3D printable 38mm motor retainer and 38 to 29mm motor adapter

[swatkat]

I was going to get started on these before the flying season here in California kicks off, but if anyone has already done them, no sense in reinventing the wheel. I've seen 29mm retainers and 29 to 24 mm adapters, but nothing in the 38mm flavor. Anyone out there know if these already exist?

 

[cavecentral]

Guessing an adapter would melt. 29mm can get pretty hot.

 

[Cl(VII)]

You could just take the existing 29mm version and scale it in your slicer program.

29mm retainer on Thingiverse: https://www.thingiverse.com/thing:882815

 

[dr wogz]

Personally, I wouldn't.

Machined aluminum will withstand much more heat & force than a layered 'low melt' plastic assembly..

 

[Andy Limper]

Ditto


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[swatkat]

I haven't had any issues with the 29-24mm adapters printed with high temp Cheetah from Ninjatek (10's of launches with them). I don't expect any with the 38mm.
https://ninjatek.com/wp-content/uploads/2016/05/Cheetah-TDS.pdf

 

[Jdog13]

Personally, I wouldn't.

Machined aluminum will withstand much more heat & force than a layered 'low melt' plastic assembly..

I have used 3d printed retainers on high power 29mm motors multiple times. A few weeks ago I did one with an H180 and it worked fine. The plastic had no sign of melting and it was just as easy to remove the motor. I even used pla, so other plastics probably work the same or even better.

 

[Dugway]

I have multiple launches on a 29mm retainer (https://www.thingiverse.com/thing:882815) that I 3d printed in PETG. Still looks brand new. I was going to try out a 38mm or 54mm version but haven't had the chance yet.

 

[cbrarick]

Hmmmm.
A2938 is $24 and made of non-meltable aluminum

CTI 3 grain case and closure is $40 or AT hobby line case at $64

Not sure how much the 3D material is but I don't think the difference is worth it. The Aluminum is good until you leave the hobby.

 

[Tonimus]

How to find out for sure: Fire a 38mm longburn motor with a few thermocouples attached to the outside of the case and aft closure. Find out how hot it gets. Test the retainer in the oven at 20% above that heat level. If it holds, call it good.

 

[cavecentral]

A retainer just has to keep the motor from coming out during ejection. An adapter has to hold up to the thrust as well. It should start ok, might take some heat damage by the time it lands. Will it be ok for a second flight, or blow through with a thrust ring type failure?

 

[Jdog13]

Hmmmm.
A2938 is $24 and made of non-meltable aluminum

CTI 3 grain case and closure is $40 or AT hobby line case at $64

Not sure how much the 3D material is but I don't think the difference is worth it. The Aluminum is good until you leave the hobby.

The $24 adds up quickly if you need one on every rocket. A 3d printed plastic one cost <$1 in material and can be printed in Ann hour or two. I know many are still skeptical, but I swear these 3d printed retainers work just as good as the aluminum. Pla softens around 65C, but remember that the motor is only hot for a few minutes. There is no where near enough heat to deform the plastic quickly and since it is only hot for a brief period there is no signs of heat deformation.


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[cbrarick]

I don't know why you'd have more then one. You can only fly one rocket at a time.
I fly a lot and have only owned one.

Motors get hot and stay that way, until you pull them out of the rocket. When I did my "tightbond only" rocket I spoke with Jeron and we talked about how long the heat could remain - from burn, getting hotter and hotter, until ejection charge. There tends to be a bit of a soak after that until you get the motor out - via conduction and convection. There's little chance to get meaningful cooling until you recover the rocket and remove the motor.

 

[Jdog13]

I don't know why you'd have more then one. You can only fly one rocket at a time.
I fly a lot and have only owned one.

Motors get hot and stay that way, until you pull them out of the rocket. When I did my "tightbond only" rocket I spoke with Jeron and we talked about how long the heat could remain - from burn, getting hotter and hotter, until ejection charge. There tends to be a bit of a soak after that until you get the motor out - via conduction and convection. There's little chance to get meaningful cooling until you recover the rocket and remove the motor.

