Using Lexan for clear fins on a rocket

Discussion in 'Low Power Rocketry (LPR)' started by Bill S, Sep 10, 2019.

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  1. Sep 10, 2019 #1

    Bill S

    Bill S

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    I recently picked up some 1/16" Lexan online to use for clear rocket fins. I saw that Micromeister said that he has used 1/16" Lexan on some builds with good results. When I picked up the sheet of Lexan, it seemed to have a lot of flex, and I have to wonder if it might be too flexible to make good fins or not. I've never tried to make clear fins before, so I wanted to make sure I haven't made a mistake and need to get something thicker.

    Fins should be 1.25" at the base, 1" at the tip, length is 1.75".
     
  2. Sep 10, 2019 #2

    rharshberger

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    What motors are you expecting to fly the rocket on?
     
  3. Sep 10, 2019 #3

    JohnCoker

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  4. Sep 10, 2019 #4

    Bill S

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    I'm using A through D engines only.
     
  5. Sep 10, 2019 #5

    K'Tesh

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    One thing that you can do is create a method of replacing the fins should one break during a hard landing. Another thing would be to make sure that you are not using a shape that is likely to impact the ground first... Something like the Cherokee-D/Optima shape would improve the odds that the bottom of the rocket would impact first. Additionally, if the motor protrudes even further below the body tube, it could help absorb the forces of a hard landing.
     
  6. Sep 10, 2019 #6

    James Harechmak

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    Years ago I scratch built one to fly on D-12's and used 3/16 lexan for the Nike style fins without any issues.
     
  7. Sep 10, 2019 #7

    K'Tesh

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    But that is 3X thicker than the OP's fins.
     
  8. Sep 10, 2019 #8

    rharshberger

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    1/16" should be good for A-D motors.
     
  9. Sep 10, 2019 #9

    Bill S

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    I did think of using a fin shape that wasn't swept back, but that is a good idea.

    I had planned to use the drill holes in the bottom side of the fins for the epoxy to get a better grip on the material, but is there a better way that allows for fin replacement? I suspect it would be a lot of work trying to replace a broken fin that was epoxied to the body tube, without causing more damage to the tube.
     
  10. Sep 10, 2019 #10

    neil_w

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    Can you cut these sheets with the score and break method, assuming you want straight cuts? Is there a thickness limit to doing that?
     
  11. Sep 10, 2019 #11

    K'Tesh

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    Slotted body tube, with an internal separate fin can... Here's what I've got in my mind... Let's presume you're using a BT-50 motor mount and your fins have a 3" long root edge, you could glue in your motor tube (say length of fins +2" long) with a pair of CRs... One at the top of the fin (plus an extra CR length), and the other at the top of the motor tube... Glue a couple of pieces of balsa to a piece of BT-52 that is the length of the fin *PLUS* the length of two extra CRs. The balsa acts as an internal slot for the fins. The fins slide into their slots, and are ziptied in place (you'll need to drill a couple of holes for the zip ties), the BT-52 is friction fit to hold in place (possibly pinned?). If a fin breaks, you pull the fin can out, remove and replace.

    BT-52 is available at eRockets.biz
     
  12. Sep 10, 2019 #12

    burkefj

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    I'm using 1/16" triangle shaped plycarbonate fins for my lightweight 10" atlas missile, works fine, I use thinner for low power, for my 8" pershing I used 3/32" because the weight was not an issue so much.
    It is flexy but that's good, it doesn't break easily unless you score it, so be careful at notches/square corners for fin tabs etc and you should be fine. I personally think removable fins on a low power rocket is overkill and added weight, I've done removable only on my highpower 20# titan II with 3/16" fins that had hardwood slots on root and forward/rear centering rings with a plastic screw that went into the rear CR and up into a slot in the fin to retain it, never needed to remove them but it was there. I had a non ejection that came in ballistic, only thing left were the unbroken fins.

    Frank
     
  13. Sep 10, 2019 #13

    beantownJPL

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    Yeah, Lexan (polycarbonate) is really strong stuff. It doesn't surprise me that the only thing that survived is the fins.

    My dad makes sailing kits for inflatable boats and kayaks and such (www.sailboatstogo.com ... shameless plug...). He uses Lexan for leeboards and rudders and such. They never break.
     
