Fin encapsulation?

Discussion in 'The Watering Hole' started by John Williams, Dec 4, 2019 at 3:56 PM.

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  1. Dec 4, 2019 at 3:56 PM #1

    John Williams

    John Williams

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    So I've decided to build a rocket out of some hunks of spare PML quantum tubing and fins that I have hanging around. I'm gonna reinforce the qt with a full length phenolic coupler from the upper centering ring forward..and I'm thinking about fin encapsulation with foam . Never done it before what do you guys think? Foam or go with regular epoxy fillets? I'm also trying to decide between a 54 and a 75mm mount again any advice?
     
  2. Dec 4, 2019 at 7:05 PM #2

    Nytrunner

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    If you want to fly much past mach, the coupler reinforcement is a good idea (75mm you say?).

    Up to you on the foam. Some folks swear by it, others say it's more weight where you don't want it on the rocket.
     
  3. Dec 4, 2019 at 9:11 PM #3

    AfterBurners

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    I use both. I'll do internal fillets and then foam the fin can. As far as weight I would have to look how much it really added to my 3" PML Spitfire, but I like the idea of filling the can with foam. Makes a solid rocket. IDK personal preference I guess? I think it added a few ounces? When I do stuff like this I always make sure I record it in RS on the rocket so I get an accurate weight of it and know how much weight to add etc. I also like to foam the nose cone. I drill a hole near the tip of the nose cone to insert a wooden dowel and then add the weight and estimate what the foam might come in add.
     

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  4. Dec 4, 2019 at 9:25 PM #4

    cerving

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    I've used "fin dams" on some projects (including my L3), i.e. strips of balsa between the centering rings that you fill up with epoxy then simply insert the fins into. Once they cure, the fin is pretty much never going anywhere... it's completely encapsulated between the dams and the motor/body tubes.
     
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  5. Dec 4, 2019 at 9:38 PM #5

    John Williams

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    I think I'm gonna go fillet and foam and then reinforce with a full length coupler she will be heavy but she's gonna be built like a tank. Im gonna call her "Dumpster Fire"
     
  6. Dec 5, 2019 at 3:14 AM #6

    blackjack2564

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    What material are fins, and thickness. Just as important as beefing airframe.If it's 4in. quantum waste of money/time going 75mm motor unless just 2-4 grain size.
    3 or 4 fins?
     
  7. Dec 5, 2019 at 9:34 AM #7

    cbrarick

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    foam adds weight - in a place we really don't want it. It's contribution to strength is questionable - especially since our rockets are vastly overbuilt anyhow. CJ's questions are spot on....as usual
     
  8. Dec 5, 2019 at 3:02 PM #8

    John Williams

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    It's 4 fins G10 why not the 75mm even if it's reinforced with the couple you don't think it will hold up?
     
  9. Dec 5, 2019 at 3:03 PM #9

    John Williams

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    Fins are.093
     
  10. Dec 5, 2019 at 4:38 PM #10

    John Williams

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    So because this rocket is being largely made of spare parts I was kinda using it as an excuse to try some crazy stuff I think the 75 mm mount is probably too big but I'm gonna go for it anyway. I'm gonna weight the nose if I have to to keep it stable. I'm not sure it will hold together but I think it will be fun. At the very least it will help me learn more about the limitations of the materials right? I like you guys advice Im gonna ditch the idea of foaming especially with the big motor mount it adds too much weight to the back of the rocket I think and I think stability is gonna be the big challenge of the day.
     
  11. Dec 5, 2019 at 4:49 PM #11

    richP

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    Need more info.
    Are the fins of a larger profile? Anticipated speed?
    Just quickly thinking about it, and 3/32" fins on a 75mm motor is asking for a shred unless you really stick to the tame stuff.
    Otherwise, you won't be getting much benefit by going with both internal fillets and foam, just choose one or the other.
     
  12. Dec 5, 2019 at 4:55 PM #12

    John Williams

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    The fins are big they are 9 inches long with a 6.5 inch span
     
  13. Dec 5, 2019 at 5:09 PM #13

    John Williams

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    Also I have no idea how fast it will go I'll put together a sim sim later this afternoon and check it out.
     
  14. Dec 5, 2019 at 5:17 PM #14

    BackSeatBryce

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    Hey John,
    For what it's worth, I can share my experience with my L1/L2 build that was based on a 4" PML Tethys using quantum tube and .093 G10 fins. It has a 54mm motor mount, and I used large fillets between the fins and the body tube (both inside and outside surfaces of the body tube) and between the fins and the motor mount. I didn't use any filler foam. My fillets are pretty generous, maybe 0.5-0.75" radius? I've flown this rocket up to around 0.75 mach, and it has survived all flights and all landings with a pretty high descent rate (like 40-45 ft/s). I have noticed that 5-minute epoxy is fairly easy to peel up from the quantum tube if you get a knife under an edge, but if you rough up the quantum tube pretty well and clean it properly, I haven't had any structural failures.
    -Bryce
     
  15. Dec 5, 2019 at 5:24 PM #15

    Nytrunner

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    That is very thin indeed. I'm leaning towards C-Jim's point
     
