75mm help

Discussion in 'The Watering Hole' started by AshleighCMcFadden, Jan 25, 2020.

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  1. Jan 25, 2020 #1

    AshleighCMcFadden

    AshleighCMcFadden

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    I'm building a 75mm fiberglass minimum-diameter dart and I need a "mandrel" to insert into the fuselage so the through-the-fuselage fins don't intrude into the motor's space. I usually use a motor casing wrapped in waxed paper for this purpose, but I've yet to purchase a 75mm casing. ($$$)

    An expended 75mm single-use motor would be perfect. Anyone got anything like that lying around?
     
  2. Jan 25, 2020 #2

    3stoogesrocketry

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    You just need a coupler for your tube .
     
  3. Jan 25, 2020 #3

    AshleighCMcFadden

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    Unfortunately, the coupler fits "too well". I need something I can get some waxed paper around.
     
  4. Jan 25, 2020 #4

    AshleighCMcFadden

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    I MIGHT be able to use it if I could find a way to grease it up well enough that the epoxy for the fin joint didn't stick to it. Suggestions for easily-removeable grease?
     
  5. Jan 25, 2020 #5

    Maxwelljets

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    Is there a reason you're putting the fins through the airframe? Typically this isn't done on minimum diameter rockets since it compromises the strength of the airframe near the bottom and doesn't provide a significant increase in fin bonding strength. I'd recommend a tip to tip layup instead. It will drastically increase fin bond strength.
     
  6. Jan 25, 2020 #6

    AshleighCMcFadden

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    I already bought a slotted airframe, so I'm stuck with it. Enough fiberglass and Kitty Hair should cover any underlying structural weaknesses.
     
  7. Jan 25, 2020 #7

    Titan II

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    Mold release agent? I have used wax before.

    BTW...agree with you leaving the slots. Common practice and the topic has been beat to death.
     
  8. Jan 25, 2020 #8

    Maxwelljets

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    Gotcha, fair enough. If nobody is able to get you a used casing, it is possible to buy fiberglass casings from RCS for $56. Probably a bit much for something you'd only use for one small step in the build process, though you could probably find a buyer in the EX community.
     
  9. Jan 25, 2020 #9

    MikeyDSlagle

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    Maybe an Estes 3" coupler (BT whatever) will be a loose enough fit to wrap wax paper around. If not just sand the heck out of a paper or phenolic coupler to get the fit you want. What's the OD of PVC?
    Using some sort of grease...you may get it in the slots and that wouldn't be helpful.
     
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  10. Jan 25, 2020 #10

    prfesser

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    I've never done this but one could cut a length of the airframe, a bit longer than the fin's root edge, saw out a thin lengthwise section and glue the ends together with cloth etc. on the inside as backing, making a coupler. For insurance make sure it's a bit smaller than "tight" so polyethylene or silicone paper (backing from labels) can be used as a mold release.

    Best -- Terry
     
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  11. Jan 25, 2020 #11

    prfesser

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    Another alternative: if you can find a hardware store that has scraps of 2 1/2" (nominal) PVC pipe, it should be 2 7/8" o.d. A few wraps of polyethylene or silicone paper and Bob's your uncle.
     
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  12. Jan 25, 2020 #12

    AshleighCMcFadden

    AshleighCMcFadden

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    MikeyDSlagle, good idea, but I'd love to get something I can really push the fins against when I set them in their slots and an airframe coupler, though tight, does fill that bill.

    Titan,

    (why am I typing in black now? Nevermind). Glad you agree about the slots. I'm stuck with 'em. And I'd play heck trying to fill them with Play Dough!

    Seriously, the structural weakness of a slotted airframe is easy to overcome - andthensome. I use enough cloth & resin to cover over lots of Kitty Hair to make an RLF (Ridiculously-Large Fillet). A fillet worthy of an Airbus! That philosophy has carried a few of my 54mm birds through Mach and beyond undamaged. For 75mm, I figure anything worth doing is worth overdoing.

    Mold Release Compound: Bejaysus, there's only 101 types of the stuff. But, yeah, I get the idea.

    I actually think I may get up my nerve and rub the coupler with the same bar of soap I use to slicken my launch rail. All I need to do is keep the epoxy I want in the seam out of the inside of the fuselage. Giving the coupler the occasional twist whilst it hardens isn't forbidden.
     
