Why do rockets require helium?

Sub: Sci

Sec: Space sector

Context:

Two NASA astronauts aboard Boeing’s Starliner will stay on the International Space Station for months because of a faulty propulsion system whose problems included helium leaks.
SpaceX’s Polaris Dawn mission has been delayed due to helium issues on ground equipment.
Past missions affected by helium leaks include ISRO’s Chandrayaan 2 and ESA’s Ariane 5.

Use of Helium in rockets:

Helium is used to pressurise fuel tanks and in cooling systems.
As fuel and oxidiser are burned in the rocket’s engines, helium fills the resulting empty space in the tanks, maintaining the overall pressure
This pressure ensures that fuel flows to the rocket’s engines without interruption.
Since Helium is non-reactive, it can safely mingle with the tanks’ residual contents.

Why Helium is preferred:

Inert gas:
- Helium is inert, it doesn’t react with other substances or combust.
Lighter:
- Helium is the second lightest element after hydrogen.
- A heavier rocket requires more energy and fuel, while also needing more powerful engines, which are more expensive to develop, test, and maintain.
Boiling point:
- Helium has a very low boiling point (-268.9°C), allowing it to remain a gas even in super-cold environments, an important feature because many rocket fuels are stored around that temperature.
Non-toxic:
- The gas is also non-toxic, but if breathed, it will displace the oxygen humans need for respiration.

Helium leaks:

Helium is highly prone to leaks. Helium’s low density means its atoms can escape through small gaps or seals in storage tanks and fuel systems.
However, since there is very little helium in the Earth’s atmosphere, leaks can be easily detected.

Alternatives to Helium:

Some rocket launches have experimented with argon and nitrogen, which are also inert and can sometimes be cheaper.
Europe’s new Ariane 6 rocket switched helium for a novel pressurisation system that converts a small portion of its liquid oxygen and hydrogen propellants to gas, which then pressurises those fluids for the engine. But the system failed in space.
At present, Helium continues to be the most prevalent in rockets.