Tripoli prohibition on the use of 3D printed fin cans for L3 certification

Discussion in 'High Power Rocketry (HPR)' started by Steve Shannon, Jun 22, 2019.

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  1. Jun 22, 2019 #1

    Steve Shannon

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    At our monthly BoD meeting we discussed the proliferation of 3D printing. We all agree that the technology is exciting and will undoubtedly lead to some great designs. However, for two reasons the board has decided to prohibit the use of 3D printed fin cans for Level 3 certification flights. The two reasons are:
    1. It’s impossible to know whether the L3 candidate designed the fin can. Simply printing a shared file doesn’t demonstrate expertise or knowledge, which is what certification represents.
    2. The strength of most 3D printed parts is still not strong enough.

    The board has not prohibited the use entirely. We would like it if actual launch experience with 3D printed fin cans gets reported so we can better understand what the risks are from this technology. Our L3 certification procedure will be updated with this prohibition.
    Thanks,
    Steve
     
    Last edited: Jun 22, 2019
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  2. Jun 22, 2019 #2

    jimzcatz

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    FWIW, I agree with that decision.
     
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  3. Jun 22, 2019 #3

    cwbullet

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    I, for one, 100% agree. I think we can print one to work for an M motor, but I do not think it is a good way to get your level 3.
     
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  4. Jun 22, 2019 #4

    rharshberger

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    I too concur with the BoD's decision, I have seen 3D printed parts fail in flight, not once but twice (both were nose cones).
     
  5. Jun 22, 2019 #5

    gldknght

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    Although I'm not Tripoli, I also agree with this. But, I thought pre- manufactured (store bought) fin cans were already prohibited for cert flights at all levels?
    From the Nar Certification rules:
    3. The member attempting certification must build the rocket that they wish to use for their certification attempt.

    Using a pre-made fin can, either printed or molded, seems to me is not building a substantial portion of the rocket.
     
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  6. Jun 22, 2019 #6

    TurbulentSphere

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    Does this prohibit just one-piece 3D printed fin cans, or does it also include 3D printed fins and components? I'm working on designing a 3D printed level 3 project, but the fins are individual pieces that are bolted to an internal structure. I haven't finished the design yet, but the construction will be similar to this (but with a different motor mount): https://www.thingiverse.com/thing:3164819. I have a few more iterations before the L3 rocket (a K-class and L-class envelope expansion), but should I start looking at other methods if it isn't allowed?
     
  7. Jun 23, 2019 #7

    gldknght

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    I think J, K, and L motors are actually level 2, not level 3. Did I miss something?
     
  8. Jun 23, 2019 #8

    TurbulentSphere

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    Those are just the rockets that I have planned before my 3D printed L3 rocket. I've been incrementally expanding the performance of my 3D printed rockets to see what they can handle, rather than jumping straight to an M motor. My fin can designs are printed in multiple pieces and bolted together, so I was wondering if the ban just includes one-piece fin cans, or 3D fins in general. If the ban prohibits 3D printed fins in general, I'll probably just make a tradition L3 rather than continuing my envelope expansion.
     
  9. Jun 23, 2019 #9

    prfesser

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    That was an excellent idea, Steve! Being pro-active is usually better than being re-active.

    As far as 3DP fins go, I think that's going to be up to the TAPs. I'm not on the TAP but personally, the idea of 3DP fins on an L3 leaves me slightly uncomfortable. The jump from L2 to L3 is quite a bit bigger than the jump from L1 to L2. Construction techniques and materials that would be fine for a J motor may fail on an M motor.

    A thought: print a fin that is porous enough to allow epoxy to fill the gaps, using a material that adheres well to epoxy. Soak in epoxy, allow the excess to drain off, cure. Someone else has probably thought of this and done it already, so if you have...how did it work? Inquiring minds want to know!

    Best -- Terry
     
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  10. Jun 23, 2019 #10

    Steve Shannon

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    This is specifically 3D printed fin cans that are glued or bolted to the outside of the airframe or motor case, whether printed in a single piece or screwed together like the Hawk Mountain fin can. We did not prohibit individual printed fins assembled to a traditional rocket body, and we would be very interested in the strength of such fins.
     
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  11. Jun 23, 2019 #11

    Steve Shannon

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    You’re exactly correct, this is an extension of that. We have allowed people to make their own metal fin cans for L3 certification flights with the idea in mind that the skills needed to do so are relevant to L3 certification flights.
    The proliferation of 3D printers makes it possible for anyone to say “ Yeah, I made it myself” without actually having rocket skills.
     
  12. Jun 23, 2019 #12

    cwbullet

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    Hoe did they fail? I think nose cones would not fail often if they are printed with the right material and with the right infill. I have had one rocket fail with 3D printed parts. I made a BT60 rocket with a fin can and nose cone. It flew great with an F so I got brave and flew it with a G or H. The rocket flew to pieces after the body tube bent. The nose cone and fin can survive to fly again despite a hard landing.

    The key to a successful 3D printed rocket for high power is the material you use to print it with and the infill. You would be foolish to use PLA, but ABS or PETG with CF should hold up to most G-J motors with the right infill.
     
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  13. Jun 23, 2019 #13

    cwbullet

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    Concur 100%
     
  14. Jun 23, 2019 #14

    TurbulentSphere

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    I might be ok then. I was planning on bolting the fins through-the-wall to an internal 3D printed structure. So far, predicted stress in the components is similar to the stress in components that I have already flown. I think I'll continue to work up to L-class, and then make a decision where to proceed from there.
     
