Recently, I’ve been working on what materials we should use for O-rings inside our propellant tanks in my rocket club on campus. We’re building a bi-propellant rocket using IPA (isopropyl alcohol) and nitrous oxide (N2O). There are some of the notes I made trying to understand how a pressure vessel, essentially the tanks housing the two fluids we are using, should be designed. Theoretically, there were a lot of things I learned, about how the placement of O-rings should be, and how they prevent the outflow of fluids from within the gap of a piston inside a tank (not mechanical, essentially just a thick slab of metal which weighs down on the fluid).
For safety reasons, HalfCatRocketry suggests using redundant O-rings, so as to not rely on solely one since this could count as a single point failure; if the fluid starts flowing out of the required volume and through the gap of the piston and the tank wall, the process will not go accordingly to plan and possibly kaboom. N2O is a notoriously difficult fluid to work with because of its self-pressurizing capabilities, but we also have to take into account its chemical properties when selecting materials for parts such as O-rings. For example, two possible O-ring materials are Viton and Buna-N. Viton can swell and degrade in the presence of N2O but Buna-N holds well. These types of small difference could be the difference between a working rocket, and everything else. This is also what fascinates me about rocketry: to get something to fly you have to get pretty much everything right (unless the gods bless you and give you amazing luck in every stage of flight). Luck always plays a role, but luck can’t do its job unless it has something working to help.
My research of HalfCat’s document “How to Design Pressure Vessels” also taught me several more things. NPT (national pipe taper) thread screws require a tape to actually be used to work ideally. NPT screw threads literally deform when they are screwed into an object, making them such a tight and strong seal. This is also why HalfCat recommends not to reuse screws too many times because the threads during reuse might not give the seal they would have if they were screwed in for the first time. Furthermore, Teflon (PTFE), the same material which makes pans non-stick, is used as the material for the tape required for these types of screws.
As I learn more, I aim to keep sharing what I find interesting and what challenges me. My next plan of action is to learn how to size bulkheads, i.e. thickness required, and number of threads the bolts require for the weight and forces they experience. At first glance this sounds dull, but these calculations lay the foundation for a working rocket. I find that amazing.
References:
- HalfCatRocketry, how to design a pressure vessel: https://thediscoverer.blog/wp-content/uploads/2025/11/5c7fe-howtodesignpressurevessels2cpropellanttanks2candrocketmotorcasings.pdf
