3D Printing Printing Tips for Fins and Other Structural Components

So, over in this thread, it was brought up that there is not a lot of documentation on how to build 3D printed fins. Rather than join in the argument over there, I thought it might be helpful to some people to start a public repository of information on 3D printed fins and other components. I know a few people on here have flown them, so please feel free to add tips/tricks/rules/guidelines/suggestions/flight data/etc.

I’ve flown a number of rockets with 3D printed fins (listed here). The low power ones are single piece fin cans, but the larger ones have bolt on fins. I use separate fins so that I can have a fin span larger than the print bed. I bolt them on for three reasons: it is easier (for me) to analyze, glue is messy, and I can replace the fins if they break (this was very important with my grid fin rockets).

For structural analysis, I use a copy of Solidworks to do FEA, but fins are simple enough that hand calcs are easy to do as well. The tricky part is the structural properties of the plastic. I print with PETG, which has a listed tensile strength of about 6 to 7 ksi depending on where you look. Sources usually don’t say if that is the base material or as printed, so I have been using 3 ksi as my maximum allowable stress to be on the safe side. Based on some load testing that I have done, the max stress is at least 1 ksi.

For flutter analysis I use the Apogee newsletter article: https://www.apogeerockets.com/education/downloads/Newsletter291.pdf. The hard part again is the plastic properties. I have found a few different values for the shear modulus of PETG, but the lowest I have found so far is ~73 ksi. Based on the second flight of Checked Baggage, the lower limit is around 23 ksi.

As far as infill, layer height, etc, I generally use a 0.2mm layer height with a 0.4mm diameter nozzle on a Prusa I3 MK2s. I try to model my parts so they can be printed solid or nearly solid as this makes the analysis a lot easier to do. If somebody does want to print something non-solid, I found this article talking about strength as a function of infill: https://my3dmatter.com/influence-infill-layer-height-pattern/. I know a lot of people worry about the strength between the layers. I have broken a few parts and they rarely break along the layer lines, but it is probably safer to treat the layers similar to a wood grain. As far as how much weaker the layer adhesion is, I don’t have an answer.

I rarely do analysis for the smaller rockets and use the traditional cargo cult “that looks thick enough” engineering. I started doing some simple analysis once I got to mid power, and I am doing a lot more now that I am in the level 1 and 2 range. I hope people find my ramblings helpful, and I look forward to what other people share.

I have made bulkheads, fins, fin cans, centering rings, nose cones, and body tubes. No issues, except I have found that you probably should line them with a motor tube.

I’ve taken to 4 wall thickness and 40% cubic infill. We’ll see how fins hold up. Life intervened, so I won’t be flying it this weekend.

TurbulentSphere said:

I know a lot of people worry about the strength between the layers. I have broken a few parts and they rarely break along the layer lines, but it is probably safer to treat the layers similar to a wood grain. As far as how much weaker the layer adhesion is, I don’t have an answer.

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Does that imply that the layer lines should run parallel to the leading edge of fins, just as the grain direction, when using Balsa Wood, for example ?

Dave F.

Ez2cDave said:

Does that imply that the layer lines should run parallel to the leading edge of fins, just as the grain direction, when using Balsa Wood, for example ?

Dave F.

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I don't know, it depends on how strong the layer adhesion is. It is possible that it is weaker but still "strong enough" that it doesn't matter. All of mine are roughly perpendicular to the airflow rather than parallel to the leading edge. Once my next spool of filament comes in I might print a couple with the layers in different directions and try breaking them to see how they compare.

Ez2cDave said:

Does that imply that the layer lines should run parallel to the leading edge of fins, just as the grain direction, when using Balsa Wood, for example ?

Dave F.

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Nope, the laters alternate so it is often stronger than woods.

I ran 3D printed canards on my Vertical Trajectory system.






Some of the strength test results are down the page on this thread here: https://forum.ausrocketry.com/viewtopic.php?f=56&t=5324&start=45

Print strength is very material and process dependent, as can be seen in the results.

OverTheTop said:

I ran 3D printed canards on my Vertical Trajectory system.

View attachment 387773

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Some of the test results are down the page on this thread here: https://forum.ausrocketry.com/viewtopic.php?f=56&t=5324&start=45

Print strength is very material and process dependent, as can be seen in the results.

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Well said; That's exactly why blanket statements concerning printed parts do not work. I do understand why it's easier to prohibit them in certain situations than attempt to validate them. As far as tips go.... well that's another quandary. Every printer/software package, operator and material used has variations that can't be easily quantified. Personally I prefer to slightly over extrude at a higher temp for better layer bonding (material dependent). What works for me and my "stuff" may not work at all with yours (since we are mostly tinkerers, most printers are modded in more than one way). Best we can hope to do is offer "advice" that can be tested by each individual and that in itself is usually enough to get someone on the right track. It is then up to them to follow up, not blindly accept and run with it.

Thanks for this thread, it seemed like too many arm chair quarterbacks in the other one who as we all know are just spectators with opinions but have no skin in the game
Any manufacturing method can be faulty or wonderful
I have seen some really sloppy glassed materials that flew and failed

3D printing requires a lot of skin in the game as even a small printer can be $300-$500
fins can be made with a hand saw and sandpaper with a lot of labor

carbon fiber was not used in rocketry until a few years ago and I make a lot of parts from carbon fiber

jbr said:

Thanks for this thread, it seemed like too many arm chair quarterbacks in the other one who as we all know are just spectators with opinions but have no skin in the game

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I've seen several "real-life" quarterbacks who think they are the whole team . . . That's usually when the blocking gets a little "sloppy" and the QB suddenly "discovers his limitations".

