Build thread - ‘3DEX’, a mostly 3D printed rocket

Discussion in 'Low Power Rocketry (LPR)' started by matthewdlaudato, Nov 29, 2018.

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  1. Nov 29, 2018 #1

    matthewdlaudato

    matthewdlaudato

    matthewdlaudato

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    A quick post hoc thread on a rocket I designed and built. Parts printed on my Prusa i3 MK3 3D printer. Designed in Autodesk Fusion 360 and Tinkercad (also from Autodesk). 3 fins and a nose cone built around an 18” Fliskits BT-60 tube. More to follow. Much of this was discussed on a dedicated 3D printing thread (link below), but thought build thread couldn’t hurt:

    https://www.rocketryforum.com/index.php?threads/3D-printer-plunge.146985/
     
    Last edited: Nov 29, 2018
  2. Nov 29, 2018 #2

    matthewdlaudato

    matthewdlaudato

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    Here’s the fincan design in Tinkercad. I’ll get the files posted somewhere - maybe just make them public on Tinkercad. I used an airplane wing that was available in Tinkercad for the fins. Simple shoulder for mating with the body tube. Walls are 2mm thick.

    IMG_3249.jpg
     
  3. Nov 29, 2018 #3

    matthewdlaudato

    matthewdlaudato

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    The finished fincan. Printed with Amazon Basics PETG filament. Early version of the motor mount shown, but I redesigned it later. Also shown is the shock cord mount, through which Kevlar thread will be tied.

    IMG_3238.jpg
     
  4. Nov 29, 2018 #4

    matthewdlaudato

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    Redesigned the motor mount to have a threaded retainer cap. The thread tool in Fusion 360 plus some YouTube instructional videos made this very straightforward to design. Motor tube also has an integrated engine block. Here’s a picture of the motor mount and shock cord mount as they appear in Slic3r, the slicing program that I use. My printer is on the home network via a Raspberry Pi, so I can send jobs directly to the printer with ease.

    IMG_3250.jpg
     
  5. Nov 29, 2018 #5

    matthewdlaudato

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    The printed motor tube and retainer cap. The threads are perfect and the cap screws on/off easily.

    IMG_3247.jpg
     
  6. Nov 29, 2018 #6

    matthewdlaudato

    matthewdlaudato

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    Shock cord mount shown at top of the fincan. All my new low power builds will use this approach.

    IMG_3239.jpg
     
  7. Nov 29, 2018 #7

    matthewdlaudato

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    Assembled everything in a few minutes. Used Loctite Super Glue Ultra Control, a thick CA gel that is known to work well on PETG plastic. Everything seems to be holding together.

    IMG_3252.jpg
     
  8. Nov 29, 2018 #8

    lowga

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    I've constructed and flown a couple of 3-D printed rockets from Boyce Aerospace Hobbies.

    http://boyceaerospacehobbies.com

    Certainly 3-D printed parts and complete kits are going to be part of the future for this hobby. Specialized applications for a very limited quantity market are where 3-D printing really shines.

    There is a learning curve for anyone interested in building and flying these types of rockets. Sanding and finishing these rockets is more involved than for most airframes. Wet sanding is a must if you want a nice looking finish.

    These rockets also react differently to the forces involved as well. Recently, I suffered a shock cord failure when the kevlar shock cord came loose from its attachment point on the rocket. The shock cord was glued with CA between two parts of the airframe.

    The failure was not in the design, but all mine. I used kevlar cord that was too thick, and probably not long enough. (3-5 times the length of the body tube is a must with these rockets.)

    Attempting a field repair, I opted to drill a small hole in the airframe and attach new kevlar through this hole. That resulted in a "horizontal zipper" on the next flight. Which made total sense when you think about how 3-D printing works. The "layers" of the rocket separated under the force of ejection.

    Lesson learned for future builds.

    Also, these rockets are heavy. Keep that in mind when running your simulations and selecting motors.

    I'm in the process of building a new rocket that will test 3-D printed fin cans on a carbon fiber frame at mach+ speeds. We'll see how the material reacts to those stresses.

    Learning stuff and having fun. Thanks for sharing this build thread. Can't wait to see how it flies.
     
  9. Nov 29, 2018 #9

    matthewdlaudato

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    Finished rocket weighs in at 146g minus chute. Good candidate for an Estes C or Q-Jet C or D. Very happy with the result, and got some good design and printing experience that should make future 3D builds go more smoothly.

    IMG_3253.jpg
     
  10. Nov 29, 2018 #10

    matthewdlaudato

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    Those are great comments and experience, thanks. See below, it’s on the heavy side so larger motor will be used. Proof will be in the flying as always. Total length of Kevlar plus elastic is around 4x rocket length.
     
  11. Nov 29, 2018 #11

    matthewdlaudato

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    I plan on finishing the body tube only on this rocket. I also expect to just use colored filament in the future and dispense with painting the printed parts altogether. But good tips.
     
  12. Nov 29, 2018 #12

    lowga

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    Matt,

    Nice looking rocket. I'm very interested in hearing how your shock cord attachment works. No reason to suspect it won't be able to handle the strain. My failure was based on my own errors, not that of the design or recommendations from Boyce.

    Did you consider going with a traditional shock cord mount inside the tube rather than the 3-D printed solution? I'm assuming the weight of the nose cone was why you opted for something more secure.
     
  13. Nov 29, 2018 #13

    matthewdlaudato

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    I did consider it, but thought the integrated mount with an ‘easy to replace cord’ feature would be interesting and functional. I’d be surprised if it failed, but then again, rockets do have a habit of surprising us!
     
  14. Aug 10, 2019 #14

    matthewdlaudato

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    Update: I flew the rocket once on a QJet C. Great up flight but the combination of a too small chute and hitting the metal leg of someone’s canopy cracked a fin. I did a small redesign to make the fincan fully modular and easy to replace. More later.
     
  15. Aug 11, 2019 #15

    matthewdlaudato

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    The mod was to use a screw-in approach. I lengthened the motor tube and threaded the top end. Then I designed and printed a threaded ring and mounted it with thick CA into the body tube. The new fincan unit just screws in. So if I ever need to replace it, I print the parts, assemble (which takes about 5 minutes), screw it in and off to the flying field.
    IMG_3710.jpg IMG_3711.jpg
     
  16. Aug 11, 2019 #16

    OverTheTop

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    Nice rocket Matthew.

    One point where you can add a little finesse to your design is to add fillets to the 3D print designs, usually as the last step in the drawing process (it just makes the modelling easier that way). Sharp corners increase mechanical stresses and can be a point of weakness or failure. The stress increase is inversely proportional to the radius:
    https://en.wikipedia.org/wiki/Stress_concentration
    Here is an area in point that is visible. If you add a radius, at each circled location, it is really easy to reduce the stress :).
    IMG_3250.jpg

    The same thing applies to anywhere where there is a sharp corner, like inside the screw-on cap.

    Keep up the good work!
     
  17. Aug 11, 2019 #17

    matthewdlaudato

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    Thanks, that’s a good tip.
     

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