Spruce Goose Build Thread

Discussion in 'Scratch Built' started by boatgeek, Mar 23, 2020.

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  1. Mar 23, 2020 #1

    boatgeek

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    I've been teasing a few things in the "what did you do" threads, and I finally have enough done to kick off a build thread. This is another rocket from Boatgeek's House of Weirdball Rockets(tm): a mid-power rocket with a spruce airframe. I'm also taking a crack at making this rocket have no metal parts whatsoever, which (IMHO) is pretty tricky for a mid-power bird. Whatever else, this needs to fly FAR 101 so that I can fly it regularly. Keeping it under 1500 grams fully loaded will be a trick.

    The airframe is made with a boatbuilding method for making hollow spars: the bird's mouth. I'm using an 8-sided tube that looks like the one shown in Calculator #8 on the page linked above. I won't round this one, so it will stay octagonal. That lets me use a thinner wall, in this case, about 5/16" (0.31").

    I completely blew the calculations for how much wood I needed, so I grossly overbought. This is a truly magnificent piece of Sitka spruce in the back of my van. The unfinished wood is about 2" x 7" x 10'. I'm using about 4' of the wood for body tube, nose cone, and couplers. Another 2' went to my daughter's boyfriend for Valentine's Day, and the remainder will become 2 paddle blades for our canoe.

    Spruce.jpg

    A friend who also works with our high school rocketry club has far better woodworking equipment than me. After trips through a bandsaw to cut into strips, a jointer to smooth, a thickness planer to get even thicknesses, and a table saw with dados for the bird's mouth, we have 8 pieces that look like this:

    IMG_2545.JPG
    and here's my friend working hard at the jointer:
    IMG_2533.JPG

    I taped the outside faces with masking tape to protect them from the glue and then turned them into a roll sort of like a roll top desk. This let us just paint the glue on to the bird's mouth and then roll it all up at once instead of trying to do things piecemeal. That was a good thing, because we nearly ran up against the cure time of the epoxy.

    Here's the roll of wood, followed by it all glued up. The white is parchment paper. The pens taped to the side help push all of the wood tight, and are held in by electrical tape since you can pull tension on electrical tape pretty well. That worked out well though I had some spots that didn't quite get enough glue. I probably should have applied the glue with a syringe instead of a little paintbrush.

    IMG_2549.JPG IMG_2550.JPG
     
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  2. Mar 23, 2020 #2

    boatgeek

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    And here's the final tube all glued up and pretty. The cardboard wrapped in parchment paper went in each end and helped keep the tube in shape until we got everything taped up for curing.

    IMG_2551.JPG IMG_2552.JPG

    The 48" tube is about 900 grams right now. I need to cut off about 6" from one end where there were some issues with the dado cutting. The final length will be determined by how well I can keep the weight down. The real unknown right now is the nose cone, which will be next.
     
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  3. Mar 23, 2020 #3

    EXPjawa

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    That's really cool, but I'm somewhat disappointed that the output isn't a rocket-powered Hughes Hercules... :D
     
  4. Mar 23, 2020 #4

    boatgeek

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    One step at a time, man. Let's get this thing flying and then see what other fun things people want to do. :)
     
  5. Mar 23, 2020 #5

    neil_w

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    Pretty!
     
  6. Mar 23, 2020 #6

    boatgeek

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    Thank you! The plan is to finish it bright (why paint such beautiful wood?!). I have laminated some cherry veneers to plywood, but I think they're likely too thick and therefore heavy to use for this. We'll see how it looks.
     
  7. Mar 24, 2020 at 4:23 AM #7

    tsai

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    Oooohh... you're having too much fun! I'm seriously jealous! :)
     
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  8. Mar 26, 2020 at 9:42 PM #8

    boatgeek

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    One good thing about everything being locked down here is that I can call on my high school junior to help me glue stuff up whenever I want (well, after 10 anyway). I followed the same basic playbook for the nose cone as for the body tube, except that the nose cone panels are all mitered instead of bird's-mouthed. Here's the nose cone parts plus a 3-D printed jig that will hold everything in place. To get the dimensions right, I made a CAD model in Rhino of the nose cone, then developed the 3-D print off of that. My coworker printed this up for me well before the pandemic hit.

    IMG_2555.JPG

    The original plan was to put glue on each piece, stick it in the fancy jig, and then go on to the next one. Dry fitting demonstrated that was a terrible idea, so we went back to the roll top desk method used for the body tube. T-88 was liberally applied, we rolled up the cone, and cleaned up the inside a little bit. Now we're at this point:

    IMG_2558.JPG

    Once the glue cures a bit, I'll peel out the masking tape and weigh the nose cone shell. I'll also post up some pictures of the body tube, which has corners all planed smooth and then sanded uniformly.

    Next stop is to put some straight epoxy on a scrap piece of spruce and then a thin layer of glass on another piece. I need to reinforce it to make it more durable (you can dent the spruce with your fingernail), but I'd like to do the minimum to minimize weight added. The nose cone will likely get glass no matter what since it'll take more abuse.
     