So far I have had no issue with 3D printed components in model rocketry. Low power up to F at this point.

The only issue with 3D printed fin cans that I have is balancing CG and CP. More aft weight means longer rockets and/or heavier payloads. So there is an efficiency consideration.

I have flown small rockets that are entirely 3D printed, minimum diameter, on D motors.

There is a lot of work and skill to design, prototype and arrive at a production solution for each mission. As others note above, the expense and maintenance of the equipment is a whole other consideration.

It seems unfair to consider 3D printed rocketry as 'easy' or unqualified for serious consideration.

From my perspective, I will not likely buy another rocket kit ever again unless it has some historical significance. I will continue to progressively explore 3D printed model rocketry, all the way to high power.

The bolt together concept mentioned above is definitely on the to do list soon.

Ez2cDave said:

I've seen several "real-life" quarterbacks who think they are the whole team . . . That's usually when the blocking gets a little "sloppy" and the QB suddenly "discovers his limitations".

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Yup, wait, what? Ya had me, then Ya lost me...

Wallace said:

Yup, wait, what? Ya had me, then Ya lost me...

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Try reading it in context to the text I quoted . . . If you're still lost, good luck !

Ez2cDave said:

Try reading it in context to the text I quoted . . . If you're still lost, good luck !

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The only "way" I can take "the blocking gets a little sloppy" means intentionally allowing someone to fail. That should never happen, especially when there are safety concerns?

How many posts does it take to go off topic? 13 or so... Ha ha.

jeffjblack said:

...

There is a lot of work and skill to design, prototype and arrive at a production solution for each mission. As others note above, the expense and maintenance of the equipment is a whole other consideration.

It seems unfair to consider 3D printed rocketry as 'easy' or unqualified for serious consideration.

From my perspective, I will not likely buy another rocket kit ever again unless it has some historical significance. I will continue to progressively explore 3D printed model rocketry, all the way to high power.

...

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(Thumbs-up emoji)

jbr said:

carbon fiber was not used in rocketry until a few years ago and I make a lot of parts from carbon fiber

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As wikipedia would say "citation needed." Most of the HPR guys have been aware of carbon fiber and many of us have used it for decades. (I rolled my first C.F. airframe in 2000 and used it for reinforcement even earlier.)

It's easy to make blanket statements in many ways. It's clear to me that:

1. additive techniques will eventually take over all low-volume manufacturing
2. current hobby printers aren't capable of printing parts with the same mechanical properties as other techniques

3D printing is a huge boon to hobbies of all kinds. I jumped in myself a several years ago (2013) and have been printing cosmetic parts for my rockets since then. There are waves of printers making their way down-market that look very promising. At least one metal printers dropped below $100,000 and there are printers that can make fiberglass-reinforced parts for under $10,000. I'm guessing within 10 years the only significant limitation for hobby purposes will be build volume. (We'll be buying airframe tubes and printing everything else.)

https://www.flightglobal.com/pdfarchive/view/1971/1971 - 1784.html

keep in mind few is a relative term as opposed to many

This is really strong filament
https://atomicfilament.com/collecti...rbon-fiber-black-petg-pro?variant=26891393281

jbr said:

This is really strong filament
https://atomicfilament.com/collecti...rbon-fiber-black-petg-pro?variant=26891393281

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How do they do that without clogging?

jbr said:

https://www.flightglobal.com/pdfarchive/view/1971/1971 - 1784.html

keep in mind few is a relative term as opposed to many

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Huh? That makes no sense, relative or otherwise.
You said carbon fiber has only been used for a few years. John asked for a citation, explaining that he has been doing it for at least nineteen years (more than a few by any relative comparison) and you produced an article from 1971 describing the first five years of CF use. Simple addition means CF construction began 53 years ago.
I think that most would agree that “a few” generally refers to two, three, or occasionally four.

we can have a debate on what the definition of words are

Contending that his statement that "there's nothing going on between us" had been truthful because he had no ongoing relationship with Lewinsky at the time he was questioned, Clinton said, "It depends upon what the meaning of the word 'is' is.

https://www.merriam-webster.com/dictionary/few

2007 is the earliest reference google can find
https://www.rocketryforum.com/threads/carbon-fiber-model-rocket.109275/

and it appears I should have said several which is much more vague than a few, but is more than a few and way less than many

Don't always trust Google. I found one a few () months earlier than that: https://forum.ausrocketry.com/viewtopic.php?f=10&t=9

I didn't think it was important to do a MANY hours search and perform a statistical analysis

OverTheTop said:

Don't always trust Google. I found one a few () months earlier than that: https://forum.ausrocketry.com/viewtopic.php?f=10&t=9

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I saw Anthony Cesaroni fly a beautiful carbon fiber rocket named the Hyperion at Roc Lake in Canada in 2002, 2003, or 2004.

Materials are evolving quickly. Here is a switch mount bracket I just had printed in glass-reinforced resin. That is straight out of the printer. Very stiff and strong. For scale, walls are 2mm (80mil) thick.

Fascinating. It what manner is it glass reinforced? Small fiber pieces?

Is there material data for it?
(Attempting to get this thread back on track)

Since you asked nicely...
https://formlabs.com/materials/engineering/

It was printed in the "rigid" resin. They are not very forthcoming with details of what is in the resin, but there is a datasheet that has the material properties on that web page. I expect it to be small fiber pieces as you have said, although I can't feel anything like that on the surface of the print. Could be something like microballoons or crystals I suppose. Maybe I should put it under the microscope at work!