A rocket tank needs pressure, not just fuel
Liquid propellant does not flow to an engine simply because gravity pulls it down. In space, there may be little or no gravity to keep liquid settled at a tank’s outlet, and an engine needs propellant delivered at controlled pressure. If pressure inside the tank drops as fuel leaves, the flow can falter—and the tank itself can be put under damaging stress.
Helium fills the space the fuel leaves
Rocket systems store helium as a compressed gas, then feed it into the propellant tanks. As liquid drains, helium expands into the growing empty space above it. That keeps the tank pressurized and helps maintain a steady push of liquid through the feed lines toward the engines.
This is tank pressurization: the gas supplies pressure so the liquid can move where it is needed. It is not the same as helium burning to provide thrust. The engine’s thrust comes from burning propellants; helium is part of the plumbing that gets those propellants there.
Why use an unreactive gas?
Fuel and oxidizer are chosen to react vigorously in the engine. A pressurizing gas must not accidentally join that reaction inside the tank or contaminate the propellants. Helium is chemically inert, meaning it does not readily react with other substances, making it useful in this demanding role.
The same gas that lifts a balloon can therefore help keep a rocket engine fed—not because the two jobs are alike, but because helium is light, compressible, and reluctant to react. In a rocket, its less visible task is to keep liquid moving reliably as the tanks empty.
