NASA and Boeing move forward with Starliner test flight after propulsion problems

LOS ANGELES – NASA and Boeing are preparing for the June 1 launch of the company’s CST-100 Starliner on a crewed test flight after analysis of a helium leak led to the discovery of a “security vulnerability.” design” in the spacecraft propulsion system.

In a briefing on May 24, officials said they believe a helium leak detected in a reaction control system (RCS) booster in the spacecraft’s service module is caused by a faulty seal on a flange. which is in itself an isolated problem. None of the other boosters have shown evidence of helium leaks after extensive testing.

Engineers discovered the leak shortly after the May 6 launch for the Crew Flight Test (CFT) mission caused by an unrelated valve issue in the Centaur upper stage of the Atlas 5 rocket. Tests conducted in the days after the washout made the leak would get worse.

Steve Stich, director of NASA’s commercial crew program, said engineers believe a seal, a rubber ring the size of a shirt button and the thickness of 10 sheets of paper, in the flange had a defect that worsened during the tests. He said other possibilities included an error installing the seal or debris from foreign objects rubbing against the seal.

NASA and Boeing concluded that Starliner can fly with the leak as is. “If we removed the seal completely,” said Mark Nappi, Boeing vice president and commercial crew program manager, “the leak rate would not exceed our ability to manage that leak. “That made us feel comfortable that if this leak got worse, it would be acceptable to fly.”

The lack of leaks in other boosters led NASA to conclude that no systemic problem exists. “We have analysis to back that up, which says we don’t expect the others to leak,” Stich said. “That’s how confident we are that we don’t have a common cause failure mode.”

“This is really not a flight safety issue for us and we believe we have a well-understood condition that we can handle,” Nappi concluded.

While the helium leak study was ongoing, Stich said engineers performed a review of the rest of the propulsion system “just to make sure we didn’t have other things we needed to worry about.”

That review revealed something it called a “design vulnerability” with Starliner’s propulsion system where, in a rare circumstance, the spacecraft would not be able to perform a deorbitation if two adjacent “doghouses” containing RCS and orbital maneuvers and attitudes larger control thrusters (OMAC) failed. That failure, he said, would disable enough OMAC and RCS boosters to prevent existing backup plans from being implemented to conduct a deorbit burn.

Engineers developed a new deorbit reentry mode that would use two burns of four RCS thrusters in that scenario. Such a failure mode would be “very remote,” Nappi said, appearing in less than one percent of possible failure combinations.

“It’s a pretty diabolical case,” Stich said.

However, it raised the question of why the design vulnerability was not found in reviews during Starliner’s long development. “The helium leak itself made us look a little more closely” at how elements of the propulsion system interact, Stich said. “It just took us a little while to realize this now. The question is, should we have seen this before? Maybe in a perfect timeframe we could have identified this sooner.”

Officials emphasized that this was part of the knowledge gained during a test flight program. Nappi said Boeing was looking at several permanent solutions to the problem, using some combination of hardware changes and software modifications, for later Starliner missions.

After two weeks of intensive work on Starliner, teams are taking a break over the Memorial Day holiday weekend before resuming final preparations for launch, scheduled for June 1 at 12:25 p.m. , Eastern Time. This includes a further review of flight test readiness on May 29, followed by the launch of Atlas 5 carrying Starliner to the launch pad on May 30.

There are backup launch opportunities for the mission on June 2, 5 and 6. However, limited life elements on the Atlas 5 rocket could cause a much longer delay if Starliner doesn’t launch for much longer after that.

Gary Wentz, vice president of government and commercial programs at United Launch Alliance, said the company regularly tests munitions used in flight termination and other systems on the rocket, but they “expire” later in June and July and would have to be replaced.

He suggested the work could lead to a further delay as the ULA reorganizes its launch manifesto. “Part of this will come down to mission prioritization for the rest of the year,” she said, as the company works with NASA and the Space Force to determine which missions should fly and in what order.

The two NASA astronauts who will fly Starliner on the CFT mission, Butch Wilmore and Suni Williams, are back in Houston but remain in pre-flight medical quarantine as they await their next launch opportunity. Stich said they were taking the delay in stride. “They’re in good spirits,” he said. “I think sometimes they care more about us than we care about them.”

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