In the first stages of most rockets, e.g. Atlas V, Delta IV and Falcon 9, the oxidizer tank is stacked on top of the fuel tank, with the usual explanation that such an arrangement gives the rocket a more fore center of gravity and thus lever for control force at the bottom of the rocket. Then why is this arrangement inverted when it comes to upper stages design e.g. Centaur?

  • $\begingroup$ @JCRM I suggest you take a closer look at any Atlas V core first stage diagram. $\endgroup$ Sep 15, 2019 at 8:38
  • $\begingroup$ ... except LOX is more dense than rp1. $\endgroup$
    – user20636
    Sep 15, 2019 at 11:19
  • $\begingroup$ counterexample: In the shuttle External Tank the denser LO2 was on top of the LH2. $\endgroup$ Sep 15, 2019 at 11:21
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    $\begingroup$ You asked for an example, I gave one. $\endgroup$ Sep 15, 2019 at 11:24
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    $\begingroup$ Having the weight forward in atmosphere helps control by keeping the CoG ahead of the CoP, there aren't such interactions on upper stages as they generally operate where atmospheric affects are near negligible. $\endgroup$
    – user20636
    Sep 15, 2019 at 11:25


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