  9. Mar 26, 2020 at 10:06 PM #9

    Greg Furtman

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    Love it. I've built two small scows (sailboats) and used sitka for the masts & booms. Nice stuff. And it will look good naturally finished. It is beautiful wood. I have a piece left over from way back then that I'm going to turn some nosecones with.
     
  10. Mar 26, 2020 at 11:25 PM #10

    neil_w

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    Love that jig, very cool!
     
  11. Mar 26, 2020 at 11:41 PM #11

    Greg Furtman

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    The nice thing like the bird's mouth & some other joinery is that when you put them under gradual equal pressure around the outside they closely approch round.
     
  12. Mar 28, 2020 at 4:39 AM #12

    Nytrunner

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    8 fins or four?

    Im very curious where youll get an 8 sided motor:cool:
     
  13. Mar 28, 2020 at 4:01 PM #13

    boatgeek

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    4 fins. I'll get some pictures up before long, but I laminated cherry veneers on to 1/8" plywood, so I now have a piece of 1/4" cherry-finished plywood. Should look pretty hot.

    Let's just say I have an interesting motor mount planned. :)
     
  14. Mar 28, 2020 at 4:02 PM #14

    boatgeek

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    I was really surprised when I measured the ID and OD (inscribed circles) yesterday that all of the measurements were with a couple of hundredths of each other.
     
  15. Mar 28, 2020 at 4:10 PM #15

    boatgeek

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    This is the current state of play. I will trim a little more off of the nose cone before final fitup, then will round the corners off a bit so that there's no discontinuities. Next stop will be to test a couple of options for stiffening up the spruce, glassing and just epoxy coating. We'll see which one works better and what's in my weight budget.

    IMG_2561.JPG

    [edit] Nose cone mass is 75 grams at this point. [/edit]

    Now I'll go write on the blackboard "I will use multi-quotes. I will use multi-quotes..." about 500 times. :)
     
    Last edited: Mar 28, 2020 at 4:19 PM
  16. Mar 28, 2020 at 4:27 PM #16

    Greg Furtman

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    I would imagine that the spruce is already pretty stiff & wouldn't need any reinforcing. I know that the mast & boom I made of sitka was pretty stiff. And an oil based spar varnish will give it a nice golden tone & bring out the grain.
     
  17. Mar 28, 2020 at 4:36 PM #17

    boatgeek

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    I want to protect it from normal dings and hangar rash. The tube is quite stiff, but I can easily mark the surface with my fingernail.
     
  18. Mar 28, 2020 at 4:58 PM #18

    Greg Furtman

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    A coat of Epoxy, like WEST, will definitely harden it up.

    Are you a boat geek? Own boats? Make boats?
     
  19. Mar 28, 2020 at 5:14 PM #19

    boatgeek

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    Own a canoe, because I know that the best boat is a borrowed/rented boat. :) My daughter sails competitively for her high school team. Professionally, I design boats, mostly commercial ones like tugboats, fishing boats, and barges.
     
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  20. Mar 28, 2020 at 8:49 PM #20

    Nytrunner

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    Hey that looks like a certain giant pencil rocket nose thats being constructed
     
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  21. Mar 28, 2020 at 9:05 PM #21

    boatgeek

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    Yabbut @neil_w is cheating and using a solid nose. Plus I have 33% more faces and about 50% less craftsmanship in filling and fairing. :)
     
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  22. Mar 28, 2020 at 9:07 PM #22

    lakeroadster

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    Impressive joinery. That should be super strong, lots of area.

    For the finish, how about using something like a spar urethane? I used 3 coats of it on the balsa fins of my lifting rocket and was surprised at how it made the wood grain pop and at how hard the surface was on such a soft wood.
     

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  23. Mar 28, 2020 at 9:12 PM #23

    boatgeek

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    It will absolutely be finished with spar urethane or an equivalent clear. Even if I glass it, the glass will largely disappear making for a really nice wood grain finish. Between the spruce and the cherry it will really pop.
     
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  24. Mar 28, 2020 at 9:12 PM #24

    neil_w

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    You may have 33% more faces, but I have only 25% fewer faces. So there!

    That nose "cone" looks amazing. Can't wait to see what this thing looks like finished.

    (got me thinking that a 5-sided rocket would be cool)
     
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  25. Mar 28, 2020 at 9:32 PM #25

    mbeels

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    Pretty cool! Looks great!
     
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  26. Mar 28, 2020 at 10:24 PM #26

    boatgeek

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    Coated one side of the fin stock with epoxy and made two test pieces for hardening the spruce. The one In the middle is straight epoxy and the one on the bottom has a layer of very thin fiberglass. I think it’s 0.75 oz but not 100% sure.

    CBD0CD1E-261A-4F62-92C7-CBBA65A42D3E.jpeg
     

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