WASHINGTON — NASA and Boeing have once again extended the CST-100 Starliner’s stay on the International Space Station as engineers complete an analysis of the crewed spacecraft’s thrusters and helium leaks.
At a June 18 briefing, NASA announced that the 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 postponed again. Starliner is now scheduled to detach from the station at 10:10 p.m. ET on June 25 and land in White Sands, New Mexico at 4:51 a.m. ET on June 26.
NASA and Boeing officials said at the briefing that the extended stay on the station would give them more time to study two key problems the spacecraft encountered during its flight to the station nearly two weeks ago: thruster malfunctions and helium leaks in the spacecraft’s propulsion system. System.
This work included a brief firing of several thrusters of the stern reaction control system (RCS), five of which were shut down by the spacecraft’s computer as the Starliner approached the station. The four thrusters were restored by the controllers to allow docking to proceed.
One thruster that had not been restored before docking showed a “strange signature” that produced almost no thrust, said Steve Stich, NASA’s commercial crew program manager. This thruster will no longer be used by the spacecraft during disengagement and deorbital maneuvers.
The other nozzles, including those that failed during approach and others that behaved normally, showed the expected chamber pressure profile during the brief quarter-second burn. The thrusters also performed as expected during a longer burn of 1.2 seconds, when controllers measured their performance by testing the response of the station’s flight control system.
“It means we feel very confident in the jets and the team is just going to make sure we’re going to take a close look at them throughout the flight,” he said. This includes comparing their performance to that of an unmanned test flight in May 2022 called OFT-2, where two thrusters failed during approach but were restored before the end of the mission.
Stich said engineers are studying what caused the jets to disengage during the approach, which may be related to heavy use. “We have several theories about what’s going on inside the nozzle where the nozzle gets very hot,” he said, such as high temperatures preventing the fuel and oxidizer from mixing properly.
Engineers also used a nozzle test to check for five helium leaks found in the propulsion system. In each case, he said, the leak rate was reduced, in one case by 50%.
“It appears to be related to jet activity,” he said of the helium leak. This could be related to heat from the nozzles or sliding surfaces wearing out the seals. Three of the larger leaks likely have similar causes, he noted, while the two smaller ones may be similar to the leaks seen on 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 decoupling and deorbital maneuvers. “The requirement for the jets is much, much less” in the later stages of flight, he said.
He said the helium leaks were occurring in various “doghouses” on the service module from malfunctioning nozzles. However, he said that “dynamic operations” during the Starliner’s approach to the station may have caused jet failures as well as helium leaks.
NASA and Boeing engineers plan to continue analyzing the Starliner until June 22, after which they will focus on preparations for disconnection and return to Earth. This work was a factor in the decision to extend the Starliner’s stay on the station. “We’re not getting the service module back, so it’s an opportunity to fully understand system performance without schedule pressure,” said Mark Nappi, Boeing vice president and commercial crew program manager. “We have time.”
He noted that the CFT mission achieved 77 of the 87 pre-launch flight test objectives, with the remainder associated with disengagement and landing. Engineers included an unspecified number of additional test objectives to take advantage of the additional time on the station, such as filming Starliner hatch operations and collecting additional cabin air temperature measurements.
After repeated questioning at the briefing, both Stich and Nappi emphasized that they believed the Starliner was safe enough for NASA astronauts Butch Wilmore and Suni Williams to use it to return to Earth. The extra time, Stich said, allows engineers more time to study the vehicle’s performance, even in a powered-down state, for use during future long-duration stays on the station.
He added that NASA had authorized the use of the Starliner to bring Wilmore and Williams back in an emergency. “We’ll have 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 is changing
Wilmore and Williams were busy on the station helping to test the Starliner systems. “They love the Starliner and are so happy to be a part of the mission,” Stich said.
Both also helped out at the station. “We used some of their extra time and extra helping hands,” said Dana Weigel, NASA’s ISS program manager, for example, conducting the research.
Both assisted in a planned spacewalk on June 13 by NASA astronauts Tracy C. Dyson and Matt Dominick. However, that spacewalk was called off just before it was scheduled to begin due to what NASA called a “spacesuit discomfort issue.”
Weigel said Dominick was an astronaut who experienced discomfort in the suit, but did not specify the specific problem that led to the delay. “We couldn’t work it out that day,” she said.
NASA has since revised plans for upcoming spacewalks, with Dyson and Mike Barratt now planning a June 24 spacewalk that has the same tasks as the postponed June 13 spacewalk, including retrieving the malfunctioning electronics box and collecting samples from the exterior stations that they will use to detect any microorganisms.
Weigel said Barratt was already prepared to perform the upcoming spacewalk and had the suit ready. “We decided it made sense to use Tracy and Mike,” she said.
This will be followed by a July 2 spacewalk for further station maintenance, although NASA will not assign astronauts to it until after the June 24 spacewalk. The agency had planned a series of three spacewalks, but Weigel said that would be cut to two because of the oxygen used in preparation for the aborted June 13 launch.