NASA update on Starliner thruster issues: This is fine

Before clearing Boeing's Starliner crew capsule to depart the International Space Station and head for Earth, NASA managers want to ensure the spacecraft's problematic control thrusters can help guide the ship's two-person crew home.

The two astronauts who launched June 5 on the Starliner spacecraft's first crew test flight agree with the managers, although they said Wednesday that they're comfortable with flying the capsule back to Earth if there's any emergency that might require evacuation of the space station.

NASA astronauts Butch Wilmore and Suni Williams were supposed to return to Earth weeks ago, but managers are keeping them at the station as engineers continue probing thruster problems and helium leaks that have plagued the mission since its launch.

“This is a tough business that we’re in," Wilmore, Starliner's commander, told reporters Wednesday in a news conference from the space station. "Human spaceflight is not easy in any regime, and there have been multiple issues with any spacecraft that’s ever been designed, and that’s the nature of what we do.”

Five of the 28 reaction control system thrusters on Starliner's service module dropped offline as the spacecraft approached the space station last month. Starliner's flight software disabled the five control jets when they started overheating and losing thrust. Four of the thrusters were later recovered, although some couldn't reach their full power levels as Starliner came in for docking.

Wilmore, who took over manual control for part of Starliner's approach to the space station, said he could sense the spacecraft's handling qualities diminish as thrusters temporarily failed. "You could tell it was degraded, but still, it was impressive," he said. Starliner ultimately docked to the station in autopilot mode.

In mid-June, the Starliner astronauts hot-fired the thrusters again, and their thrust levels were closer to normal.

“What we want to know is that the thrusters can perform; if whatever their percentage of thrust is, we can put it into a package that will get us a deorbit burn," said Williams, a NASA astronaut serving as Starliner's pilot. "That's the main purpose that we need [for] the service module: to get us a good deorbit burn so that we can come back."

These small thrusters aren't necessary for the deorbit burn itself, which will use a different set of engines to slow Starliner's velocity enough for it to drop out of orbit and head for landing. But Starliner needs enough of the control jets working to maneuver into the proper orientation for the deorbit firing.

This test flight is the first time astronauts have flown in space on Boeing's Starliner spacecraft, following years of delays and setbacks. Starliner is NASA's second human-rated commercial crew capsule, and it's poised to join SpaceX's Crew Dragon in a rotation of missions ferrying astronauts to and from the space station through the rest of the decade.

But first, Boeing and NASA need to safely complete the Starliner test flight and resolve the thruster problems and helium leaks plaguing the spacecraft before moving forward with operational crew rotation missions. There's a Crew Dragon spacecraft currently docked to the station, but Steve Stich, NASA's commercial crew program manager, told reporters Wednesday that, right now, Wilmore and Williams still plan to come home on Starliner.

"The beautiful thing about the commercial crew program is that we have two vehicles, two different systems, that we could use to return crew," Stich said. "So we have a little bit more time to go through the data and then make a decision as to whether we need to do anything different. But the prime option today is to return Butch and Suni on Starliner. Right now, we don’t see any reason that wouldn’t be the case."

Mark Nappi, Boeing's Starliner program manager, said officials identified more than 30 actions to investigate five "small" helium leaks and the thruster problems on Starliner's service module. "All these items are scheduled to be completed by the end of next week," Nappi said.

"It’s a test flight, and the first with crew, and we’re just taking a little extra time to make sure that we understand everything before we commit to deorbit," Stich said.

Thrust of the problem

After the thruster trouble last month, NASA and Boeing officials decided to test a Starliner thruster at a test facility at White Sands, New Mexico, and run it through the same sequence of firings that one of the overheating thrusters saw as the spacecraft flew to the space station. Ideally, engineers would like the thruster to reach the same high operating temperature that led to its degraded performance in orbit. Then, once the test sequence is complete, engineers will physically examine the thruster to see if the overheating caused any damage.

