Build Thread MIM-23 Hawk

Discussion in 'Low Power Rocketry (LPR)' started by Bruiser, Sep 14, 2018.

  1. Sep 14, 2018 #1

    Bruiser

    Bruiser

    Bruiser

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    Since my DIY nose cone lathe is stalled while I figure out some hardware issues I decided it's time to start on what may turn out to be a new collection of rockets for me.

    This is an Army Hawk based around a BT-20 body tube. It's designed around the Estes 13mm engine and if all goes well I hope to also build a Little John, a Patriot, and a Nike Smoke along with others.

    Hawk MissileS.jpg

    I purchased an Estes Mini Comanche kit to use for parts. It has enough body tube and engine mounts to build at least two, maybe three, rockets this size. I just need nose cones, that's why I am also building a diy mini lathe :)

    Back when I scratch build R/C airplanes, I would usually start off by building a kit, so that is where I started with this.

    Hawk Parts1.jpg

    The fins are going to be layered with a center core of 1/32 bass sandwiched by 1/16th balsa. Then the fins will be sanded to the correct profile similar to those of an Honest John.

    To be continued...

    -Bob
     
    Last edited: Sep 14, 2018
  2. Sep 20, 2018 at 10:00 PM #2

    Bruiser

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    I've been struggling with the fins on this one, more than my last two builds of BT-60 sized rockets. While waiting for the primer to dry again I went to work on the motor mount.

    I glued in the engine block so 1/4 inch of the engine would protrude for removal. The rear centering ring is positioned where it is because of the boat tail.
    Hawk Mount.jpg


    Next I glued the motor mount assembly in the body tube. I left a little of the rear centering ring sticking out to glue the boat tail to.
    Hawk Mount Installed.jpg


    Next I was able to come up with the pattern for the boat tail after a few tries.
    Hawk Boat Tail.jpg


    I've never done a boat tail before. I guess I use a little cwf to smooth it all out. I confess that I should have positioned the seam where it would be covered by the conduit that runs up two sides of the rocket but I only just thought of it :(

    -Bob


    Tomorrow, it'll be back to the fins :)
     
    Last edited: Sep 20, 2018 at 10:07 PM
  3. Sep 21, 2018 at 12:13 AM #3

    Rex R

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    did you make the transition(boat tail) from card stock or...? I've made mine from cardstock using a 'glue behind tab'(not an overlap joint), followed up with ca and glazing/sealing putty then sanded smooth (320 grit).
    Rex
     
  4. Sep 21, 2018 at 1:44 AM #4

    Bruiser

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    It was from card stock (um, paper plate) and I did use a glue behind tab, but it still "peaked" at the glue joint. So CA it first, then the filler? Thanks for the help.

    -Bob
     
  5. Sep 21, 2018 at 1:50 AM #5

    rharshberger

    rharshberger

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    To help mitigate the "peaking" I use a butt joint with backer strip instead of a lap joint. It helps reduce the peak so that CA and sanding are usually all thats required to make as close to perfect as its going to get.
     
  6. Sep 21, 2018 at 1:55 AM #6

    Rex R

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    an old mouse pad and a 1/4 - 5/16" diameter rod help to 'roll' your paper to the right shape.
    Rex
     
  7. Sep 21, 2018 at 4:32 PM #7

    Bruiser

    Bruiser

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    I've been working on the fins but for some reason I am having trouble with these ones. They are balsa with a bass core. I am having to apply cwf several times to fill the grain. I had a separate thread going about cwf and fins. I have two fins done now, one just got sprayed and I can see it will need some cwf touch up and the fourth just got a major coating of cwf. I did a little experiment with the cwf on the first coat that just didn't work out. I used alcohol to thin the cwf in a effort to have it dry faster. Well I think it was too thick to actually get in the grain to fill it. I know it went on a lot thicker. I have decided to try papering the fins on my next rocket. Here's the two that are done:

    Hawk 2 Fins Done.jpg


    Argh, maybe not. I just realized that you can see the "line" too well so I need to do a little masking and a quick spray to accent one "face' from the other. Anybody know what to call them? Facets maybe?

    -Bob
     
  8. Sep 21, 2018 at 5:31 PM #8

    neil_w

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    For what it's worth, you didn't need the through-hole fins here. Those fins have a *lot* of root surface area, and would have been fine, especially with 13mm motors.

    The TTW construction certainly doesn't hurt, though, just making an observation.

    It's funny, when I've tried using a backer strip I get worse results, because I can't get the curve of the double-layered part to match the rest of the shroud. I must be doing something wrong.

    Rule #1: whenever possible, position the seam so it'll be underneath something (mine seem to most often find themselves underneath fins)
    Rule #2: you can smooth out anything. Don't worry, it'll be fine. :)
     
  9. Sep 21, 2018 at 5:51 PM #9

    Bruiser

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    The tabs on the fins are there more for alignment more than strength. My bi-focals trick me into making things look true from one angle and then off true if I shift my view just a little. I usually take them off to do close up work. The distance I'll be working on the fins at while gluing them is to far to see with my glasses off so the tabs will help :)

    I am going to apply some CA to the transition to stiffen it up some and then determine the next step.

    -Bob
     
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  10. Sep 21, 2018 at 5:59 PM #10

    kuririn

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  11. Sep 21, 2018 at 6:01 PM #11

    Drumguy89

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    Looking good! I really like this design.
     
  12. Sep 21, 2018 at 10:33 PM #12

    neil_w

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    I like this answer so much I'm going to adopt this on my Alcubierre build. I have two veeeeeeery long wings to attach, and am going to put small TTW tabs on them just to help with alignment. Thank you sir. :)
     
  13. Sep 24, 2018 at 3:37 PM #13

    Bruiser

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    I had a little set back this morning. I was planning on gluing the conduits to the side of the body tube. I was reviewing my pictures when it occurred to me again that either my fins looked to large or my body tube is too short. So I compared my model to the drawing and determined that my body tube was .375 of an inch short. I started to agonize over what to do, leave it alone or make it longer... since it's a small rocket and the cg is close to the cp as designed, requiring nose weight I realized that leaving it shorter was a step in the wrong direction. I cut off a section so I could use a coupler to join in the new corrected length body tube without having the coupler interfere with the shoulder of the nose cone.

    Hawk Body Tube longer.jpg


    I attached the new length of body tube with the coupler and Elmer's Carpenter's Wood Glue followed by an application of CWF.

    Hawk Tube Lengthened.jpg


    All this for 3/8ths of an inch. But I think it was the wise thing to do. It would have been over an inch if I was building a BT-60 size rocket. The coupler will also add some weight to the nose which should help with moving the CG in the correct direction.

    -Bob
     
  14. Sep 24, 2018 at 3:47 PM #14

    kuririn

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  15. Sep 24, 2018 at 6:39 PM #15

    Bruiser

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    Thank you! I'm not sure what happened as I was working off the drawing that I measured it against. All I can think of is that I measured from the wrong lines when I cut the body tube.

    I have the conduits on and have been working the bead of glue for a fillet on them and the two fins that are glued on. It's pretty tough getting my finger in there between the fin and the conduit. I tried a q-tip but it allowed the glue to squeeze out "above it" on the fin and the radius of the fillet was small.

    Hawk Conduit Glue.jpg

    -Bob
     

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