Starliner’s uncertain futureby Jeff Foust
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| “We can’t totally prove with certainty what we’re seeing on orbit is exactly what’s been replicated on the ground,” Stich said of Starliner’s thrusters. “People want to understand the physics of what’s going on.” |
It wasn’t until nearly six hours after liftoff that NASA issued a press release stating that the Cygnus was healthy but had missed its first two burns because of technical glitches. “Cygnus is at a safe altitude, and Northrop Grumman engineers are working a new burn and trajectory plan,” the agency said. That was, at least, more detail than what Northrop Grumman provided: its press release about the launch, issued at the same time as NASA’s, made no mention of any thruster issues.
It was nearly 24 hours later before NASA provided another update, confirming that the burns had been performed and the spacecraft’s arrival at the ISS remained on schedule for early August 6. The spacecraft indeed arrived as originally scheduled and was grappled by the station’s robotic arm, berthing the spacecraft to the station as if nothing had gone wrong.
Dana Weigel, NASA ISS program manager, did discuss the “engine burn execution issue” at an August 7 briefing. “The spacecraft’s onboard limit-detection system flagged an out-of-limit calculated parameter,” she said. “That violated a limit and it did was it was designed to do and it cancelled the burn.”
She said Northrop Grumman engineers reviewed the data and concluded there was no problem with the spacecraft hardware itself. “They reworked the far-field rendezvous plan to keep us on time with our planned arrival time. It was pretty impressive to watch them do that,” she said. Notably, she didn’t identify the parameter the onboard computer flagged as being out of limits, although NASA’s initial release mentioned a “slightly low initial pressure state” in the engine.
The incident might have attracted more attention and scrutiny had NASA not been grappling with a far more serious issue: the ongoing investigation into the thruster problems that Boeing’s CST-100 Starliner experienced during its approach to the station two months earlier on the Crew Flight Test (CFT) mission. Five reaction control system (RCS) thrusters were shut down by Starliner’s computers when their thrust dropped; four were later restored, allowing the docking to proceed, but with little insight into what happened to the thrusters.
In the weeks that followed, NASA and Boeing have been trying to figure out what caused the thrusters to malfunction, while examining a separate, unrelated issue with leaks in the helium pressurization system used by the thrusters. What was originally scheduled to be a stay of as little as eight days on the ISS for astronauts Butch Wilmore and Suni Williams has stretched, in days and weeks at a time, into more than two months, leading to headlines that the astronauts were “stuck” or “stranded” in space that Boeing in particular complained about (see “Starliner struggles”, The Space Review, July 1, 2024.)
In late June, NASA and Boeing announced plans to test an RCS thruster at NASA’s White Sands Test Facility in New Mexico, putting it through the same “uphill” and “downhill” profiles, mimicking its use going to and ISS and departing to return to Earth, to see if they could replicate the diminished thrust and better understand what was causing it. Those tests in early July were followed by test-firings of the RCS thrusters on Starliner itself in late July, while still docked to the ISS.
The good news from those tests was that the thrusters on Starliner performed normally, with the exception of the one that was not recovered during the spacecraft’s approach to the station in June.
“The interesting thing when we hotfired the thrusters is all the thrusters now, across the whole vehicle, are essentially at nominal thrust levels,” said Steve Stich, NASA commercial crew program manager, at the August 7 briefing. One thruster, whose performance had dropped to 80% of its rated thrust, was back to 98% in that test, he said.
| “The testing at White Sands and the discovery that the poppet Teflon was extruding after the testing was a bit of a surprise to us,” Stich said. “That, I would say, upped the level of discomfort.” |
The bad news was that NASA was still not sure what had caused the performance of the RCS thrusters to degrade. In the White Sands tests, the thruster went through two uphill profiles of firings and five downhill profiles, which resulted in a loss of thrust similar to what was seen in orbit. Inspection of the thruster revealed a Teflon seal on a poppet in the thruster’s oxidizer line had swelled, extruding into the line and partially blocking the flow of oxidizer.
But, Stich added that the propellant in the thruster can vaporize when it gets hot in the thruster, as is the case for the RCS thrusters located in housings called “doghouses” mounted on the service module, which can also degrade the thrusters’ performance. “There’s really two key things that we think are happening,” he said.
