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  1. #31
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    15th October 2016
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    Looks salvageable. mist with superglue to harden the remnants, then sand clean. Hi power guys have to expurgate fillet detritus similarly when doing repairs (minus the superglue on non-cardboard stuff)


  2. #32
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    14th July 2015
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    It's not bad. The balsa is sanded off, then I just need to sand off the remaining TBII. When finished, the tube is not compromised at all, I just apply a thin layer of diluted CWF to the area to smooth out the scuff-up surface, and then ready to glue on the next piece. Key is to do the sanding as carefully as possible, to avoid sanding the exposed BT.

    I have learned a great deal about how to build and install these pieces more successfully, even since I did the last repair. Almost tempted to pull all of them off and start again. *Almost*. I expect this new (and hopefully, final) one to be really good. We shall see.


  3. #33
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    14th July 2015
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    Even better than I thought!
    Click image for larger version. 

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    In other news, I am building “pockets” for the main fins to mount into, both for strength and to insure alignment. More on this later on when I get to the whole main fin project, which is surprisingly extensive.
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    Also another good use for that 1/16” basswood square dowel.

  4. #34
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    Main fins (aka "wings"), part 1

    I hope there's someone out there, because now we are coming to the first bit of what I would humbly call "the good stuff": constructing (but not yet mounting) the main fins (i.e. the long ones in the middle of the rocket.) Each one of these fins is a mini-project unto itself. I encourage clicking on the images to embiggen them and see the important details.

    [edit: these are the "wings']

    These took a lot of planning and experimenting before I nailed the technique. If you go back and look at the pictures of the prototype, you'll notice three things about the main fins:
    1) They are very long
    2) They span across three of the body bands
    3) They have long straight "skirts" (for lack of a better name) near the root, rather than traditional fillets. The skirts taper back toward the fin surface at the front and back.

    One could easily build the model without the skirts and it would still look fine. But I was determined to recreate the real profile as well as I could. Here is what I came up with.

    The basic fin shape is cut from 1/16" balsa. At first I was going to go with basswood (and made a full set) before I realized that I would not need the extra strength, and so I switched to balsa to save weight.

    To accommodate grain direction, two pieces needed to be glued together end to end. These initial pieces are oversized, not cut exactly to shape. other than a good root edge and the leading edge:
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    Then the fin shape is traced on...
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    ...and cut out. Note the notches for the body bands, which are cut slightly oversize because they'll be hidden by the skirts and so no need to make them exact. Also note the small piece in the corner that broke off three of the four fins, and needed to be glued back on. No biggie.
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    At this point the wood piece is very flimsy (requiring careful handling) and also not nearly flat. This will all be fixed shortly.

    The skirts were fabricated out of 65lb card stock. I created a template including both cuts and scores:
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    This took a lot of planning and thought to get just right. Here's one piece that has been cut out, very slowly and carefully with a new sharp blade, scored along the blue lines, and then folded. We need eight of these.
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    Now one piece is glued to each side of the fin. A coating of TBII was applied to the back of the cardstock, being careful not to get the glue on the skirts:
    Click image for larger version. 

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    Then it is applied to the fin. Getting placement just right was tricky, but eventually I got it pretty good. This piece was clamped for a few minutes to let the glue grab, and then I glued up the piece for the other side, applied it, and then clamped the whole assembly. I have it clamped to a piece of aluminum angle, with a piece of basswood on the other side to even out the pressure and protect the fin.
    Click image for larger version. 

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    After waiting a while the clamp is removed. The resulting fin is very stiff and dead straight. Repeat four times and the set is complete:


    This is a major milestone in the build. It'll still be a while before they're attached, but it feels very good to get to this point.
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    Last edited by neil_w; 4th April 2018 at 02:48 AM.

  5. #35
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    That's some intense boardery.

  6. #36
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    14th July 2015
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    Finally got the last platform (make that “actuator fairing”) on. So much better.

    Click image for larger version. 

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    Also learned that the main fins are called the “wings”, thanks to the ever helpful eljefe over on YORF:
    General missile naming convention is canards are forward of the CG, wings at the CG, and tail fins aft of the CG.

