Helium Leak Delays Boeing’s Astronaut Launch Attempt for the Second Time; Crew Debut Set on May 21 After Spacecraft Repair

The launch of a space rocket carrying Boeing's Starliner capsule has been postponed for four days to give engineers and officials time to resolve a technical problem in the propulsion system.

Postponed Crewed Launch

On May 14, aerospace company Boeing announced that its first Starliner mission set to carry astronauts into space has been delayed again for the second time. The crewed debut of the capsule has been pushed back for four more days.

The spacecraft had been scheduled to lift off on May 17 on Crew Flight Test (CFT), carried by United Launch Alliance (ULA) Atlas 5 rocket. The mission aims to send NASA astronauts Butch Wilmore and Suni Williams to the International Space Station (ISS) for an eight-day stay.

However, the Starliner team detected a small helium leak in the spaceship's service module. The leak was traced on one of the 28 control thrusters of the propulsion system used for maneuvering in Earth's orbit.

Last week, sensors initially spotted suspicious traces of helium inside the propulsion system while the spacecraft was on the launchpad, but they did not raise an alarm to engineers at the time. Engineers investigated the helium detections while ULA was replacing the faulty valve and decided that more testing was needed to meet the strict launch safety criteria of the mission.

Engineers will run tests and try to address helium leak before the next possible launch window scheduled on May 21 at 4:43 pm ET. The latest postponement makes the program years behind schedule and over $1.5 billion over budget.

For more than a decade, Boeing has been developing Starliner to provide NSA with a second American spacecraft capable of transporting astronauts to and from the International Space Station. Under the same NASA program, SpaceX's Crew Dragon capsule first launched astronauts to space in 2020.

In 2022, the aerospace company completed an uncrewed Starliner trip after years of technical and management issues. The Crewed Flight Test is due to be the final test before NASA certifies the spacecraft to fly routine crewed missions to the ISS.

Handling Helium in Pressure-Fed Engine

Helium is the gas chosen by Boeing to pressurize the fuel, which powers the thrusters of Starliner for orbital maneuvering. The vehicle uses a class of rocket engine design called pressure-fed engine.

This type of engine uses a separate supply of gas, usually helium, to pressurize the propellant tanks and force oxidizer and fuel to the combustion chamber. Tank pressures need to exceed the combustion chamber pressure in order to maintain adequate flow.

During long burns, care must be taken to prevent excessive cooling of the pressurizing gas because of adiabatic expansion. Cold helium would not qualify for this condition, but it could decrease tank pressure, freeze a propellant, or damage engine components which are not designed for low temperatures.

Pressure-fed engines are known for their limitations on propellant pressure, which in turn limits combustion chamber pressure. Propellant tanks with high pressure need thicker walls and stronger materials, making the vehicle tanks heavier and reducing performance and payload capacity. In order to address this issue, the lower stages of launch vehicles typically use pump-fed liquid fuel engines or solid fuel, since high pressure ration nozzles are more desirable.