However, it’s not clear how much each of the factors contributes to the thruster problems and what it means for the ability of Starliner to safely depart the ISS and return home. “We can’t totally prove with certainty what we’re seeing on orbit is exactly what’s been replicated on the ground,” he said, noting that the thrusters have experienced problems at different temperature and a different number of pulses.
“People want to understand the physics of what’s going on,” he said, “and that’s what the team is off trying to understand and then look ahead to the downhill phase and the heating on the downhill phase, seeing if we can model that on the downhill phase and ensure we have good thrusters.”
Stich acknowledged that the White Sands tests may have raised more questions about thrusters than they answered. “The testing at White Sands and the discovery that the poppet Teflon was extruding after the testing was a bit of a surprise to us,” he said. “That, I would say, upped the level of discomfort.”
That discomfort led NASA to acknowledge at the briefing what had been rumored in recent weeks: it was considering alternative ways of bringing back Williams and Wilmore. Stich said the agency decided to give itself some more time to decide what to do when it announced, a day before the briefing, that it was delaying the launch of the Crew-9 mission to the station on a SpaceX Crew Dragon from August 18 to no earlier than September 24.
“Our prime option is to return Butch and Suni on Starliner. However, we have done the requisite planning to make sure that we have other options open,” he said.
In that contingency scenario outlined at the briefing, Starliner would undock from the station and return to Earth without a crew on board. The Crew-9 Crew Dragon would then launch to the station with only two of the currently assigned four people on board, freeing up seats for Wilmore and Williams. After Crew-8 departs, Wilmore and Williams would join the Crew-9 astronauts for a full expedition on the ISS, returning with them on that Crew Dragon in February 2025. What was to be an eight-day stay on the ISS would instead last more than eight months for the CFT astronauts.
NASA officials at the briefing declined to say how likely that scenario is, but the fact that they would publicly discuss it would indicate they are taking it seriously. They added that Williams and Wilmore have gone through training to allow them to perform a full expedition at the station, including conducting spacewalks; both have served on previous ISS expeditions.
The agency also declined to say which of the current Crew-9 members—NASA astronauts Zena Cardman, Nick Hague, and Stephanie Wilson, and Roscosmos cosmonaut Alexsandr Gorbunov—would be retained if NASA opted for the contingency scenario. Cardman is the commander but both Hague and Wilson have spaceflight experience. One industry source speculated that if Gorbunov is not included, interrupting seat barter agreements between NASA and Roscosmos, the Russian space agency might replace NASA astronaut Don Pettit on next month’s Soyuz flight with a Russian cosmonaut.
That decision of whether Wilmore and Williams will return in the coming weeks on Starliner or early next year on Crew Dragon is coming soon. “I don’t think we’re too far away from making that call,” said Ken Bowersox, NASA associate administrator for space operations, said on the call. Stich said later a decision by mid-August is needed to perform training and other preparations should NASA choose the contingency scenario. (As this article was being prepared for publication August 12, NASA announced the decision on Starliner’s return would be made next week, not later this week.)
That decision could come from the top of the agency. “The administrator ultimately takes responsibility,” Bowersox said.
What’s working, what isn’t
In the call, Bowersox and Stich said there have been long discussions within the agency about whether it is safe for Wilmore and Williams to return on Starliner. Stich said the concerns focused on the potential for an “integrated failure mechanism” involving thruster failures and helium leaks that could keep the RCS thrusters from maintaining the proper orientation of the capsule during its deorbit burn and reentry.
“The team wants to understand the worst-case ramifications of getting the thruster hot,” he said. “They want to make sure there’s no particular failure mode if the thruster gets too hot that we haven’t seen yet.”
| “I have to admit that sometimes, when we get disagreements, it’s not fun. It can be painful having those discussions,” Bowersox said. “It’s what makes us a good organization and it’s what will get us to a good decision.” |
To that end, he said the commercial crew program has called in propulsion experts from across the agency to help review data while additional testing is ongoing. “I think the team would like us to understand that physics a little bit better to understand why this Teflon is swelling,” he said, “and to make sure there’s not a really serious, catastrophic failure mode of the thrusters.”