  13. Jan 25, 2020 #13

    AshleighCMcFadden

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    I'm going to go play with my coupler and a bar of soap. If that sounds weird, I applegize from the bottom of my gizzard.

    There's time enough, anyway. The plumber's own kitchen sink leaks and the car mechanic's wife's car always needs work and the laser cutting guy hasn't got around to cutting some baltic birch plywood into a proper fin jig. I have it in 54mm, so all I need to do is scale it up, but have I scheduled that in along with my normal commercial cutting? noooooooo......

    BTW, yeah, I do all sorts of laser work. I'd post my site, but I don't know if that's allowed. I barged in here with a problem and figured I'd be really polite and read the rules later. This might be the right time.
     
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  14. Jan 25, 2020 #14

    3stoogesrocketry

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    Hopefully im not to late to the party . Here is your solution . You have a good fitting coupler , perfect . On the inside of your bodytube , use one strip of 1/2 wide tape sticky side out per fin slot to seal the tube . Then slip your coupler back into your airframe. The coupler will then support the tape to body tube seal. Glue your fins on , do your reinforcments , then pull the coupler back out . This has worked for me on a MD Intimidator 4 that has gone mach 5 times .
     
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  15. Jan 26, 2020 #15

    blackjack2564

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    Try packing tape. I had the same issue. The 2 in wide stuff for shipping boxes, fits the handheld gun
    P-tape is very thin, just use 1 strip on coupler and rotate it to each slot as needed.
     
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  16. Jan 26, 2020 #16
  17. Jan 26, 2020 #17

    rharshberger

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    +1, and no epoxy I have found yet sticks to the stuff.
     
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  18. Jan 27, 2020 #18

    AshleighCMcFadden

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    blackjack2564,

    Great idea! My ONLY worry would be having the epoxy glue itself firmly to the packing tape and the packing tape stay firmly attached to the coupler. I like the idea of ... well, see what 3stoogesrocketry suggested.

    3stoogesrocketry suggested:

    On the inside of your bodytube , use one strip of 1/2 wide tape sticky side out per fin slot to seal the tube . Then slip your coupler back into your airframe.

    I like the idea of sticky side towards body tube, but getting the tape in is easier said than done. You can't easily put the end of the tape on a stick and get it into the right place. I tried inserting it it with my big piano player's hands and bled on the body tube (Why didn't I go for 98mm?), but that's what grandkids are for.

    And this shocker just in from rharshberger:

    Holy cow, that's a game changer. I assume this includes JB Weld, so we can forget the aforementioned mold release compound.

    All this is just a workaround for my usual method of wrapping waxed paper around a motor casing for the same purpose, but I've checked and while it may require a bit more sanding, I think I can get a half-inch width of packing tape to squeeze in beside the coupler.

    I like loose couplers, anyway - when they need to slide apart in flight, they should do so easily. Holding the rocket together is what shear pins are for.

    BTW, a note to 3stoogesrocketry:
    I remove the motor casing (or, in this case, coupler) after cementing the fins in place with JB Weld. I then use Kitty Hair to build up a BIG honkin' fillet that can go a third or more the span of each fin.
     
  19. Feb 2, 2020 #19

    Rob702Martinez

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    I use packing tape same as Jim. Works everytime. Dont compromise the slot bonding ability with foreign material such as wax grease soap etc. Packing tape.
     
  20. Feb 4, 2020 #20

    jimzcatz

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    Kitty hair??
     
  21. Feb 5, 2020 #21

    liv3mind

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    apparently its a type of fiberglass reinforced filler. Yeah I agree, who knew?
     
  22. Feb 5, 2020 #22

    bmonagle

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    I like the idea of the 2 1/2 inch PVC or something similar. Cut a lengthwise slot on a table saw, coat it 1 layer of packing tape, slide it in with the slot halfway between the fin slot. Insert a wedge in the slot. You can make it as tight as you want. When the epoxy sets remove the wedge and the tube can be squeezed down and removed. There should be little or no sanding required inside.
     
  23. Feb 5, 2020 #23

    OverTheTop

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    I don't do tip to tip, and I only slot MD fins into the airframe :). That saves having to rely on the peel strength of the epoxy too much.
     

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