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  1. #1

    Build Thread: Gene's L3 Project

    Gene's L3 Project

    I'm in the process of doing my L3 project and I thought it would be fun to use the forum to document my build. I think this is a fun venue to share my build ideas with others, as well as provide the necessary documentation to my L3 advisor James Marino (thanks James for doing this!).

    A good deal of the beginning of this thread is going to focus on composit fabrication of things like centering ring and fins. My goal is to keep the aft end of the rocket light to minimize weight. With my past experience using carbon and glass reinforcements, and my ever handy vacuum baging experience I should be able to cut 30 to 50% weight of may components.



    I'll start by posting my project introduction:

    ----------

    1) Introduction

    For my L3 project the goal was to have a fun, reliable rocket that is a solid design, relatively conventional construction, easy to load-up and fly. Pervious experience includes a range of L2 projects including a 2 stage K to K projected called the Foley a Deux. Over the last year I have had very good luck working with altimeter controlled – dual deploy rockets. Much of the experience is on my EVE rocket. It’s a cool 2.6” / 54MM rocket that just goes and goes. It’s been really fun to fly and very reliable. I have quite a few good flights on my Talon 4 (five I think) and my 5.5” Jay Hawk. My goal for my L3 project is a similarly reliable vehicle.

    Most of the components for this build were originally from Jeff Race’s Polecat Patriot rocket. He bought it for his L3 but never finished it. Also included in the purchase was an extra nose cone and coupler. He had a 75MM Aeropac retainer as well. I thought for some time if I wanted to just build it as is, but fundamentally not really liking the Patriot, I decided to change it. A few things I don’t like about it are that it has a big 7.5” airframe, but only a 75mm motor. After my L3 then what? Also the fins are actually pretty small, leading to stability concerns or the need for lead in the nose. Also construction of fins and rings are all from 3/8” ply and that causes the tail to be quite heavy. And what do I do with an extra nose cone?

    So I decided to modify the rocket to 98MM (L3 attempt will be a 75mm though) and use the extra nose cone as a boat tail – I like Boat tails, makes the rocket more efficient. Adding the boat tail also lengthens the rocket some. To lighten up the tail I’m using honeycomb composite fins. There are four centering rings. Primary load bearing rings will be ½” birch-ply with carbon reinforcement. The other centering rings will be carbon lay-up over honeycomb to save weight while having great strength. Similarly I will attempt to save weight on the av-bay using carbon-composite end caps with birch inlays.

    -----------

    Next up - how to make carbon / honeycomb composits rings and parts

    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  2. #2
    Join Date
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    Re: Build Thread: Gene's L3 Project

    [INDENT]Looking forward to working with you, Gene. This should be a fun project.

    James


  3. #3

    Re: Build Thread: Gene's L3 Project

    Awesome! Do you have a specific motor in mind the the cert flight?

  4. #4

    Re: Build Thread: Gene's L3 Project

    3) Certification Flight Overview

    The certification flight will use a M1297. Estimated altitude will be about 10K feet. Drogue deployment at apogee,
    main at 800’. Hope to do the cert flight at 2008 October Skies. Also plan to use Alan Thym’s 12’ launch tower
    Good luck with your L3 Gene.

  5. #5

    Re: Build Thread: Gene's L3 Project

    Thanks. I completely missed the link above the Rocksim picture.

  6. #6

    Re: Build Thread: Gene's L3 Project

    That looks like a great project. I will have to plan on attending October Skies.
    Mark W

    NAR #26451 L2
    LUNAR #724
    K6RZY
    Cal Pyro III

  7. #7

    Re: Build Thread: Gene's L3 Project

    Building Composite Centering Rings

    Centering rings are a good candidate to take some weight out of the tail. A 7.5" -> 98mm Giant Leap ring weighs about 160g. There are four centering rings for this build.

    1 - Aft ring - this has an Aeropac flange retainer and because it needs the screw insets I won't use composite for that. I will however put two layers of carbon on it because I can

    2 - The next two rings working forward are positioned on either side of the fin tab. These will be composite.

