L2 rocket scratch build

Discussion in 'High Power Rocketry (HPR)' started by Ben Martin, Nov 5, 2018.

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  1. Nov 8, 2018 #61

    jlabrasca

    jlabrasca

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    What Rex said. Contact the NAR Section President about bringing the exam and the scoring key. I think you have a year after you pass it to fly for certification. I am guessing you've been drilling on the questions?

    Likewise, you've confirmed with the vendor that there will be motor for you to buy?

    And bring your NAR card and the signed temporary card from your L1 application.
     
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  2. Nov 8, 2018 #62

    Ben Martin

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    I've been using the study guides, are there better resources? Might just take the test this month and do the flight next month after I test the rocket on an I.

    I confirmed that the vendor will be there and have the motors I need.
     
  3. Nov 9, 2018 #63

    jlabrasca

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    I just drilled with the questions from the pool

    http://www.nar.org/pdf/HP-question-pool.pdf

    I've seen folks recommend making up flash cards.

    Since so many of the questions rely on rote memorization, and because I am a very poor student, I printed off the questions and blacked out all the wrong answers. I recited the questions and answers -- backwards and forwards -- a bunch of times. Then I ran the practice exam bunch of times to review. It was tedious and unrewarding exercise.
     
  4. Nov 9, 2018 #64

    Ben Martin

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    Practically complete. Only thing left are the exterior fillets which I need more epoxy for. I'll be filling in the gap on the transition with epoxy putty. 20181109_001645.jpeg
     
  5. Nov 9, 2018 #65

    Faroutspacenut

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    Good luck with this! I highly recommend taking your test well ahead of your cert flight. That's what I did and it took so much of the pressure off - I only had to worry about my flight on that day. Not sure what it the rule is for NAR, but for TRA you have a year to do your flight after passing your test. Also, if you just need a small quantity of black powder for a few flights and ground tests, you can buy a ejection charge kit at buyrocketmotors.com (sorry having trouble posting a link for some reason)
     
  6. Nov 9, 2018 #66

    Rex R

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    there is something to be said about building and flying saucers, do it right and they don't need ejection charges :).
    Rex
     
  7. Nov 9, 2018 #67

    Ben Martin

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    What size exterior fillets do you guys recommend? I'm thinking small due to all the interior reinforcement, but exterior could help stiffen it even more.
     
  8. Nov 9, 2018 #68

    Rex R

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    well...unless you plan on putting a CF rod into each fillet you're not going to gain much(except mass) by applying large fillets on the outside. I would use enough to smooth things out and call it good. I've been using micro balloons mixed with epoxy (creamy peanut butter thick) to do exterior fillets.
    Rex
     
  9. Nov 9, 2018 #69

    OverTheTop

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    +1 for microballoons in fillets. Makes sanding much easier ;).
     
  10. Nov 9, 2018 #70

    DGBrown

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    You might want to shoot a quick coat of primer on the tubes to minimize water damage if there is any rain. Bare cardboard tubes like that love to soak up water.
     
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  11. Nov 11, 2018 #71

    Ben Martin

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    Results:

    L2 high powered certification.

    The rocket performed nominally. Screenshot_20181110-174000_VideoFramePlayer.jpeg Screenshot_20181110-174017_VideoFramePlayer.jpeg
     
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  12. Nov 11, 2018 #72

    jlabrasca

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    Congratulations. L3 next week? <g>
     
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  13. Nov 11, 2018 #73

    Ben Martin

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    I will be disassembling it tomorrow to see how the printed parts held up. This will allow me to make modifications and create guidelines for future builds. I know that it really melted my motor retainer so I will see how bad it is.
     
  14. Nov 11, 2018 #74

    OverTheTop

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    Congrats on the certification. I'm glad the fins held together ;).

    3D printing is a paradigm-shift that opens up so many possibilities.
     
  15. Nov 12, 2018 #75

    Ben Martin

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    3D printing made this L2 build really cheap. Only thing I had to buy were the tubes and hardware.
     
  16. Nov 12, 2018 #76

    OverTheTop

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    Nice work all around.

    I forgot to mention I think that the stiffening things with CF comes from the RC plane world where some additional stiffness on expanded-foam craft really help with performance due to reduction in aeroelasticity. That's where I have used it anyway.
     
  17. Nov 12, 2018 #77

    LawnDart2020

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    Congratulations and nice flight, sounds like things worked well, and the printed parts held up as well as you could hope for. Just out of curiosity did you consider the grain orientation of the layers in the printed fins? I understand that the CF rods added a large amount of strength perpendicular to the orientation of the layers, but could you have printed on an angle so the layers were oriented parallel to the leading edge? I am thinking of wood and how the gain of balsa wood is always oriented in this way in order to prevent breaking of the fins along the weak axis. Nice project, I'm looking forward to seeing the L3
     
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  18. Nov 12, 2018 #78

    Ben Martin

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    Thank you.

    About the orientation: My initial design without holes was printed vertically due to greater layer strength. When I added the holes printing vertically was no longer an option due to overhangs. I printed with PETG pretty slow and at 250C so that it has incredible layer strength, making it equivalent to the vertical orientation. In addition, the carbon fiber rods were epoxied in, helping to hold the layers together.

    Then again, only time will tell how well they hold up.

    L3 project will probably be in a few months as L2 already breaks the bank. Doesn't hurt to get started on the design though.
     
  19. Nov 12, 2018 #79

    LawnDart2020

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    Makes sense, most of my printing experience has been with PLA which is weak over the grain layers and substantially stronger perpendicular to the grain lines. I wonder how well your fins would handle mach speeds.
     
  20. Nov 12, 2018 #80

    Ben Martin

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    PETG is known for its excellent layer adhesion. Only one way to find out how it would handle it.
     
  21. Nov 12, 2018 #81

    Ben Martin

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    This recent launch on a J250 got it up to around 500mph (mach .7). The max motor has it going at mach 1.7, that would be quite a show.
     
  22. Nov 12, 2018 #82

    boatgeek

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    Destructive testing is the best kind of testing! :D
     
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  23. Nov 12, 2018 #83

    OverTheTop

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    Only if it is not your own rocket ;).
     
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  24. Nov 28, 2018 #84

    Ben Martin

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    Is this too much stability even for a really long rocket?
    upload_2018-11-28_0-7-59.png
     
  25. Nov 28, 2018 #85

    Nytrunner

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    Without knowing the diameter and length it could just means it's more susceptible to weathercocking. Get it going fast enough and and that shouldn't be too much of a problem ( it'll just arc over more near apogee)

    Is this your L2 rocket?
     
  26. Nov 28, 2018 #86

    Bat-mite

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    Congrats on the L2. Every time I say to myself, I'm going to try scratch-building, I end up buying a kit. :eek:
     
  27. Nov 28, 2018 #87

    boatgeek

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    It comes off the pad at 2 calibers, so that's pretty reasonable. If it's very long and skinny (20:1 L/D), it might even be a little light on stability.
     
  28. Nov 28, 2018 #88

    Ben Martin

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    Yes, this is my L2 rocket.
     
  29. Nov 28, 2018 #89

    Ben Martin

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    It's 63in long and 3in diameter. The advise I got earlier was 2cal off the pad. Keep in mind that this is OpenRocket, which tends to be less generous with stability than RockSim.
     
  30. Nov 28, 2018 #90

    Nytrunner

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    I'm guessing these higher stability numbers are due to adding electronics as you continue to experiment?

    This is true. Especially when you stick a boattail on the end.
     

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