Critical tests will be conducted this weekend to confirm that Boeing's Starliner capsule can safely carry its two-person crew back to Earth despite unexpected helium leaks and degraded maneuvering thrusters, officials said Thursday.
The problems, discovered during the spacecraft's rendezvous with the International Space Station in early June, triggered weeks of testing and analysis that have extended the spacecraft's first piloted test flight from just over a week to nearly two months.
Steve Stich, NASA's Commercial Crew Program manager, said Starliner commander Barry “Butch” Wilmore and co-pilot Sunita Williams, both veterans of previous visits to the space station, have taken the extended mission in stride and are enjoying their extra time in orbit.
As for when they might be cleared to return to Earth, Stich told reporters: “We don't have any major announcements today about a return date. We're making great progress, but we're not ready to go yet.”
Two technical hurdles remain: tests this weekend to “fire up” 27 maneuvering thrusters on Starliner’s service module to ensure they will function as expected between undocking and reentry; and parallel tests to confirm that five known helium leaks in the propulsion pressurization system have not worsened.
Pressurized helium is used to propel the thrusters toward the orbiters for ignition. The thrusters, in turn, are needed to reorient Starliner as needed after undocking and to keep it stable when larger rockets are fired to deorbit it for reentry and landing.
“We’re going to activate all of those thrusters through a series of pulses, just to make sure before we undock that the whole system is working as we expected and as it did the last time we checked it,” Stich said. “We’ll also have a chance to look at the helium system.”
“It’s been six weeks since we last checked the helium system, which was June 15. So we’ll pressurize manifold by manifold, and then we’ll fire the thrusters, and then we’ll have a chance to look at the helium leak rates and verify that the system is stable.”
Overall, “this is a very important series of tests that we will be doing over the weekend,” he said.
Assuming no major surprises emerge, a NASA flight readiness review will be conducted to outline the issues for senior management, along with a “flight rationale” — analysis showing the issues are understood and do not pose a credible threat to safety.
“There is significant leadership training required prior to this review of the agency’s flight readiness,” Stich said. “We have had a difficult time explaining everything that is happening and I apologize for that. This is a very, very complicated issue.”
Starliner launched on its first crewed test flight on June 5. During the subsequent rendezvous with the International Space Station, multiple helium leaks were detected (one of which was known before launch) and five rear-facing maneuvering thrusters failed to operate as expected by the flight software.
Four tests were subsequently carried out successfully and one was declared a failure.
The rear-facing thrusters were exposed to direct sunlight for extended periods, causing them to operate at higher than normal temperatures. That, coupled with the sequence of rapid ignitions during the encounter, likely contributed to the observed performance.
To find out, Boeing took a flight booster from another Starliner to a NASA test facility in White Sands, New Mexico, and ran it through two rendezvous sequences that replicated what the Starliner experienced in orbit along with five return-to-Earth, or “downhill,” scenarios.
Similar thrust degradation was observed, and when engineers disassembled the test booster, a Teflon seal was found to be slightly deformed, likely due to exposure to one of the propellants, nitrogen tetroxide.
“The team is … looking at that booster to see if that particular seal can survive the rest of the flight,” Stich said. “If you look at what we did with that booster, it shows that we can survive up to five descent legs. So we’re making sure that that seal stays intact.”
As for the helium leaks, Stich said flight controllers pressurized the system for earlier in-orbit tests and the known leaks did not worsen; they were all within acceptable limits.
“This weekend we’re going to do a check of the helium leak scenario and then just before we undock we’ll repressurize the system and check for helium leaks. We’re making sure that all of that is covered.”
When Starliner launched, its batteries were good for 45 days in space. Based on its actual performance in orbit, that limit has been extended to 90 days, according to Stich. Thursday marked the craft's 50th day in space, and Stich said Wilmore and Williams could possibly return to Earth by the end of August.
“I'm very confident that we have a good vehicle to bring the crew back,” said Mark Nappi, Boeing's Starliner program manager. “We need to take the next steps to get that information out to the world, and that leads to the agency's review, and that's what we're going to do over the next week.”
Leave feedback about this