    3 - The forward ring will also accommodate two U-bolts, one on either side of the ring and will be composite. But I will use birch-ply hardpoints inlayed where the U-bolds are positioned to add extra strength and make it more crush resistant. That way the U-bolt can be cinched down.

    All these composite lay-ups will use 0.3” Aramid (Kevlar is made of this) honeycomb core with 8 layers of 5.9oz / 3K carbon twill. with 4 layers on each side. All are wetted down with Fiberglast system 2000 epoxy with peel-ply and breather on the outside for vacuum bagging. Then they are put into a bag and a 60cm vacuum is drawn. They are also pressed to assure they are perfectly flat. Simple - hu!

    On to the pictorial...


    This shows the original forward ring, the original honeycomb stock on the right, and a square of it in the center. The cut stock is taped where it’s going to be trimmed, there is no way to mark on honeycomb in any way you can see so taping it works well. Also taping holds it more firm for cutting. Above this are the three hard points for the for ring. The two left and right are for the U-bolt connections. The middle is used only to reinforce the hole for the table router pivot pin.


    The position of the hard points is marked.


    The other two rings are cut, tapes and marked. The lower two are the rings that flank the fin tabs. Note where the u-bolts will go.


    The stock is now cut. The outer edge is rough cut to assure it stays larger than the final ring size. The hard-point areas are cut out. The rings will be final trimmed on a router table using a carbide bit.


    The hardpoints are in-set into the honeycomb.


    This shows the middle rings that flank the fin tab. Note the right one is smaller to accommodate the boat tail.


    This shows all rings for to aft plus the AeroPac retainer. Now to vacuum bag! Be sure to remove all the tape.


    Here is the lay-up in the bag. Sorry, no pics or cutting, I forgot...


    Times up, the layup is pulled from the bag, looking good. Humm - not looking very ring like.

    continued...
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  8. #8

    Re: Build Thread: Gene's L3 Project

    Building Composite Centering Rings - Continued


    Now the peel-ply comes up. Ain’t they a beauty! These are rough-cut using a Dremel carbide cutting wheel. Don’t even bother cutting Carbon without carbide, you won’t get far. The aft ring is shown at the top.


    Here is where the center hard point comes in. The center hole is drilled out and acts as a pivot for my table router. I made a simple jig that holds a 1/8” pin that drops into the hole. I set the size, clamp the overhead piece, and cut away the flashing.


    Next I take the cut disk and trim the honeycomb on the edge. To finish the edge I mix up some 20min System 2000 epoxy with micro-balloons and lay it into the edges. I then tape over the edged to keep it in place while it sets. I thought about using my Aeropoxy light – but it is so light and airy that it’s not as strong as this mix.


    To add further amusement I used a balloon for apply pressure to the inside epoxy and force it into the edge. This looks like a model or an exploding star picture I saw from the Hubble.


    Here is the final front ring ready to go! It weighs just 86g, 50% less that the original.

    Overall I think I will save about 9 oz by replacing birch rings with composite. But it is a lot of work! But hey, keeps me out'a trouble.

    There strong to. I put a ring between two blocks and stood on it - minimal flexing. And let's just say I weigh more than I would like to, we'll leave it at that.

    Next up - AV-bay construction and composite end caps.
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  9. #9

    Re: Build Thread: Gene's L3 Project

    Looking good so far.

  10. #10

    Re: Build Thread: Gene's L3 Project

    Building the AV-Bay

    The AV-Bay is a simple and conventional design having a 7.5” phenolic coupler and two end caps. The AV-Bay is completely removable from the rocket. For flight it is attached to the upper airframe with 6 4-40 countersunk machine screws. What might be a bit different about my AV Bay design is:

    • Coupler is reinforced with a couple of wraps of carbon to strengthen it
    • Uses four threaded rods for extra strength and support
    • Composite end caps to save weight
    • Caps Mounts inside the end of the coupler against a ply shoulder
      with a silicone gasket


    On to the pictorial...


