L2 4" Tubefin build thread

Just got underway with my L2 project, and thought I would share some build pics and get some community input.

The rocket is going to be a 4" upscale of a tubefin design I've played around with over the last couple years. The largest version I've flown to date has been a 3" diameter 5' long single deploy motor ejection rocket called the "Purple People Eater". Pic available here:

https://www.rocketryplanet.com/forums/attachment.php?attachmentid=6636

This time, I'm making several modifications to the basic design, including an Aeropack aluminum tailcone retainer, electronics for dual deployment, and the use of BlueTube for the airframe and MMT. Still not sure of a color scheme or name for this bird. The idea is to fly it naked or in primer on an I motor as soon as I can, then get it all prettied up for my L2 attempt, probably at Red Glare IX this fall.

I started construction by cutting an 18" piece of 4" BlueTube to serve as the core section of the booster, and a 25" length of 54mm BlueTube for an MMT. 3 centering rings were fabricated from 1/2" birch plywood, 2 of which are sized to fit inside the airframe tube, and 1 to fit inside a coupler. The smaller centering ring gets a couple 5/16" U-Bolts installed for recovery harness anchor points, and becomes the forward bulkhead of the booster section. I tack welded the nuts onto the U-Bolts for insurance against them ever loosening up.

The 2 larger rings get bonded onto the MMT with 30 min epoxy, so that they are exactly 14.5" center to center. the Aft ring location was chosen so that the MMT section that protrudes from the aft end of the rocket is the proper length for the tailcone when assembled into the airframe. The body section of the tailcone assembly gets bonded onto the aft end of the MMT using JB Weld.

After allowing the MMT assembly to cure overnight, the entire assembly gets glued into the 18" airframe tube using 30 min epoxy. A motor casing and the tailcone were installed temporarily to make sure that everything would align perfectly once the epoxy cured.

After allowing the epoxy to cure for a couple hours, I finished the booster core assembly by epoxying in a coupler and the forward bulkhead assembly.

A few more pics.

Up next--Fin fabrication and installation...

SCE to AUX said:

A few more pics.

Up next--Fin fabrication and installation...

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Cool!!! Where did you get the boat tail?

John

Looks like an Aeropack tailcone retainer to me.

Yes, it's an Aeropack TRA5439A

https://www.aeropack.net/motor_tailcones.html

These are usually fairly pricey (around $50), but I managed to snag one from Al's Hobby Shop's HPR liquidation sale for 30% off. Still trying to decide between painting it or leaving it with the polished aluminum finish.

SCE to AUX said:

Yes, it's an Aeropack TRA5439A

https://www.aeropack.net/motor_tailcones.html

Still trying to decide between painting it or leaving it with the polished aluminum finish.

Click to expand...

Thanks! Bob.

SCE to AUX said:

Yes, it's an Aeropack TRA5439A

https://www.aeropack.net/motor_tailcones.html

Still trying to decide between painting it or leaving it with the polished aluminum finish.

Click to expand...

I vote for the polished aluminum finish. I've used them a couple of times, and I like the bare look

The build continues with the attachment of the aft launch lug. Previous versions of this design have all used rods, but due to size and weight issues, I wanted the added security of a rail launch system for this one. Rods are easier to do with tubefins, because the spaces between the tubefins and the airframe form PERFECT places for the launch rod to pass through. A standard 80/20 rail isn't going to fit, however, so it is going to have to fit THROUGH one of the fin tubes.

I used Acme conformal rail guides, which are extruded, anodized aluminum, curved to fit the OD of the airframe. In this application, they really wanted to be attached to the ID of the tubefins. I got around this by gluing the lug to a small scrap of 4" BlueTube, trimmed to the same size as the lug base. The lug and spacer is then glued in place on the booster airframe. The extra thickness of BlueTube between the lug and the airframe raises the lug sufficiently to prevent the rail from rubbing on the inside of the tubefin. One of the tubefins gets a rectangular cutout to fit over the launch lug.

