Critical tests are planned this weekend to confirm Boeing’s Starliner capsule can safely transport his crew of two people returned to Earth despite problems that arose early in the mission, officials said Thursday.
Unexpected helium leaks and degraded maneuvering thrusters, discovered during the ship’s rendezvous with the International Space Station in early June, triggered weeks of testing and analysis that 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 space station visits, have taken the extended mission in stride and are enjoying their extra time in orbit.
NASA
As for when they might be allowed to return to Earth, Stich told reporters: “We don’t have a major announcement today about a return date. We’re making great progress, but we’re not quite ready to do that yet.”
“A very important series of tests” for Starliner
Two technical hurdles remain: tests this weekend to “hot fire” 27 maneuvering thrusters in the Starliner’s service module to ensure they will operate as designed between undocking and reentry; and parallel tests to confirm that five known helium leaks in the propulsion pressurization system have not worsened.
The pressurized helium is used to propel the thrusters to the boosters for ignition. The thrusters, in turn, are needed to reorient the Starliner as needed after undocking and to keep it stable when larger rockets are fired to bring the craft out of orbit for reentry and landing.
“We’re going to fire all of these thrusters a few times, just to make sure, before we undock, that the whole system is working as 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 on June 15. So we’re going to pressurize collector by collector, then hot 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, “it’s a very important series of tests that we’re going to do over the weekend,” he said.
Assuming no major surprises arise, a NASA Flight Readiness Review will be held to present the issues to senior management, along with the “flight rationale” – the analysis showing that the issues are understood and pose no credible threat to safety.
“There’s a lot of training that needs to be done for leaders before this assessment of the agency’s flight readiness,” Stich said. “We’ve had a hard time explaining everything that’s going on, and I apologize for that. It’s a very, very complicated subject.”
The Starliner launched on its first manned test flight on June 5. During the subsequent rendezvous with the International Space Station, several helium leaks were detected — one of which was known before launch — and five rearward-facing maneuvering thrusters failed to operate as expected by the flight software.
Four of them were then successfully tested and one was declared a failure.
The rear-facing thrusters were exposed to direct sunlight for extended periods of time, requiring them to operate at higher than normal temperatures. This, along with the rapid start sequence during rendezvous, 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 put it through two rendezvous sequences that mimic what the orbiting Starliner experienced, as well as five return-to-Earth, or “descent,” scenarios.
A similar degradation in thrust was observed and when engineers disassembled the test thruster, 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 this thruster to see if this particular seal could survive the rest of the flight,” Stich said. “If you look at what we’ve done on this thruster, it shows that we can survive up to five stages of descent. So we’re making sure that seal stays intact.”
As for the helium leaks, Stich said flight controllers had pressurized the system for earlier on-orbit tests and that the known leaks had not worsened; all were within acceptable limits.
“We’re going to do a check this weekend on the helium leak scenario, and then just before undocking, we’ll repressurize the system and check for helium leaks. … We’re making sure all of that is covered.”
At the time of Starliner’s launch, its batteries were designed to last for 45 days in space. Based on their actual performance in orbit, Stich said that limit has been extended to 90 days. Thursday marked the craft’s 50th day in space, and Stich said Wilmore and Williams could potentially return to Earth in late August.
“I’m confident 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 show that information to everyone, which will lead to the agency’s review, and that’s what we’re going to do over the next week.”
