Strengthening airfoil fins for L1 rocket

The G10 cuts fast and cleanly. Per Ahansom's warning; the edges are really, dangerously, sharp (cut myself a little while carefully peeling off the blue tape).

Getting ready to glue them up. Toying now with the idea of pinning the halves with a pair of nylon screws at about 2 cm from the tip. Easy enough to do. Additional invested time would be minimal, and these fins area already crazily over-engineered. Risk is that I will screw something up while drilling through the plywood. Nobody in my shop but me to talk me out of it.

These fins are beautiful.

dhbarr said:

These fins are beautiful.

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Thanks. My finishing skills aren't great, but I am going to try not to glop it up too badly when I attach them. 17 days to get it together.

jlabrasca said:

Risk is that I will screw something up while drilling through the plywood. Nobody in my shop but me to talk me out of it.

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I screwed something up. These fins were too thin. Less than one ply at the corner where I wanted to pin it with a screw. It split when I started to drill. Even with the fiberglass core, I am worried about what will happen if I land on a fin tip.

A quick re-design reducing the root-to-tip taper to leave a little more than 1 ply at the corners of the fin tip, and thinning the leading and trailing edges by 30% to make the airfoil shape more apparent.



I scuffed up the G10 and buttered the inside faces of the fin halves with with JB Weld. I put toughed CA in the screw holes before I tightened them down. I've got them clamped up and curing now. I'll cut the screws flush when I take the fins out of the press tomorrow. The manufacturer claims this formula will adhere fiberglass. If that claim is overstated, the wood should adhere to the wood through the vias, and the screws should provide mechanical support at the weakest point on the fin.

I kind of like the look screw coming out of the fin. If I do a 3" version of the WAC Corporal I think I'll put screws right along the edge, to suggest bolts or spot-welds.

Looks very realistic! Great work

Well, they don't look nearly as cool with paint on them.



The paint highlights the tool marks -- which I don't mind actually -- but I really do need to work on my finishing techniques. The mirror bright silver is unforgiving of my haphazard epoxy and sanding. Hopefully a coat of flat high-heat silver will cover some of my shame.

Its finding an hour or two to sand, fill, sand, prime ... without interruption or distraction, that I just cannot manage. Everything rocket-related has to be accomplished 10 minute intervals separated by hours and hours of doing grown-up stuff.

I am a little worried that I sanded the bottom corner of one of the fins a little too aggressively -- trying to get the stub of the nylon screw flush with the surface. It is noticeably thinner than the other two. No time to fabricate a new one, and it should still be strong enough to survive anything that wouldn't otherwise cause disqualifying damage to the rocket. I ought to trust my own math on this, but I suspect that I will chicken out on the day; put in a bigger chute and and hope that it doesn't blow over the horizon.

Rustoleum Filler Primer is your friend. A coat of that, sanded so all the high spots show, and then repeat until ready. A final coat with very fine sand paper will do wonders to get rid of the "tool marks" and scratches from the course sand paper.

Handeman said:

Rustoleum Filler Primer is your friend. A coat of that, sanded so all the high spots show, and then repeat until ready. A final coat with very fine sand paper will do wonders to get rid of the "tool marks" and scratches from the course sand paper.

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Yes, I probably should have used something like that. Next time.

The tool marks are actually kind of cool. If I hadn't needed to sand down the stubs of the screws, and clean up drips of wood hardener and epoxy, I might not have sanded the fins at all. I could have claimed that the surfaces were scalloped to reduce drag ^-^

I cut the airfoil profile with something Autodesk calls a Morphed Spiral. I set the step-over to about half the radius of the end-mill. The resulting pattern on the fin looked a little bit like a fingerprint.



I blew out this image with an unsharp mask to bring up the marks, as they appeared on the cut but unpainted fin.

With your concerns for the fins strength have you thought of laminating the fins with fiberglass? This might also help cover the tool marks.

A second coat, with the high-heat silver did improve the appearance. Its not up to the standard of the more experienced folks here, but I will be able to hold my head up when I present it at the RSO table. I cut the centering rings and cut the motor mount tube last night. I built a baffle for this, but I've decided to leave it out for the L1 attempt. I am printing a skin for the body tube now. I also cut a piece of BT5 for the conduit. It'll cover the seam in the skin, and also hide a pair of launch lugs (I am putting rail buttons on the other side of the airframe).

markkoelsch said:

With your concerns for the fins strength have you thought of laminating the fins with fiberglass? This might also help cover the tool marks.

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Thanks. I considered it, but I my experience with fiberglass begins and ends with the G10 cores I cut for these fins. Figuring out how to model the fin in Fusion 360, coming up with a strategies to mill it and assemble it, was as much learning as I can manage for this project. Figuring out how to cut fiberglass sheet stock (and why I must NEVER do that again. It just about ruined an expensive endmill) was a bonus. Wrapping a complex shape with fiberglass -- an unfamiliar material, and completely new technique, would mean more time, effort, and expense than I can afford. High power launches are infrequent, and far away from where I live. I need to finish this in time to launch next week.

I undertook this project now because my hours are a little reduced for the summer -- giving me an extra day and a half off every week. Still, I am squeezed for time.


Nevertheless,on the advice of friends in the club, I stole a few hours to start a second rocket -- Plan B in case The WAC Corporal does something embarrassing. I have a spare piece of BT80H and heavy plastic 2.56" ogive nosecone. I designed it to be tough and quick to assemble -- 6.25 mm plywood for the fins and all the centering rings. Long pieces of coupler stock reinforcing the top and bottom of the body tube. Surprisingly, OpenRocket says it'll go about 150 meters higher than the WAC Corporal on the same H182 motor. Take that for what its worth.

My only indulgence, because it's just a few more mouse clicks. was to design the fins with a graduated chamfer -- so that the bevel gets a little steeper closer to the edge. The most time consuming part of cutting on the CNC, after doing the CAD/CAM, is setting up the tool. Once the job is running, you can do other stuff in the shop. I was painting and cutting cardboard while the robot cut these. The fancy bevel won't do much aerodynamically, but I've got some metallic orange paint. It'll look cool.

Nice work!

Sometimes silver paint comes out better if you put a black coat of paint underneath, particularly if you want an aluminium look. Worth trying on a scrap before you apply to the rocket if you are going to do that.

OverTheTop said:

Sometimes silver paint comes out better if you put a black coat of paint underneath, particularly if you want an aluminium look. Worth trying on a scrap before you apply to the rocket if you are going to do that.

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Thanks. I'll remember that. Now that I've had them out in the sun, I notice that Rustoleum "flat" high heat silver is actually a little bit sparkly? This is a new formula, I think. New to me, anyway. There is a barbecue grill on the can. I am used to the engine enamel silver, which dries to look like the bottom of a soda can. Couldn't find that at the big-box home supply store, so I grabbed what they had. The glittery fairy dust effect isn't very noticeable, but it doesn't look much like unpainted aluminum <grin>

I just put a coat of chalkboard paint on the black fin and the nosecone. That stuff is absolutely flat, very hard, water resistant, and it fills really well. It's expensive, but I had a can left over from painting a chalkboard. I used it on a BT55 WAC Corporal, and it survived a lawn-dart.

Dry fitting.

I need to paint some more, before I can start glueing it together and I can't paint for a few more hours -- but it is going to be a rocket today.