    This shows the original end cap from the Patriot but with the threaded rod holes moved toward the center about ¾”. I also drilled for a 5/15 U-bold which is stronger than the original ¼”. Shown to the right are the hard points that will be laminated into the honeycomb.


    After cutting the honeycomb and inserting the hardpoints here is what we have. The holes for the U-bold and the 1/8” router pivot pin hole are pre-drilled. After layup I will clamp the plywood to the composite cap for final drilling. This assures the holes are evenly matched.


    Of course I need two end caps!


    After cutting and fussing here are the caps ready to go. Construction method is identical to the centering rings. Refer to that for technique.


    I fill the edges of the end caps with Aeropoxy Lite.


    The coupler is lined with two layers of 3K twill. I “balloon bagged” it to make sure I had a tight bond to the coupler inside.


    After trimming the carbon off the ends I make two plywood rings and glue these inside on each end. I CA a silicon gasket on the shoulder for the rings to seat onto. This helps seal the Av-bay against ejection gases. Using a shoulder allows the end caps to fit flush inside the end of the coupler. The plywood also allows the use of T-nuts for the 6 4-40 machine screws that will attach the Av-bay to the upper airframe. And finally it gives more support for the sheer pins between the couple and the bottom airframe.


    Here is the completed AV-bay with the end-cap installed. Later I’ll add the charge cups and build the electronics sled.

    Next up – let’s bob a nosecone to make the boat-tail.
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  11. #11
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    Re: Build Thread: Gene's L3 Project

    Cool. This is the first I've seen honeycomb used this way.
    Josh

  12. #12
    Join Date
    16th February 2009
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    Kalifornia
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    Re: Build Thread: Gene's L3 Project

    Quote Originally Posted by eengelgau View Post
    To add further amusement I used a balloon for apply pressure to the inside epoxy and force it into the edge. This looks like a model or an exploding star picture I saw from the Hubble.
    That would be the image of Eta Carinae. That is one astounding image.

    James

  13. #13

    Re: Build Thread: Gene's L3 Project

    Making the boat tail

    I like boat tails (as I already said) and since I have two nose cones, let’s bob one to make a boat tail.

    This is actually pretty simple in theory. The only catch is where to bob it. I want it just big enough to accommodate the Aeropac retailer flange. I figured I need a diameter of 5.25” at the bottom. This allows the centering ring large enough to mount the Aeropac retainer flange.

    When I measured the nosecone and figured it was a Ogive shape and Rocksim indicated that bobbing it at 14.5” below the shoulder would do the trick. But that was still a bit small. So I bobbed .5” more (14”) and it was perfect.

    On to the pictorial...


    Ok, my daughter Jan got a pic of me measuring.


    Ok, this is better, I used a paper strip with a mark so when laid on the curved surface it would yield exactly 14” from the shoulder. Using this I made a series of marks around the nosecone every few inches. From past experience it’s really hard to eye-ball this sort of cut.


    I then used a piece of trim tape and followed the markings. Here is where I used my eye to adjust for variation in the mark position and make sure I had a straight line.


    Sorry, no pics of the cutting since I needed both hands. I had my son Phil hold the cone and rotate it while I cut. Anyway it turned out great. I used me Dremel to clean up the inside layup and make it a consistent thichness where the aft ring is going to sit.

    Sorry James, no ballon pics here

    Next up – making the rear centering ring bulkhead.
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  14. #14

    Re: Build Thread: Gene's L3 Project

    Making the rear centering ring

    So I am devoting a post to the read ring since it a critical component and must fit perfectly. Once of the challenges with this component was measuring and fitting a boat tail that was not perfectly round. I solved that by cutting a series of concentric rings that I could fit over the boar tail to keep it round. Then I could measure an fit the ring no problem.

    On to the pictorial...


    Here is the boat tail with the rings that keep it round. This allowed me to accurately size the centering ring and later fit it.


