NASA has confirmed that Boeing’s CST-100 Starliner spacecraft has experienced a fifth, albeit minor, helium leak in its propulsion system. This comes as engineers are preparing the vehicle for its return to Earth next week.
Discovery of Multiple Leaks
On June 10, NASA released a statement revealing that spacecraft teams were assessing the impact of “five small leaks in the service module helium manifolds” on the remaining mission. This was the first mention of five leaks, following an earlier briefing that noted four leaks after the spacecraft’s June 6 docking with the International Space Station (ISS).
In a subsequent statement on June 11 to SpaceNews, NASA spokesperson Josh Finch clarified that the fifth leak was detected around the time of the post-docking briefing. “The leak is considerably smaller than the others and has been recorded at 1.7 psi [pounds per square inch] per minute,” Finch said.
Initial Leak and Subsequent Discoveries
NASA was aware of one leak at the time of Starliner’s June 5 launch, initially detected after a scrubbed launch attempt on May 6. At the time, NASA and Boeing officials considered it an isolated issue likely caused by a defective seal. However, after launch, controllers identified two additional leaks, one of which was significantly larger at 395 psi per minute, according to Steve Stich, NASA‘s commercial crew program manager.
A fourth, smaller leak was discovered post-docking, with a rate of 7.5 psi per minute. “Over the next few days, we need to evaluate the leak rate and determine the best approach for the remainder of the mission,” Stich stated during the briefing.
Managing the Helium Manifold Leaks
After docking, NASA closed the helium manifolds in the propulsion system to halt the leaks. These manifolds will need to be reopened for the spacecraft’s thrusters to function during undocking and deorbit maneuvers. NASA’s June 10 update indicated that engineers believe Starliner has sufficient helium to support 70 hours of flight operations, though only seven hours are required for its return to Earth.
Additional Technical Challenges
Besides the helium leaks, engineers are investigating the shutdown of one reaction control system (RCS) thruster during the spacecraft’s journey to the ISS. Additionally, flight software had temporarily turned off four other thrusters, which were later reenabled. An RCS oxidizer isolation valve in the service module is also not correctly closed.
“We have the commercial crew program, Boeing, and ISS teams integrated and working well together to devise a forward plan for the undocking and reentry,” said Dina Contella, NASA ISS deputy program manager, during a June 11 briefing.
Adjusted Return Schedule
NASA has postponed Starliner’s undocking to no earlier than June 18, from the initially scheduled June 14. This delay avoids a conflict with a June 13 ISS spacewalk by NASA astronauts Tracy Dyson and Matt Dominick. “Having an EVA followed by undocking back-to-back was not convenient,” Contella explained.
Astronauts’ Positive Feedback
NASA astronauts Butch Wilmore and Suni Williams, who piloted Starliner to the ISS, have been conducting tests on the spacecraft while engaging in other station activities, such as scientific experiments. “Butch and Suni have been an invaluable extra set of hands,” Contella noted, praising their contributions.
Wilmore and Williams have expressed satisfaction with Starliner’s performance. “The spacecraft was precise, more so than I would have expected. We could stop on a dime,” Wilmore said during a June 10 call with NASA leadership.
Mark Nappi, Boeing’s vice president and commercial crew program manager, echoed their sentiments. “Our experienced test pilots have been overwhelmingly positive about their flight on Starliner, and we look forward to learning more from them and the flight data to continue improving the vehicle,” Nappi stated on June 11.