Gemini BT 70 Build

Discussion in 'Low Power Rocketry (LPR)' started by VonRecktofen, Oct 29, 2018.

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  1. Oct 29, 2018 #1

    VonRecktofen

    VonRecktofen

    VonRecktofen

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    Hi folks,
    Putting together a Gemini BT70 size sort of scale rocket to launch this July as part of the 50th Anniversary of Apollo 11. Got a 3D printed nose from Shapeways, what would be a good way to attach Kevlar shock cord string to this nose?
    Any ideas? IMG_1870.jpg

    Thanks for any suggestions!
    Franz in NJ
     
  2. Oct 29, 2018 #2

    Brent

    Brent

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    Run a dowel through the holes on the shoulder of the capsule and sand flush with the shoulder.
     
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  3. Oct 30, 2018 #3

    zog139

    zog139

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    I used a piece of 1/2 wide by 1/8 thick spruce with the ends rounded to match the edge. Epoxied in under the lip, drilled a hole in the center of the spruce added a loop of 100 lb kevlar. Covered the end with cardstock so nothing gets pushed up into the cone.
     
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  4. Oct 30, 2018 #4

    VonRecktofen

    VonRecktofen

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    Thanks for the ideas!
    Franz in NJ
     
  5. Oct 31, 2018 #5

    JStarStar

    JStarStar

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    Using clear plastic fins, or "flame fins?"
     
  6. Oct 31, 2018 #6

    VonRecktofen

    VonRecktofen

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    What are “flame fins”?
    Was thinking about making balsa fins on the next size larger BT, then splitting the BT into 2 pieces, then sleeving it over the lower BT and holding it in place for launches with small plastic rivets...

    Franz in NJ
     
  7. Oct 31, 2018 #7

    les

    les

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    Flame fins are shaped to look like flames. Often designed to be removable. eRockets sells some laser cut versions

    [​IMG]
     
  8. Oct 31, 2018 #8

    GlenP

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  9. Nov 2, 2018 #9

    JPalmer621

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    You can find clear fin martial at you local office supply store. I use acrylic sign holders to make my clear fins
     
  10. Nov 2, 2018 #10

    Micromeister

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    We should never use acrylic plastic for fins on any flying model rocket. The Acrylic and modified Acrylics are far to brittle for the job. Neither will hold up to landings and more often then not will not endure the pressure of flight stress. This can lead to snappped shards of Aclylic plastic falling from the sky.

    Clear Polystyrene, Polyproplene and by far the best Choice for fins is Polycarbonate (G.E.Lexan). Polycarbonate can be had in thicknesses from .010" calandered/frosted clear to .063" thick for the largest MPR models. For a BT-70 Gemini-Titan I'd go with .030" clear Polycarbonate as the preferred material for clear fins.
    Any of the above Clear Plastic fins can be applied to the rocket with either 5 or 30 minute epoxy with a small radius epoxy fillet.
    I have large and heavy MPR 5-D12 clustered rockets with .063" Clear Polycarbonate fins that have more the 20 launches on them without a single flight or landing crack with butt mounted Clear fins (NOT through the wall).
     

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  11. Nov 3, 2018 #11

    JPalmer621

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    I have been using acrylics for over 10 years on my scratch Gemini Titan, about 6 launches each year. I would never use them on a mid or high power rocket but on low power they work fine Yes I have broke some fins. I have also broke balsa, basswood and plywood fins. It all depends on how the parachute comes out. Glad to hear there are other options for everyone to use.
     

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  12. Nov 5, 2018 #12

    JStarStar

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    The arrival of the new 18mm D/E's should make BT-70 Gemini-Titans much more practical.

    Because of weight resulting from clear plastic fin assemblies, BT-70 Gemini-Titans are often pretty heavy even for a 2xC6 cluster.


    It would be all kinds of awesome if 3D printing had a clear plastic option. Instant fin units!!
     
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  13. Nov 5, 2018 #13

    JStarStar

    JStarStar

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    If I was launching a Gemini-Titan model, I'd want to get a mp3 or CD boom box stereo to play the "WHOOP!!" sound effect of the turbopumps just before liftoff.
     
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  14. Nov 6, 2018 #14

    JStarStar

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    I believe Estes used to make a detachable clear plastic fin unit on one kit a decade or so ago (I forget which kit it was).

    It would be awesome if somebody could make a clear fin unit for BT-60 or BT-70 ... it would really open up the possibilities for finless scale models.
     
  15. Nov 6, 2018 #15

    kuririn

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    Like this?

    https://www.ebay.com/itm/Flying-Mod...h=item41eb1ccf54:g:qEQAAOSwYXBbT-G7:rk:2:pf:0

    Back in the 60's Estes had two models that had clear fin cans that you constructed yourself, the Thor Agena and Gemini Titan. A real PITA to construct, and not very sturdy. If I was building one today, I would buy the clear fin kits from eBay.
     
    Last edited: Nov 6, 2018
  16. Nov 6, 2018 #16

    JStarStar

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    Yep, exactly like that.
    I tried a Thor-Agena back in my kiddie days when I was 11-12 and botched it up big time. In the end I just said "screw it" and glued on some balsa fins.

    I suppose I could probably do it now -- I've learned the value of actually following instructions-- but the whole process of building the fin can with clear plastic cement as you mentioned was a complete PITFA, plus as you also mentioned the plastic fins were almost guaranteed to pop off every flight.
     
  17. Nov 7, 2018 #17

    linville86

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    I've wondered if you took a long strip of thin clear plastic from maybe a 2 liter and folded it in several spots for fins, if you couldn't make a one piece fin can that couldn't lose fins.
    drawing.jpg
    I tried it out of clear toy package for a bt-50 "proof of concept". I think it would work if you could find a glue that would hold the fins together.
     
    Last edited: Nov 7, 2018
  18. Nov 7, 2018 #18

    snrkl

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    The water rocketry guys at “It’s Rocket Science” use this principle, but with corrugated corflute that they slice along the corrugations all on one side, then fold and join, making a ring fin can that can slip onto a soft drink bottle (Soda pop bottle for the janquis).

    I’ll see if I can grab a photo at this weekends club launch...
     

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