WASHINGTON — NASA and Boeing have again extended the CST-100 Starliner’s stay on the International Space Station while engineers conclude analysis of propulsion problems and helium leaks on the crewed spacecraft.
At a press conference on June 18, NASA announced that Starliner’s return to Earth for the Crew Flight Test (CFT) mission, which had already been postponed twice to June 22, had been postponed again. Starliner is now scheduled to detach from the station at 10:10 PM EDT on June 25 and land in White Sands, New Mexico, at 4:51 AM EDT on June 26.
Officials from NASA and Boeing said at the press conference that the extended stay at the station would give them more time to study two major problems the spacecraft encountered during its journey to the station nearly two weeks ago: thrust malfunctions and helium leaks in the spacecraft’s propulsion. System.
This work included brief firings of several Reaction Control System (RCS) thrusters, five of which were shut down by the spacecraft’s computer as the Starliner approached the station. Four of the thrusters were recovered by the controllers to allow the docking process to continue.
The single thruster that was not recovered before docking showed a “strange signature” that produced almost no thrust, said Steve Stich, NASA’s commercial crew program manager. This thruster will not be used by the spacecraft during separation and deorbit maneuvers.
Other thrusters, including those that malfunctioned during the approach and others that behaved normally, showed the expected appearance of chamber pressures during a short burn lasting a quarter of a second. The thrusters also performed as expected during a longer burn of 1.2 seconds each, with controllers measuring their performance by testing the response of the station’s flight control system.
“After that, we feel very confident in the propulsion devices and the team makes sure to look at the propulsion devices in detail throughout the journey,” he said. This includes comparing its performance to that of a May 2022 unmanned test flight, called OFT-2, in which two engines failed during approach but were recovered before the end of the mission.
Engineers are studying why the thrusters stopped working during the approach, which could be related to heavy use, Stitch said. “We have some theories about what happens inside the engine where the engine gets too hot,” he said, such as high temperatures preventing proper mixing of fuel and oxidizer.
Engineers also used thruster testing to verify five helium leaks discovered in the propulsion system. In each case, dropout rates dropped, in one case by 50%, he said.
“It appears to be related to thruster activity,” he said of the helium leak. This can be linked to heat from the thrusters or sliding surfaces corroding the seals. He noted that three of the largest leaks likely have similar causes, while two of the smaller leaks may be similar to leaks seen on the OFT-2 mission.
Like the propulsion tests, Stitch said the low helium leak gives him confidence that the spacecraft will be able to perform as expected during separation and deorbit maneuvers. “There is much less demand for propulsion devices” in the later stages of flight, he said.
Helium leaks are occurring at various “dog kennels” in the service module from malfunctioning propulsion devices, he said. However, he stated that “dynamic processes” during the Starliner’s approach to the station may have caused thruster malfunctions and helium leaks.
NASA and Boeing engineers plan to continue analyzing the Starliner vehicle until June 22, then focus on preparations to separate the vehicle and return to Earth. This work was a factor in the decision to extend the Starliner’s stay at the station. “We haven’t gotten the service module back, so this is an opportunity to fully understand the system’s performance without schedule pressure,” said Mark Nappi, Boeing vice president and director of the Commercial Crew Program. “We have the time.”
He noted that the CFT mission achieved 77 out of 87 flight test objectives set before launch, while the rest were associated with docking and landing. Engineers have included an unspecified number of additional test objectives to take advantage of the extra time on station, such as imaging the Starliner’s hatch operations and collecting more cabin air temperature measurements.
Both Stitch and Nappi confirmed, after repeated questioning at the press conference, that they believe the Starliner is safe for NASA astronauts Butch Wilmore and Sonny Williams to use on their return to Earth. The extra time gives engineers more time to study the vehicle’s performance, including in the off state that will be used during extended stays at the station in the future, Stitch said.
He added that NASA has authorized the use of Starliner to return Wilmore and Williams in emergency situations, if necessary. “We are taking a little extra time to review all the data and also learn as much as we can while we have the service module in orbit.”
Spacewalk changes
Wilmore and Williams were busy at the station helping test the Starliner’s systems. “They love the Starliner, and they are very happy to be part of the mission,” Stitch said.
The two were also helping out at the station. “We benefited from some of their extra time and extra helping hands,” said Dana Weigel, NASA’s International Space Station program manager, conducting the research.
The two assisted in the planned June 13 spacewalk for NASA astronauts Tracy C. Dyson and Matt Dominik. However, this spacewalk was canceled just before the scheduled start due to what NASA called a “spacesuit discomfort issue.”
Weigel said Dominic was the astronaut who felt uncomfortable in the suit, but he did not detail the specific problem that led to the delay. “We couldn’t solve the problem that day,” she said.
NASA has since revised plans for upcoming spacewalks, with Dyson and Mike Barratt now scheduled to perform a spacewalk on June 24 that includes the same tasks as the postponed June 13 spacewalk, which includes retrieving a malfunctioning electronics box. And collect samples from the outside of the station, which will be used to detect any microorganisms.
Weigel said Barratt was already scheduled for the next spacewalk and had a suit ready. “We decided it made sense to use Tracy and Mike,” she said.
This will be followed by a spacewalk on July 2 to perform additional maintenance on the station, although NASA will not assign astronauts to it until after the spacewalk on June 24. The agency had planned a series of three spacewalks, but Weigel said that will be reduced to two due to the oxygen used in preparation for the aborted spacewalk on June 13.