3D Printing Anyone ever attempt a slim 18mm retainer?

Right now I'm building a Deuce's Wild which has two friction-fit, 18mm mounts.

I am not a fan of friction fit motor mounts for low power stuff, because the tubes are frequently flimsy and I always fear damaging them by making the friction fit too tight. I'm not saying there's anything bad about friction fit, it's just not my preference.

And so, being a fan of over-engineered solutions, I'm pondering some sort of 3D-printed slim retainer. I think anything with threads will be too bulky and hard to print cleanly at that size, and so I'm instead thinking of some sort of twist-lock mechanism that can be made very small. Anyone ever tried anything like that, or have ideas? It should be simple, *light*, and reliable.

I have some ideas on how to do it but haven't had a chance to try designing it yet, figured I'd bounce it around here first. Challenge is to make it hold in place solidly, while able to remove without too much force.

Just like the Tres the Deuce has no need to be friction fit only, the motor tubes stick out beyond the rear of the rocket far enough a simple wrap of tape around the motor and the motor tube will keep it in place.

rharshberger said:

Just like the Tres the Deuce has no need to be friction fit only, the motor tubes stick out beyond the rear of the rocket far enough a simple wrap of tape around the motor and the motor tube will keep it in place.

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That would be my preferred option. I hate friction fit and avoid it at all costs.

rharshberger said:

Just like the Tres the Deuce has no need to be friction fit only, the motor tubes stick out beyond the rear of the rocket far enough a simple wrap of tape around the motor and the motor tube will keep it in place.

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Fair. In my head I was lumping friction fit and tape wrap together, when in fact they're totally different. I wouldn't have a big problem with tape wrap method.

That said, I still think a tiny minimalist retainer would be fun, and after my success with my twist-lock nose cone retainer it seems doable. I'm still gonna hack around with it in CAD, see if I can come up with something reasonable. Gives me something to do.

What size rocket tube on the outside?

I believe it's a standard BT20, so.... .736".

No, what is the rocket diameter? How much mounting tue do I have to work with and how thin do you want the outside (motor mount to body tube)?

Ah. It's a BT60, and the motor mounts stick out.... actually I don't have a number at the moment, the rocket is out in the shed while primer is drying. My goal for this design would be to use as little of the tube as possible, which means... I dunno, would love to do it in 1/4" or less. I'm pretty sure the Deuce has more than that available. 1/4" or so would make it useful in a variety of situations I think.

The Estes Amazon has a twist-lock engine retainer, sort of built-in to a boattail.




I recall some of their long-ago OOP small mini-engine power RTF plastic rockets used these also: Vampire/Jinx, and the MoonDog. You might be able to find some pictures of those for ideas, if you were to design your own 3D printable twist-lock retainer.


Looking at the two 18mm engine mounts protruding out the sides of the Duece's Wild, kind of hard to think of how to modify those for a twist lock.

GlenP said:

The Estes Amazon has a twist-lock engine retainer, sort of built-in to a boattail.

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Got any larger pictures?

This is a file for BT56.

Ah. That's a nice design when the retainer is built into the motor mount, unfortunately I don't think that approach would work (in general) as an add-on to a cardboard motor mount.

That said, twist-lock designs tend to have a lot in common, at least the ones I've seen. I think I have a possible approach for this one, will try to bang it out in CAD as soon as I get a chance.

cwbullet said:

This is a file for BT56.

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Was there supposed to be something attached?

Maybe a locking e clip or circular ring snapping into a groove like the Slimlines?

I think the one on Thingiverse is a replacement for a lost estes.

cwbullet said:

I think the one on Thingiverse is a replacement for a lost estes.

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that particular design is the Amazon rocket ... most easy build rockets have that back engine mount nice fit just annoying when i comes to launching and you forget that the igniter has to be inserted with the engine before twisting the cap on ... also the " obsessed with rocketry" is a funny code name for you it is true though

I know your thinking 3D printing, but have you considered just using a Rocketarium 18mm retainer? https://www.rocketarium.com/Build/Motor-Retainers/18-mm

Those look nice but expensive and a bit heavy. I think they're a bit overkill for 18mm motors, but I'm guessing. My goal would be for something more minimal. We'll see how it goes, I have a design in mind to try and it may or may not work out but it'll be fun to mess around.

Any chance this would fit your needs? Could always scan and print it... change the inner diameter to eliminate the taping but keep the threads the same to use standard bottle tops.

Homer

The apogee retainer is cheap and works well.

cwbullet said:

The apogee retainer is cheap and works well.

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All I see on their site are the Rocketarium retainer (not cheap) and their vinyl tape strips, which are nice but not necessary. If I were making a substantial Apogee order I'd throw some of those in, but not worth an order unto themselves.

Homer_S said:

Any chance this would fit your needs? Could always scan and print it... change the inner diameter to eliminate the taping but keep the threads the same to use standard bottle tops.

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Ha, I forgot about the old water bottle retainer! Maybe a bit bulky for this application, but I'll fool around with it a bit. I think the water bottle caps have gotten smaller and flimsier since Chris wrote that, so I need to see how the new ones fit.

neil_w said:

All I see on their site are the Rocketarium retainer (not cheap) and their vinyl tape strips, which are nice but not necessary. If I were making a substantial Apogee order I'd throw some of those in, but not worth an order unto themselves.


