Post-Mortem on Broken Star Orbiter Fin

Discussion in 'Low Power Rocketry (LPR)' started by BlastoffDFW, Nov 19, 2018.

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  1. Nov 19, 2018 #1

    BlastoffDFW

    BlastoffDFW

    BlastoffDFW

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    This weekend, I managed to snap a TTW fin clean off my new Star Orbiter. Any advice on how to do better next time?

    The fin itself survived intact (probably because it was papered), except for a little bit of balsa that ripped off of the TTW tab. It took a good bit of paper from the body tube off with it.

    IMG_2356.JPG IMG_2357.JPG

    The construction used Titebond (original). Am I right to think that using epoxy would not have mattered, because the glue I used was already stronger than the paper/wood that failed?

    My current ideas:
    • Bigger parachute (slower descent)
    • Bigger fillets
    • Built a fin slot (walls on the side of the TTW portion) to give extra lateral support
     
  2. Nov 19, 2018 #2

    Nytrunner

    Nytrunner

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    You're right, you had paper delamination there instead of glue failure. Did you scuff the glassine surface at all prior to gluing? Removing that outermost thin layer of tube will give you a rougher surface for the glue to seep in (I use TBII). Heck, lay some wood glue on the break surface and stick them back together.
    Bigger fillets may help, try Titebond Quick&Thick (Lowes or online). I've heard 8% of fin root as a fillet baseline before.

    How did it land? Did it strike a rock or cornstalk?
     
  3. Nov 19, 2018 #3

    BlastoffDFW

    BlastoffDFW

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    I didn't see the landing, but nothing nearby it could have hit. Just a dry field, strong wind, and reefed 18" chute. I'm guessing it swung into the ground sideways.
     
  4. Nov 19, 2018 #4

    kuririn

    kuririn

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    Some landing damage is to be expected, that is the bane of flying model rockets. Per Mr. Binder, no advantage in using epoxy over wood glue for cardboard/wood models. He would know. Fiberglass rockets are more "bulletproof" than cardboard, less damage upon hard landings. For your model however, your ideas will all mitigate landing damage. To that I would add:
    * Using a JLCR along with a bigger chute will cut down wind drift while slowing descent rate.
    * Not only bigger fillets, but also doing internal fillets along the inside of the body tube/fin seam and the motor tube/fin joint.
    * The fin slots you refer to are called gussets, and they will reinforce the fin joint.
    * Try flying on grassy fields instead of hard ground.
    * Try rigging the chute so the rocket lands either horizontally or nose down. Dings on a plastic nose cone are easily repaired compared to a broken fin.
     
    Last edited: Nov 19, 2018
  5. Nov 19, 2018 #5

    Jozef

    Jozef

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    You have a clean separation at the glue joint. The paint is also peeling from the airframe. a good indication of adhesion issues. Next time you glue fins, border the glue area plus 1/8 or so on each side. Mask that area and sand with something like 100 grit paper to roughen the surface. The extra 1/8 allows for small glue fillets. Wood glue is perfect for that rocket it is light. epoxy would require the same ritual. Also, double glue your mating surfaces...fin tabs and airframe
     
  6. Nov 20, 2018 #6

    dhbarr

    dhbarr

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    Put a small spill hole in your chute so it doesn't whiplash into the ground next time.
     

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