Build thread: 1 x 54mm + 2 x 38mm in a 5.54" airframe

Discussion in 'High Power Rocketry (HPR)' started by Tim51, Oct 10, 2018.

  1. Oct 10, 2018 #1

    Tim51

    Tim51

    Tim51

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    So this will be the build thread to follow my earlier posts in the Plans section, which some of you kindly offered thoughts and feedback on. As yet I haven't come up with a name, but over the summer I've been cutting and fashioning parts, putting together a kit for a slow winter build over the coming months. Since posting in the Plans section, I've made some small adjustments which will become evident as the build progresses.

    Rendering.jpg
    I've already documented some of the basic airframe preparation in the Plans thread, so if any one is interested you can refer back. What follows is basically a summary of the preparation of the other components.
    First off, the centering rings were laser cut, then drilled for inserts and retainers:

    Drilling thrust plate.jpg

    Thrust plate with inserts (outward facing).jpg

    My aim with this build is to attempt airstarting, so each CR was drilled for a aluminium conduit. Also shown here are the bulkplates for the Av bay coupler.

    Centring rings and Bulkheads.jpg

    For the AV Bay coupler itself, I'm using a standard LOC coupler, with a postal tube inserted as a stiffener. bonded with Gorilla Wood Glue, reinforced inside with fibre glass. Wearing a respirator I painted the outside with CA...
    AV coupler.jpg
    And then squared off the edges with a sander: Squared off AV coupler.jpg

    The fins were cut from 4mm birch ply, then laminated with 200gsm fibreglass. This is an initial dry fit of how it will roughly look:
    Dry Fit 2.jpg

    I say 'roughly' because, of course, I've yet to make the cuts for the outboards, which, as in the colour rendering above, bulge from the sides of the main airframe. The outboard 38mm MMTs are designed to take up to 3G casings. Each motor mount and retainer will be streamlined by a 54mm shroud and fairing as indicated: Rendering with outboard detail.jpg

    To make the fairing for the outboards I first made a pattern using balsa and MDF. This was a balsa NC cut and sanded down half way:
    Mould blank.jpg
     

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    Last edited: Oct 10, 2018
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  2. Oct 10, 2018 #2

    Tim51

    Tim51

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    From the pattern I made a mould using epoxy clay and tooling gelcoat, from which I've now pulled two carbon fibre fairings for the outboards. I'll trim, fettle and fine sand them once I'm closer to fitting them. That's all for tonight... more to follow in due course. Thanks for watching.

    mould and pulled fairing.jpg
     
  3. Oct 11, 2018 #3

    Steve Shannon

    Steve Shannon

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    For some reason this made me think of guacamole …
     
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  4. Oct 11, 2018 #4

    Tim51

    Tim51

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    Indeed! It's a very bright green in daylight, but more like an over ripe avocado under the lamp light...
     
  5. Oct 11, 2018 #5

    Banzai88

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    Where do you get tooling gel coat?
     
  6. Oct 11, 2018 #6

    Tim51

    Tim51

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    I ordered one of these little starter kits
     
  7. Oct 11, 2018 #7

    Andrew_ASC

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    Love the artistic depictions of the concept. Good luck with your build.
     
  8. Oct 11, 2018 #8

    Tim51

    Tim51

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    Thank you.
     
  9. Oct 11, 2018 #9

    viciouspeanut

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    Love the design and impressed with your custom molding. Will be watching.
     
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  10. Oct 11, 2018 #10

    boatgeek

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    The outboard fairings are a nice touch. Did you paint the artist's rendition?

    I've never heard the word fettle used in this context before, but I like it.
     
  11. Oct 11, 2018 #11

    Tim51

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    Thanks! Yes I did the painting - just acrylic on paper. In 'English English' (if you get my drift :)) the verb 'to fettle' does actually mean to trim off sharp edges from a metal casting.
     
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  12. Oct 11, 2018 #12

    Tim51

    Tim51

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    Thanks. I should have noted in the original post that I used 4 layers of 2/2 twill 3k carbon.
     
  13. Oct 13, 2018 #13

    Tim51

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    Ok - time for a little update on this, Ye Olde Englishe Fettling thread, from right here in London's olde East End..

    Yesterday evening I did a quick dry fit of the basic motor mount assembly. As indicated in bright pink on the photo, a section of the airframe will be removed on each side to accommodate the 38mm outboard tubes.

    MMT dry fit.jpg

    But before I get to that, I've decided to epoxy the front and aft CRs to the 54mm motor tube. That will allow me to use it as a dry fit sub frame to support the cut airframe, and make any adjustments whilst dryfitting the two 38s and the fin tabs before installation.

    For the 54mm mount, I'm using a 40cm length of seamless phenolic from Black Cat. I used some little paper stickers to ensure the insert threads were not fouled by the epoxy, and allowed to cure.

    MMT 1.jpg

    This MMT will comprise a total of 4 tubes: the central 54mm, the two 38's and of course an aluminium conduit to allow for airstart igniters. Like the thrust plate / rear CR, the middle CR has two half moons cut out to accommodate the 38s.
    The 54mm tube was marked with a line using an angle, and the two remaining CRs were pushed on to it. The conduit was then inserted, and all three CRs were aligned, using the drawn line:
    MMT 2.jpg

    Fore CR fillet being prepared:

    MMT 3jpg.jpg

    Before the epoxy set, I used an envelope at various points to ensure everything was square:

    MMT 4.jpg

    For the moment I'm leaving both the middle CR dry, so that I can adjust if needed to accommodate the fin tabs. The conduit will also be trimmed down and epoxied later.

    That's all for now. Thanks for watching.
     

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