  7. #37
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    15th October 2016
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    This thing has the stubbiest "wings" I've ever seen.

  8. #38
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    Quote Originally Posted by Nytrunner View Post
    This thing has the stubbiest "wings" I've ever seen.
    As long as they don't flap I should be OK.

  9. #39
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    Test fit of a wing looks pretty good. Hard to hold it in place with one hand while taking the picture with the other (you can't tell but I'm really squeezing the life out of the thing with my left hand, else the skirts want to push it away from the BT).


    Pretty nifty.

    As you can see, the front of this skirt doesn't quite rest on the BT The means I'll need to trim a bit from the part that rests on the front body band, to allow the skirt to settle down a bit more. Probably will need plenty of that sort of thing to get all four optimally fit (it actually looks like all my body band cutouts are a bit too small, which is going to be a pain if true.)

    I think I didn't make the skirts stick out quite far enough, but I don't have the heart to make a whole new set. They should still look good.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  10. #40
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    14th July 2015
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    Randolph, NJ
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    I am fretting over how to handle the hangers/lugs. I'll be launching off a 3/16" rod.

    Here are the scale locations of the hangers, shown on my own OR render:


    My plan was (is) to put the lugs where the hangers go, since that seems to be the scale-ish way to do it. This still leaves a number of questions:

    1) If I put lugs in all three locations, would I be risking binding on the rod? Should I be worried about this?
    2) If I don't put a lug up front, I'll need to at least make sure the front hanger assembly doesn't interfere with the rod. This might be difficult to do while simultaneously making it look vaguely scale.
    3) Do I try to make hanger-looking structures around the lugs? I'm not sure how successful this would be, but I could give it a shot.
    4) Do I dispense with lugs altogether, and just put appropriate-sized holes through the hangers? I think this might force me to make the hangers too tall. Can't decide which would look better.

    One problem I have is that I lack dimensions for the hangers. I'd like to have a better idea before I try to actually model them. Anyone have an idea? I've searched online to no avail thus far. Measurements for the AIM9 Sidewinder should be equivalent, since they're designed to be compatible.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  11. #41
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    More on the wings and pockets

    Today I enlarged the body-band slots in the wing skirts. Interestingly, I found that the most effective method was to whittle, or carve, the cardstock with my snap-off blade. Pulling towards me slowly, I was able to shave off very very fine strands of material until I got it to where I wanted. End result was very clean, better than my original results cutting normally. Here's one example:
    Click image for larger version. 

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    With the slots enlarged, the fit is better; the skirt now rests on the airframe more consistently, without gaps.
    Click image for larger version. 

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    There might actually be a small gap here and there, but I'll just do a final bit of whittling before gluing them in (which is still a ways off).

    While I was at it, I finished installing the "pockets" for the wings to mount into:
    Click image for larger version. 

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    As mentioned previously, each pocket will hold a single wing in perfect alignment and provide an anchor to take the place of fillets, since it'll be very difficult or impossible to squeeze fillets under the skirts when gluing the fins. Each pocket is made just slightly wider than the fin stock, and does not cover the entire root edge but should be more than enough. My plan is to run a bead of glue into each pocket and just shove the fin in there. The ends of the fin without pockets will be glued down normally, and there should be room for just a bit of fillet at the front and back where there are no skirts.

    To ensure correct placement of the pocket pieces (which are 1/16" square dowel, and a bit finicky to work with) I build an alignment piece out of a discarded wing core. I glued strips of basswood to each side of it, perpendicularly, to ensure the piece is absolutely straight and rigid. Then, as I glued the pocket pieces down, I'd put the alignment piece in and make sure that (a) it would fit properly in the pocket (not too loose but not too snug), and also that the front and rear tips of the fin rest exactly on the pencil line on the BT. When the pocket is complete, it is sufficient to hold the alignment piece in the correct position, like so:
    Click image for larger version. 

