Boeing said Friday that it will remove its Starliner spacecraft from atop of a rocket to fix valves that have remained stuck, a decision that will probably force yet another months-long delay in its do-over of a test flight without astronauts aboard.

Boeing engineers have been trying since Aug. 3 to fix the problem, one in a series of significant issues that has plagued its troubled spacecraft program and become another symbol of Boeing’s woes in the wake of the 737 MAX scandal.

Boeing had been hoping to restore functionality to the valves and get a launch off to the International Space Station this month under its contract with NASA. But the decision to move the spacecraft into the factory means the issue is a troublesome one. Because of other missions to the space station, a rescheduled launch might not happen until next year.

In 2014, NASA awarded Boeing and SpaceX contracts to develop a spacecraft to fly astronauts to and from the space station. Boeing received the majority of the money, $4.2 billion, and was expected to fly first. But it has run into a series of delays and setbacks, and last year SpaceX became the first company to fly NASA’s astronauts to the space station in what is known as NASA’s commercial crew program.

Boeing had been hoping to fly astronauts by the end of this year. But it must first complete a test flight without anyone on board to demonstrate the autonomous spacecraft can catch up with the station in orbit, dock with it and then return safely to Earth. So far it has struggled with that mission.

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The company botched its first attempt in December 2019, when Starliner suffered a major software breakdown that forced controllers on the ground to end the mission before the spacecraft could dock with the station.

After 18 months, the spacecraft was finally mounted on top of an Atlas V rocket and rolled out to the launchpad at Cape Canaveral, Fla., last month, for what Boeing hoped would be a triumphant return to flight after its previously attempt went so horribly awry.

But hours before the scheduled launch on Aug. 3, Boeing engineers discovered that 13 valves in the service module’s propulsion system were stuck in the closed position when they should have been open. Crews scrambled to fix the problem and moved the spacecraft and rocket to a nearby assembly building, where technicians were able to reopen nine.

The company had held out hope that it would be able to get the launch off this month but on Friday conceded that it would not be possible.

“We’re obviously disappointed that we were unable to get these issues resolved in time to make this launch window,” John Vollmer, vice president and program manager for Boeing’s Commercial Crew Program, said in a briefing with reporters. “The launch window, while important to us, was not the driver. The driver was safety.” He added that the company is “determined to get the spacecraft back and ready to fly at the next available opportunity.”

Vollmer is it was unclear if Starliner is likely to be launched in 2021.

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“It’s probably too early to say whether it’s this year, or not. I would certainly hope for as early as possible, and if we could fly this year, it would be fantastic,” Vollmer said. “But at this point I think that would be too much to speculate.”

The United Launch Alliance, the joint venture of Boeing and Lockheed Martin that operates the Atlas V rocket used to launch Starliner, has a launch scheduled for October to fly a NASA spacecraft to research asteroids associated with Jupiter. That will tie up the launchpad, forcing a wait.

The space station also has a heavy schedule planned through the end of the year. SpaceX is scheduled to fly a cargo resupply mission to the station at the end of this month that would occupy the docking port that Boeing was planning to use. It also has another flight with NASA astronauts scheduled for the end of October.

Boeing and NASA officials said they were focusing on fixing the problem and then would determine when they might try to launch again.

“It’s pretty early to speculate on where the flight might end up,” said Steve Stich, the manager of NASA’s commercial crew program. “We really need to get the vehicle back into the factory and get our hands on some valves and then figure out what the problem is and how to correct it.”

Boeing and NASA are exploring whether the cause of the problem was moisture from Florida’s humid air interacting with a chemical that helps the rocket fuel burn. The resulting mixture damaged the valves and kept them from opening, officials theorize.

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“That interaction, we believe, created some nitric acid,” Vollmer said. “And that nitric acid resulted in some corrosion which resulted in stiction of those valves. So that is primarily what we’re looking at right now, as the most likely cause for the issue.”

It was unclear whether the company would need to redesign the valves or whether “it’s just some preventative measures that we need to take,” Vollmer said. He added that the company would work with its partner, Aerojet Rocketdyne, on solving the problem. A spokesman for Aerojet declined to comment.

Vollmer added that the valve system worked fine in the weeks leading up to moving the spacecraft out of the climate-controlled factory to the launchpad. He said there had been no issues with the valves in the December 2019 flight, and “no changes, either to the valves or the methodology with how we operate them.”

In 2018, however, Boeing had a problem with valves in its launch abort system that failed to close properly and resulted in a fuel leak. The company said it had since remedied that issue, and Stich said Friday that it was in a separate system that operates differently and was “totally unrelated to what we saw” with the most recent problem.

In 2019, Starliner had another issue, this time with its parachutes during a test of its launch abort system. One of the three main chutes failed to deploy because a pin wasn’t securely fastened to a smaller, pilot chute that was meant to pull out the larger, main chute. Engineers overlooked it because the pin was hidden beneath a protective sheath but said that on future flights they would go over the connection manually.

Then in December of 2019, when it first attempted the flight test without crew on board, the spacecraft ran into trouble as soon as it reached orbit. A software problem made the computer think it was at an entirely different part of the mission and as a result started firing its thrusters to put itself in the orientation it thought it should be in.

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Controllers on the ground had trouble communicating with the spacecraft, and by the time they were finally able to get a software patch to the flight computers, the spacecraft had burned through too much fuel to dock with the station.

Engineers also discovered a second software problem that could have caused the service module to collide with the crew capsule, a potentially very dangerous situation that NASA officials said could have caused a loss of the vehicle. Controllers were able to fix that issue while the spacecraft was in orbit, and it landed safely two days after launching.

Boeing spent more than a year-and-a-half preparing for its do-over flight, a mission that it said would cost the company $410 million. In the days leading up to the flight, it said it was confident that the craft was ready to fly again and that it hoped to be able to launch astronauts by the end of the year.

Now, all of that is on hold again. And Vollmer could not say what the additional delay would cost the company, or who ultimately would pay for it.

“I would tell you we’re a little sad,” said Kathy Lueders, NASA associate administrator for human exploration and operations. “But I want to emphasize that this is another example of why these demo flights are so very important.”

“These are the kinds of things you want to find on the ground.”