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Does someone knows why H-IIB rocket can stay fuelled on launchpad for many minutes, just venting excess gaseous H2 and O2, while Ariane 5 must be connected to umbilicals almost to the very last moment before lift-off?

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  • $\begingroup$ It's possible the first stage umbilicals aren't visible. The SpaceX Falcon 9 is fueled through one of the 4 support frames at the bottom of the first stage, for instance. $\endgroup$ – Hobbes May 27 '15 at 18:59
  • $\begingroup$ yes, I assume that H-II must have a low-point, bottom, link for LH2 fuelling to the 1rst stage, but there is still a lack of fuelling point for the LOX tank, wich is located on the top of 1rst stage. Unless that LOX is also feed by the bottom - wich I doubt, though. $\endgroup$ – Junior Miranda May 27 '15 at 19:03
  • $\begingroup$ There's a LOX line from the tank to the engine, anyway. They could use the same line for fueling. $\endgroup$ – Hobbes May 28 '15 at 6:48
  • $\begingroup$ Yes, there is a line linking the tank to the engine bay, but would it be feasible to pump the LOX all that way up? Think about the temperature change along the tube, and all the pressure required to push it up. $\endgroup$ – Junior Miranda May 29 '15 at 18:23
  • $\begingroup$ Pressure due to gravity is not a problem. The LOX tank is under pressure due to boiloff, so the entire circuit must be built to withstand that anyway. The temperature change is accounted for in the design as well: temperature differences occur not only during filling, but during the launch as well. $\endgroup$ – Hobbes Sep 22 '15 at 15:33
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Payload margins most likely. The reason to stay connected is to stay topped up to the last second, to have the most available fuel for the mission.

HII possibly has sufficient margin for the flight even with venting.

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  • $\begingroup$ It would be cool if they could top off the tanks of the Ariane, while the main engine is already running, because they run it for 7 seconds before igniting the boosters, so they can check for malfunctions (and shut down if necessary). The rocket stays completely put during those 7 seconds, but evidently the umbical is already disconnected at that point. $\endgroup$ – Rikki-Tikki-Tavi Sep 23 '15 at 13:13
  • $\begingroup$ However, it would be VERY uncool if your disconnect valves didn't close. $\endgroup$ – Organic Marble Sep 27 '15 at 21:12
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According to the H-IIA user manual (PDF page 219, figure 5.7.12), in a normal launch, propellant loading stops at 3 hours before launch.
But there is an option ("98% additional loading") to continue propellant loading until a few minutes before liftoff. So Geoff's answer was correct.

H-IIA launch sequence

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