Ah, I missed that detail about the pins (along with several others in your CAD drawings. Thanks for filling it in.
I think the big issue with AL vs Composite is fin attachment. With a fin can like yours you can easily and very securely attach the fins using metal fasteners that require little technical expertise. But attaching composite fins using an adhesive (whether using a layup or not) requires far more expertise and careful research to materials. Not doubt that CF plate may be stronger than AL but it is also far harder to machine as well. At least in my experience many folks won't touch it due to the conductive nature of the CF and the worrisome effects of the CF dust and fibers. I can have just about any good CNC shop knock out a set of alum fins but not so with CF plate. And being able to just screw (or pin) them in and knowing they aren't coming off is a huge difference.
After reading through this thread:
https://www.rocketryforum.com/showt...rame-build-for-the-Loki-54-4000-Fins-Attached
especially the response to Bob Kretch's post (he made a series of incorrect assumptions as to the cause of the warped plated) made me realize I would never have the time or technical expertise to deal with really high end composite work. But alum is something that I can easily understand and handle, and more importantly, can have someone do the hard work of milling it.
The fact is that most of us don't have the expertise or technical knowledge to build a composite structure that holds up to Mach 3+ while many of us can do so with an alum fin can. And you are correct that a lot of that feeling comes from watching many flights at BALLS and seeing what works and what doesn't. My skills are far poorer than many of the other flyers at BALLS so I've decided that the odds that I can outbuild any of them using composites are slim to none.
Please don't misunderstand my comments - this is a great thread and has been very informative. I'm just one of those guys whose composite skills will never be up to the demands of a super high speed flight so alum looks like the way for me to go. And your technique holds a lot of promise for both simplicity and strength.
Again, thanks for taking the time to post the photos and write-up.
Tony
You bring up a good point about fin attachment. I do consider that the Achilles's Heel of composite fin cans. Almost ALL composite fins will fail right at the joint to the tube (unless you're doing really stupid like 1/32in carbon and massive fillets).
Now I'm really getting off on a tangent, but fin attachment is also something I've thought a lot about.
The standard epoxy fillet method is far from ideal, and epoxy alone doesn't have nearly the tensile and compressive strength of a composite. There isn't really a solution that's easy and efficient. You can increase the fillet size to an insane radius. You can try alternative methods such as attaching them with screws from the inside of the tube. You can slot the tube to add a little rotational resistance. All of those options come with trade-offs, and none I'm willing to take.
I've played with CF/epoxy hairball fillets, which are neither easy to deal with or particularly strong considering the random fiber orientation. A few years ago I did a few tests where the fillets were made completely of small layers of unidirectional CF fabric, staggered to create a fillet shape. An adhesive failure sent me back to the drawing board.
I believe tip to tip (T2T) reinforcement is an answer to a lot of these problems. A single piece of material that stretches from one side of the fins to the other both reinforces the fillets and adds stiffness to the fins. However, I dont think the full benefit of T2T reinforcement is realized in "hobby" style building.
I want to do some tests where the entire fincan is T2T. You would bond very thin (1/32) fin cores to a styrofoam or mylar wrapped mandrel. These would serve to give structure to the layup and serve as a guide while trimming the carbon. You would then essentially add layers and layers of carbon between each fin tip (like regular T2T) until you built up the thickness you want. Once it's cured it'll be one solid piece of material. The only way it could break is if the actual carbon fiber fails. This would eliminate the adhesive problem I described with most "hobby" methods.
It would probably have to be done with unidirectional PrePreg for the sake of precision and working time.
Anyway, this is a tangent but it's also something I've put a lot of thought into.
With regards to machining, I definitely think you're right. CF is substantially harder to machine using common methods. I've seen it done, but it requires weird(expensive) endmills and weird speeds and feeds. I machine mostly by sounds, surface finish and chip color, but all of those dont work with CF. There's a few unorthidox ways to machine carbon, but it's not as easy and throwing it on a Mill. A great thing about carbon is that the shape can be largely determined by layup schedule, so machining is kept to a bare minimum. On my Aeronaut project a year or so ago I tried creating a tapered profile on the fin by making a finco...you know what. I'll just leave a link to it. I essentially built my bevels into the layup and didn't really touch a sander.
My "Playin With Fire 2.0" Build.
Post 38.
Tony, I dont think I'm misunderstanding your comments. I'm relieved to finally be having a civil discussion about ROCKETS on the ROCKETRY forum. Most other people are arguing about who's GF is hotter.
I'm not going to tell people how to build their rockets, so If AL works for some people, then go at it. It's all just for fun.
Alex
