Boeing’s Starliner is set to complete its Crew Flight Test without its crew with an undocking planned for Friday evening from the International Space Station and a trip back to Earth.

The spacecraft launched from Cape Canaveral atop a United Launch Alliance Atlas V back on June 5 with NASA astronauts Butch Wilmore and Suni Williams docking with the ISS the next day for what was supposed to be an eight-day visit.

Issues with failing thrusters and helium leaks on Starliner’s propulsion module, though, led to a series of delays and eventual call by NASA to keep its two astronauts safe on board the ISS and send Starliner home without them.

Undocking is slated for 6:04 p.m. after which it will have about a six-hour trip back and aim for a desert landing at White Sands Space Harbor, New Mexico at 12:03 a.m. Saturday. Its deorbit burn for the final run home would come at 11:17 p.m. with NASA coverage available on its social channels. Backup opportunities fall about every four days.

“I’m personally looking forward to getting Starliner back,” said NASA Commercial Crew Program Manager Steve Stich on Wednesday. “We’ve learned a lot on this test flight. We’ll continue to learn more I’m sure through the undock and deorbit phase. We have a lot of flight test objectives still ahead of us on the flight, and we’re looking forward to getting those. The teams are ready. … It’s been a journey to get here.”

Wilmore and Williams finished packing Starliner with cargo, including their unneeded Boeing spacesuits, on Thursday and closed the hatch for the final time before its departure. The duo will remain on board no longer part of the CFT crew, but officially joining the ISS Expedition 71 crew.

They won’t fly home until next February as they await the arrival of their rescue ride in the form the SpaceX Crew Dragon Freedom flying on the Crew-9 mission slated to launch from Cape Canaveral as early as Sept. 24. It will be flying up with only two instead of four passengers — NASA astronaut Nick Hague and Roscomos cosmonaut Aleksandr Gorbunov — to make room for Wilmore and Williams on the way home.

Despite Starliner leaving without them, NASA had been OK for Starliner to be used as the duo’s emergency ride home during their last three months on board. Before Crew-9’s arrival, in case of emergency, they would join the four crew of Crew-8 on their Crew Dragon.

“They were never really stuck or stranded,” Stich said. “They always had a way to depart the space station. And to me, when somebody is stranded, there’s a location where they cannot leave. And so they had Starliner as the vehicle that they could depart from for a period of time. Now the Crew-8 is their emergency return vehicle, and when Crew-9 gets there, that will be their return vehicle.”

Starliner’s departure allows for its arrival as the ISS only has two docking ports for the likes of the SpaceX and Boeing spacecraft at a time. The Crew Dragon Endeavour used for Crew-8 remains docked and that spacecraft’s quartet of passengers that have been on board since March won’t fly home until Crew-9’s arrival.

For NASA and Boeing, the mission is not a storybook ending and the future of Starliner depends on whether or not NASA will certify the spacecraft for Boeing’s six contracted operational flights as part of the Commercial Crew Program contract it was awarded alongside SpaceX back in 2014.

The trip home will include some more hot fire tests of some of the 28 problematic reaction control system thrusters. Five had shut down during the trip up, but four of those have come back online. Testing over two months including ground tests to recreate heat conditions, showed the likely reason for the thruster issues, as well as the likely reason for the helium leaks, but nothing could have been done with Starliner in space to fix the issues.

Starliner’s propulsion module will be discarded before the spacecraft’s deorbit and burn up in the atmosphere.

As an uncrewed departure, NASA will perform a more powerful thruster breakout burns soon after undock to get up and away from the ISS, which is different than the slow separation that would have been done with crew on board. That may relieve some of the heat-related thruster issues seen on arrival, though, with less frequent thruster use allowing for cooling.

“It really puts less stress on the thrusters,” Stich said. “There’s a lot fewer thruster firings, essentially about 30 seconds. After undock, we’ll start this small series of burns using primarily the forward thrusters, not putting the stress on the aft thrusters. And it really just takes about five minutes or so to actually execute that whole sequence. … So it’s a quicker way away from station, way less stress on the thrusters.”

He said he doesn’t expect any issues near the station, but heat stresses could still be seen for the 60-second deorbit burn.

“That’s the critical thing that needs to happen to get the crew module on a safe trajectory, to enter into the atmosphere and then land at White Sands,” Stich said.

Once the capsule has landed, it will go through months of post-flight analysis of the trajectory and how the thrusters performed, Stich said.

“We’re also already working hand-in-hand with Boeing to look at modifications to the system. Is there ways that we can fly the vehicle differently? Do we need to do some more thruster testing out at White Sands to fill in some of the gaps that we have, perhaps, in qualification,” Stich said. “So all that’s going to start taking place. And there’s teams starting to look at what we do to get the vehicle fully certified in the future.”

Stich said the helium leak issues might be fixed by changing material or increasing the size on a seal they think was being degraded because of exposure to oxidizer propellants. And the thruster failures were likely due to overheating that caused a Teflon “poppet” to swell and block the flow of propellant. The thrusters enclosure within what Stich refers to as “doghouses” may also contribute to overheating.

“Clearly, the way we fire the thrusters causes the thruster to overheat,” Stich said. “So we need to go understand what kinds of pulses in particular cause that swelling, the number of pulses, the pulse train itself, and what temperatures it gets to on the thruster itself. And then, how do we then tell the vehicle not to fire the thruster in that manner that caused the extra heating?”