“What we have found in this flight is we have fired the thrusters more than expected, and I would say more frequently," said Stich. "When I say frequently, I’m talking about how close you fire an individual thruster pulse to the next pulse of that thruster.

"What we’re trying to do at White Sands is really replicating exactly what those pulses were that those thrusters saw, and then understand the heating effects from those pulses, and then make sure there’s no unintended consequences of those pulses," Stich said.

Astronauts Suni Williams (left) and Butch Wilmore (right) inside a Starliner simulator at NASA's Johnson Space Center in Houston.

Credit: NASA/Robert Markowitz

Their thinking is that if the thruster overheats on the ground and engineers are able to prove it wasn't damaged from the extra thermal stress, officials will gain more confidence in the thruster system's ability to control the spacecraft after it leaves the space station. Also, it's important to better understand the performance of the thrusters for future Starliner flights to know whether it's necessary to change settings in control software, or how often the thrusters are fired to keep them from overheating.

"The way that we fly the flight, do we want to change that a little bit? Are there limits that we want to set differently in our software? And how it de-selects the thrusters. Those are the types of things that we want to learn," Nappi said.

The test sequence began July 3 with a series of firings mimicking what one of Starliner's thrusters experienced in orbit. But test engineers couldn't get the thruster on the ground as hot as the Starliner thrusters got in space, so officials have paused the tests this week to see if there's a way to better simulate the actual thermal conditions on the spacecraft. They hope to resume testing later this week and wrap it up over the weekend.

Starliner's control thrusters are located in four propulsion pods, known as doghouses, around the circumference of the service module. It turns out these doghouses act like a thermos, and their design exacerbates the overheating problem with the thrusters.

"We had a great deal of thermal kickback from the doghouse itself, so not all the heat is coming directly from the thruster or through the injector of the thruster," Nappi said. "We’re getting quite a bit of heat soak from the doghouse, so that’s good information to have as we move forward and determine what the future mission profiles are going to look like."

Engineers are also confirming their estimates of how much extra helium the spacecraft carries to pressurize its propulsion system after Starliner undocks from the space station. Managers believe there is an ample margin in the helium system to handle the several hours of flight time from undocking until Starliner jettisons its service module just before reentry.

The service module burns up in the atmosphere, so Boeing won't get it back for inspections to fully diagnose the causes of the leaks and thruster issues. Starliner's reusable crew module will parachute to a landing at one of several possible sites in the Western United States.

Still no return date

The initial plan was for Starliner to stay at the ISS for at least eight days, with the possibility of a short extension. When it launched, the spacecraft's batteries were certified for a 45-day flight, which the mission will reach on July 20. It appears all but certain Starliner will stay in orbit for more than 45 days.

Stich said NASA would like to have Starliner back on the ground by mid-August when the next SpaceX crew mission is set to launch to the space station. The SpaceX Dragon capsule will need to dock at the port currently occupied by Boeing's spacecraft. There's a possibility Starliner could be cleared for undocking by the end of July, but officials haven't made any final decisions.

"I think some of the data suggests, optimistically, maybe it’s by the end of July, but we’ll just follow the data each step at a time and then at the right time figure out when the right undock opportunity is," Stich said.

The chances of flying the first operational Starliner mission early next year, as it is currently scheduled, appear to be fading. Engineers will need time to address and resolve the helium leaks and thruster issues before the next Starliner flight.

“I think the longer we go, probably, the more risk there is to that date in February," Stich said. "But we’ll make the decision once the vehicle returns and we lay out the work.”

Nappi said Boeing engineers are considering replacing seals in the next service module to reduce the risk of helium leaks recurring on the next mission, known as Starliner-1. Boeing has six operational Starliner missions on contract with NASA.

“We have to understand these helium leaks and get that fixed for Starliner-1," Nappi said. "I would say that’s probably the largest threat to when we end up flying that Starliner-1 vehicle."