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During lift-off and landing, the methane liquid fuel inside the Starship is being consumed. How constant fuel pressure to the Raptor engines is maintained when the fuel is being depleted and during flip maneuvers?

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    $\begingroup$ Engines are throttled, thrust is varied, so pressure and flow rate are not exactly constant. I don't know much about engines, but don't the tanks just have to provide a sufficient flow rate to some pumps, and it's the pumps that have to worry about what the engine sees? $\endgroup$ – uhoh Mar 25 at 2:22
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Each propellant has its own turbopump and preburner. The liquid propellants are pumped into their respective turbo pump/preburners where they are gasified and pass on to the combustion chamber.

To speed the intake of propellants and help prevent cavitation both propellant tanks are pressurized. If nothing further was done as the liquid levels reduced, the pressure in the tanks would slowly decrease as the headspace above the liquids increased.

To prevent this the intention is to use autogenous pressurization. When the Raptor is fired up hot gaseous methane and hot gaseous oxygen are tapped off and fed back into their respective tanks at an appropriate temperature and pressure to maintain the pressure in the tanks. See red and blue up arrows in the diagram

Some of the prototype Starships have used Helium as a pressurant as an interim measure while SpaceX are attempting to solve the sloshing / condensation issue during landing.

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  • $\begingroup$ Nice answer +1. Unless a better comes up, I am ready to accept this as the answer $\endgroup$ – seccpur Mar 25 at 13:52
  • $\begingroup$ thx - the interesting bit is when the hot gases are pumped into the space above the cryogenic liquids and then the liquids are sloshed around. $\endgroup$ – Slarty Mar 25 at 13:59
  • $\begingroup$ I guess the fuel will no longer be able to maintain cryogenic temperature during re-entry and effect the thrust performance. During lift off maximum thrust and hence keeping at cryo temperature is essential. How these pressure differences are maintained. $\endgroup$ – seccpur Mar 25 at 14:11
  • $\begingroup$ Another important bit that is missing is that the landing burn is fueled from the header tanks, which are much smaller, and are not used during ascent. Therefore, they will be mostly full with less tendency to slosh. $\endgroup$ – Jörg W Mittag Mar 25 at 16:03
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    $\begingroup$ It's true that the landing burn is powered from the header tanks, but as soon as all 3 Raptors fire up each tank loses around 1.5 tones/sec so the head space grows very quickly in a spherical tank as does the need to keep it pressurized. At that point they must carry out a rapid change in orientation and that creates slosh in the head space and when the cryogenic sloshing liquid hits the hot gas the gas condenses and the pressure goes down. The whole thing is far from equilibrium and very difficult to model hence the current difficulties. $\endgroup$ – Slarty Mar 25 at 16:22

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