Starliner to Perform Uncrewed Return Flight From International Space Station This Week

September 4, 2024

Woodrow Bellamy III

Boeing's Starliner spacecraft is scheduled to make an uncrewed return flight to Earth on Friday, Sept. 6, while the two astronauts who flew the spacecraft to the International Space Station (ISS) in June will remain there until late February 2025, according to updates provided by NASA officials during an Aug. 24 press conference.

NASA astronauts Butch Wilmore and Suni Williams completed a successful space flight and docking onboard  the ISS on June 6, 2024, and were originally scheduled to stay there for eight days. However, as Starliner began its approach to the ISS, the NASA mission team monitoring the flight identified three helium leaks and "five reaction control system thrusters failed off during flight," according to an update on the mission released by the agency.

Since June 6, engineering teams from NASA and Boeing have been reviewing data about the performance of Starliner’s 28 total thrusters and the five that malfunctioned, while also trying to understand if the performance of those systems are safe enough for a crewed return of Starliner to Earth. Ultimately, the engineers found that there was too much risk associated with the performance of the thrusters in certain phases of flight.

"The uncertainty and lack of expert concurrence does not meet the agency’s safety and performance requirements for human spaceflight, thus prompting NASA leadership to move the astronauts to the Crew-9 mission," NASA writes in an Aug. 24 press release .

During their press conference, leadership from NASA said that they had exhausted all available options and spent countless days and hours simulating the return flight before deciding to return Starliner to Earth uncrewed last week. Steve Stich, Manager, NASA Commercial Crew Program, expanded on their observations.

"The White Sands testing did give us a surprise," Stich said, referring to NASA and Boeing's plan to return Starliner to White Sands Space Harbor in New Mexico.

"We did five total simulations with that thruster of a downhill de-orbit burn sequence, and so that's when we saw this swelling of the poppet on the oxidizer side," Stich said. "In other words, a piece of Teflon that swells up, and it gets in the flow path and causes the oxidizer to not go into the thruster the way that it needs to. That's what caused the degradation of thrust."

Stich describes the structural housing that contains Starliner's thrusters as a "doghouse," and that temperatures inside of the doghouse were higher than they were designed to experience. On-ground testing and simulation of the thrusters under those higher temperatures lead the engineers to determine that there was too much uncertainty and risk associated with the performance of the thrusters while trying to navigate Starliner during some phases of the de-orbit process.

During Starliner's flight to the ISS in June, after five of Starliner's 28 total maneuvering thrusters malfunctioned, NASA's mission teams performed a series of "hot-fire tests" that re-enabled four of those malfunctioning thrusters while the crew manually piloted the spacecraft at the station’s 200-meter hold point, before eventually docking.

According to Stich, moving forward, the NASA team will continue to work with Boeing to understand what causes the overheating.

"What we have to do now, moving forward for Starliner, is to learn how do we avoid firing that thruster in a manner that would cause the heating, that would cause that oxidizer poppet Teflon piece to swell,” Stich said. "We also have learned recently that the environment in the dog house is hotter than we thought. When the other thrusters fire in the dog house, some of that heat soaks back into an individual thruster and that causes the Teflon to swell. It also causes some vaporization in the propellant."

On Aug. 29, NASA released an additional update  about the return flight, stating that the Starliner will return uncrewed on Sept. 6 to the landing zone at White Sands Space Harbor. The return needs to occur prior to the Sept. 24 SpaceX Crew-9 flight of the Dragon spacecraft to the ISS so that the docking port will be open.

NASA has also reduced the number of astronauts traveling to the ISS on the next Dragon mission  by two, to accommodate a return flight home for the Starliner astronauts. NASA Astronaut Nick Hague and Roscosmos Cosmonaut Aleksandr Gorbunov will operate a two-crew flight on Sept. 24. NASA Astronauts Wilmore and Williams will fly home with Hague and Gorbunov in February 2025.

“Decisions like this are never easy, but I want to commend our NASA and Boeing teams for their thorough analysis, transparent discussions, and focus on safety during the Crew Flight Test,” said Ken Bowersox, Associate Administrator for NASA’s Space Operations Mission Directorate. “We’ve learned a lot about the spacecraft during its journey to the station and its docked operations. We also will continue to gather more data about Starliner during the uncrewed return and improve the system for future flights to the space station.”

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