WASHINGTON — NASA and Boeing have again extended the CST-100 Starliner’s stay at the International Space Station as engineers complete an analysis of thruster issues and a helium leak on the crewed spacecraft.
During a June 18 briefing, NASA announced that the Starliner Crew Flight Test (CFT) mission’s return to Earth has been postponed a further time, having already been postponed twice, to June 22. Starliner is currently scheduled to undock from the station at 10:10 pm ET on June 25, and land at White Sands, New Mexico, at 4:51 am ET on June 26.
NASA and Boeing officials said at a press conference that the extended stay at the space station will allow more time to investigate two major issues the spacecraft encountered on its flight to the station about two weeks ago: a thruster failure and a helium leak in the spacecraft’s propulsion system.
This involved briefly firing several aft-facing Reaction Control System (RCS) thrusters – five of which were shut down by the spacecraft’s computers as Starliner approached the station – and controllers restored four thrusters so docking could continue.
One thruster that wasn’t repaired before docking exhibited “odd characteristics” such that it produced very little thrust, said Steve Stich, NASA’s commercial crew program manager. That thruster will no longer be used during the spacecraft’s undocking or deorbit maneuvers.
The other thrusters, both those that had failed during approach and those that had operated normally, exhibited the expected chamber pressure profiles during their short quarter-second burns. The thrusters also performed as expected during the longer 1.2-second burns, during which mission controllers measured thruster performance by testing the response of the station’s flight control system.
“Following these results, we have great confidence in our thrusters and the team is ensuring that they are monitored closely throughout the flight,” he said, including a comparison with their performance during an unmanned test flight, called OFT-2, in May 2022. During OFT-2, two thrusters failed during the landing approach but were repaired before the end of the mission.
Stich said engineers are investigating why the thrusters went offline during the approach, which could be related to overuse. “There are a few theories about what’s going on inside the thrusters that make them so hot,” he said, including that high temperatures prevent the fuel and oxidizer from mixing properly.
Engineers also used the thruster tests to verify five helium leaks detected in the propulsion system, and in each case the leak rate has decreased, in one case by 50 percent.
“This appears to be related to thruster activity,” he said of the helium leak, which could be related to thruster heat or sliding surfaces wearing down seals. He noted that three of the larger leaks are likely from a similar source, while two of the smaller leaks may be similar to leaks seen on the OFT-2 mission.
Stich said the reduced helium leak, as well as the thruster tests, gave them confidence that the spacecraft could perform as expected during undocking and deorbiting operations. “Later in the flight, there are much fewer demands on the thrusters,” he said.
He said the helium leak was in a separate “doghouse” in the service module from the failed thruster, but that “dynamic maneuvers” during Starliner’s approach to the station could have caused both the thruster failure and the helium leak.
NASA and Boeing engineers will continue analyzing Starliner through June 22, then focus on preparing for undocking and return to Earth. That work was a factor in the decision to extend Starliner’s stay on station. “The service module is not coming back, so this is our chance to fully understand the system’s performance without the pressure of schedule,” said Mark Nappi, Boeing’s vice president and commercial crew program manager. “We have time.”
He noted that the CFT mission accomplished 77 of the 87 flight test objectives set before launch, with the rest related to undocking and landing. Engineers included an unspecified number of additional test objectives to take advantage of the additional time aboard the space station, including filming Starliner’s hatch operation and gathering more cabin air temperature measurements.
After being questioned multiple times during the briefing, Stich and Nappi stressed that they were confident Starliner was safe for NASA astronauts Butch Wilmore and Suni Williams to use for their return to Earth. Stich said the extra time will give engineers more time to study the spacecraft’s performance, including in a powered-off state that could be used during future long-duration missions.
He added that NASA has approved the use of Starliner to bring Wilmore and Williams home if necessary in an emergency. “We’re going to take some time while the service module is in orbit to review all the data and learn as much as we can.”
Spacewalk Changes
Wilmore and Williams have been busy helping test the Starliner system on the space station. “They love Starliner and are excited to be a part of the mission,” Stich said.
The two also help out on the ISS, and “we’ve taken advantage of their extra time and help, including conducting research,” said Dana Weigel, NASA’s ISS program manager.
The two were assisting NASA astronauts Tracy C. Dyson and Matt Dominik in a spacewalk scheduled for June 13, but the spacewalk was canceled shortly before it was due to what NASA called a “spacesuit discomfort issue.”
Weigel said it was Dominique who experienced discomfort in the spacesuit, but did not elaborate on the specific issues that prompted the postponement. “It couldn’t be resolved that day,” she said.
NASA has since revised plans for future spacewalks, with Dyson and Mike Barratt now scheduled to perform the same tasks on June 24 as the postponed June 13 spacewalk, including retrieving a faulty electronics box and taking samples from the space station’s exterior to detect microorganisms.
Weigel said Barratt had already scheduled his next spacewalk and had his spacesuits ready. “We decided it would make sense to use Tracy and Mike,” she said.
A spacewalk will follow on July 2 and some more maintenance on the station, but NASA won’t send astronauts there until after the June 24 spacewalk. NASA had planned three spacewalks, but that will be cut to two because oxygen was used to prepare for the canceled June 13 spacewalk, Weigel said.