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    During all this excitement I noticed that some of the wings had picked up a bit of warp. Not too bad, but not as arrow-straight as I wanted them. I'm working on straightening them out, mainly by just bending them in my hands, but there's only so much I can do. I doubt it'll be visible on the final model unless you sight down the length of the tube from the front or back.

  12. #42
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    11th April 2017
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    Brisbane Australia
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    711
    This is looking amazing.. loving the build thread!! =^)
    Reasonably new to rocketry and hailing from the land down under.. I speak metric... I know not of these feet and inches you speak of...

    QRS: #193
    AMRS: #148

    AMRS L1 (2018-03-18 - Mk4 Rocket Propelled Companion Pod)

  13. #43
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    14th July 2015
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    More fun with actuator fairings

    Where we last left the actuator fairings, the structure was in place but there are still gaps around the tail cone:
    Click image for larger version. 

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    Time to deal with that. First, I cut the following piece of cardstock:
    Click image for larger version. 

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    The precise size and shape is a result of measure, trim, test, trim some more, lather rinse repeat. Once I'm satisfied with the fit, it is glued in place.
    [picture missing, sorry about that]

    After being glued in place, there's a bit of a ridge at the front edge of the cardstock. This is addressed by running a nail file gently around the edge, as shown:
    Click image for larger version. 

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    After this, although the edge of the cardstock is still visible, running my finger over it I can't feel it at all. There's still a bit of CWF to be applied; these seams should be invisible eventually.

    Finally, some Quick and Thick is applied to fill and smooth the gaps. Here's a single finished quadrant:


    Pretty sweet. You can see the effect of the sanding at the front of the cardstock. Would have been a nicer picture if I didn't do the pieces around the motor hook, oh well.

    Two down, six more still to do.
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  14. #44
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    15th October 2016
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    That's another brainteaser worked out well.

    Love the new profile photo

  15. #45
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    Quote Originally Posted by Nytrunner View Post
    That's another brainteaser worked out well.
    So many fun brainteasers on this build. Still a few more to come.

    Love the new profile photo
    Thanks. Annoyed I completely overlooked it for so long....

  16. #46
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    14th July 2015
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    It has occurred to me that this rocket, at about 23:1 aspect ratio, might qualify as a "long skinny rocket". And in that case I might have short-changed the stability margin a bit, at least with a D12. What is the aspect ratio that marks the crossover from "normal" to "long and skinny"?

    In the current OR model, .41 oz in the nose puts stability around 1.66 with a D12 (who knows how accurate calculated CG is; I don't know for sure until more construction is finished). I am thinking of enlarging the rear fins just a tad; a very small increase (not noticeable to the average onlooker probably) could get it right to 1.8. Or I could add more weight to the nose.

    With smaller engines margin is well over 2, so I don't think there's any issue there.

    Anyone have thoughts?

  17. #47
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    15th October 2016
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    Caliber has always seemed a bit arbitrary to me, and definitely breaks down in extreme cases (think mean machine vs fatboy)

    I found an old paper by nasa about sounding rockets which I can't locate currently, but I clearly remember the static margin being desi b ed to be between 8-15% of the body length. On that metric I try and kept mine at least 12% and stability has never been my problem.

  18. #48
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    Quote Originally Posted by Nytrunner View Post
    I found an old paper by nasa about sounding rockets which I can't locate currently, but I clearly remember the static margin being desi b ed to be between 8-15% of the body length. On that metric I try and kept mine at least 12% and stability has never been my problem.
    Well, for this rocket, 10% would be 3" = 2.3 calibers, so I'm somewhat below that with the D12. It's worth noting though that OR's CP calculation is really a rough guess with this rocket; there's a lot of junk all over it that is probably not accounted for accurately. It's probably over-calculating the effect of the tailcone, which is significantly covered by the actuator fairings, and probably not factoring in at all the base drag created by the fairings. So maybe the CP is a little pessimistic. Then again, I haven't figured out yet how it is accounting for the actuator fairings in general.

    I'm sort of leaning a bit towards enlarging the rear fins a tad. I don't think think there's much downside to it.