The fins themselves are quite simple to fabricate. I started with 16" lengths of 4" BlueTube. I drew 2 alignment lines along the length of each tube, 180 degrees apart. One of these lines will end up being the "root edge" bonded to the airframe, and the other is used to assure alignment when cutting the leading and trailing edge angles.

The angles are cut using a standard power miter saw, with a fine tooth carbide blade. Leading edges are cut to 60 degrees, trailing edges to 30 degrees. Stops are set on the saw to position the tubes so that all of fins end up the same overall length, just slightly less than 16".

Each fin gets 2 small holes drilled in it for a pair of #6 flathead woodscrews, one 3/8" from the rear end, and the other 14.5" forward of the first. This spacing corresponds to the spacing of the aft and middle centering rings, into which the screws are driven to secure the fins. The screwholes are drilled along the root edge alignment line on each tube fin.

An alignment line is drawn on the airframe tube, and the first tubefin is set in place, pilot holes drilled into the centering rings, and screws installed. then the remaining 5 fins get installed the same way, being banked up against the previous fin, and rotated into alignment before drilling. once all 6 fins are installed, they get numbered 1-6, and corresponding numbers are marked on the airframe at each fin position.

The fins are then removed, and the airframe and each fin root edge is roughed up with 80 grit paper. Starting with Fin #1 (the one cut out for the launch lug), each fin is coated with 30 minute epoxy along the root edge and screwed into place. The BlueTube is soft enough that the screw heads countersink themselves when tightened down.

The fin installation is completed by pouring epoxy fillets into the 6 "valleys" formed between adjacent pairs of fins.

More pics.

Up next--Finishing touches on the fincan...

Looking nice!

Looks great

This makes me kind of want to build a large tubefin rocket.

Moving right along, the fincan gets a finished off with a couple small details.

4 press-in stainless threaded inserts (8-32 thread) get installed in the sides of the forward bulkhead, to provide a strong but removable attachment for the aft airframe section. 4 corresponding holes are drilled into the airframe section, and the screws installed temporarily to check alignment.

The fincan is finished up by pouring a thin layer of laminating epoxy (West System 105/205) into the recesses at the front and rear centering rings. This protects the exposed plywood from damage and provides a little additional bonding between the airframe, centering rings, and MMT.

Next up--Avionics Bay construction...

Looking good. I really like what you are doing. It's kind of nice to watch you build. My wife and I are doing a similar project together for LDRS 30. I suppose I could let the cat out of the bag, since I have the fin tubes ordered. we are upscaling the super neon to a 6". It was here dream not mine, but its fun to do a build with the wife.
Please keep posting as it helps me with the design of the 6xl super neon.

John

G2Rockets said:

Looking good. I really like what you are doing. It's kind of nice to watch you build. My wife and I are doing a similar project together for LDRS 30. I suppose I could let the cat out of the bag, since I have the fin tubes ordered. we are upscaling the super neon to a 6". It was here dream not mine, but its fun to do a build with the wife.
Please keep posting as it helps me with the design of the 6xl super neon.

Click to expand...

Thanks!

I hope your are going to post some pics of your build--a 6" SuperNeon would be really sweet! Are you going to include the little "finlets" or not? I've built a couple of those kits, and haven't noticed any difference in stability with or without.

What kind of motor are you planning on?

SCE to AUX said:

Thanks!

I hope your are going to post some pics of your build--a 6" SuperNeon would be really sweet! Are you going to include the little "finlets" or not? I've built a couple of those kits, and haven't noticed any difference in stability with or without.

What kind of motor are you planning on?

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Yup! I will post pics. I am just gathering components right now. I am going to include the finlets. This is going to add a little bit more challenge, due to strength issues. I am thinking Big J or small K. A CTI J330 carries enough umph to get around 2000 feet. I would like to do a K, but the conservative side of me wants the first launch to be a J. I almost have all the Engineering drawings done. Maybe when I am finished I will post those, until I get all the supplies. The wife wanted it built by airfest this year, but I had too much going on.

