The astronauts lifted off on June 5. The setbacks started the next day.
As Starliner approached the space station, five of its 28 reaction control system thrusters malfunctioned, delaying the docking process by almost an hour.
Those thrusters are on the spacecraft’s service module and are used to move the capsule around in orbit, particularly to adjust its position as it docks and undocks with the space station.
Four thrusters were eventually restored, which allowed the astronauts to complete their arrival at the space station, but the problem prompted NASA to launch an investigation into what caused the thrusters to shut down.
Engineers with NASA and Boeing used a test engine at the space agency’s White Sands Test Facility in New Mexico to study the performance of the thrusters. Teams subjected that engine and its thrusters, which were developed for future Starliner flights, to conditions similar to those the capsule experienced on its way to the space station.
Mission managers also conducted a “hot fire test” in space, firing Starliner’s thrusters in short bursts while it stayed docked at the space station.
Preliminary results indicated that all but one of the 28 reaction control system thrusters performed well, but NASA said various tests had shown that a tiny Teflon seal seemed to swell under high temperatures, which could block the flow of propellant into the thrusters. The space agency said the expanding seals may be to blame for the thruster problems that cropped up during docking.
However, Steve Stich, NASA’s Commercial Crew Program manager, said in August that the team “can’t totally prove with certainty what we’re seeing on orbit is exactly what’s been replicated on the ground.”
Separately, teams also monitored slow helium leaks in the spacecraft’s propulsion system.
Mission managers knew about one helium leak before Starliner’s launch but said at the time that the slow leak was manageable and unlikely to affect the mission or compromise the astronauts’ safety.
Not long after liftoff, however, officials detected two more helium leaks at the top side of the capsule’s service module.
In late July, NASA said testing had verified that Starliner’s propulsion system was stable and that helium leak rates had not increased in a way that might jeopardize a return trip to Earth.
The space agency brought in additional propulsion experts in August to conduct further testing and analysis.
NASA’s decision to send Wilmore and Williams home on a SpaceX capsule required some adjustments to other planned missions. An upcoming SpaceX launch, known as Crew-9, was previously supposed to take four new crew members to the space station, but instead only two will go. That way, Wilmore and Williams can fly back in the open seats at the end of the Crew-9 mission in February.
