The diffuser introduces the pressurant gas into the propellant tank as a critical component of the pressurization system in a liquid propulsion engine. Element present in both pressurization systems (self-pressurization and by inert gases), the diffuser makes the pressurant enter the propellant tank at a desired direction and velocity to keep the pressure inside the tank at the design level during the pressurization activities without the engine working (on-ground operations or during coasting phase) and to avoid the creation of zones where the operating pressure falls below a threshold value of NPSH during engine firing.
Imma have to defer to CSI Starbase for this one 😅
I found some info! It is indeed part of the autogenous pressurisation system.
Diagram from mcrs987:
Photo from megagoose11 / Kevin Randolph:
Someone also dredged up a 189-page PDF on Designing and Optimization of Pressurizing Gas Diffusers in Cryogenic Methane Tanks: A Technical Approach if you’re really interested :)
Edit: Also, direct links to the Flight 9 and Ship 36 report (archive) and the Flight 10 webpage (archive).
Well, I took the plunge. From the thesis:
Hell yeah, great work. Thanks for reporting back, I was very curious about this too!