https://en.wikipedia.org/wiki/SpaceX_Super_Heavy
The oxygen tank has four chines attached. These protect the various COPVs on board, as well as providing lift during flight.
First I've heard of these things. No explanation as to what they are or what they do.
There are a number of different pressure vessels on Starship and there placement and purpose are not entirely fixed. However I believe the principle COPV's under the chines on the side of Superheavy are to pressurise the methalox header tanks.
When on the ground the main tanks are pressurized from the Ground Support Equipment. Whilst in flight tank pressurization is maintained autogenously (pressurized oxygen gas and pressurized methane gas are tapped from their respective turbopumps and fed back into their respective tanks to ensure that the tanks remain at the appropriate pressure as propellant is depleted.
But after engine cut off autogenous pressurisation is no longer available, so when Superheavy needs to relight its engines for the boost back burn and the landing burn the header tanks have to be pressurized from the COPV cylinders.
Due to the problems experienced with the previous Starship landings involving pressurization and engine anomalies SpaceX have been experimenting with variations on pressurization system and these are not usually made public.
So it is possible that COPV helium may also be used for some pressurisation purposes in the main and/or header tanks during some operations on some Starships. The exact current status is unclear to me at the moment, but the goal is to eventually move to entirely autogenously pressurised tanks with the COPV being re-pressurized when the engines are running ready for the next engine start up.
The main purpose of the COPVs is to provide high-pressure helium to spin start the turbo pumps in flight.
The 20 Raptor Boost engines on the outside ring can only be started on the ground. They all have their own individual little quick-disconnect which supplies the high-pressure helium to spin start the turbo pumps.
The 10 Raptor Center engines in the middle ring can only be started on the ground. They might receive high-pressure helium through the main quick-disconnect or through the 20 outer quick-disconnects. I don't think that is publicly known at this point.
The 3 Raptor Center engines in the center cluster are started on the ground using high-pressure helium supplied by the ground service equipment. But they also need to be started in flight for the landing burn, and in that case, they receive their high-pressure helium from the COPVs.
The COPVs might be used for other purposes as well, but I don't that is publicly known at this point. On Falcon 9, the COPVs contain helium for spin-starting the turbo pumps (just like Starship) but also for tank pressurization, as well as nitrogen for the cold-gas thrusters. However, Starship uses autogenous pressurization and ullage thrusters, so it has no need for either.
Note that Elon Musk mentioned in one interview that being able to put the COPVs inside the chines is a double-edged sword. On the one hand, it is a nice way to put them outside the tanks without any negative aerodynamic effects. On the other hand, the fact they are so conveniently hidden reduces the incentive to get rid of COPVs. Musk believes there are too many COPVs on Super Heavy, and when they were just hanging off the side of the booster (before chines were added to the design), they were immediately visible and I would say pretty ugly, which was a nice incentive to remove some of them.
