As mentioned in previous posts, my main goal with this filament winder project is to manufacture motor cases. I still have some improvements to make to the winder itself before I can wind the 3"/4", L-O cases I'm picturing, such as lengthening the main rails and increasing the speed of winding by tuning it better. Still, I wanted a check to make sure I was on the right track for winding pressure vessels, so I kicked off a (hopefully) quick project to prove that concept. First, a hydrostatic test article.
I started by winding my longest tube yet, using a 54mm liner as the mandrel. 1 hoop layer, 3 layers of 55 degree helical winds. It went about as smoothly as the last 3" wind and the mylar meant that the resulting tube was a perfect fit over the liner, snug without being too tight. The primary reason I'm excited about using custom CF tubes for motor cases is the weight savings, but I've also been burned a couple of times by buying liners/motor cases that didn't fit well with their counterpart and either required an incredible amount of sanding or just buying a replacement. These tubes are made to the ground exterior of the liner, so they fit perfectly. You can also fix the liner-fit problem with case bonding, but that's for another time...
I then turned closures from circles cut out of a sheet of 1" LE phenolic, one solid and one with a hole for a tapered aluminum NPT fitting that I also made. The closures have steps on them for interfacing with the liner and taper on the other end to give more space to pour glue into.
I cut about 6" from the tube to test, hoping to use the rest for a motor. I chose to include a liner even in the hydrostatic test article because it will provide some amount of hoop strength in the motors themselves, so I want it present in the test too. If I switch over to casebonding these motors I will have to redo the test without the liner, but that's not a problem because I don't think I'll try that until 3" or 4" motors and I'd want to test those anyway. I RTV'd the aluminum fitting into one closure, and glued the endcaps into the liner with Scotchweld DP420NS. I chose this epoxy because I had a fresh tube of it around for another project.
While that part cured I used a flap wheel and acetone to clean the inside of the case. The entire outside of the liner/closure assembly was coated with the same epoxy and slid into the tube, then the tapered regions were filled in with more epoxy. The DP420NS was not the right choice here because the "NS" stands for "no sag", and you want it to flow into the narrow space. In the future I'll use something much less viscous, maybe the Stycast 3050 that AT/RCS includes for their DMS motors.
Once cured, I filled the case up with water and rigged it up to the pump. In the process, I encountered a setback. I had intended to mill a feature into the aluminum part to allow me to grab it with a wrench when torqueing the fitting in, but I didn't get around to it and figured that the RTV would be enough. Nope, right as I was done screwing it in, it started spinning in the closure, breaking the seal. This made it a struggle to get the case started pressurizing and there was a constant slow leak out of the fitting. I decided to try anyway, because the alternative would just be throwing all of this away and making a new one. This single-use hardware is going to require me to not cut any corners.
Still, with enough pumping to account for the leak, the case held pressure, and actually reached ~1500 PSI before the I heard a pop and pressure dropped. It seems the epoxy cracked and the closure shifted. Seems like the closure attachment was the weak link, and the tube itself was unfazed by 1500 psi!
Post-mortem cross section. I was pleased by just how thin this liner+case assembly ended up. Of course, there's room for mass savings on the closures.
That's it for now. My next step is another hydrostatic test with a few tweaks (and no leaks), then a K motor with a silly mass fraction (that might work). I'll probably start a thread over in the research section for that to keep this one focused on the winder.