Test Stand Engineering Pt 2
The second set of considerations are all about plumbing and in this I include valves, fittings, o-rings, and making sure all my bends fit together. The first set of plumbing I had to do was the hydro test setup which was a hand pump connected to a check valve and from there connected to the test article, drain valve and soon multiple transducers. One thing to note is that where straight threads meet, teflon tape is not necessary but an o-ring is. Flares do not require o-rings. One important thing to note in future systems is to allow for whatever bend radius is formed by your bending tool. My bending tool had at max a 9” bend radius for 1” tubing which is enough to cause problems and some…interesting looking plumbing designs. A smaller radius comes with the risk of ruining thinner walled tubing and a higher pressure drop but that is something that each system designer has to weigh accordingly ( or just wing it, I completely overlooked this entirely). Another thing to consider is the fittings used, whether AN or NPT, and if the latter having a tap on hand is a good idea. NPT is often painful to seal and not repeatable as well as being a FOD (literal garbage) generator. I picked mostly AN because it was what Ken suggested and it’s worked out well. A small note about galvanic corrosion, some things will oxidize and CTE differences need to be considered. Most of my system is aluminum, but there are some brass and steel fittings here and there. I’ve yet to see any destructive failures in my own system but many others have reported galling, seizing and other inconvenient consequences to mixing and matching materials. Plumbing wise, I’ve got to pressurize, burn, and purge my system of residual propellant. My pressurant during testing is going to be gaseous nitrogen because it’s cheap. I anticipate massive ullage collapse using nitrogen but I’ll employ a brute force approach of just using more gas to offset it. I’ve got an unregulated blowdown system that is pressurized using a Marotta solenoid valve. Filling solutions for each tank must be considered and the gas should be remotely filled due to the nature of pressurized systems and the need to be distant from non-human rated tanks under pressure. Many engineers use various forms of quick disconnects and magnet umbilicals and various other solutions for remote detachments on flight vehicles but I’ll just plumb it normally with no umbilicals. Not totally sure on the specifics of that yet. Check and ball valves are used to fill the propellants. Pretty much everything is connected with various sizes of aluminum tubing. One thing to pay attention to is the fluid velocities of the oxygen. Over 70 ft/s it will ignite the aluminum due to the massive amount of activation energy available in the fluid. This is counteracted by using bigger piping. Filling, and draining are done from the same valves, venturis are inserted in the propellant lines to stabilize mass flow and the two propellant tanks are connected to the gas tank. There’s a lot of small details in the testing process that need to be accounted for. I’ve certainly missed an untold amount of them but this is sufficient for now.