Officials will also be examining whether the parachute system, which Boeing had to redesign after an earlier test flight without anyone on board, provides a safe landing in what would be the final act of Starliner’s first flight with humans on board.
After delays caused by a faulty valve on the rocket and helium leaks in the spacecraft, Starliner launched from Cape Canaveral, Fla. on June 5 and reached the station a day later. As Starliner approached the station, five of its thrusters, used to make small adjustments to its trajectory, went offline, forcing Boeing to back the vehicle away from the station and troubleshoot the issue.
NASA and Boeing were able to bring four of the five back online and dock successfully.
The teams test fired the thrusters on Saturday while the spacecraft was attached to the station, and all of them worked well, NASA said. They did not try to test fire the one thruster that didn’t come back online during the flight and won’t try to use it during Starliner’s return flight “out of an abundance of caution,” Steve Stich, NASA’s commercial crew program manager, said during a briefing Tuesday.
In addition to the thruster problems, Starliner has suffered a series of helium leaks in its propulsion system. NASA and Boeing discovered a new one — the fifth — since Starliner has been on the station. That leak is small and won’t pose a problem for the return, NASA said. Helium is used to pressurize the propellants through the propulsion system.
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NASA originally said Starliner would come home June 18, then pushed the landing back to June 22. The thruster problems and helium leaks are contained in the spacecraft’s service module, which is used to maneuver the capsule during flight. Before the capsule reenters Earth’s atmosphere, the service module is jettisoned and burns up. That means engineers won’t be able to study it after the flight, which is one of the reasons they said they were taking more time to understand the issues now.
“We’re taking our extra time given that this is a crewed vehicle, and we want to make sure that we haven’t left any stone unturned,” Stich said. “We also want to look at the systems, and potential interaction between the systems, and make sure we haven’t missed something before we return. And we’re getting a lot of great data while we’re at the space station for not only this flight but for the next flight.”
NASA and Boeing think the thrusters went offline because of the extreme heat generated while they were firing “in rapid succession” to keep the capsule on course with the space station, Stich said.
“In some cases, we think the heating may have caused the propellants to vaporize a little bit and we didn’t get good mixing [of the propellants], and that’s why the thrust was a little bit lower,” he said. Engineers still don’t understand what is causing the helium leaks, he said.
While on the station, Williams and Wilmore have prepared for their return, as well as rehearsed using Starliner as a safe haven in the event of an emergency on the space station. They’ve also worked alongside the other astronauts “installing research equipment, maintaining the lab’s hardware, and helping station crew members Matt Dominick and Tracy Dyson prepare for a spacewalk,” NASA said in a statement.
Despite the problems, NASA expressed confidence in Starliner. Officials said they expected to discover issues during the mission, a test flight designed to see how Starliner operates with humans on board.
“We’ve always said this as a test flight and we’re going to learn some things,” said Mark Nappi, a Boeing vice president who oversees the Starliner program. “So here we are. We’ve learned that our helium system is not performing as designed, albeit manageable. … So we’ve got to go figure that out.”
Once the mission is complete, NASA would certify Starliner for regular crew rotation flights of a full contingent of four astronauts to the space station. SpaceX, the other participant in NASA’s commercial crew program, which outsourced human spaceflight to the private sector after the space shuttle was retired, has been flying astronauts for NASA since 2020.
Given the problems Starliner has faced on this test flight, it’s not clear when Boeing, which was awarded a $4.2 billion NASA contract in 2014, would fly its first regular crew rotation mission.
“We’ve got to go address the helium leaks,” Stich said. “We’re not going to go fly another mission like this with the helium leaks.” The teams also need to find out what’s “causing the thrusters to have low thrust,” he added. “So we’ve got some of that work to do after this flight.”
