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At 1:40 on the SpaceX launch commentary, the announcer said that after hot staging, “the three engines still firing on Super Heavy will flip the booster around”.

With empty tanks, the center of mass is at the engine end of the booster. The grid fins are at the other end. The engines weigh over 50 tons and the density of empty tanks is less than an empty Budweiser. Aerodynamically, isn’t the booster flying backwards at 5600+km/hr after separation? Isn’t it going to flip regardless? For that matter, could the 3 lit engines stop it from flipping?

enter image description here

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  • $\begingroup$ Note that the booster tanks aren't actually empty, as there is still fuel there to attempt a (in this specific case a relatively soft practice landing in the ocean), return to launch zone soft capture/landing. So although there isn't a huge mass of fuel/oxidizer, there is enough to have an effect on the center of mass of the booster. $\endgroup$
    – Milwrdfan
    Nov 21 at 22:14
  • $\begingroup$ Good point. And a very lively center of mass it would be. $\endgroup$
    – Woody
    Nov 21 at 22:28

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Aerodynamics don't matter. Staging takes place at an altitude of roughly 74 km, in an atmospheric pressure of about 4 Pa (0.004% of surface pressure). Even at the speed Super Heavy is going, that's effectively a vacuum.

(For entertainment purposes, I put an approximation of Super Heavy into a drag calculator I wrote for another question. It told me that the air is applying a force of about 1400 N (310 pounds) to the rocket.)

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  • $\begingroup$ Interesting. What force is generated by the grid fins in your simulation? $\endgroup$
    – Woody
    Nov 21 at 3:41
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    $\begingroup$ @Woody, the simulation doesn't have enough precision to tell. I put in the numbers for a long cylinder, then added a fudge factor for the fins. $\endgroup$
    – Mark
    Nov 21 at 3:51
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    $\begingroup$ 310 pounds actually sounds like a lot, applied continuously. Is there reason to think that's not enough force to significantly disturb its orientation, especially since the fuel is empty at this point? I'm thinking of how it's possible to move even a large boat by just pushing steadily against the dock. And I can't bench press 310 pounds. $\endgroup$
    – TypeIA
    Nov 21 at 14:37
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    $\begingroup$ @TypeIA, Super Heavy weighs somewhere upwards of 450,000 pounds at the point of turnaround, and the moment of inertia for flipping is huge. I get a lower bound of 100,000,000 ft*lb*s^2, and a probable number somewhere around 2,000,000,000 ft*lb*s^2. $\endgroup$
    – Mark
    Nov 21 at 20:00
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    $\begingroup$ @Mark ok, so it's a really really large boat indeed ;) After looking at the numbers more carefully you've convinced me it is indeed negligible. Thanks. $\endgroup$
    – TypeIA
    Nov 21 at 20:39
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At this speed and altitude, nothing matters more than inertia. As others noted, air density at this altitude is almost nonexistent; its effect is almost negligible. Tip-off forces from the upper stage separation mechanics and engines igniting may induce some additional tumble, but this stage is a BIG vehicle. Yes, it will eventually tumble but not right away. Also, the remaining fuel inside will begin to tumble about inside the tanks, hence the need in part to have control jets to reorient then produce some thrust to force the liquid propellants back into the bottoms of the tanks so the pumps won't be starved or choke on bubbles. It's a bit of gymnastics at very high speed.

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  • $\begingroup$ Three Raptor engines remain firing during the flip, wouldn't they provide enough ullage thrust? Even though the booster is rotating, the acceleration would always be forward towards the top of the booster with the resulting force pressing rearward. $\endgroup$
    – Steve Pemberton
    Nov 24 at 12:57
  • $\begingroup$ @StevePemberton, leading third-party theory about the explosion of Super Heavy is that they didn't provide sufficient ullage thrust. $\endgroup$
    – Mark
    Nov 25 at 23:32
  • $\begingroup$ @Mark - just seems like in a near vacuum at 70 km and with a near empty booster that three half-million lb Raptors would be enough to keep the fuel at the bottom. But the booster did apparently get knocked back pretty good during hotfire, so in theory there may not have been enough ullage for a brief moment. But it didn't seem to affect the three running engines just the ones that were trying to restart. But starting a rocket engine is tricky and those are some pretty extreme conditions which makes it hard to predict what the problem was. This was the first Raptor restart in a vacuum. $\endgroup$
    – Steve Pemberton
    Nov 26 at 2:23

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