid of it completely, the smooth circle would be 17.95mm in circumference, just under the 18mm of a BT-20. Next time I make a BT-20 rockets I'll give this a shot. Alternatively, if anybody has one they want to try this on let me know and I can send you a file to print!
 
BT-20 OD is actually 18.7mm. So add that meat back on.

Also, you would be much better off having fewer threads on the cap and a nice radius for the transition from the female threads to the thrust flange that retains the motor. Even put a little straight shoulder on the male part to make clearance. The vast majority of the load is transferred in the first few threads, moreso the lower the stiffness of the material.

You might want to put a wedge on the ID of the opening where the fire goes.

Be sure to consider the strength at temperature of your materials. I once tried thinning the cap on a 24mm Estes retainer by putting it in the lathe. It let a D12 punch out the back of the rocket, through the cap, at the same time it was pushing the chute out the front. Successful, normal appearing recovery until I picked it up and there was no motor in it. It's somewhere out there on the playa. By the time I got there, the lip of the cap had cooled enough that it was stiff and would retain the motor, but there is no other plausible scenario than the cap being softened by the heat and letting the motor out.
 
I worked for a while on the design of an *extremely* low-profile twist-lock design... I think it would have worked (and could definitely have fit inside a BT20 with a 13mm mount) but I just never had the motivation to go all the way through with implementation and test. It's an interesting problem.
 
BT-20 OD is actually 18.7mm. So add that meat back on.

Also, you would be much better off having fewer threads on the cap and a nice radius for the transition from the female threads to the thrust flange that retains the motor. Even put a little straight shoulder on the male part to make clearance. The vast majority of the load is transferred in the first few threads, moreso the lower the stiffness of the material.

You might want to put a wedge on the ID of the opening where the fire goes.

Be sure to consider the strength at temperature of your materials. I once tried thinning the cap on a 24mm Estes retainer by putting it in the lathe. It let a D12 punch out the back of the rocket, through the cap, at the same time it was pushing the chute out the front. Successful, normal appearing recovery until I picked it up and there was no motor in it. It's somewhere out there on the playa. By the time I got there, the lip of the cap had cooled enough that it was stiff and would retain the motor, but there is no other plausible scenario than the cap being softened by the heat and letting the motor out.
I think I understand some of what you're saying but can you draw it out? Even if it's just like in MS paint over my screenshot, I'd be happy to make any changes that makes it stronger! I'm by no means a mechanical engineer so I'm pretty ignorant when it comes to that kind of stuff. For the most part I go with the spaceX "let it blow up and learn from it" approach to engineering (hence why starting at 13mm motors seems like a pretty good idea to me lol).
 
I think I have 2 issues with this approach.

First, especially where I live, dry grass is an issue. Now, I'm not sure that a tiny spent A motor being ejected at 100' and falling into dry grass is going to cause a fire but I don't really want to take that risk. I'd so much rather KNOW that the motor is going to be in the tube. I've had motors eject from friction fit mini mounts in the past so I know this is not an unfounded concern.

My second worry is stretching. I really like to see how many times I can launch a rocket. I think half the fun of these little mini guys are the fact that most of them you can get like 15 flights out of, maybe more. If every time I'm launching the rocket I'm having to add tape to the motor to squeeze it into the motor tube, that likely will cause some motor tube stretching. While I'm sure it's minimal, I like the idea of a motor retention system that doesn't leave any permanent change to the rocket.

Not saying the friction fit doesn't work or that you shouldn't use it though! Like I said at the beginning of this thread, this is really just me liking the screw on retention system, thinking it's cool and wanting it on my mini rockets too!

For those of you who like to go deep into the regulations, what are the regulations around motor retention? Let's say hypothetically you did a friction fit motor and it did eject and start a fire. Are you held liable for that? Where would I even find that information?

Thanks for the discussion everyone!
Perfectly valid and good points, both of which I share your sentiments with. I don't like the idea of motor ejection, even if the rocket is designed for tumble recovery. And I especially don't like the idea of constantly stretching out a MMT or body tube of a minimum diameter rocket.

As for your liability question, you'd have to go to case law. There might be regulations and ordinances, but they would be subservient to actual cases where a precedential opinion was published.
 
I think I understand some of what you're saying but can you draw it out? Even if it's just like in MS paint over my screenshot, I'd be happy to make any changes that makes it stronger! I'm by no means a mechanical engineer so I'm pretty ignorant when it comes to that kind of stuff. For the most part I go with the spaceX "let it blow up and learn from it" approach to engineering (hence why starting at 13mm motors seems like a pretty good idea to me lol).
13mm cap markup.png

Also, I reiterate my recommendation to adopt a standard thread form - oh, wait, that was in the other thread.
 
IMHO a full threaded retainer like that is overkill for a 13mm ejection charge. There are ways to pare it back. The trick, I think, is to ensure it stays tight in place, rather than survive the ejection kick.
 
Heat resistance might be addressed by making the cap with an ID a little smaller than the threads and cutting a thin aluminum washer to fit in it, with an ID small enough to retain the motor. Maybe just the aluminum from a soda can. It wouldn't take much. The finer the thread, within the capabilities of the printer, the less extra diameter is added. I was going to suggest cutting the top part off a plastic jar, but the cap on the cough syrup jar I have is a bit fat. I think the neck of the jar is 16 point something mm ID, and the outside of the cap is around 25mm, AFTER the adult-proof safety cap is popped off.

An M16X1 might work with a BT-20, if you could keep the walls thin enough. If standard threads are too coarse, one could always make some up.

Is there anything wrong with the old flat spring retainers that Estes used, or just extending the motor tube slightly and putting a pin through it? In the former case, there's nothing to lose.
 
I'd replace the engine block with a thin, fine, self-threading nut and just screw the motor in.
 
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I'd replace the engine block with a thin, fine, self-threading nut and just screw the motor in.

Does anything like that exist that would fit in a BT5 or BT20? I think it would definitely have to be metal (aluminum?) to reliable 'cut' into the thick cardboard motor case. The engine case would have to be turned down for clearance, and possible the case would have to be threaded.

If there was a market for it, special reusable 13mm/18mm cases could be made to thread into a dedicated motor mount/engine block.
 
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The motor retainer flew for the first time last weekend! As I said, this was going on a mini honest john rebuild. It performed great!

1CE00AC4-B4F5-4925-8AA3-CEE423F7B077_1_105_c.jpeg


Minus some VERY mild charing, it is 100% fine.

IMG_5656.JPG

I was a little worried about thermal management and warping but it looks like that fear is not a concern at all! I've got a few things I wana tweak and then I'll publish these. For reference, I printed these in PETG.
 
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