  16. Dec 5, 2019 at 5:26 PM #16

    John Williams

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    That's great to hear I have a PML endeavour and an explorer. I used to have a 1/4 patriot but it zippered during a flight at Midwest power but all three rockets have survived some very hard landings including the Patriot taking one ad a descent of 50 ft/s main chute tangled no fin damage though. Unfortunately my beloved patriot is now this monstrosity
     

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  17. Dec 5, 2019 at 5:32 PM #17

    John Williams

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    Awww well unfortunately I already ordered the fins I guess I was so worried about the airframe I didn't consider the fin issue. Well I'm gonna throw it together with what I have and maybe I'll get lucky and she will stay together. Thanks so much for everyones advice seriously I love hearing everyone's experience and knowledge about the builds ! I'll keep you guys posted no matter what it's gonna be alot of fun
     
  18. Dec 5, 2019 at 5:40 PM #18

    blackjack2564

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    There are 2 ways a new flier learns to build.
    1. just throw stuff together and hope for the best.
    2. ask experienced fliers that have "been there...done that" for advice and listen to what they say. Read, study, learn the failure points of material used, build around that. It's all there on google.

    The path you choose will have a dramatic impact on your wallet and enjoyment of the sport.:)

    Ps if you using a thinner fin with good bevels and Mach angle leading edge....you may get way with it, BUTa 6.5 span on Quantum 4in. airframe with 75mm mount, is just asking for naming your rocket...''shread me too" or flutter me gone.

    Good luck, have fun
     
    Last edited: Dec 5, 2019 at 6:47 PM
  19. Dec 5, 2019 at 5:54 PM #19

    richP

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    :eek:

    There are a few excel calcs on TRF that can help you calculate fin flutter velocities (estimations).
     
  20. Dec 5, 2019 at 6:24 PM #20

    jd2cylman

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    No one has mentioned the fact that temperature also plays a huge part in QT survive-ability. QT is only good to about 35° to 40°. After that it gets pretty brittle. And if you use the PML pistons, there's the dreaded shrinkage in cold weather... Personally, I'd use a 54mm mmt or switch to Phenolic w/glass overlay or WM FG tube.
    I finally retired my PML Endeavour 4" because I got tired of repairing the QT. Although to be honest, the fin can never gave me issues, just the upper tubes. But it only has the 54mm mmt.
     
  21. Dec 5, 2019 at 8:21 PM #21

    CoyoteNumber2

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    Don't forget the 1/2" all-thread between centering rings.
     
  22. Dec 5, 2019 at 8:47 PM #22

    boatgeek

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    Thin wide fins might be a good time to play with tip to tip glass/carbon. A few layers should stiffen the fins up nicely, particularly if you aren't crossing Mach. If you are going that fast, check with PML about reinforcement since they say QT shouldn't got past 0.85M.
     
  23. Dec 5, 2019 at 10:39 PM #23

    John Williams

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    Thanks for your advice I will look at other options for fins. In my defense I ordered a predesignated fin size using a calculator on PML s website and that is what it spat out it just had me enter the size of the body tube and the motor mount and what shape I wanted and it generated the dimensions.....so I am trying to listen but I feel like sometimes I'm getting different information from different places. As for the quantum tube I was planning to reinforce the tube with a phenolic coupler. Maybe I can find better fins some where maybe carbon fiber or something....or maybe I'll just order a smaller motor mount and step it down to a 54
     
  24. Dec 5, 2019 at 10:44 PM #24

    John Williams

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    So in my defense I didn't just arbitrarily come up with this big giant thin fin I just used a predesignated fin from PML I entered some numbers picked a shape and it gave the dimensions. I wasn't aware of the issues I will look into different fins or I will change the engine mount thanks everyone
     

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  25. Dec 5, 2019 at 11:19 PM #25

    Nytrunner

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    Don't worry, your fins aren't a complete loss. You can build a Q-tube rocket, omit the tube reinforcement, use the exact fins you've ordered, and still have a nice, light, flying rocket. You just can't push it as fast as some motors will (see Jim's "only 1 or 2 grain 75s")

    Not every rocket has to be a mach-buster.

    Get your parts, throw your design into OpenRocket, download FinSim or another calculator to learn about flutter, and keep flying!
     
  26. Dec 6, 2019 at 12:39 AM #26

    cbrarick

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    or trash the quantum tube, go to loc tubing (yes you will have to pay for it) and then no problems...
     
  27. Dec 6, 2019 at 1:38 AM #27

    AfterBurners

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    You are building a tank for a rocket and you are worried about adding a little expanding foam?
     
  28. Dec 6, 2019 at 1:40 AM #28

    John Williams

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    I'm worried about the cg not the weight o guess I could just weight the nose
     
  29. Dec 6, 2019 at 1:43 AM #29

    John Williams

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    Right now I'm working on the fin flutter calculations rock sim has it over Mach 1 on an l800 but I don't have finsim so I'm doing the numbers manually
     
  30. Dec 6, 2019 at 2:04 PM #30

    MikeyDSlagle

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    When and where is this thing flying? Just so I won't be there. :D

    You can modify the fins. Fiberglass isn't a mystical material that can only be machined elsewhere. Cut them down if need be. Reach out to PML and change the order of possible.

    There is only one commercial 1grain 75mm I think, and it is pretty aggressive. A good fun challenge would be to keep it light enough to fly it on 1grain 54mm, those are fun little motors.
     

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