WASHINGTON — NASA confirmed that Boeing’s CST-100 Starliner spacecraft has suffered a fifth, although minor, helium leak in its propulsion system as engineers work to prepare the vehicle for its return to Earth next week.
In a June 10 statement, NASA mentioned that spacecraft teams were examining “what impacts, if any, five small leaks in the service module helium manifolds would have on the remainder of the mission.” That was the first reference to there being five leaks in the spacecraft; NASA had mentioned there were four in a briefing hours after the spacecraft’s June 6 docking with the International Space Station.
In a June 11 statement to SpaceNews, NASA spokesperson Josh Finch said the fifth leak was detected around the time of that 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,” he said.
NASA was aware of one leak at the time of Starliner’s June 5 launch, having been detected shortly after a scrubbed launch attempt May 6. At the time of launch, NASA and Boeing officials considered that a one-off problem, likely caused by a defect in a seal. However, hours after launch controllers said they had detected two more leaks, one of which was relatively large at 395 psi per minute, said Steve Stich, NASA commercial crew program manager, at the briefing.
A fourth leak was found after docking, although it was much smaller at 7.5 psi per minute. “What we need to do over the next few days is take a look at the leak rate there and figure out what we go do relative to the rest of the mission,” Stich said at the briefing.
NASA closed the helium manifolds in the propulsion system after docking to stop the leaks, although they will have to be opened to use the spacecraft’s thrusters for undocking and deorbit maneuvers. NASA said June 10 that engineers estimate that Starliner has enough helium to support 70 hours of flight operations, while only seven hours is needed for Starliner to return to Earth.
In addition to the helium leaks, engineers are studying one reaction control system (RCS) thruster that shut down during the spacecraft’s flight to the ISS. Four other thrusters were turned off by flight software but later reenabled. An RCS oxidizer isolation valve in Starliner’s service module is also not properly closed.
“We have the commercial crew program, Boeing, ISS teams all integrated, working very well together in order to come up with a forward plan for getting us in the best posture for that undock and reentry,” Dina Contella, NASA ISS deputy program manager, said at a June 11 briefing about a series of upcoming spacewalks at the ISS. “The teams are still working through what are the best ways to go about testing and preparing for undock and reentry.”
Those teams have some time to complete that work. NASA had initially scheduled a June 14 undocking for Starliner, but NASA said June 9 it was delaying the undocking to no earlier than June 18. That delay was to avoid a conflict with a June 13 ISS spacewalk, or EVA, by NASA astronauts Tracy Dyson and Matt Dominick.
“To have it back to back, were we had an EVA followed by undock, was not the most convenient,” Contella said. There are undocking opportunities every few days, governed by the orbital mechanics that set up a landing in the southwestern United States.
The two NASA astronauts who flew Starliner to the ISS, Butch Wilmore and Suni Williams, have been busy both conducting tests of Starliner while at the station while also performing other work, such as science experiments. “Butch and Suni are an extra set of hands,” she said, particularly as other ISS crewmembers prepare for the upcoming spacewalks. “Having Butch and Suni available to perform some key critical science has been outstanding.”
Wilmore and Williams have publicly praised the performance of the spacecraft. “The spacecraft was precise, more so than I would have expected. We could stop on a dime, so to speak,” said Wilmore during a June 10 call with NASA leadership, discussing how the spacecraft maneuvered.
“Our experienced test pilots have been overwhelmingly positive of their flight on Starliner, and we can’t wait to learn more from them and the flight data to continue improving the vehicle,” Mark Nappi, Boeing vice president and commercial crew program manager, said of the astronauts in a June 11 statement.