John

Okay, another progress report on the L2 build:

The aft airframe section has been finished off with the addition of 2 more launch lugs and a 1/4" vent hole to prevent premature separation due to differential pressure.

The construction of the avionics bay started with the fabrication of 2 end caps from 3/8" thick FRP sheet. The pieces were turned in a lathe, and stepped to fit into the ends of a 4" BlueTube coupler. Holes were drilled into the end caps to accommodate the tie rods which hold the avbay together, binding posts for the ejection charge connections, a 5/16" forged eyebolt for the recovery harness, and a screw to secure the ejection canister at each end.

The ejection canisters were turned from 6061-T6 aluminum barstock. They are 1" OD, 3/4" ID, and 1.25" high, with a 1" deep internal volume for the powder. A small hole and groove machined into the mounting end accommodates a Quest Q2G2 igniter, and an 8-32 tapped hole provides a mounting point. The canisters were designed to make a snug fit with the same snap-on plastic caps (McMaster-Carr p/n 9567K13) that I use to hold the powder in the forward closures of 29 and 38mm AT reloads.

The 3"x6" altimeter sled is made from single-sided copper clad G10 sheet, usually used for making PC boards. 7" lengths of 9/32" soft brass tubing are soldered along the edges, and slide over the 1/4"-20 threaded tie rods for quick removal and installation.

More pics.

Could you post a few pics showing specifically how you attach the ematches and altimeter leads to the binding posts?

Will try to snap a few pics tomorrow. The connections are standard "5-way binding posts", as used on all kinds of electrical/electronics equipment. The cap is loosened, the stripped wire end from the ematch is inserted through a hole in the post, then the cap is tightened down, clamping the wire. These posts will also accept spade lugs, phone tips, alligator clips, or banana plugs, hence the "5-way" designation.

https://en.wikipedia.org/wiki/Binding_post

https://www.superiorelectric.com/PDF/Connector-catalog.pdf (Various wire connection methods shown on Pg. 3)

https://www.htd.com/cabinet-speakers/installation-instructions

The altimeter wires are connected by soldering them to the turret terminal on the inside end of the post. As this is a threaded part, a spade or ring lug could also be used.

I prefer binding posts to the screw terminal blocks that many others use for this purpose because they give a neater installation, need no tools to connect wires to them, and mount in a single hole, without need to thread wires through the end caps from the altimeter, then seal the penetration to keep ejection gases out.

I may become a binding post convert.

Wouldn't it be easier to crimp a spade on the altimeter wire so that you could take them off and completely remove the bulkplate any time you want?

Jeff said:

I may become a binding post convert.

Wouldn't it be easier to crimp a spade on the altimeter wire so that you could take them off and completely remove the bulkplate any time you want?

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I put a 2 pin miniature series Molex connector between the altimeter and each end plate. That way, the end plates are removable without tools. I put a similar connector on the wiring connected to the safe/arm switches, so the altimeter sled itself can come out of the housing easily.

Pictures of another AvBay I built a while back here, showing the Molex plugs:

https://www.rocketryforum.com/attachment.php?attachmentid=22602

Gotcha - that's what I used on my I5 (on one end anyway).

Just to close out the thread, here's a couple shots of the completed rocket after primer, paint and decals. Ended up with an overall length of 92", 13.4 Lbs at liftoff with a J350 motor (and 54-38 mm adapter). Paint is good old rattlecan Krylon gloss red and flat black. Vinyl decals by Stickershock23. Used a 30" drogue at apogee, 96" main at 400', both by TopFlight. Deployment via PerfectFlite HA45K altimeter.

"The Red Menace" flew its first flight for my L2 certification at Red Glare IX.

Looks great. Congrats on your L2. I just started making the cone for my tube fin L2 attempt this spring. What kind of altitude did it get?

El Cheapo said:

Looks great. Congrats on your L2. I just started making the cone for my tube fin L2 attempt this spring. What kind of altitude did it get?

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Kept it "low and slow" for the cert flight. Simmed to ~1600', altimeter said 1325'.

Planning to shove a few bigger motors in there next season...