Quote:
Originally Posted by redpoint5
It's the lightest weight inert gas, is my guess. Helium is also used to force propellant to motors as the tanks are rapidly depleted.
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Last I heard they have massive pumps (turbines) feeding the thrust motors. The helium should be used to prevent cavitation and tank collapse because there's no appreciable atmosphere to vent the tank. Only reason to use helium that I can think of is that it's readily available as either a compressed gas or liquid all over the launch site. Not so sure about inert.