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  15. Jun 23, 2019 #15

    Steve Shannon

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    Please do keep me updated.
     
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  16. Jun 23, 2019 #16

    gldknght

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    I wonder about those nose cone failures also. When I started my L3 project, I was advised to fill the nose cone with expandable foam. Were those 3DP cones filled with anything?
     
  17. Jun 23, 2019 #17

    gldknght

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    Oh, ok, that makes sense.
     
  18. Jun 23, 2019 #18

    cwbullet

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    No foam in mine. Then again, the highest motor I have used is an "I" so far. I am slowly building up with 3d printed parts.

    I do 10 layers of 100% infill on the outside with 20% on the inside. I learned very early that eye screws would not hold up so I now build in my shock cord attachment.

    Don't get me wrong, I have had low power failures with printed parts so I learned from them. I do not use eye screws or standard epoxy with ABS or PETG. Both are a recipe for disaster.
     
  19. Jun 23, 2019 #19

    Charles_McG

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    What do you use with PETG?

    And not being an L3 flier, what’s so different between a 10G J boost and a 10G M boost? Torque and lateral forces on components because the parts are bigger?
     
  20. Jun 23, 2019 #20

    gldknght

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    I agree, epoxy is generally not compatible with plastic under stress. Sometimes I use epoxy on plastic if the plastic can be roughed up, but I've always thought that was "iffy" at best.
     
  21. Jun 23, 2019 #21

    Locksmith

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    My only suggestion is to be careful with broad words and statements like 3d printed. There are hundreds of different types of plastics nylons and hybrid materials that can be "3d printed" and thats with relatively cheap machines some of which are stronger then aluminum. There are also extreme cases and machines which you can "3d print" metals like steel and titanium again those are ridiculous examples but you get the idea.
    I would leave it up to the TAPS and RSO to determine if something is strong enough or should be allowed.

    Also please dont take a handful of samples and make your decisions on that. 3d printing is just as much an art as it is a science. You can take the same brand and type of filament give it to 10 different people that the ending product will vary drastically in quality and strength. Same could be said with anything related to are hobby.

    Just me 2 cents
     
  22. Jun 23, 2019 #22

    cwbullet

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    This a agree with, but I think an M or N flight might be pushing the limits of safety with a 3D printed fin can.

    This I sort of disagree. This is not the old wild west. You have to have some limits in the name of safety. It can be revisited annually if printers change or if some of the research flights prove the materials have progressed.
     
  23. Jun 23, 2019 #23

    gtg738w

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    I don’t disagree with the general intent but I do hope the ruling is a little more specific.

    Is this just thermoplastic FFF/FDM? Metallic DMLS & LENS? Any additive process? What about an additive core with composite layup?

    Given the rate at which the technology is progressing, it seems safer to have it be subject to TAP approval or a committee review instead of flat out prohibited. At the same time, it is probably not the worst idea to restrict technology development to already certified flyers and require proven construction methods for new cert flights.
     
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  24. Jun 23, 2019 #24

    cwbullet

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    PETG? I use with PVC Pipe Glue or Plastic Epoxies. Both will weld the parts.
     
  25. Jun 23, 2019 #25

    TurbulentSphere

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    It's just my opinion, but I would think that the overall rocket performance would have a stronger impact than just the motor impulse. For example, I've flown 3D printed fins on a J to ~430 mph. My 3D printed L3 project sims out to just slightly faster (~450 mph), the fin span is only and inch wider (6" vs 5"), and the fin thickness is increased. As long as the fin isn't taking the motor loads, the fin doesn't know or care about how big the motor is; it only cares about acceleration and aerodynamic loads that it sees. That being said, a mach 2+ N flight with unreinforced printed fins will probably fail, but I feel that a low-and-slow M flight is a much different discussion. Basically what I'm trying to say is, I think speed and span matters more than impulse.
     
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  26. Jun 23, 2019 #26

    rharshberger

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    Layer seperation, layers were perpendicular to the axis, not sure what the infill percent was but the interior looked like a honeycomb.
     
  27. Jun 23, 2019 #27

    Steve Shannon

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    And that last sentence is all that this does.
     
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  28. Jun 23, 2019 #28

    rharshberger

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    I concur, not all TAPS and RSO's are familiar/experienced (with 3D printing) enough to make informed calls on 3D printed parts.

    I just encountered my first 3D printed rockets being brought to be RSO'd at our June 3 day launch, they were very well done and he flew them (3 models) on progressively more powerful motors (successfully up to H or I). Then as a counterpoint we had a college rocket team (and their very knowledgeable mentor) fly a 4" rocket with 3d printed nose cone that snapped off about a 1/2" forward of the airframe/nosecone shoulder at 500'+AGL (probably closer to 1k), which impacted on the sod two hundred or so yards from the pads.
     
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  29. Jun 23, 2019 #29

    MikeyDSlagle

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    While Steve does mention the strength of the printed parts, the first thing mentioned was not knowing if the flyer actually designed the part himself/herself. There are several manufacturers of aluminum fin cans. They are proven to be strong enough but are not allowed because it doesn't display the knowledge of skill that is looked for in an L3 candidate. Special considerations may can be made if one can prove they have done the homework, designing, testing, and building, such as TurbulentSphere, though his aren't one piece designs either.
     
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  30. Jun 23, 2019 #30

    cwbullet

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    I would say they used the stock setting with a 20% infill. That will fail at high speed. I will keep testing them.
     

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