How do you remove the retainer after epoxying it on?


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[Locksmith]

Might want to look into some of the new High temp PLA's they can withstand over double the heat of standard pla and even more if you anneal them. Unfortunately there might be alot of trial and error since they are relatively new and have bad habit of slightly shrinking during the Anneal process so might have to compensate and print it 5% bigger.
Just google or youtube search for high temp PLA there are alot of info out there.

 

[cbrarick]

One adapter, many retainers. I generally fly 38's on J's so the "not worry about the motor" part of this is worth it. Without being too much of a dork, I have to ask if a $25 retainer breaks the bank, then what about the $26 and up reload you're throwing in the expensive case? On the other hand, sure am glad you guys push the limits. Hopefully success and failure are reported, so we can establish where the limit really is. Drogue charge size and expected pressure would be interesting data too....

 

[Jdog13]

One adapter, many retainers. I generally fly 38's on J's so the "not worry about the motor" part of this is worth it. Without being too much of a dork, I have to ask if a $25 retainer breaks the bank, then what about the $26 and up reload you're throwing in the expensive case? On the other hand, sure am glad you guys push the limits. Hopefully success and failure are reported, so we can establish where the limit really is. Drogue charge size and expected pressure would be interesting data too....

I agree with you on the adapter. I wouldn't try to make one of those. About the cost of the retainers, $25 is a whole other reload that could have been used to launch the rocket. Also, retainers can either work or not work, there is no in between. So, if you can spend a fraction of the cost and get the same functionality it makes sense to go with the plastic. Then again, a professionally made retainer looks and feels better which could convey some to prefer them.


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[qquake2k]

I would love to see a $5 38mm retainer. I can make an adapter.

 

[BDB]

I'd love to see a plastic 38 mm retainer. I upgraded my Scion to fly on Loki 38 mm G motors. The most expensive part of the build was the Aeropack.

 

[MikeyDSlagle]

I've been working on some clip style retainers. So far manufacturing cost is higher than I would expect folks to pay for them. I can get the cost down, but I don't have the capital to get the ball rolling. Too high a risk ATM.

I have some printed from some high temp stuff, don't know exactly what but the guy said it was high temp stuff. Was looking to try Onyx's HTHS (high temp high strength) fiberglass material but haven't moved forward with it. I have three prototypes for 54mm, one of which will be going up Saturday just to see if it will deform and how soft it will be when I get to it. I'm using other positive retention as well so if it doesn't hold I won't lose my casing and rocket.

I'd love to see a plastic 38 mm retainer. I upgraded my Scion to fly on Loki 38 mm G motors. The most expensive part of the build was the Aeropack.

For Loki I use an aluminum ring and two threaded inserts. 100 8-32 inserts for 13 bucks at McMaster and use your favorite 8-32 screws and it can be retrofitted to any rocket. I have plenty of scrap aluminum so I am out less than 50 cents a rocket. Grab a 1/8" x 3" x 12" piece of flat bar from McMaster for less than 4 dollars. Same principle as kaplow clips or washers and bushings but easier. Slide the motor in, slip on the ring and put in two screws. And don't dare say you prefer tool-less retainers when you need specialty tools to assemble your Lokis! LOL.

I prefer the flush aft end rather than having a large Aeropack sticking out there. I use them, but I recess them to get the aft end as flush as possible. This makes thrust plates impossible though.

 

[swatkat]

I would love to see a $5 38mm retainer. I can make an adapter.

I'll bring some 38mm retainers (screw type) to the first SARG launch. Shouldn't be more than about a dollar in materials. I've got some 38mm G's to launch there as well, so we can test how they hold up. I've used the adapters there in Ninjatek all last year for the little 29 to 24mm (Pro series) launches and never had an issue with softening, etc. I'd agree that for the high thrust/long burn a more significant adapter is needed (which I have), but for the 29mm hobby line motors in the 38mm motor mount, the concerns above are unwarranted with the right materials.