Essentially, NASA and Boeing will see if adjusting how the spacecraft uses its thrusters may be enough to avoid the same issues it saw on this flight as opposed to replacing the thruster design.

“We haven’t really rolled anything out yet, but an easier fix would be to not change the thruster design itself, but to try to understand the kinds of pulses we can get into where once you pulse it a number of times, after it stops pulsing, then the heat will soak back into the thruster and cause the poppet to swell,” Stich said. “So the easiest thing to do is figure out, how do we lower the temperature the thrusters operating at and maybe not firing it in a manner that causes it to have this overheating phenomenon.”

This is based on all the test seen both on the land at White Sands and what has been seen during flight.

“Many parts of the flight went extremely well. And Starliner is a is a great spacecraft. We know that it performs well,” he said. “What we really need to go do is look at the things that didn’t perform the way we expected.”

He said the thrusters remain a good component.

“We know the thrusters are working well when we don’t command them in a manner that overheats them,” he said. “Many of the thrusters, we never saw any degradation at all. So we know that the thruster is a viable thruster. … We just have to go look system by system as we move forward toward certification, hand-in-hand with the Boeing team. And we have a process that we understand very well, and we’ll start to work on that as soon as the vehicle comes back.”

Reports of agitation between NASA and Boeing leading up to the decision was addressed by Stich.

“I would say the last meeting we had where we talked detailed technical data on the different positions and the different analyzes, I would not characterize it as heated. I would say, anytime you’re in a meeting of this magnitude where there’s this kind of decision, there is some tension in the room,” he said.

Boeing pushed for the return of the astronauts relying on their modeling that predicted thruster degradation, but Stich said NASA saw limitations to that modeling.

“And the NASA team, due to the uncertainty in the modeling, could not get comfortable with that,” he said. “So I wouldn’t say it was a yelling, screaming kind of meeting. It was a tense technical discussion where we had both sides listening intently to all the data and in the end to make a decision whether to return crewed or uncrewed, and NASA chose to allow Butch and Suni to stay on station.”

Boeing has not made comment since NASA’s decision to return Starliner home without crew, but NASA Administrator Bill Nelson said he spoke with new Boeing CEO Kelly Ortberg, who told him the company intended to continue with the program.

The original fixed-price contract of $4.2 billion, which has since risen to a max value of $4.6 billion, was the larger of two awards going to the long-time NASA mainstay Boeing. Only $2.6 billion was awarded to the relatively new SpaceX. The contract called for both providers to fly an uncrewed test mission to the ISS followed by a crewed demonstration followed by six flights each to ferry astronauts to and from the ISS and help end the U.S. reliance on Russian Soyuz launches to staff the station.

SpaceX accomplished their crewed mission in 2020 and have since flown its fleet of four Crew Dragons 13 times taking 50 humans into space. That includes all six of the first contract’s missions as well as two more since that have launched and at least two more on tap including Crew-9 and next February’s Crew-10 mission.

Boeing, though, was not able to rendezvous with the ISS on its uncrewed test flight in 2019 during a mission plagued with issues that led to NASA labeling it a “high visibility close call” that led to an independent review citing 80 things NASA and Boeing needed to change before moving forward. Boeing took 2 1/2 years to get to a reflight of the uncrewed spacecraft, but finally docked with the ISS in May 2022.

More issues popped up, though, leading up to what was supposed to be the CFT mission one year later forcing further delays into 2024 so that its launch with Wilmore and Williams came more than four years after SpaceX’s successful Dragon flight.

To date, Boeing has reported nearly $1.6 billion in cost overruns because of the issues during Starliner’s development, as NASA’s contracts had put the onus for delays on SpaceX and Boeing. That includes the redo of the uncrewed test flight that Boeing said came at no cost to NASA.

NASA, though, still has paid out to Boeing about $2.7 billion of the $4.6 billion awarded, meaning Boeing has nearly another $2 billion it could earn if it gets certified and actually flies its contracted missions.

SpaceX, meanwhile, was awarded in 2022 a new contract for three additional flights beyond its first six for $776 million, and then won an additional contract that year worth $1.44 billion for Crew-10 through Crew-14. That makes SpaceX’s take just shy of $5 billion since the original 2014 award.

From a per-seat price point, SpaceX has contracts to fly 56 passengers across 14 flights to the ISS, which breaks down to $88 million per passenger. Boeing would fly 24 across six flights, which breaks down to about $200,000 per passenger. Russian Soyuz flights had climbed to more than $90 million by the end of NASA’s reliance on its ISS partner, which was the sole provider for crew flights to the ISS from the end of the space shuttle program in 2011 until SpaceX’s success in 2020.

NASA and Roscosmos now simply trade off seats with cosmonauts flying normally on SpaceX flights including Crew-9 and astronauts flying up as one of the three on board Soyuz, including the planned arrival next week of the Soyuz MS-26 with NASA’s Don Pettit and cosmonauts Alexey Ovchinin and Ivan Wagner.

NASA hopes to keep the ISS up and running through 2030, and the six Boeing flights and 14 from SpaceX should be enough to finish out the station’s needs.

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