High-Temp O-ring material

[elovekin]

My university rocket club is working on designing an O-class KNSB motor. We're picking O-rings and we're unsure about which material to use. Our simulated combustion temperature is 2250 deg F and our burn time is about 5 sec. The Parker O-ring handbook gives ranges of operating temperatures for different materials, and all of these are below 572 deg F. Considering that our temperature is well above the maximum for all the listed materials, what are ways to thermally isolate the O-rings? Considering the short burn time, is this even necessary? Finally, is there a significant advantage to using expensive but more heat resistant FFKM over cheaper materials (EPDM, Silicone), considering that the operational temps are still way above the suggested maximum?

 

[G_T]

Does anybody on your team have either a Tripoli or NRA certification? And what goes along with it, experience working with commercial solid propellant motors? Also, does anybody on your team have experience working with KNSB in smaller motors, at least up to size M? Or at least APCP EX motors of similar size? I would consider that a minimum experience level before starting on a DIY O motor.

We get questions like this from new members a lot, so please don't take my answer personally. I don't know you. We encourage people to learn and develop their knowledge and skills. But making solid propellant of any type and working with rocket motors does pose some danger. Knowing the various dangers, and how to deal with them is good. Having the experience doing so is even better. Many learn from youtube, and I'm sorry to say, there's more to the story than a youtube video.

To your specific question, very few and rather specific materials are truly suited for exposure to the environment that resides inside a rocket motor for more than just a short time. O rings are used in locations where they are not exposed to flow of hot gas, just pressure from hot gas (and liquids/solids collectively slag). The heat transfer is slower when the gas is more static (less convection). O rings are to block the gas from flowing past them, thereby protecting portions of the motor structure from exposure to the high heat. They can be partially destroyed in the process, or they can come out essentially unscathed. Often two rings are used in series so if the first fails there is a more protected second ring to back it up. Necessary? Not generally. But sometimes. Depends.

KNSB has very high slag percentage. Slag has higher heat capacity than gas and could be a leading cause of damage in a KNSB motor since otherwise the combustion temperature is fairly cool as rocket motors go.

There are going to be challenges making a KNSB O motor. If you have not successfully worked up through smaller KNSB motors at least to size M and preferrably N, you are not IMHO ready.

Gerald

 

[Brainstormz123]

The gold standard oring for that would be the high temp red silicone orings, but normal silicone or viton or buna or whatever else you wish will all work fine.