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The Starship second stage has 3 sea level engines and 3 vacuum engines.

I don't imagine that all 6 will be used for trans Mars injection, as it isn't very efficient.

And for the Mars landing which ones will be used? Mars has a thin atmosphere, So maybe the vacuum ones? Or a combination?

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The 3 vacuum engines will be used for Trans Mars Injection (TMI) since they are more efficient in vacuum. While each one is off centered, firing all 3 will make it balanced, so this makes the most sense.

For landing, it will be a bit trickier. I haven't seen anything publicly, but I suspect they will use the vacuum engines for part of the time, and the 3 sea level engines for the final landing. The 3 sea level engines will give maximum flexibility for the most critical point, but the vacuum engines can be used for parts of the decent if required. The sea level engines are more central, and can gimble, allowing for this flexibility.

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  • $\begingroup$ Why will the 3 sea level engines give maximum flexibility for the most critical point? $\endgroup$ – uhoh Aug 5 at 12:55
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    $\begingroup$ @uhoh they can Gimble. $\endgroup$ – moshevi Aug 5 at 13:00
  • $\begingroup$ Gimbal, not gimble.. $\endgroup$ – SF. Aug 5 at 14:27
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    $\begingroup$ When doing the atmosphere entry burn how many Bars do the engines experience? Because of the speed of entry, might the pressure be high enough for the sea level engines to be efficient? $\endgroup$ – moshevi Aug 5 at 14:42
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    $\begingroup$ @uhoh your'e right it deserves it's own question $\endgroup$ – moshevi Aug 5 at 16:01

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