Could someone explain to me how, during the descent, the pumps pull the oxidizer and propellant? I could be wrong, but during the burn the propellant should move inside an almost empty tank.


3 Answers 3


Falcon first stages also have cold-gas thrusters at the top of the stage; you can see them firing for thrust perpendicular to the stage during some landings. Among other things these thrusters can be fired downwards (parallel to the stage) to provide thrust for ullage. A few seconds of thrust will settle propellants at the bottom of the tanks immediately prior to engine ignition (this is probably necessary prior to the boostback burn, for example, as the rocket is clear of the vast majority of the atmosphere by then).

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    $\begingroup$ I was a little dubious about this about the existence of downward facing/ullage cold gas thrusters (I don't know why, I guess because I'd never considered it before) but I did some poking and it looks like you're right. You can see the downward facing nozzle on the rcs pod in this image and you can also see them firing in the crs6 mission landing. $\endgroup$ Mar 7, 2017 at 23:14
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    $\begingroup$ During videos of reentry, we see brief puffs from the gas thrusters as the stage reorients itself, but we don't see thrusts long enough to settle ullage. I think the other answer, that air drag provides the ullage force, is more likely correct. $\endgroup$ Feb 12, 2018 at 2:52
  • $\begingroup$ @RussellBorogove For re-entry and landing burns, air resistance provides ullage. I think the boostback burn does use the thrusters for ullage, though. In the video feeds, they switch to a camera either on the ground or the second stage camera (and pointing at the Mvac engine) immediately after separation, and neither one would show the cold gas plumes. However, the ground-based views do show that the stages spend several seconds before igniting for boostback, and separation (plus S2 ignition) would tend to settle propellants upward. Pretty sure they use ullage thrusters there. $\endgroup$
    – CBHacking
    Feb 12, 2018 at 8:48

During descent, the stage is flying with the rocket nozzle in the flight direction, which means the "bottom" of the tank is also oriented in the flight direction. Since the stage is decelerating due to atmospheric drag (and engine burns) the fuel is "pressed" to the bottom of the tank and can thus be pumped like on ascent.

SpaceX Falcon 9 Flight Profile

  • $\begingroup$ Is that graphic correct? I believe they only need the boost back burn if they are returning to the launch site. If they are landing on the drone ship, they can just let the rocket continue on it's ballistic trajectory. There is the case when they are attempting to recover the first stage from a high orbit (Like geo stationary ) launch and the velocity is very high. In that case they have a burn to reduce the reentry speed & heating. But I don't think that is called a boost back. $\endgroup$ Mar 7, 2017 at 2:42
  • $\begingroup$ @JohnnyRobinson There was a (partial) boostback done on some launches where the margins were better than from GTO but RTLS was not possible (either because fuel or pad readiness, paperwork...) - CRS-8, Jason 3, Iridium. It seems to be done for slowing the re-entry even more (and to not land do far away from the coast) $\endgroup$
    – jkavalik
    Mar 7, 2017 at 6:12
  • $\begingroup$ I do know that those upper fins are a relatively new addition - the first few flights didnt have them. They apparently are used both for steerage as well as settling fuel towards the bottom of the tanks. $\endgroup$
    – SDsolar
    Mar 7, 2017 at 9:48

A graphic of a barge landing shows an arc.

Landing on the drone ship is a ballistic arc with only a reentry burn.

  • $\begingroup$ I can't even imagine the precision location and stability of the ship. $\endgroup$
    – SDsolar
    Mar 7, 2017 at 9:49

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