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At the moment, the design of Spacex's Starship is roughly, from bottom to top:

  1. Engines
  2. Liquid Oxygen (LOX) tank
  3. Liquid Methane (CH4) header tank
  4. Liquid Methane tank
  5. Payload
  6. Liquid Oxygen header tank

(See the diagram below for an illustration)

In this configuration the LOX header tank is quite far from the engines. Isn't this detrimental to the quick restarts the header tanks are designed for?

In addition, the LOX header tank at the front seems quite vulnerable to micrometeorites and other impacts, while the CH4 header tank is better protected as there are multiple sheets of steel separating it from space.

Why aren't both header tanks below the payload?

It seems to me that there is sufficient space to have both header tanks next to theirs main tanks counterparts, all while ensuring the header tanks are at a safe distance from each other.

Cross section diagram of the Spacex Starship where the LOX header tank is at the top Image source

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The only official source we have is a tweet from Elon Musk:

It’s mostly to balance the ship during entry. After delivering satellites, the front is light & back is heavy due to engines & landing legs.

Putting all the weight at the back is okay for a vehicle that enters engines first like Super Heavy, Falcon 9, or Electron. But for a vehicle that enters belly first, you want the center of mass further forward nearer to the middle.

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    $\begingroup$ But won't the crewed version of Starship still have payload mass on entry? Will a crewed Starship have the header tank in a different position? $\endgroup$
    – Speedphoenix
    Jan 14, 2021 at 2:03
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    $\begingroup$ As I wrote, to the best of my knowledge, this one tweet is all the information that exists. Everything else would be speculation. Keep in mind, though, that a crew vehicle, even fully loaded, is still mostly empty space for people to move around and live in. It's not packed to the brim like a cargo or tanker vehicle would be. $\endgroup$
    – Jörg W Mittag
    Jan 16, 2021 at 19:21
  • $\begingroup$ Still, being able to enter belly first only depends on relative positions of centrer of gravity and centre of pressure. Aft and forward fins should be meant to deal with different payloads and allow to move actual front header LOX tank somewhere more convenient $\endgroup$
    – jkztd
    Jan 25, 2021 at 21:21

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