On aspect of this design that worried me was the required precision of the fin cuts, particularly the three that hold the ring, and the general curviness of all the fins would present some challenge as well. Furthermore, my recent attempts at hand cutting even simple fins accurately were not great, so at
jqavins' urging I decided to go ahead and get the fins laser-cut. This would be my first time trying this.
Unfortunately, I'm not one of the lucky ones that has access to a cutter, or knows someone who does. So I had to find a cost-effective way to do it commercially. After querying a bunch of services, I decided to go with
Laser Cutting By John, mostly because they had *by far* the smallest minimum, and were therefore the only one I found that could do this at manageable cost. It actually turned out that the minimum was still more than I needed for this rocket, so I bundled in a set of fins (and other goodies
) for a forthcoming build of my modified version of
Gary Byrum's APRO Lander. Needing to get *that* design finalized before I could get any fins cut delayed the start of this build by several weeks.
Getting the files prepared for cutting turned out to be an non-trivial adventure. Most services wanted DXF files, and I didn't have a program that would generate them. I wanted something free, so I eventually settled on
Inkscape, which presented a fairly steep learning curve for me but ultimately got the job done (mostly). The Mac version runs under X, which is pretty painful, but I muddled through.
I started by exporting the PDF fin templates from OpenRocket, and then importing that file into Inkscape. Inside Inkscape I then made the following adjustments:
1) cleaned up the curves (i.e., turned them into real smooth curves rather than the segmented approximations I used in OR)
2) split the ring-holder fins into two pieces
3) added TTW tabs to the rear fins (something else I probably wouldn't have done without the laser cutting)
4) laid out the fins on a sheet
For reasons that will probably never be clear, John had some problems with resulting DXF file, but I was eventually able to give him something he could work with after many (!) back-and-forths. I'm not even sure which file he ended up using to do the cutting, but this is a reasonable approximation of the final sheet layout:
Note that I made four pieces of each, to allow for screw-ups; that turned out to be a smart move. I fiddled with grain direction quite a bit; it's hard to know exactly how to do it when all the edges are curved. Ultimately I took my best guess, and everything will be papered anyway (there will be lots of papering in this build!) so they should end up plenty strong.
John was extremely patient with me and eventually shipped me some beautiful fins. I recommend his service highly, although I don't know if he would recommend me as a customer.
Fins for two rockets were delivered to my door for $20, can't beat that. Here's the finished product:
You don't get to see what crazy stuff I got made for the APRO Lander until I get around to that build thread.
Epilogue: It turns out that a good deal of the problem I was having cutting my own fins was due to the ungraded 3/32" Revell balsa I bought at Michael's being
outrageously dense and hard. I asked John for medium-weight balsa, around 10 lb/ft^3, and it is the perfect balance of light and stiff and easy to work with. If I can get my hands on some like-weighted balsa sheets in the future, I'll be ready to dive back into hand-cutting. But I'm still glad I got the parts for these two particular rockets laser-cut. Thanks jqavins for the motivation!