  14. Sep 10, 2019 #14

    Alan15578

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    Yes! Follow this link and grab the tech note 017, and read it.
    Use what you have and you will be fine, at your size and power, even polystyrene should work OK.

    My preference is acrylic (Plexiglass). It is stiffer, clearer, and more scratch resistant. Polycarbonate (Lexan) is stronger, and tougher, but is easy to scratch and mar the surface. If you crash land in parking lot, the acrylic may well break, but the rest of the model will also be destroyed.

    BTW, CD and DVDs are also polycarbonate, and can be a cheap and readily available source for small fin stock.
     
  15. Sep 10, 2019 #15

    GaryT

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    Agreed with JohnC and the other's, Lexan will prove quit resilient for fins. I used to build saltwater fish tank filtration systems with it, its not to hard to flame polish the edges to a high gloss clear finish as well.
     
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  16. Sep 10, 2019 #16

    aerostadt

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    I have used 1/16" polycarbonate on small models. I've made slots and pins, so that they can be removable. If I go slow and careful, I can cut them with a scroll saw.
     
    Last edited: Sep 10, 2019
  17. Sep 10, 2019 #17

    mikec

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    I've flown a 4xG cluster with 0.090" Lexan rectangular fins with no issues.
     
  18. Sep 10, 2019 #18

    Bill S

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    I was planning on using tin snips/shears, as I've read some people here use them on thin Lexan.
     
  19. Sep 10, 2019 #19

    Nytrunner

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    Which G's? That makes a difference lol
    ( I have no doubt the fins were fine)
     
  20. Sep 10, 2019 #20

    burkefj

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    I use svore and break up to 1/16", otherwise scroll saw. 1/16 is too thick for shears, thats for very thin rc car body material
     
  21. Sep 10, 2019 #21

    moondawg

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    If you know anyone with a K40 or other laser cutter, you can probably talk them into cutting some lexan/acrylic fins for you. Only takes a few minutes.
     
  22. Sep 10, 2019 #22

    aerostadt

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    I have used the scroll saw to cut 0.18" polycarbonate fins for my HPR 1/12 scale Atlas/Mercury. As I recall it took a little bit of practice to get the hang of it. If the blade went too fast and dwelled in the same place too long, the plastic could actually melt and the blade would have a tendency to get stuck.
     
    Last edited: Sep 11, 2019
  23. Sep 11, 2019 #23

    Bill S

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    Okay, thank you for the info. I'll figure out something else then. Maybe a hobby saw (fine toothed), go slowly?
     
  24. Sep 11, 2019 #24

    K'Tesh

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    For the fin tabs, you'd probably want to replace the traditional corner with a rounded edge... You could do that by drilling the corner, then cutting to it. The idea is to avoid the chance for concentrating the forces into a single point... Kind of like why aircraft windows are round shapes, instead of square/rectangular, it took several crashes and a lot of people getting killed for aviation engineers to learn that one (and it killed the British commercial aviation industry).
     
  25. Sep 11, 2019 #25

    Bill S

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    Good idea, thanks. :)
     
  26. Sep 11, 2019 #26

    mikec

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    But that was metal fatigue in the aluminum around the windows, not the windows themselves, caused by pressure cycling. See https://aerospaceengineeringblog.com/dehavilland-comet-crash/ I don't think the forces on fins are going to affect square corners very much. But you can do it if you want, I don't think it will hurt.
     
  27. Sep 11, 2019 #27

    K'Tesh

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    I *was* referring to the metal fatigue caused by the square windows that were used in the De Havilland Comet. It was the metal that failed, and not the "window panes" themselves. It was caused by the sharp corners of the window's shape which concentrated the forces on the metal of the fuselage leading to the loss of the aircraft involved.

    Of course here we're talking about plastic. However, if the fins flex too much, too often, (or suffer from a hard impact) a sharp corner could yield a significant reduction in strength and ultimately lead to eventual failure. I don't have any to test/demonstrate what I'm thinking of, but I do think that it could have a profound affect on the performance capabilities of the fins. The best option of course would be not to have them at all.

    So, in the image below, presuming all things being equal, I would posit that design "A" with 90 degree cuts for the fin tabs would be the weakest option, "B" with 45 degree cuts would be significantly better than "A", but worse than "C", "C" would be drilled at the corners, and the cuts would be made into the holes would be the strongest, save for option "D".

    upload_2019-9-12_4-4-48.png
     
    Last edited: Sep 11, 2019

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