Rocket Ship 27: An L3 Build Thread

Discussion in 'High Power Rocketry (HPR)' started by JLebow, Nov 25, 2019.

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  1. Jan 26, 2020 #31

    JLebow

    JLebow

    JLebow

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    The target launch is Oregon Rocketry's AP Showers in late April, in Brothers.

    Thanks, Rich. I'll be at as many Oregon Rocketry events as I can make and Sod Blasters, but I will have to adapt down in motor to stay under the waiver at the Sod Farm. Will you make it to NXRS this year?

    I don't see why not. You will need wet rated sand paper. There will be quite a bit of water involved if you start wet at this stage. I usually sand dry until 400 grit, and then it becomes worth the hassle because the water keeps the dust from clogging the paper.

    Jim, thank you for volume 2. I like the detail and generous sharing of your techniques. I saw your flight at Balls last year. What an incredible project.
     
  2. Jan 26, 2020 #32

    Ez2cDave

    Ez2cDave

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    I am thinking more in terms of safety than clogged sandpaper. Those airborne particles can kill !

    Dave F.
     
  3. Jan 26, 2020 #33

    rharshberger

    rharshberger

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    I will not be at NXRS this year, if extremely lucky the only event I will attend away from TCR is LDRS. Hopefully we pay the car off soon and get a travel trailer, that will open up rocketry destinations as the wife will actually enjoy it too.
     
  4. Jan 26, 2020 #34

    JLebow

    JLebow

    JLebow

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    Time to glue some fins onto the air frame.

    I wrapped a sheet of paper around the rocket tube to serve as a guide to trim and square up the tube ends. I cut with a hack saw and cleaned up with a sanding block. I checked the finish against a spent DMS rocket motor.
    10 trim the tube.jpg

    Because the fin has quite a bit of thickness at the root, the root edge is not flat (in a plane). The fin sits on a cylinder, that falls away from where a traditional thin fin would sit. My math came out that the would be a 1/4" height difference between the thick and thin portion of the fin. I included this feature in the fin molds. See how the fin doesn't sit flat:
    20 root not flat.jpg
    The fin is a pretty good fit, but to get a great fit, I sanded the fin with some sand paper stuck to the mandrel.
    30 sanding the root.jpg
    I wanted to do something better than "eyeballing it" to make sure my fin is straight (pointing radially outward). So I used my height gauge to indicate the tip of the fin, relative to some setup blocks. Then I rotated the fin 180 degrees. If the fin is leaning to one side or the other, the tip of the fin will be offset to the left or right from the tip of the gauge:
    40 vert jig.jpg 50 vert jig 2.jpg

    So I sand the fin root, check the tilt of the fin, sand some more, and continue until the fins are straight and all the same height:
    60 height gauge.jpg

    I like the shape of this compound curve:
    70 curved root.jpg

    Then I made up some alignment jigs from foam core poster board and tacked the first fin in place with some rocketpoxy.
    80 first fin in place.jpg
    I don't know if it is required, but my fins are hollow, and my flight profile will have a 6 psi differential in air pressure at apogee vs at the pad. I decided to drill three tiny vent holes into the airframe, placed 4 inches above the motor thrust ring.
     
    Last edited: Jan 26, 2020
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  5. Jan 26, 2020 #35

    mbeels

    mbeels

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    Those fins look great, very methodical fitting. Is that an electric Porsche in the background?
     
  6. Jan 26, 2020 #36

    JLebow

    JLebow

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    Yes, that is an electric Porsche. I converted it about 8 years ago. If you are interested in the build thread, google eporsche911.blogspot.com
    rear no plates-1 compressed.jpg
     
    Last edited: Jan 26, 2020

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