Why did Boeing Starliner scrub and when will it launch?

A pair of NASA astronauts were ready to go, but a single valve caused a scrub of their ride on Boeing’s CST-100 Starliner on Monday night. The next shot to fly won’t be until Friday but could be delayed further.

“I know everybody was eager to see a launch,” said NASA’s Ken Bowersox, associate administrator of the Space Operations Mission Directorate during a Monday night news conference after the scrub. ” … But all I want to say first is that good things are worth waiting for, and we’ll get a chance to see that rocket and spacecraft lift off the pad here soon.”

Butch Wilmore and Suni Williams were strapped into Starliner sitting atop a United Launch Alliance Atlas V rocket at Cape Canaveral Space Force Station but with just over two hours before the planned liftoff, teams called off the launch attempt.

The reason was a technical issue with a valve within the upper Centaur stage on ULA’s rocket designed to regulate pressure on a liquid oxygen tank.

Teams at the pad reported unexpected sounds from the rocket after NASA’s astronauts had already entered the spacecraft.

“We saw the self-regulating valve on the (liquid oxygen) side had a bit of a buzz and so it was moving in a strange behavior,” said NASA Commercial Crew Program manager Steve Stich. “The flight rules had been laid out for this flight ahead of time with the crew at the launch pad. The proper action was to take the scrub and the United Launch Alliance team did a great job of assessing the data, talking through various options and put us into a scrub condition.”

ULA President and CEO Tory Bruno said the decision to scrub was more about flight rules in place vs. the actual problem.

“Our philosophy is we don’t change the fueled state of the vehicle when the crew is present,” he said. “You can do it otherwise from that, and other people do, but that’s our philosophy. So we built our flight rules around that.”

He noted the issue was something they had seen before on ULA rockets, and had humans not been on board, the fix in place was a simple one.

“It’s not dissimilar to many other valves like that, and you have one in your home on your hot water tank that’s not all that different,” he said. “Every now and again in rare occasions, a valve like that can get into a position where it’s just off the seat. Its temperature, its stiffness, everything is just right and it’ll flutter or it’ll buzz in this case, in cycle.”

He said the fix is to force the valve closed to cycle it.

“Once we had the crew off, we cycled the valve and it stopped buzzing,” he said. “If this were a satellite, that is our standard procedure, and the satellite would already be in orbit.”

But with humans on board, ULA’s rules meant not changing the fueling state of the volatile cryogenic propellants.

“I promised Butch and Suni a boring evening,” Bruno said. “I didn’t mean for it to be quite this boring. But we’re going to follow our rules and we’re going to make sure that the crew is safe.”

But another issue has to do with the acceptable lifespan of the valve. Bruno said it had been qualified to open and close 200,000 times at full pressure. Based on the Monday night data, it’s possible the fluttering on the valve, if it was, in fact, opening and closing fully, it would be nearing that 200,000 limit.

But Bruno said it’s possible the pressure exerted was not full capacity, meaning only a partial debit against that 200,000 limit. Bruno said there was no instruments actually measuring the valve in question and that data had to be figured out from surrounding hardware. That process had to continue overnight into Tuesday and was not complete by midday.

But early on Tuesday, it was evident ULA would need more time to determine whether to replace the valve, so it announced it would not try for the 24-hour turnaround, meaning no Tuesday launch attempt was possible.

Already on the calendar were possible attempts on Friday and Saturday based on orbital dynamics that would let Starliner meet up with the International Space Station at the right time after launch.

So for now, the Eastern Range has set aside Friday at 9 p.m., as the next possible launch.

But that would be taken off the board if ULA determines it has to replace the valve. ULA would have to roll the Atlas V back from the pad to the nearby Vertical Integration Facility to do that. The rocket would then be “stretched” to allow access to the valve, but would not need to be destacked.

“We have spare valves. We know how to do it. We’ve done it before, but it would take several days,” Bruno said.

That would mean at least Sunday before the rocket would be ready again. And NASA and Boeing’s windows of opportunities would then push into the middle of next week, likely no earlier than late May 14 or early May 15, although NASA has not announced exact windows yet.

The good news for NASA is that the normally busy ISS has some time with no new cargo or crew missions on tap,

“We’re not in a rush to fly from a station standpoint,” said NASA ISS manager Dana Weigel. “We did clear our summer schedule intentionally to give us plenty of runway for the CFT mission. Our next docking vehicle comes up in August so plenty of time.”

When Starliner does launch, it would mark the final required qualification flight for the spacecraft to be used for regular missions to the ISS sharing duties with SpaceX’s Crew Dragon.

Wilmore and Williams will test manual operations of the spacecraft on approach to the ISS before an eight-day stay on board.

After that they will depart and test out more manual operations on the return trip with an eventual landing in the desert in the western United States.

If all goes well, Boeing could be in line to fly its first regular mission, Starliner-1, as early as February 2025, the first of six contracted flights to the ISS that would fly once a year through 2030 after which NASA plans to decommission the station.