    Shown above is the ring before cutting. It was a stock 6” to 98MM ½” centering ring I got from Giant Leap. Of course this ring has 2 layers of 6oz carbon vacuum bagged on it. Inside the ring is a wooden disk with a 1/8” centering hole for the router pivot. I taped the edge of the centering disk with tape and press-fit it into the centering ring. I then cut the ring to the proper size on my router table.


    Here is the disk after cutting. I then carefully aligned the Aeropac retainer on the disk and marked the first two mounting holes. Here I am screwing in the second threaded insert. Once these are set I bolted on the retainer and used the retainer holes as a jig to assure I drilled the rest of the holes in exactly the right place. I used my drill press for this.


    Here is the ring with the Aeropac retainer mounted. Perfect alignment!


    Another view. Now I need to sand the sides to match the boat tail profile for a perfect fit.


    I used my overhead router pivot and clamped it to my bench sander table to cut the sides at an angle to match the boat tail profile. Also important detail – make sure to do the taper the right direction.


    Here is the final ring ready to be epoxied to the motor mount.


    Now I epoxy the ring to the motor tube. I used Aeropoxy for this for max strength. Before I glued it on I put a little square of tape over each mounting hole to keep epoxy out – this saved me a bunch of grief.


    All done... and it fits perfectly in the boat tail.

    Before proceeding further on the tail assembly I want to make the fins. So next up – Making the fin patterns, and then the fins.
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  15. #15

    Re: Build Thread: Gene's L3 Project

    Make the fins

    Now for the fins. For these I will use a combination of GL 3/8” honeycomb with balsa and air-ply edging. The GL honeycomb is already laminated with .01” fiberglass and is very flat – ideal for this purpose. If I vacuum bagged my own honeycomb it would not be ideal – the vacuum bagging process causes little dimples to form. So while just as strong the surface would need a lot of filling.

    To further keep the fin weight down I chose balsa for the leading and training edge. Balsa is very light and easy to shape. Also the fins will have reinforcement laminated over them so there will be plenty of strength. Lamination over the fin from inside to out consists of:

    1. One layer of carbon from the root out to 5”. This makes the root tab very strong.
    2. Additional two layers of carbon over the entire fin
    3. 8oz glass
    4. 3oz satin weave glass to provide a good finishing surface.

    The entire structure is laid up in a single session and vacuum bagged.

    The construction starts with printing the fin pattern from Rocksim – actually two of them. The first pattern sets the overall dimensions as well as I add outlines for the 1” wide balsa leading edge and 1.5” balsa training edge. I also use a .5” air-ply root to increase strength. The inner boarder of balsa and air-ply sets the size of the honeycomb. The second pattern is cut down in size by this much and used for cutting out the honeycomb board.

    On to the pictorial...


    Here are the two fin patterns. The top is the entire fin. You can just make out the outline of the edging. The bottom is just the honeycomb core. Oh – and that’s Phil my 13 year old son as spokes-model.



    The honeycomb pattern is transferred to the material.



    I cut the honeycomb using my old Dremel scroll saw. It cut really easy.



    Here are three cores ready to go.



    I stripped 1” and 1.5” balsa and taped then to the core. I also ripped .5” air-ply for the root.



    I applied tape over the entire back edge.



    Using the tape as a hinge I opened up the joint and applied Airpoxy. The edging is then closed and tape applied. Both sides of the joint are sealed with tape.



    I placed a piece is shelving material that is very flat over the fin layup and put a heavy weight on it. This keeps the entire assembly flat.



    After bakeing over night and removal of the tape the edging is done.

    Continued...
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  16. #16

    Re: Build Thread: Gene's L3 Project

    Make the fins – Cont


    I use the fill size pattern as a cutting guide to trim the edging.


    The fin edging is now ready to shape.


    Before shaping I mark a center line on the balsa edging. I use this as a guide to assure the sides are symmetrical.


    I use my bench sander disk and belt to do the rough shaping. I use a t-bar sanding tool for the fine sanding. Results speak for themselves. Note the centerline still visible. These fins have near perfect symmetry.


    I cut the material and do the layup in a single session. Sorry – no detail pics here, it was just too hectic!


