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I was watching the live stream of the Iridium-5 / SpaceX launch and they mentioned that the strongback would outgas a minute before launch. I assume it was to prevent the strongback from exploding when the rocket launches?

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They meant the umbilicals that feed LOX and RP1 into the vehicle, connect from the vehicle to the strongback, will outgas.

That is, there is pressure in the flexible pipe that connects to the booster. There is pressure downstream from the tank farm, feeding the strongback.

You will see the vehicle vent LOX as it warms up and builds pressure that needs to be released. Once they stop filling the tanks, very close to launch, the pipes that were connected have pressure in them that needs to be vented as well.

If you want to fly often, you need to preserve the equipment on the pad, like the umbilicals. So although there are certainly cut off valves on the umbilicals, you still need to release the LOX inside them, which will warm up and possible rupture them, if not managed.

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