The astronauts, who departed for the International Space Station last month aboard Boeing’s Starliner spacecraft, don’t yet know when they’ll return to Earth.
Astronauts Butch Wilmore and Suni Williams served as ground engineers troubleshooting problems with the Starliner’s propulsion system and spent 51 days in space, six weeks longer than originally planned.
The problem was twofold: the spacecraft’s reaction control thrusters overheated, causing some of them to shut down as Starliner approached the space station on June 6, and there was also a separate, and possibly related, problem with a helium leak in the spacecraft’s propulsion system.
On Thursday, NASA and Boeing officials said the plan to return Wilmore and Williams aboard the Starliner spacecraft remains unchanged. In recent weeks, ground teams have completed thruster testing on a test stand at White Sands, New Mexico. This weekend, Boeing and NASA plan to fire the spacecraft’s thrusters in orbit and confirm their performance while docked at the space station.
“I think we’re getting close to finalizing the flight grounds to make sure we can return safely, that’s what we’re focused on right now,” Stich said.
These issues have led to speculation that NASA might return Wilmore and Williams to Earth on SpaceX’s Crew Dragon spacecraft. One Crew Dragon is currently docked at the station, and another is scheduled to launch next month with a new crew. Steve Stich, NASA’s commercial crew program manager, said the agency is considering alternatives to returning the Starliner crew on a SpaceX capsule, but its primary focus remains on returning astronauts on Starliner.
“Our first option is to complete the mission,” Stitch said. “There are many reasons to complete this mission and bring Butch and Suni home aboard the Starliner. The Starliner was designed as a spacecraft, with a crew in the cockpit.”
Starliner launched on June 5 from Cape Canaveral Space Force Station in Florida. Wilmore and Williams are the first astronauts to fly in space aboard Boeing’s commercial spacecraft, and the test flight is intended to pave the way for future operational flights flying rotating four-person crew members to and from the International Space Station.
If NASA fully certifies Starliner for operational missions, the agency will have two crewed spacecraft available to fly to the station. SpaceX’s Crew Dragon has been flying astronauts since 2020.
Test, test and some more testing
NASA is extending Starliner’s test flights to conduct tests and analyze data to gain confidence in the spacecraft’s ability to return crew members safely and to better understand the root causes of the overheated thrusters and helium leaks. The issues arose inside Starliner’s Service Module, which detaches during re-entry and burns up in the atmosphere while the reusable crew module carrying the astronauts parachutes to land on airbag cushions.
Chief among these tests was a series of test firings of Starliner thrusters on the ground. The thrusters were taken from the set of hardware that will fly on future Starlink missions, and engineers stress tested them, firing them multiple times to replicate the sequences of pulses that would occur during flight. The tests simulated two flight sequences to the space station, and five sequences that the thrusters would perform during the deorbit burn for undocking and returning to Earth.
“This thruster experienced more pulses than we would expect during flight. It was a more intense pulse in terms of two uphills and five downhills,” Stich said. “What we’ve actually seen with the thruster is the same type of thrust reduction that we’re seeing on orbit. Many of Starliner’s thrusters have seen thrust reductions, which is significant.”
Starliner’s flight computers shut down five of the spacecraft’s 28 reaction control system thrusters, made by Aerojet Rocketdyne, during a rendezvous with the space station last month. Four of the five were retrieved after overheating and losing thrust, but officials declared one thruster out of commission.
Thrusters tested on the ground exhibited similar behavior. Inspection of the thrusters at White Sands revealed bulging Teflon seals in the oxidizer valves, potentially restricting the flow of nitrogen tetroxide propellant. The thrusters produce approximately 85 pounds of thrust each, consuming nitrogen tetroxide (NTO) oxidizer, which is mixed with hydrazine fuel and burned.
The poppet valve is similar to a tire inflation valve and is designed to open and close to allow nitrogen tetroxide to flow into the thruster.
“There’s a Teflon seal on the end of that poppet,” Nappi said, “and the heat and natural vacuum created by the thrusters firing caused that poppet seal to deform and actually bulge a little bit.”
Stich said engineers are evaluating the integrity of the Teflon seals to determine whether they will remain intact through the undocking and deorbit burn of the Starliner spacecraft. The thrusters will not be needed while the Starliner is docked to the space station.
“Would the seal have survived the rest of the flight? That’s the important part,” Stich said.