    Another view of the layup under vacuum. I get a 24in (65cm) of vacuum out of my pump, or about 10lb / sq foot I would guess.


    Here is a pic of my rig. At the top is the pump which I bought I think at a garage sale years ago. It is a centrifugal pump and draws a vacuum very fast. I also use a 10gal compressor tank as a vacuum reservoir. The bags almost always leak a little and the reservoir really slows down the loss of vacuum by giving a much larger volume to draw down. The layup has a small volume and even a tiny leak would cause the vacuum to fall very fast. After I pump down the bag I close a valve on the pump. I monitor the vacuum (see gauge on tank) and run the pump again as needed. So I only need to periodically run the pump – keeps the energy use down!


    After baking for 8 hrs and while it’s still green (not sticky but not fully cured) I peel off the peel ply – beautiful!


    Sorry – pic is sideways, but you see all three fins attached by the “flashing”.

    Continued...
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  17. #17

    Re: Build Thread: Gene's L3 Project

    Make the fins – Cont 2


    I use a jig saw with a carbide blade to cut the fins apart. Do this before it’s fully cured to make it easier. Once fully cured it’s really tough going. I position my shop-vac to suck up the cuttings. Carbon dust is nasty and causes your skin to itch.


    Sideways again A fin cut and ready for shaping. Now I let the fin cure the rest of the way (24hrs in this case). Sanding before this clogs the paper.


    After much shaping, sanding and fussing there are the three fins ready to go. Each fin weighs just 10oz – very light. And they pass my standing test with absolutely no flexing. They are incredibly stiff!

    next up – Vacuum bag the lower airframe.
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  18. #18
    Join Date
    19th January 2009
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    5,211

    Re: Build Thread: Gene's L3 Project

    Not sure what happened but I don't get any pics. Just the little white box and red X indicating a missing photo link.
    Handeman

    TRA #09903 L2

    "If you don't use your head, you have to use your feet!" my Dad

    Tripoli Central Virginia #25 - BattlePark.org

  19. #19

    Re: Build Thread: Gene's L3 Project

    Quote Originally Posted by Handeman View Post
    Not sure what happened but I don't get any pics. Just the little white box and red X indicating a missing photo link.
    Same here. Way keep us in suspense!! Looks like a Great L3 build. Good luck!!

    Ben

  20. #20
    Join Date
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    7,406

    Re: Build Thread: Gene's L3 Project

    I get no pictures either, and no white swuare with a red 'X' in it..Just a "Heres the picture" and blank space...
    Chute Happens!!
    NAR 86940 L2
    TRA 12270 L3 09-01-12
    KF4GUL
    TeleTubby Fan, Unofficial King of Namby-Pamby Land

  21. #21
    Join Date
    8th February 2009
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    Re: Build Thread: Gene's L3 Project

    If he was using photobucket and moved the photos into a new folder, all the links to the images will be broken. Looks like that's what happened.
    TRA 10762, Level 3

    12" Talon on a P build:

    http://www.rocketryforum.com/showthread.php?t=3797

    On Twitter: @Sparta_Chris

  22. #22
    Join Date
    18th January 2009
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    1,089

    Re: Build Thread: Gene's L3 Project

    He's using the Fruity Chutes site, which looks to be down.





  23. #23

    Re: Build Thread: Gene's L3 Project

    Quote Originally Posted by Gym_Class_Hero View Post
    He's using the Fruity Chutes site, which looks to be down.
    You're right - I have boat loads of pics in my build thread, and more to come. I host on my Fruitychutes site... I just upgraded the plan... Shouldn't have any more issues...
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  24. #24
    Join Date
    19th January 2009
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    Hole in the wall near Savannah, GA
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    Re: Build Thread: Gene's L3 Project

    We have pictures now! Looks like an awesome build!
    Chute Happens!!
    NAR 86940 L2
    TRA 12270 L3 09-01-12
    KF4GUL
    TeleTubby Fan, Unofficial King of Namby-Pamby Land

  25. #25

    Re: Build Thread: Gene's L3 Project

    Quote Originally Posted by Pantherjon View Post
    We have pictures now! Looks like an awesome build!
    Hey I was going to say that!!

