WASHINGTON — NASA and Boeing have again extended the CST-100 Starliner’s stay at the International Space Station as engineers complete analysis of propellant problems and helium leaks on the crewed spacecraft.
During a briefing on June 18, NASA announced that Starliner’s return to Earth as part of the Crew Flight Test (CFT) mission, which had already been postponed twice to June 22, had been pushed back again . Starliner is now scheduled to undock from the station at 10:10 p.m. Eastern on June 25 and land in White Sands, New Mexico at 4:51 a.m. Eastern on June 26.
NASA and Boeing officials said during the briefing that the extended stay at the station would give them more time to study two key issues the spacecraft encountered during its flight to the station nearly two weeks ago: thruster malfunctions and helium leaks in the spacecraft’s propulsion. system.
This work included brief firings of several rear-facing Reaction Control System (RCS) thrusters, five of which were shut down by the spacecraft’s computer as Starliner approached the station. Four of the thrusters were restored by controllers to allow docking to continue.
The only thruster that was not restored before docking had a “strange signature” producing almost no thrust, said Steve Stich, manager of NASA’s commercial crew program. This thruster will no longer be used by the spacecraft during undocking and deorbit maneuvers.
Other thrusters, including those that malfunctioned during approach and those that behaved normally, showed the expected profile of chamber pressures during a brief burn lasting a quarter of a second . The thrusters also operated as designed for a longer operating time of 1.2 seconds each, during which controllers measured their performance by testing the response of the station’s flight control system.
“Coming out of this, we’re very confident in the thrusters and the team is just making sure to look at the thrusters in detail throughout the flight,” he said. This includes comparing their performance to that of an uncrewed test flight in May 2022, called OFT-2, during which two thrusters malfunctioned during approach but were restored before the end of the mission.
Stich said engineers were investigating why the thrusters disconnected during approach, which could be related to heavy use. “We have a few theories about what’s happening inside the propellant, where the propellant gets very hot,” he said, such as high temperatures preventing proper mixing of fuel and oxidizer.
Engineers also used the thruster test to check for five helium leaks detected in the propulsion system. In all cases, he said, leak rates decreased, in one case by 50%.
“It appears to be related to thruster activity,” he said of the helium leaks. This could be related to the heat of the thrusters or the sliding surfaces which wear down the seals. Three of the larger leaks likely have similar causes, he noted, while two smaller ones could instead be similar to leaks observed during the OFT-2 mission.
As with the booster tests, Stich said the reduction in helium leaks gives him confidence that the spacecraft will be able to perform as intended during undocking and deorbit maneuvers. “The demand on propellants is much less” in later phases of flight, he said.
He said the helium leaks were occurring in different “niches” on the service module due to faulty thrusters. However, he said “dynamic operations” during Starliner’s approach to the station may have caused both propellant malfunctions and helium leaks.
NASA and Boeing engineers plan to continue analysis of the Starliner until June 22, then focus on preparations for undocking and return to Earth. This work was a factor in the decision to extend the Starliner’s stay at the station. “We’re not recovering the service module, so this is an opportunity to fully understand the performance of the system without the pressure of schedule,” said Mark Nappi, Boeing vice president and manager of the commercial crew program. “We have the time.”
He noted that the CFT mission achieved 77 of the 87 flight test objectives set before launch, with the remainder associated with undocking and landing. Engineers included an unspecified number of additional test objectives to take advantage of the extra time spent at the station, such as filming the Starliner’s hatch operations and collecting more air temperature measurements in the cabin.
Stich and Nappi emphasized, after repeated questions during the briefing, that they believed Starliner was safe for NASA astronauts Butch Wilmore and Suni Williams to return to Earth. The additional time, Stich said, gives engineers more time to study the vehicle’s performance, including in a powered-off state that will be used during future long-term stays at the station.
He added that NASA had authorized the use of the Starliner to bring Wilmore and Williams back in an emergency, if necessary. “We’re taking a little extra time to look at all the data and learn as much as we can while the service module is in orbit.”
Spacewalk modifications
Wilmore and Williams have been busy at the station testing Starliner systems. “They love Starliner and they are very happy to be part of the mission,” Stich said.
The two also helped out at the station. “We took advantage of some of their extra time and extra help,” said Dana Weigel, manager of NASA’s ISS program, such as conducting research.
The two men participated in a spacewalk planned for June 13 by NASA astronauts Tracy C. Dyson and Matt Dominick. That spacewalk, however, was canceled just before it was scheduled to begin due to what NASA called a “spacesuit discomfort issue.”
Weigel said Dominick was the astronaut who experienced discomfort in the suit, but did not elaborate on the specific problem that prompted the postponement. “We couldn’t resolve the problem that day,” she said.
NASA has since revised its plans for upcoming spacewalks, with Dyson and Mike Barratt now scheduled to perform a June 24 spacewalk that features the same tasks as the postponed June 13 spacewalk , including the recovery of a defective electronic box and the collection of samples outside the station which will be used to detect possible microorganisms.
Weigel said Barratt was already scheduled to perform an upcoming spacewalk and had a suit ready. “We decided it just made sense to use Tracy and Mike,” she said.
This will be followed by a spacewalk on July 2 for additional maintenance of the station, although NASA will not assign astronauts there until after the June 24 spacewalk. The agency had planned a series of three spacewalks, but Weigel said that number would be reduced to two because of the oxygen used to prepare for the aborted June 13 spacewalk.