High temperature epoxy glue necessary?

Discussion in 'High Power Rocketry (HPR)' started by Crumb fire, Aug 23, 2019.

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  1. Aug 23, 2019 #1

    Crumb fire

    Crumb fire

    Crumb fire

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    Hello folks,

    I'm building a two stage 98mm rocket with a minimum diameter booster.

    Do I need high temperature epoxy glue for the fins, since they are "directly" over the motor? I will t2t the fins with carbon fiber, but was wondering if it is necessary.

    What are your experiences with 75/98mm MD fin epoxies? Anyone had problems?

    Thanks,
    Steeve
     
  2. Aug 23, 2019 #2

    plugger

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    What epoxy system are you using for your t2t layup? What's the max V of our two stage stack?
     
  3. Aug 23, 2019 #3

    Crumb fire

    Crumb fire

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    Should be West System, around 1600 mph. 900mph only for the booster. I'm wondering if the casing's heat can alter the strength of the fin fillet.
     
  4. Aug 23, 2019 #4

    plugger

    plugger

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    Given you're only approaching Mach 2 I don't think heating will be much of an issue but if I were you I'd choose a better epoxy system for the layup. Wests is a bit suboptimal imo. And I'm not sure what you mean by the fins being "directly over the motor" but I wouldn't expect motor heating to have any real impact on your fins, even a MD or sub MD configuration.
     
  5. Aug 23, 2019 #5

    Rob702Martinez

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    Aeromarine 21/300 or 21/400
     
  6. Aug 24, 2019 #6

    Crumb fire

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    Aeromarine 21/300 or 21/400

    Rob702Martinez, have you tested it on a minimum diameter? I can't find data about temperature resistance for this epoxy.
     
  7. Aug 24, 2019 #7

    Rocket501

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    I don't know what you are making, but if it is under Mach 2, temperature probably won't cause the fins to fail in flight. However, if directly attached to the motor, and depending on the motor, the whole thing may be softened and more easily damaged upon landing. If you could give us a bit more detail, that would be helpful. However, I usually use high temp epoxy for pretty much everything, because I have it on hand for free. However, you can most likely get away without it.

    Basically, if you will be flying it a lot, high temp epoxy will certainly improve the longevity. However, 98mm motor are expensive, so you might not be planning on doing this. It is up to you.
     
  8. Aug 24, 2019 #8

    Rob702Martinez

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  9. Aug 24, 2019 #9

    Crumb fire

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    Thanks Rob702Martinez, I'll give it a try. Amazon has the trial size kit.

    Sorry if it wasn't clear. What I meant is that I'm afraid heat temperature from the 98mm casing will pass through the airframe to the fin. I don't fear the heat cause by speed, but the one cause by the motor, since it is a minimum diameter.

    Could it happen and melt the fin fillet during ascension?
    The sustainer is a stretched Formula 98, and the booster is a 5ft G12 airframe with slightly bigger fins. All fins are tip-to-tip with carbon fiber.

    upload_2019-8-24_7-56-53.png
     

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    Last edited: Aug 25, 2019
  10. Aug 26, 2019 #10

    plugger

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    I think your fears are misplaced. The heat caused by your motor is a non issue. The heat and possibility of flutter with your sustainer going Mach 2 is much more of a concern. But realistically as long as your fins are an appropriate thickness for your flight profile your fins and fillets should be fine, especially considering you'll be leveraging t2t.
     
  11. Aug 26, 2019 #11

    OverTheTop

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    Are you slotting the airframe and carefully fitting the fins into the slot and gluing? It gives you a good deal of strength and rigidity improvement (higher resonance frequency) over just sticking them onto the surface. It also means the epoxy is working mainly in compression and tension, and the reliance on peel-strength (aka tip-to-tip ;)) of the joint is reduced.

    FWIW if you are pushing M2 then I would look at something better than West Systems, just out of caution, but I suspect it will be OK anyway. I have flown M1.8 and not even scorched the paint. A recent flight to M2.14 was also only Wests for the fins. Casing heat should not be such an issue due to a fair amount of convective cooling during flight ;).
     
  12. Aug 26, 2019 #12

    G_T

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    The nozzle region of the case is where the greatest and fastest internal heating will occur. Consider relocating the fins just a bit forward so the fins do not overlap the nozzle?

    If you separate upon burnout, there isn't much time for the heat pulse to cook the fin joint.

    Gerald
     
  13. Aug 27, 2019 #13

    Crumb fire

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    Thank you all,

    Apparently, the motor don't produce enough heat to soften the epoxy joint of the fins. At least, not during the thrust. It will also be surrounded by an airflow that will help cool the airframe. I will not risk, and probably try Aeromarine 21/300 or 21/400, since it can resist to 250°f. I've been told Cti 98mm is not supposed to go over 200°f.

    As for the sustainer's fins, they are made of 5/16'' G12, glassed each side with 3x6oz carbon fiber, and then tip-to-tip with 2x 18oz of CF.

    Since the fins on the booster are made but not glued, I tried to relocate them in open rocket, with bigger reloads, to see if the CG/CP relation change dangerously :
    upload_2019-8-26_21-4-48.png

    The fins are now at 3'' from the end, and the motor is a CTi 6 grains, instead of a 4 grains. Not much space for the chute (1ft). Stability is now 3.3.
    I won't fly it the first time with a six grains, but I'd like to build it in way I can use these reloads.

    What do you think?
     
  14. Aug 30, 2019 #14

    DavidMcCann

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    Remember, even with a motor mount buried in a larger tube, those mounts are still glued in and people aren't blowing motor mounts out of rockets. I wouldn't worry. But I never worry....
     
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