  19. #49
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    Finished cardstocking (a new verb I just invented) the rest of the actuator fairings today. The tail end is looking really good I think. Painting will be interesting, lots of nooks and crannies back there.

  20. #50
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    Gluing the wings

    Having CWFed all the exposed wood parts, I decided it was as good a time as any to start gluing on the wings. Hoo boy, this is tough.

    I laid glue in the pockets, and put a small bead on the other areas where the root would contact the airframe, especially the front and back:


    I had to hold the fin in place for like a year, because the skirts were pushing back surprisingly hard. So I stood there like an idiot, my fingertips white from gripping, and *eventually* the glue was set and I was able to let go. Then there was a bunch of Quick and Thick all around the skirt to seal it down and fill the gaps. One very lovely finished wing:


    I thought I'd be smarter with the second one, by bending the skirts out a bit beforehand, so they wouldn't push. Well it solved that problem, but the second one really didn't want to sit all the way flush to the BT (either the front or the rear wanted to lift. After holding it for a *long* time, but still not long enough, I realized that I had tools to handle this sort of thing:


    You can see that on the top wing (the second one), the skirts are not sitting down all the way due to my bending them up beforehand. They should sit down OK once I attack with the Quick and Thick.

    Anyway, I hope that second wing is solid in place tomorrow. I have no easy way to inspect what's going on in and around the pockets (would need to really lift the skirt way up and look underneath and that sort of thing is frowned upon these days (and, well, always)). So I'll just let it dry and keep my fingers crossed. My concern is that while I was holding it, I periodically would slip and shift the position of the whole fin. My hope is that the big glop of glue in the pocket was still wet enough at that point, so it would re-set in the new position. Really, once the skirt is filleted, the whole thing should be pretty strong. That's my story and I'm sticking to it.

    So that's 1.75 wings done so far. Definitely enough for one night.
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  21. #51
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    The second wing seemed healthy when I removed the rubber bands tonight. I applied fillets, so now two wings are completed.

  22. #52
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    Quote Originally Posted by neil_w View Post
    I'm sort of leaning a bit towards enlarging the rear fins a tad. I don't think think there's much downside to it.
    So, after doing some further measurements on some IRIS-T pictures, I think that my existing dimensions for the rear control fins are already somewhat oversized... so I don't know how much bigger I want to make them. Maybe I'll leave them as is, and just hang a bit of additional weight off the eyelet when I fly a D (or save the D for very calm conditions.)

  23. #53
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    Quote Originally Posted by Nytrunner View Post
    Caliber has always seemed a bit arbitrary to me, and definitely breaks down in extreme cases (think mean machine vs fatboy)

    I found an old paper by nasa about sounding rockets which I can't locate currently, but I clearly remember the static margin being desi b ed to be between 8-15% of the body length. On that metric I try and kept mine at least 12% and stability has never been my problem.
    Im with Nytrunner stability margins over one are a preference. Most of my rockets I aim for a stability margin of 1 plus or minus .2 doesnt worry me much usually, mach capable rockets I usually aim for a a bit more but not usually exceeding 2 ( unless the sim shows instability during the flight then additional weight beyond optimizing for altitude may be necessary).
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  24. #54
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    Quote Originally Posted by neil_w View Post
    So, after doing some further measurements on some IRIS-T pictures, I think that my existing dimensions for the rear control fins are already somewhat oversized... so I don't know how much bigger I want to make them. Maybe I'll leave them as is, and just hang a bit of additional weight off the eyelet when I fly a D (or save the D for very calm conditions.)
    Lol, you're still convinced my rear fin measurements are large.

    I agree with a smidge of noseweight as appropriate. (if it works for the O3400,......I bet it'll work for the D12)
    "I'm at least 70% confident about whatever I say (90% of the time)"- college me

    NAR 101195
    Level 1: Big SAM, 9/10/16

  25. #55
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    Quote Originally Posted by Nytrunner View Post
    Lol, you're still convinced my rear fin measurements are large.
    I am, although I'm OK with it. But it leads me to not want to make them any larger. If I need more noseweight for a D12, I'll just add more noseweight.


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