Ha, I forgot about the old water bottle retainer! Maybe a bit bulky for this application, but I'll fool around with it a bit. I think the water bottle caps have gotten smaller and flimsier since Chris wrote that, so I need to see how the new ones fit.

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You could eyeball the soda ones. They still have to retain pressure. I've saved a few in my components box to try.

Homer

neil_w said:

All I see on their site are the Rocketarium retainer (not cheap) and their vinyl tape strips, which are nice but not necessary. If I were making a substantial Apogee order I'd throw some of those in, but not worth an order unto themselves.


Ha, I forgot about the old water bottle retainer! Maybe a bit bulky for this application, but I'll fool around with it a bit. I think the water bottle caps have gotten smaller and flimsier since Chris wrote that, so I need to see how the new ones fit.

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It is the one from the link to their newsletter - they use soda bottles and lids. The perfect price - free.

neil_w said:

Right now I'm building a Deuce's Wild which has two friction-fit, 18mm mounts.

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Funny you should bring this up, as I just looked at my build pile and wondered what should be next: Deuce's Wild, or the Screaming Eagle ...

neil_w said:

I am not a fan of friction fit motor mounts for low power stuff, because the tubes are frequently flimsy and I always fear damaging them by making the friction fit too tight. [...] I'm pondering some sort of 3D-printed slim retainer. I think anything with threads will be too bulky and hard to print cleanly at that size, and so I'm instead thinking of some sort of twist-lock mechanism that can be made very small. Anyone ever tried anything like that, or have ideas? It should be simple, *light*, and reliable.

Click to expand...

The challenge with Deuce's Wild MMTs are the highly constrained packaging requirements.
The two canted motor mounts are, effectively, minimum diameter, merging into the main BT-60 airframe at the top. Plus, they stick out the aft end by about 1/2" :
https://archive.rocketreviews.com/reviews/all/flis_deuces_wild.shtml

Thus, retainer options are:
1). Tape over the exposed motor + MMT ends.
2). Retainers that attach to the outside of the exposed MMT BT-20 tubes. Cosmetics will matter, Rocketerium's 18mm aluminum retainer is very pretty and very easy to operate. DIY soda bottle will do, but will be uglier, and you are will be substituting time for $$ to achieve the same outcome.
3). Retainers that embed inside a wider MMT tube (upgrade to a larger diameter MMT (BT-50?). Estes Amazon-style inner retainer with a boat-tail screw is also pretty, but the stock one is needlessly big (BT-55 diameter, with lots of wasted space), so packaging into BT-50 diameter tube would be doable, but tight.

neil_w said:

That said, I still think a tiny minimalist retainer would be fun, and after my success with my twist-lock nose cone retainer it seems doable. I'm still gonna hack around with it in CAD, see if I can come up with something reasonable. Gives me something to do.

Click to expand...

#1 would be easiest, by far.
#3 would be the trickiest to 3D-printer, but if you are looking for a challenge, this would certainly qualify!

You are welcome to borrow my kid's Amazon RTF rocket, if you need hand-on retainer inspiration.

a

P.S.: I think I will go with #1.

I'm gonna give #2 a try, and if it doesn't work then I'll resort to #1. 1/2" exposed should be plenty.

Have you looked at the flip clip? I use these a lot for LPR. Works great. I will put them in fillet for MD builds. Only cost is the tube which you can get at Hobby Lobby. You get 4 12” pieces for a few bucks with coupon. You probably have paper clips.

https://www.apogeerockets.com/education/downloads/Newsletter326.pdf

I’ve had good success with a threaded retainer. It is about the OD of a 24mm tube. The two halves together weigh about 2.5g in ABS. The threads aren’t super durable but I have about a dozen flights on this set so far. I think a bayonet style would probably make more sense at this scale but I haven’t had time to try it. The picture is a bt55 tube for reference.

not bad. I got stuck at work.

Here is what I had in mind. Not saying that it would definitely work, but it's an accurate representation of what I originally envisioned. Design is not final, still have some chamfers and things to add to help ensure smooth mating. The transparent bits represent the end of the motor mount and a motor sticking out 1/4".

Here are the two clips that glue onto the end of the MMT. No dimensions are shown but the whole thing needs just over 1/4" of MMT to stick out the back. I put a small lip at the edge so they'd be easy to get on straight, just push them to the edge of the MMT tube. Also they span slightly *less than* 90 degrees each, so there's a bit of slop available in their placement.


Here is the retainer:


Next three pictures show the mating sequence:


The hard part here is getting the right amount of friction so it'll stay in place but not be too hard to get on or off. The retainer is designed to squeeze the top and bottom of the purple pieces, not to squish the motor mount tube.

I haven't had a chance to calculate weight yet but it should be very light. Sadly 3DHubs changed their business model and are no longer a viable printer for me, so I'll probably end up here begging Chuck or someone to print for me. Not quite there yet though.

What does everyone think, other than that a wrap of tape would be a heck of a lot simpler?

Interesting design. I would have to see it in action.