 

[ttabbal]

I'll dig around, I started modifying an OpenSCAD script to build a motor retainer. I had to tinker with clearances and such, but I think I have a usable version now. If I can find the file, I'll post it here. It looked like it should work with most 38mm motors, and being SCAD, would be able to be scaled to whatever size you like. For longer burn/hotter cases I was thinking of using a couple O-Rings to insulate the plastic from the case. It would still get a bit hot, but even simple Buna-N rings can take the heat. It might not even be necessary though.

 

[qquake2k]

I'll bring some 38mm retainers (screw type) to the first SARG launch. Shouldn't be more than about a dollar in materials. I've got some 38mm G's to launch there as well, so we can test how they hold up. I've used the adapters there in Ninjatek all last year for the little 29 to 24mm (Pro series) launches and never had an issue with softening, etc. I'd agree that for the high thrust/long burn a more significant adapter is needed (which I have), but for the 29mm hobby line motors in the 38mm motor mount, the concerns above are unwarranted with the right materials.

I look forward to seeing them. I'm not planning on flying any 38mm G's at the SARG launch, but I will be flying at least one 38mm H at the Snow Ranch launch.

 

[swatkat]

I look forward to seeing them. I'm not planning on flying any 38mm G's at the SARG launch, but I will be flying at least one 38mm H at the Snow Ranch launch.

Me too I was just referring to the 38mm G's as that is what is allowed at the launch. They aren't particularly long burn, (G67R's) but 1.4 seconds of heat should give a good idea on what to expect. I'll also try a few hobbyline 29mm G's with the printed 38-> 29 adapters. Longest burn would be the G53. I've got a few of those and a Single use G38 we can also use.

 

[ttabbal]

Here's the SCAD and 38mm STLs for a simple threaded retainer. You might need to adjust the ID to match whatever MMT you use.

Note that it has not been flight tested, so if you do try it, please post and let us know how it goes.

View attachment Retainer.zip

 

[ecarson]

This is something I've been doing a bit of work on as well. Here in this photo, is sort of a progression of design.

On the left is basically the Thingiverse 29mm scaled up to 38mm. I used PLA on this, as just a proof of concept and fit for the
tube. I decided it took up way too much real estate on the tube end (a full inch), and decided to shrink it down on just the
Z-axis. I used Blender to do the resizing work, rather than use the scaling routine in the slicer.

In the middle is the "Z-shrunk" design. The threads squeezed down on both parts, and it took quite a bit of hand finishing
to get the threads to mate and engage properly. This takes up about 5/8" of the tube end. I also made this from ABS
plastic, after having built an enclosure for the printer, which worked very well.

On the right is the latest iteration. I added material to the base of the cap. After "Z-shrinking" the cap, there was less than
2mm in the web to resist the kick back of the motor. The new web is about 4mm thick, which was more than the 1st iteration
of just a bit over 2mm. This cap and thread body are made of ABS as well. It fits my 38mm motor tube very nicely.

I'm still debating on whether or not to actually try this on my first Level 1 rocket, but it is encouraging to read above that even a PLA
retainer works. If anyone is interested in actually testing this, before I get my Level 1 rocket completed, I can upload the .stl files.
No guarantees of course. The two zipped files are 725 KB in size.

 

[Eric]

Aaron, I can help you with any field testing of your printed items. I have a couple rockets that are still seating motor retention, if I don't just friction fit.

 

[JohnCoker]

It's not just the heat coming off the motor case; you also have to consider the heat from the exhaust that will linger at the aft plate due to base drag (or not use them with much-above-minimum-diameter rockets).

 

[ttabbal]

Quick thought for testing. If you can, use electronic deployment and a threaded forward closure. Then put an eyebolt in the FC and tether it to the rocket or even use it as the main recovery attachment point. Then, if the plastic fails, you still have something holding the motor case and the rocket together.

 

[swatkat]

Would love to print these out too. I'll bring a few to the next launch and we can run the gamut. I only have four 38mm G motors though, so likely 1 or 2 each.