Elon Musk tweeted that Starship SN8 failed to land due to insufficient header tank pressure:

Fuel header tank pressure was low during landing burn, causing touchdown velocity to be high & RUD [...]

In this answer, it is explained that header tanks are separate tanks to be used exclusively for landing, to avoid all the problems that would arise from using the partially empty main tanks (ullage, insufficient pressure..). It that answer is correct, I would expect that the header tank is not used during ascent, so it should keep the pressure that it had upon launch (maybe even a little more if it got warmer?). If that's the case, then having insufficient pressure at landing could be easily be solved by pressurizing the header tank more before launch. This answer to SN10 failure instead talks about adding helium as pressurant as a fix. Why would that be needed if the header tank can be simply pressurized with CH4 (or LOX) before launch?

In this other answer about methane header tank having many holes, it is said that the header tank is not separate from the main tank, but it actually gets fuel from it, possible even when the rocket is leaning to one side (during belly dive?). Given that Starship might have to make two consecutive landings on its own (one on Mars, then one back to Earth) with autonomous refuelling in the middle, it might make sense to have the header tanks receive fuel (or LOX) from the main tanks; however, why not make it only when landed and then close the connection while in flight, to ensure that you will have the proper pressure on landing?

  • $\begingroup$ Keep in mind that by necessity, header tanks lose pressure because the fuel is used during landing. $\endgroup$
    – Polygnome
    Mar 20 at 12:50
  • $\begingroup$ @GremlinWranger yes, thanks for pointing it out. It was due to the fact that I actually answer the title question in the middle of the text by finding the second quoted answer (althought I am not sure if it is correct). I have changed the title to better reflect where the question headed to. $\endgroup$
    – BlueCoder
    Mar 20 at 12:50
  • $\begingroup$ @Polygnome so SN8 problem could be a failure of autogenous pressurization of header tanks while they are used? $\endgroup$
    – BlueCoder
    Mar 20 at 12:52
  • $\begingroup$ "it should keep the pressure that it had upon launch (maybe even a little more if it got warmer?)": the propellants are subcooled well below their boiling points and pressurized (in SN8) with gaseous propellant, so this is not a valid assumption: the pressurizing gas is constantly condensing out and has to be replenished before the engines can be started. $\endgroup$ Mar 20 at 17:12
  • $\begingroup$ @ChristopherJamesHuff Good point thanks, I forgot that most of it is liquid so it won't get bigger, actually it will cool and reduce the gas part. Would it still be possible to pressurize the header tanks enough before ascent to account for the condensing and have it still enough pressurized for landing? Maybe not and your point is the key to the answer? $\endgroup$
    – BlueCoder
    Mar 20 at 17:34

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