Scratch Building a L3-class rocket with outboard motors

Discussion in 'High Power Rocketry (HPR)' started by BDB, Nov 30, 2019.

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  1. Nov 30, 2019 #1

    BDB

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    My daughter wants me to build an upscaled version of her scratch-built QuadPod rocket. The easiest way to do this would be to buy a commercial airframe, but she wants to use 10" Sonotube and 4" mailing tubes. The rocket will probably be about 16' long. I'm intrigued by the idea of building a rocket with most of the supplies coming from Lowes, so I might be up for the challenge. Here's a pic of the BT60 version that she designed and built, and the smile that I can't say no to.

    the_before_pic_20190122_1171563012.jpg

    IMHO...mccordmw wrote the Bible on how to build and L3 rocket with Sonotube: https://drive.google.com/open?id=1i4anunZXsiG7dyIP78nmFN7WJrSK-7uAgVvensHU9ok, but this will be even more complicated. I'm assuming that there would be a central 75 mm MMT with 38 mm pods--Likely flying on an 3-grain L motor in the center and some DMS H or I motors in the four pods.

    For starters, I have two questions (there will likely be more to come):

    1) Attachment of the pods: They will stay fixed to the central airframe, but what's the best way to attach them. Of course I can use epoxy, but I was also considering rivets or some other mechanical method.

    2) Nosecones: I'm sure I can 3D print suitable nosecones for the pods, and my printer is large enough to print a stubby 10" nosecone. I can fill the cone with expanding foam for some added strength. Is it possible to wrap a 3D printed nosecone in fiberglass?
     
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  2. Nov 30, 2019 #2

    rharshberger

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    1) Use TTW splines, like fins but they only are internal to the pods, they would allow bonding two sides of the pod (kind of like a fin inside the tube).

    2) Use the 3D printed cone as a plug to make a mold. If you can print a 10" diameter nosecone, do it in sections, then you can have a longer ratio. I believe some slicer programs will allow breaking a print into smaller sections.
     
  3. Nov 30, 2019 #3

    BDB

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    Thanks, Rich. I really like the idea of the spines. I need to watch John Coker’s video about making an FG nosecone from a mold again.

    Ok second question....does anyone have advice on the fin material? I’ll probably laminate some plywood with CF, but the 3/8” maple stuff at Lowe’s is the only 3 ply. Is that sufficient as a substrate?
     
  4. Nov 30, 2019 #4

    rharshberger

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    The Chinese ply at Lowes/HD is crap, it has delaminations, voids and glue starved areas, find a local wood supplier or cabinet shop and purchase some good Baltic Birch ply. I used a skeletonized plywood fin with endgrain balsa filling the holes then several layers of CF for skins on my 5 5" Cherokee-D. The fins were both lighter and stiffer than ply alone.

    https://www.rocketryforum.com/threads/5-5-cherokee-d-l3-build-thread-for-rharshberger.131649/

    There is also Dan's ( last name escaping me) huge Dragonfly upscale, he used 1/2" Endgrain balsa panels with carbon fiber sheet (1/16"?) bonded to each side under pressure, a light and stiff result again.
     
    Last edited: Nov 30, 2019
  5. Nov 30, 2019 #5

    Dave A

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    I buy all my ply from Woodcraft Store. I pick out sheets that are straight, obviously.
     
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  6. Nov 30, 2019 #6

    David Schwantz

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    Aircraft Spruce and Specialty has the best ply. Also Midwest, Balsa USA. If you get the aircraft hard ply, 1/4" stuff is like 9 plys. Will never warp.
     
  7. Nov 30, 2019 #7

    rharshberger

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    Whether plywood stays straight/flat or not has more to do with storage method than anything else, 1/4" ply can be bent around some pretty tight radii just using gravity, time and some moisture. Ply is resistant to warpage but like nearly every wood product is subject to natural expansion/contraction. Aircraft ply while definitely superior to cabinet grade real baltic birch ply is not a necessity for most of our uses in rocketry ( its also quite a bit more expensive. I have been using cabinet grade baltic birch (1/4" 5 ply and some 1/8" ply) often laminated with FG or CF with zero issues, if pushing past transonic then I usually move to a composite sandwich of some sort or solid FG or CF plate fins.

    A full sheet of aircraft ply 1/4" aka 6mm 12 ply is $129 per sheet (4'x4' sheet size), iirc my local Windsor Plywood $25 for the same thickness and 5 ply but 5'x5' (finnish baltic birch 6mm). A full sheet will make quite a few CR's and fins.
     
  8. Nov 30, 2019 #8

    David Schwantz

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    Actually warping has everything to do with this. It will never flutter. A full sheet is 4'x8', way spendy for making fins. But if you look at the other vendors, you can get 12"x12" sheets, even 6"x12", can buy just what you need to cut fins. Seal the surface, no need to glass of laminate.
     
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  9. Dec 1, 2019 #9

    David Schwantz

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    As far as attaching the pods, I am building a sonotube rocket now also. I have found that you need to sand very well for glue. The inside has a waxy surface. I sanded with 36 grit till I got down to paper. Glue now rips the paper off.
    the NC, I have never used 3D printed stuff for glassing, some one else might chip in there. But as rharshberger has said, it would probably make a fine mold. I molded my NC and tail cone with a paper mold. If you do, use several layers with the weave going different directions. this is where the strength will come from. I had a layer of carbon cloth between layers of glass. If nothing else, looks cool. But it is getting covered with a .0015 skin.
     
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  10. Dec 1, 2019 #10

    BDB

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    David, what are you doing for the nosecone on your sonotube rocket?
     
  11. Dec 1, 2019 #11

    David Schwantz

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    I made a mold from thick construction paper,a cone, then layered fiberglass, carbon cloth, and then fiberglass again. Cut it out, sanded, all that good stuff. Covered the outside with aluminum foil tape, added rivets. I also had thought about using foam for support inside, do not know if it would need it, but I will need weight up front anyway. Might as well be usable weight. But have also considered making a rafter style frame inside.
     

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  12. Dec 1, 2019 #12

    llickteig1

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    Having dealt with many sonotube rockets, I have a few tips:
    1) sonotube is heavy
    2) avoid sonotube brand sold at big box home centers. The layers of cardboard have wax in between and can delaminate. We use Concrete Accessories brand.
    3) peel out the inner wax paper layer inside. Better than sanding. It peels pretty easily.
    4) you can peel outer paper layers to get smoother surface and reduce weight. Helps bonding too.
    5) lay up the tube with two wraps of fiberglass or carbon cloth.
    6) nominal diameter sonotube comes in 2-3 nested actual diameters. Watch this if you buy more than one piece. The sizes aren't close enough to make couplers, though.
    7) stacked construction foam disks can be used to make large nosecones. Cut a set of disks in decreasing diameters. Bond together. Shape with hot wire cutter or Sureform tool and lay up with fiberglass.

    These projects are a blast. There can be frustrations with imprecise materials but eventually you get it figured out. Cool rocket. Enjoy!

    --Lance.
     
    Last edited: Dec 1, 2019
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  13. Dec 1, 2019 #13

    rharshberger

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    Looks fantastic
     
  14. Dec 1, 2019 #14

    BDB

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    Thanks for the extra details. This looks very cool.
     
  15. Dec 1, 2019 #15

    JohnCoker

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  16. Dec 2, 2019 #16

    David Schwantz

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    Thanks guys.
     
  17. Dec 2, 2019 #17

    BDB

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    John, this is the most incredible thing I’ve seen lately! GREAT work!
     

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