MMT Hard Point

Discussion in 'High Power Rocketry (HPR)' started by Brian-Tampa, Feb 15, 2020 at 11:21 AM.

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  1. Feb 15, 2020 at 11:21 AM #1

    Brian-Tampa

    Brian-Tampa

    Brian-Tampa

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    Hello,
    Just ordered a Madcow Super DX3 Fiberglass kit.

    In this build, I prefer not to drill into the top CR to attach a shock cord eyebolt for recovery. Seems like drilling a 1/4" TO 5/16" hole in the top fiberglass CR will dramatically weaken it.
    And I'm not too fond of the method of epoxying the end of the shock cord along the MMT, as it doesn't allow for the future should the shock cord need to be replaced.

    I am thinking of using a Giant Leap 54mm vented hard point epoxied to the top of the MMT for recovery attachment. Has anybody used these hard points and is this an acceptable practice?

    I'm setting up the DX3 as follows:
    Single deploy.
    54mm MMT
    Aeropack tailcone retainer w/38mm adapter

    I currently use SU motors in my builds, but I like having the option of using reload casings in the future.
    I am extending the stock MMT to 16" so that I can run 14" motors. The stock tube is a little short once the hard point gets epoxied in place.

    Any input will be greatly appreciated!!!

    Brian
     
  2. Feb 15, 2020 at 11:25 AM #2

    Flyfalcons

    Flyfalcons

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    With an eye bolt or U-bolt installed into the centering ring, you won't need to lengthen the motor mount tube to accomodate longer motors. If you are concerned about strength then you could laminate a ply backing on the centering ring where the attach point is located. Keep in mind the TTW fins will transfer the bulk of the power load to the airframe so the upper CR just needs to be able to handle recovery load.
     
  3. Feb 15, 2020 at 11:42 AM #3

    Brian-Tampa

    Brian-Tampa

    Brian-Tampa

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    Thanks for the input. I really appreciate it. What you are saying makes good building sense.

    I really enjoyed your Youtube Drago build.
    I have watched each episode 2X.
    The Drago will probably be my next rocket after the DX3 gets built.
    I am actually flying today in Plant City Florida with my latest build.
    A Loc Precision EZI-65 set up for single deploy. I built it extra strong to handle an L2 certification next month.
    It's set up for 38 or 54mm motors.

    Thanks.
    Best Regards,
    Brian
     

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  4. Feb 15, 2020 at 12:23 PM #4

    David Schwantz

    David Schwantz

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    Brian, I have seen others epoxy a pin through the main air frame tube for recovery attachment. I have not done it myself, but it seemed to work well.
     
  5. Feb 15, 2020 at 4:33 PM #5

    blackjack2564

    blackjack2564

    blackjack2564

    Crazy Jim's Gone Banana's

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    First 15 or so rockets I built used E-bolts through Cr's, then I switched to gluing kevlar Y harness to MM. in 17yrs of flying over 150 rockets...NEVER had either fail. My oldest and first one with harness glued to mm has 116 flights in 15 yrs.
    Why you would not want to follow instructions or proven methods is beyond me.
    Google is your friend try looking up strength of materials, then decide.
    You can always use E-bolt in top closure and eliminate both or modify closure in single use by drilling hole through side of forward closure and using kevlar loop for recovery attach. Done this also.
    Good luck have fun...don't overthink or overbuild stuff as many starting out do.:)
     
    afadeev likes this.
  6. Feb 15, 2020 at 10:28 PM #6

    Brian-Tampa

    Brian-Tampa

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    Jim,
    Thanks for the input. As this is my first fiberglass build I am just a little concerned about the top CR on this kit.
    It is only 1/16" thick and only 13/16" between inner and outer circumferences.
    It is just so much thinner the the wooden centering rings I am used to. I guess I am probably worrying over nothing.
    (54mm MMT to 4" airframe)
    I was just worried that drilling a 1/4" to 5/16" for an eyebolt I would be drastically reducing the strength of that CR.
    I was just looking for an alternate way of attachment.

    I greatly value your advice as a seasoned flier and I will opt for the harness epoxied to the MMT as per your advice.

    I would have replied sooner, but I was at the field today in Plant City Florida.
    I had 2 great flights on my latest build despite the wind. (See included pics)
    I140W 1900'
    I280DM 4400'

    Again, thanks for the feedback!
    Best Regards,
    Brian
     

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  7. Feb 16, 2020 at 12:33 AM #7

    blackjack2564

    blackjack2564

    blackjack2564

    Crazy Jim's Gone Banana's

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  8. Feb 16, 2020 at 3:37 AM #8

    John Kemker

    John Kemker

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    If we're talking G-10 or G-12 fiberglass for the CR, then 1/16" isn't going to be significantly weakened by drilling a hole for an eyebolt. You should be fine.
     
  9. Feb 16, 2020 at 9:04 AM #9

    o1d_dude

    o1d_dude

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    I have used the 38mm Giant Leap hard point to rebuild a zippered tube fin rocket. I have since launched it quite a few times and recently inspected the kevlar recovery train. No obvious damage seen.

    I use the y-harness method which Crazy Jim explained to me. The "legs" of the Y are laminated to the motor tube. My thinking is that I will either 1) lose the rocket long before the kevlar becomes unusable or 2) will no longer be able to hike over hill and dale to retrieve rockets due to my geezerly-ness. Getting old sucks but it's still better than the alternative.
     
  10. Feb 16, 2020 at 2:46 PM #10

    Brian-Tampa

    Brian-Tampa

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    Jim,
    Wow! Thanks for this link.
    It will provide a wealth of info for my FG build.
    Best Regards,
    Brian
     

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