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  1. #1
    Join Date
    15th June 2011
    Location
    Macon GA
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    2,376

    Right-angle joint reinforcement

    A couple of my existing rockets have fin structures with right-angle joints, most notably:


    Those angles take a real beating on landings. Even with generous epoxy fillets, and sufficient parachute and landing on grass, I've had these angle joints break on landing - usually separating off a layer of plywood!

    Does anybody have a suggestion on how to retrofit something to strengthen this joint? I'm leaning toward using some triangle stock, but I'm open to other ideas.

    Keep in mind, I am referring to already built and painted models.

    Thank you in advance for your input.

    Last edited by LW Bercini; 11th October 2017 at 03:08 PM.
    LW Bercini
    TRA #00134 / NAR #18121
    Seeing "the box" from the rear view mirror since 1970


    I don't measure my enjoyment of the hobby in Newton-Seconds

    It never ceases to amaze me how so many otherwise intelligent people have such poor reading comprehension.

  2. #2
    Join Date
    19th January 2009
    Posts
    1,228
    Triangle stock or dowels on the inside, layer of paper or light fiberglass over the outside. You can also make them stronger by making tabs to interlock the two parts to give more surface area for the glue.

    kj


  3. #3
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
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    1,030
    Triangle stock or dowels on the inside, layer of paper or light fiberglass over the outside. You can also make them stronger by making tabs to interlock the two parts to give more surface area for the glue.
    +1. All great advice.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  4. #4
    Join Date
    2nd May 2011
    Location
    Frisco/Plano TX
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    4,547
    Quote Originally Posted by kjohnson View Post
    Triangle stock or dowels on the inside, layer of paper or light fiberglass over the outside. You can also make them stronger by making tabs to interlock the two parts to give more surface area for the glue.

    kj
    Yep what he said!. I've had the same problem with one of my boost gliders (box wing). I ended up putting a glass strap on the inside and outside of the joint. It's a fix but not clean when done after the fact--next time I'll just build it in so it's clean.---H
    Anyboby see that caboose go by----I lost my train of thought again! There's leaders and managers, those who can't lead just manage

  5. #5
    Join Date
    27th January 2015
    Location
    Rochester, NY
    Posts
    1,971
    I built an upscale Orbital Interceptor, which has a T-tail configuration. I tabbed it together and used triangle stock on the inside. Its held up well. I built a slight upscale of a BSG Laser Torpedo, which is similar to the Photon Disruptor. Tabbed fin plates, but no reinforcing stock. I got several flights before a fin end plate broke off, through the tab hole. And I'm still occasionally flying a Pro Series Jayhawk, but I seem to break a wing tip fin near every flight, despite heavy epoxy fillets. Bottom line is, the more you can reinforce the joint the better. Most of these are also good candidates for using a Chute Release and oversized parachute...
    Rick
    URRG, MARS
    Tripoli #15706
    http://www.rocketreviews.com/rick-barness-page.html

    There is a theory that states that if anyone figures out why the universe is for or why it is here, it will suddenly disappear and be replaced by something even more bizarrely inexplicable.
    There is another theory that says this may have already happened...



  6. #6
    Join Date
    15th June 2011
    Location
    Macon GA
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    2,376
    Quote Originally Posted by EXPjawa View Post
    I built an upscale Orbital Interceptor, which has a T-tail configuration. I tabbed it together and used triangle stock on the inside. Its held up well.

    <snip>

    Most of these are also good candidates for using a Chute Release and oversized parachute...
    I flew my upscale Orbital Interceptor at my last launch. It hit directly on the T-tail on landing. The horizontal piece split in half where the tab went through. The repairs will feature not only triangle stock, but glassing as well.

    But yeah...big chutes and JLCR for these puppies from now on....
    Last edited by LW Bercini; 12th October 2017 at 05:05 PM.
    LW Bercini
    TRA #00134 / NAR #18121
    Seeing "the box" from the rear view mirror since 1970


    I don't measure my enjoyment of the hobby in Newton-Seconds

    It never ceases to amaze me how so many otherwise intelligent people have such poor reading comprehension.

  7. #7
    Join Date
    27th August 2011
    Location
    West Tennessee
    Posts
    2,649
    Thinking outside the box. Is there a way to place a shock cord anchor on the OUTSIDE of the body tube, either at or just tailward of the rocket CG MINUS post deploy (CG with expended engine without nose cone and recovery) so the rocket dangles from the shock cord either horizontal or slightly tail up. Some sort of small external ring or loop. At launch you will have a short length of Kevlar attached will you run up and stuff into the tube with nose cone over it. So at launch might not look perfect (although if it is on the rail button or launch lug side, no one will see it) and will be a little less aerodynamic (but these are show birds not altitude or speed demons.)

    All the previous ideas are also good, but may be hard to implement on an already painted rocket.
    It is amazing what you can do when you don't have a choice.

    Smart people learn from their mistakes.
    REALLY SMART PEOPLE learn from OTHERS' mistakes.

  8. #8
    Join Date
    5th February 2009
    Location
    Montreal, Qweebec
    Posts
    3,142
    What about adding some thin plexi? What I'm suggesting is a 1" long (ish) piece of 1/16" plexi, and added from the free edge of the vertical fin (the edge that isn't joined at 90) to, say, the 'turbine motor tube' (the tube fin) or the joint thereof. Essentially, you're adding a wide strut from the fin tip to body tube (or somewhere convenient), making the 90 fin a triangular tube fin.. box it in..

    -paul

    NAR# 101258 - L1
    www.CRMRC.org
    I don't know the same things you don't know..

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