Not silly question at all. To my knowledge the Falcon uses helium to pressurise the tanks. So the gas you can see is helium. It has several benefits: does not b

Not silly question at all. To my knowledge the Falcon uses helium to pressurise the tanks. So the gas you can see is helium. It has several benefits: does not burn, and does not freeze at the temperature of the liquid oxygen. It is also a relatively simple system. They “just”[1] have liquid helium in a composite overwrapped pressure vessel inside the tank and most likely they have a regulator on it to release gas as the oxygen/fuel is used up. 1: this is not easy at all, they lost a rocket due to the helium bottle breaking free once: https://sma.nasa.gov/SignificantIncidents/assets/spacex-falcon-9-failure-investigation-focuses-on-copv-struts--nasaspaceflight.com.pdf https://sma.nasa.gov/SignificantIncidents/assets/spacex-falc... > wouldn't pulling a vacuum in the tank on the way up make it a lot harder to pump the liquid It would make it impossible to pump. But that is not the only reason you need a pressurant. The tank wall is relatively thin. It would implode in the lower atmosphere if you would have vacuum in it.