The lack of consensus so far, Bowersox said, was a good thing: a willingness to examine the data and debate their implications. “I have to admit that sometimes, when we get disagreements, it’s not fun. It can be painful having those discussions,” he said. “It’s what makes us a good organization and it’s what will get us to a good decision.”
At an August 1 meeting of the Aerospace Safety Advisory Panel, committee chair Susan Helms noted that NASA allowed her committee to sit on commercial crew decision boards. “There isn’t a better group of people to be working these extremely hard problems than the people who are working them now,” she said.
Another panel member, Kent Rominger, also highlighted what he saw as a positive “safety culture” in those meetings. “NASA’s commercial crew program appears to be following disciplined processes working through the helium leaks and the thruster fail-off issues,” he said, noting that, at the meetings, ““people are willing to speak up with varying opinions and are being listened to.”
At the same time, there appears to be a widening rift between NASA and Boeing. The Friday before the latest NASA briefing, Boeing issued a statement highlighting its confidence in the ability for Starliner to return astronauts safely. It mentioned both the ground tests of the RCS thruster and in-space tests of the thrusters on Starliner as well as various other reviews and simulations.
“Boeing remains confident in the Starliner spacecraft and its ability to return safely with crew. We continue to support NASA’s requests for additional testing, data, analysis and reviews to affirm the spacecraft’s safe undocking and landing capabilities,” Boeing said.
(That statement, though, was not without its own problems. An initial version emailed out late on a Friday said that the company performed “roughly 1 million computer model simulations” of the RCS thrusters. A half-hour later, it emailed a correction: the actual number of simulations was about 100,000.)
“Boeing asserted their vehicle was ready to come home on Friday,” Stich said, referring to the Boeing statement at the briefing. “I think it depends on your experience base a little bit as to how you view the risk.”
“The Boeing team, because of their experience and their belief in their hardware, would take them to being very confident that the vehicle could bring the crew home, even right now with the uncertainty that we’ve got,” added Bowersox. “But we’ve got other folks that are probably a little more conservative. They’re worried that we don’t know for sure, so they estimate the risk higher.”
Notably absent from the briefing, though, was Mark Nappi, Boeing’s commercial crew program manager who had regularly appeared at previous briefing about the CFT mission, or any other representatives from the company.
| “Boeing remains confident in the Starliner spacecraft and its ability to return safely with crew,” the company said. |
That heightened speculation that Boeing, having already recorded $1.6 billion in losses on Starliner so far, including $125 million in its last fiscal quarter, might walk away from the program if the CFT mission doesn’t end as planned with a crewed return. NASA has already delayed the first operational mission, Starliner-1, from February to August 2025, adding that it will prepare SpaceX’s Crew-11 in parallel with Starliner for that August 2025 launch opportunity. With Kelly Ortberg joining Boeing earlier this month as the company’s new CEO, Starliner is likely to get a fresh look along with many other programs at the company.
“I think the path forward for Starliner is to complete the rest of the test flight,” Stich said, leaving open the question of whether an uncrewed but successful landing would be sufficient to certify the spacecraft for operational missions. “I see a bright future for Starliner. Either way, we really need two crew transportation systems.”
NASA also faced renewed criticism on the call for a lack of updates about progress on Starliner. In late June, the agency said it would try to provide more frequent updates, but held just two briefings in July to discuss updates on testing and other aspects of the mission. Williams and Wilmore have also had just one brief telecon with reporters.
At the latest briefing, Stich said the agency was providing “daily and weekly” updates to the two astronauts, who have been able to call into meetings about Starliner. “I think Butch and Suni are ready to do whatever we need them to do,” he said. “They are professional astronauts and test pilots and they understand the difficulties of the decision that we’re facing.”
The Cygnus that arrived at the station last week did include some supplies specifically intended for Wilmore and Williams, including clothes that were originally intended to fly with them on Starliner in June but had to be removed to make room for critical spare parts for the station’s urine processor.
That Cygnus is scheduled to remain at the station until January, when it will be unberthed to perform a destructive reentry. It remains to be seen if Butch Wilmore and Suni Williams will be there to see it depart.
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