Shouldn't be a problem. One thing I'd advise is to bevel the edges of the caps so you get a full-penetration groove weld, instead of a simple fillet weld. Groove welds have more structural integrity since there's guaranteed fusion through the entire thickness of the material. Also groove welds require less heat input for fusion which will reduce the potential for distortion and burn-through. This is obviously a failure-critical weld. Have it done by someone with lots of experience TIG welding aluminum. Don't do it yourselves since you obviously don't know what you're doing. What is the oxidizer? Compressed N2O? Sch 40 seems a bit excessive to me, if the walls are only going to be sch 10. Have you done tank stress calculations? It would also be wise to pressure test your tanks with water to check for leaks, before fielding them. Also, the type of alloy you use comes into play with regard to weldability. AA 6061 is very common for applications like this. However 6061 is a precipitation strengthened alloy. Heat causes the weld + heat-affected zone to be in the solutionized condition- which means they will lose about 70% of their ultimate strength..... only slightly stronger than pure aluminum. The best procedure in this case would be to purchase materials in the all-solutionized condition, weld the parts, then have them be precipitation heat-treated *after* welding.