    Ben

  26. #26

    Re: Build Thread: Gene's L3 Project

    Vacuum bag the Airframe

    So I have been absent for a few weeks from posting, but things are progressing. In this installment I will vacuum bag the lower airframe.

    I like GL phenolic tubing since its light however somewhat brittle. But if I add a few wraps of fiberglass that makes all the difference. I will use one layer of 9oz E glass and top that with 3oz satin weave for a nice surface finish. Using a vacuum bag allows is tight efficient layup with minimum resin and very flat. It finishes well and is a great surface to prime and paint. Before I glass I will incorporate a Kevlar anti-zipper feature into the airframe.

    But before I do any of this that there is quite a bit of tooling and a few jigs that need to be made. One important feature I build into my airframes is a Kevlar anti-zipper band. For this I need to route a 5/16” shoulder into the end of the airframe using my router. I remove about ½ the thickness of airframe. For this I need a jig to stabilize the tube on the router table.

    Other tooling needed is end-caps, and a series of reinforcing disks to go inside the airframe to assure I don’t crush the tube once I pull the vacuum.

    On to the pictorial...


    First I make the end caps. For this I cut a couple of ½” ply end caps the exact diameter of the outer airframe. Then I glue a 1.5” long piece of coupler centered on the ring so there is a shoulder to keep the cap from being sucked into the tube. I coat the coupler and ply with a coat of epoxy to seal it. I then paint these surfaces with PVA mold release so I can get them out after glassing. Don’t forget this step!


    To reinforce the airframe against being crushed I made a series of disks cut to the inner diameter of the airframe. I position these on a piece of threaded rod (several of them) at regular intervals. This shows all the disks and the end caps in place.


    The Kevlar anti-zipper reinforcement is made by winding Kevlar tow onto a shoulder routed into the airframe end. This shows the cradle I made to stabilize the airframe on the router table. The airframe nestles right into the cradle and the airframe end butts up against the upper cross piece just over the router bit.


    This is clamped at the right location for a 5/16 wide cut.


    Now the airframe is placed in the cradle. Phil stabilizes the aft end.


    Turn on the router and move it onto the head. Turn the tube slowly to cut the shoulder. Of course you first want to make sure you set the router bit to the proper depth!


    Presto! A perfect shoulder is routed in the airframe. Kevlar will be wound into the shoulder before the glass goes on.


    The reinforcing disks and end caps are installed and tightened. This is mounted on a couple of stands to get the airframe up off the workbench so it can be rotated easily.

    Continued...
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  27. #27

    Re: Build Thread: Gene's L3 Project

    Vacuum Bag the Airframe - Continued...


    I use CA to tack glue the start of the Kevlar tow. I use a heavy thread for this I got from some guy on Ebay. The airframe is then rotated while the Kevlar is wound on the airframe nice and neat. Two layers just fills the shoulder back to level.


    Once the shoulder is filled up the end is again tacked with CA.


    Next I did the layup and got it into the gag. Sorry, no pics. My hands were covered with resin and I was under the gun to finish it in 1 hr. It was a warm day as well so I had to hustle.


    The next day I pulled it from the bag. Beautiful! Final steps are to cut the fiberglass just past the airframe edges. Then knock out the caps and trim. Don’t forget your mold release or this last step may take surgery...


    Here is the final result. The Kevlar anti-zipper reinforcement is incorporated directly into the airframe.

    next up – Make the fin alignment jigs.
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  28. #28

    Re: Build Thread: Gene's L3 Project

    Looking good Gene. I'll try to make it to October Sky.

  29. #29

    Re: Build Thread: Gene's L3 Project

    Fin Alignment Jig and Fin Slots

    The fins are mounted in the boat-tail before the boat tail is attached to the lower airframe. After mounting the entire boat tail fin assembly can be reinforced. After reinforcement is applied then the airframe is attached.

