WASHINGTON – NASA and Boeing have once again extended the CST-100 Starliner’s stay on the International Space Station as engineers complete analysis of thruster problems and helium leaks on the crewed spacecraft.
At a June 18 briefing, NASA announced that Starliner’s return to Earth on the Crew Flight Test (CFT) mission, which had already been postponed twice until June 22, had been postponed again. Starliner is now scheduled to disconnect from the station at 10:10 PM on June 25 and land in White Sands, New Mexico at 4:51 AM on June 26.
NASA and Boeing officials said at the briefing that the extended stay on the station will give them more time to study two major problems the spacecraft encountered during its flight to the station nearly two weeks ago: thruster malfunctions and helium leaks in the spacecraft propulsion. system.
That work included the brief firing of several rear-facing reaction control system (RCS) thrusters, five of which were disabled by the spacecraft’s computer as Starliner approached the station. Four of the thrusters were repaired by controllers to allow docking.
The one thruster that had not been repaired before docking had a “strange signature” and produced almost no thrust, said Steve Stich, NASA’s commercial crew program manager. That thruster will no longer be used by the spacecraft during docking and deorbit maneuvers.
The other thrusters, including both those that malfunctioned during the approach and others that behaved normally, exhibited the expected chamber pressure profile during a brief quarter-second burn. The thrusters also worked 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.
“Coming out of this, we have a lot of confidence in the thrusters and the team is just making sure they look at the thrusters in detail throughout the flight,” he said. That includes comparing their performance to that of an unmanned test flight in May 2022, called OFT-2, in which two thrusters malfunctioned during the approach but were repaired before the end of the mission.
Stich said engineers are investigating what caused the thrusters to go offline during the approach, which may be related to heavy use. “We have some theories about what’s happening in the thruster, where the thruster gets very hot,” he said, such as high temperatures that prevent proper mixing of fuel and oxidizer.
Engineers also used the thruster test to check the five helium leaks detected in the propulsion system. In all cases, he said, leak rates had fallen, in one case by 50%.
“It appears to be related to the activity of the thrusters,” he said of the helium leaks. This could be due to the heat of the thrusters or sliding surfaces wearing out seals. Three of the larger leaks likely have similar causes, he noted, while two smaller ones may instead be similar to leaks observed during the OFT-2 mission.
As with the thruster tests, Stich said the reduced helium leaks give him confidence that the spacecraft will be able to perform as expected during docking and deorbit maneuvers. “The demand for the thrusters is much, much less” in the later stages of flight, he said.
He said the helium leaks are occurring in several “doghouses” on the service module of the failed thrusters. However, he stated that the “dynamic operations” during Starliner’s approach to the station may have caused both the thruster failures and the helium leaks.
Engineers from NASA and Boeing plan to continue Starliner analysis until June 22 and then focus on preparations for undocking and return to Earth. That work played a role in the decision to extend Starliner’s stay at the station. “We are not getting the service module back, so this is an opportunity to fully understand the system’s performance without the pressure of scheduling,” said Mark Nappi, Boeing vice president and Commercial Crew Program Manager. “We have the time.”
He noted that the CFT mission achieved 77 of the 87 flight test objectives set before launch, with the rest related to undocking and landing. Engineers have added an unspecified number of additional test objectives to take advantage of the extra time on station, such as filming Starliner’s hatch operations and collecting more cabin air temperature measurements.
Both Stich and Nappi, after repeated questions during the briefing, emphasized that they believed the Starliner was safe for NASA astronauts Butch Wilmore and Suni Williams to use for their return to Earth. The additional time, according to Stich, gives engineers more time to study the performance of the vehicle, including in a powered-off state, which will be used during future extended stays at the station.
He added that NASA had authorized the use of Starliner to return Wilmore and Williams if necessary in an emergency. “We’re taking some extra time to look at all the data and also learn as much as we can while we have the service module in orbit.”
Changes in the spacewalk
Wilmore and Williams have been busy at the station testing Starliner systems. “They love Starliner and are so 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 helping hands,” said Dana Weigel, NASA ISS program manager, such as conducting research.
The two assisted a planned June 13 spacewalk by NASA astronauts Tracy C. Dyson and Matt Dominick. However, that spacewalk was canceled just before the scheduled takeoff due to what NASA called a “space suit discomfort issue.”
Weigel said Dominick was the astronaut who experienced discomfort in the suit, but did not elaborate on the specific issue that led to the postponement. “We couldn’t solve it 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 with the same tasks as the postponed June 13 spacewalk, including retrieving a faulty electronics box and collecting samples from the outside of the station. are used to detect any micro-organisms.
Weigel said Barratt was already scheduled to do a spacewalk and had a suit ready. “We decided it made sense to bring in Tracy and Mike,” she said.
That will be followed by a spacewalk on July 2 for additional station maintenance, although NASA won’t assign astronauts to it until after the June 24 spacewalk. The agency had planned a series of three spacewalks, but Weigel said this will be reduced to two due to oxygen use in preparation for the aborted June 13 spacewalk.