According to most sources, the Starship Mk.1 had its LOX main tank (not talking about the header tanks here) on top and its main methane tank below. But it seems the SN1 revision, and all subsequent versions, have the exact opposite arrangement: CH4 on top, O2 at the aft.

I thought you usually don't want LOX tank at the bottom to avoid having some fuel going in a frozen pipe through cold LOX, and that you want to keep the denser part (i.e. CH4) near the bottom – rather than up top – for stability.

Why did they change this? To my knowledge, very few rockets have the LOX at the bottom; it’s even rarer to find an arrangement where the fuel manifold goes through the LOX tank. And it's the first time I’ve heard about the two main propellant tanks being swapped in this fashion.

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    $\begingroup$ "I thought ... that you want to keep the denser part (i.e. CH4) near the bottom than the top for stability." You want the denser propellant forward for aerodynamic stability (think of a pub dart with its heavy metal tip. However, LOX is almost 3x as dense as liquid methane, so the question is still valid. $\endgroup$ Mar 31, 2020 at 0:02

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Just a thought, but since LOX has a higher density than liquid Methane (1.140 g/cc vs 0.424 g/cc) SpaceX may have swapped the tanks to increase stability during landing, placing the heaviest tank closer to the aft.

  • $\begingroup$ The density isn’t relevant, but the atomic weights (which are) argue in the same direction. Mol.wt. of methane is 16, and it requires 4 atoms of oxygen for complete combustion, a total atomic weight of 64. Thus there has to be 4 times as much mass of oxygen as methane - not allowing for intentional deviations towards rich or lean combustion $\endgroup$ Jun 27, 2020 at 20:10
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    $\begingroup$ @MartinKochanski why would the densities not be relevant? It has a big effect on the location of the rocket's center of mass which is an important parameter for stability. One might argue that when active thrust vectoring is occurring passive stability is not important, but during the descent and landing phase thrust is usually off. Related but unanswered: Why is the relative axial position of oxidizer vs fuel tank inverted in the upper stages of most rockets? $\endgroup$
    – uhoh
    Jun 27, 2020 at 23:12
  • $\begingroup$ It might also be a head pressure thing at the pump inducer, this is as I understand it why LH2 propellant tanks are usually at the top, it raises the pressure at the turbopump intake on the pad and reduces the tendency to cavitiate. $\endgroup$
    – Dan Mills
    Mar 5, 2021 at 15:30

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