    To assure proper alignment several pieces of plywood are used to make an alignment jig. These are slotted to assure the fins are straight. Holes are cut in the jig for the boat tail to slip into. The holes also assure the boat tail is kept exactly round.

    On to the pictorial...


    First on a large piece of pattern paper the slots and boat tail cutouts are drawn. There are two sets of plywood, but just one pattern is used to make both pieces. This assures the slots are exactly aligned.


    Here is the completed pattern. Note there are two sets of concentric circles. One will set the aft boat tail cutout, and the other the front cutout.


    The pattern is transferred to the plywood. Both pieces of plywood are clamped together to assure the slots are precisely aligned. A circular saw is used to cut the slots.


    After cutting the pieces are separated. Since the slots approach the edges an extra piece of ply is glued to the tips to stiffen these up.


    Next the center holes are cut and threaded rod used so they can be joined. Here are the jigs mounted to the boat tail.


    Another view. The center holes assure the boat tail is exactly round then the fins are installed. The forward ply rests against the boat tail shoulder so the entire assembly can be tightened. The boat tail on it’s own is not exactly round.

    Cutting the Fin Slots

    The fin alignment jigs are used to set the position of the fin slots. Again this assures precise alignment of the slot to the jig.


    Pieces are ply wood are used to insert into the slots so the positions can be exactly marked.


    Here is another view. The wood extends up the airframe (not yet attached) and is used to make sure the fins are lined up with the center line of the airframe. The jig positions are adjusts to assure precise alignment. After hours of fussing and once everyting is set the slot positions is scribed onto the boat tail using a knife blade. The boat tail is then removed from the jig and the fin tabs used to outline the slots. I like to use tape for these outlines. I find a tape edge more precise than a pencil line or marker.


    The slots are then cut using a carbide cutting wheel. Sorry I don’t have pics of the cutting Note the tape outline.

    Next up – attaching the fins...
    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

  30. #30

    Re: Build Thread: Gene's L3 Project

    Attach the fins

    The fins are glued to the tail cone using the jigs to guarantee alignment. But before the fins can be glued in the centering ring just aft of the fins must be glued in. After this the fins are attached. Next the fin tabs are reinforces with fiber glass through the fin can opening. Since reinforcement is done inside there is no messy reinforcement done outside the airframe – keeping it clean and easy to finish. After all the inside work is done the top centering ring is put in place.

    On to the pictorial...


    Before the fins are attached the centering ring just aft of the fin tab needs to be adjusted and glued into place. Before pushing in for test fitting I attached 3 strings in a triangular pattern that can be used to pull the rind out if the test fit isn’t good. Otherwise once the ring is inside the tail cone you can imagine how hard it would be to remove. After several rounds if test fitting I glued it in using Aeropoxy.


    Next using the alignment jig I glued in the top fin only with Aeropoxy. The other two fins are inserted to keep alignment. Note my daughter Jayne thinks it’s pretty cool.


    The airframe is also used to help keep the engine mount exactly centered. Jayne thinks she might be able to ride the rocket ala Slim Pickens in Dr Strangelove.


    After the first fin is glued the other two are as well.


    Here is another view with all three fins in place.


    After 8 hours – perfect!

    Now the fin attachments need to be reinforced. To do that I make a few little tools that let me apply fiberglass to the fin tab through the tail cone opening.


    I lay some parchment paper on the airframe in case I drip glue. I used a long slick to place pre-wetted glass on the tab root and inside the tail cone. All reinforcements are done inside the tail cone leaving the outside clean.


    Here you can see the glass reinforcement on the fin tab to motor mount connection.


    Once the reinforcements are in place for the fins the upper centering ring can be positioned and adjusted. Again note the strings so it can be pulled out if the fit is not correct.


    Once the fit is perfect I glue it in. I add some glass roving to the Aeropoxy to make the joint stronger.

    Next up – attaching the aft airframe...

    Gene Engelgau
    KI6IBL, NAR 86770 / TRA 12243 - L3
    http://fruitychutes.com - Aerospace Recovery Solutions

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