SpaceX plans to use autogenous pressurization for Starship. Autogenous pressurization means using the same consumable that is in the tank in its gaseous state to pressurize the tank. So, for the liquid methane tanks, you use gaseous methane for pressurization, etc.
The landing failure of Starship SN8 was caused by low pressure in the methane header tank, according to Elon Musk:
Fuel header tank pressure was low during landing burn, causing touchdown velocity to be high & RUD, but we got all the data we needed! Congrats SpaceX team hell yeah!!
As a quick fix, SpaceX switched from autogenous pressurization to using Helium as the pressurant:
SN9 will press CH4 header tank with helium. Long-term solution is under debate. Not clear what is lightest/simplest.
Helium pressurization is used by many rockets, including Falcon 9. For Starship, however, SpaceX is trying to
- aggressively reduce the number of different consumables required and
- only choose consumables that can easily be replenished on Mars by in-situ resource utilization.
For comparison, Falcon 9 with Dragon uses RP-1 as fuel, liquid oxygen as oxidizer, Helium as pressurant, TEA-TEB as igniter, some form of hydraulic fluid for thrust vectoring and grid fin control, and nitrogen for the cold-gas attitude control thrusters for Falcon 9, and Monomethylhydrazine as fuel and Dinitrogen tetroxide as oxidizer for the Dragon. That's eight different consumables of which only oxygen can be easily produced on Mars. Starship only uses methane as fuel and liquid oxygen as oxidizer. Pressurization is intended to be autogenous, ignition is done via spark ignition, the body flaps are actuated with electric motors (from a Tesla, actually), and the attitude control thrusters (which are yet to be developed) will be hot-gas thrusters derived from the Raptor, thus also using methane and oxygen.
At least, that's the plan. At the moment, pressurization is done with Helium and the attitude control thrusters haven't been developed yet, so they are currently using nitrogen cold-gas thrusters. (Probably the ones from Falcon 9, it wouldn't make sense to use anything else.)
Unfortunately, it appears that it was exactly the quick fix of using Helium pressurization which then caused the landing failure of SN10:
SN10 engine was low on thrust due (probably) to partial helium ingestion from fuel header tank. Impact of 10m/s crushed legs & part of skirt. Multiple fixes in work for SN11.
Chris Bergin from NASASpaceflight.com pointed out that the Helium pressurization system which caused the failure of SN10 was added to fix the failure of SN8:
This is a tricky one given that I believe said helium pressurization was added to the CH4 header tank to mitigate what happened with SN8.
To which Elon Musk replied that if methane had been used for pressurization, the bubbles would probably not have caused a problem:
Fair point. If autogenous pressurization had been used, CH4 bubbles would most likely have reverted to liquid.
He further explained that the specific problem the Helium pressurization system was meant to fix was to prevent "ullage collapse from slosh":
Helium in header was used to prevent ullage collapse from slosh, which happened in prior flight.
Elon Musk also took responsibility for the decision:
My fault for approving. Sounded good at the time.
Tim Dodd the Everyday Astronaut asked whether future Starships will have slosh baffles:
Are there baffles in future designs to prevent slosh?
Elon Musk replied there were already slosh baffles at least in SN10 but that one of the slosh baffles may actually have caused the problem:
There were baffles, but one may have acted like a straw to suck bubbles in from above liquid/gas level.
Elon Musk further remarked that SpaceX had actually seen something similar with Falcon 1:
Something similar happened on an early Falcon 1 flight, resulting in unexpectedly high liquid oxygen residuals at main engine cutoff.
Interestingly, shortly after the test of SN10, Elon Musk remarked that SpaceX had never seen the behavior before:
Thrust was low despite being commanded high for reasons unknown at present, hence hard touchdown. We’ve never seen this before.
It is quite interesting that after a longer investigation, it turned out that they had in fact seen something similar before in one of their very first tests ever.