(This is from an ARocket post by Joel)
For Xoie we needed bigger tanks than on Xombie, but smaller than the 36inch tanks we had in house that were originally for a four-engine design. So we kicked off two science projects. We knew we really needed to squeeze for both weight and pressure to get Xoie off the ground. For the LOX tank we went really high tech – Alustar Aluminum (5059) spun into 25.5 inch hemis. Had to have it custom shipped (by air!) from the Netherlands. This is the current state of the art aluminum alloy (mostly used for yachts and high performance marine construction), and seemed to have all the properties we needed. After welding we sent our first tank off to be burst, with a design goal of an 1,100 psi burst – didn’t pop all the way to 1,400! That was a good day – we thought we were done thinking about the tanks. Then the next two (the ones we wanted to actually use) failed their basic hydro at 850 psi. That was a bad day.
After a lot of back and forth with our welders, we decided to go to the instant replay, and had everything x-rayed. Turned out the 2nd and 3rd welds (done at different times from the first one) had simply not had the penetration needed. We worked with them to change the welding process a little, and also made x-ray before hydro a standard part of our new tank process. Fortunately we’d started with 12 hemispheres, so we had a little bit of margin. The current batch x-rayed clean, passed hydro with flying colors, and one of them is now on Xoie.
For the IPA, we went a different route. Our fallback was two aluminum tanks, but we really wanted to see if we could push the weight down. So we found a supplier to build us an all composite carbon fiber IPA tank. I was worried that we were really pushing the state of the art here, but turns out this isn’t that big a deal, and we’ve got a gorgeous black spun tank sitting on Xoie right now, maybe 15 lbs lighter than the equivalent aluminum. The fiberglass attachment skirt actually came in a little heavier than we’d planned, so we should be able to shave a few more pounds off the next version as well.