The astronauts who traveled to the International Space Station last month aboard Boeing's Starliner spacecraft still don't know when they will return to Earth.
Astronauts Butch Wilmore and Suni Williams have been in space for 51 days, six weeks longer than originally planned, as engineers troubleshoot Starliner's propulsion system.
The problems are twofold. The spacecraft’s reaction control thrusters overheated and some failed as Starliner approached the space station on June 6. A separate, though possibly related, problem involves helium leaks in the spacecraft’s propulsion system.
On Thursday, NASA and Boeing managers said they still plan to bring Wilmore and Williams home on the Starliner spacecraft. In recent weeks, ground crews have completed tests of a booster on a test stand in White Sands, New Mexico. This weekend, Boeing and NASA plan to fire the spacecraft's booster into orbit to test its performance while docked to the space station.
“I think we're getting into the final details of the flight logic to make sure we can get home safely, and that's our primary focus right now,” Stich said.
The problems have led to speculation that NASA may decide to return Wilmore and Williams to Earth in a SpaceX Crew Dragon spacecraft. There is currently one Crew Dragon docked at the space station, and another is scheduled to launch next month with a fresh crew. Steve Stich, manager of NASA's commercial crew program, said the agency has looked at backup plans to bring the Starliner crew home in a SpaceX capsule, but the main focus remains on flying the astronauts home aboard Starliner.
“Our first option is to complete the mission,” Stich said. “There are a lot of good reasons to complete this mission and bring Butch and Suni home on Starliner. Starliner is designed, as a spacecraft, to have the crew in the cockpit.”
Starliner launched June 5 from Cape Canaveral Space Force Station in Florida. Wilmore and Williams are the first astronauts to fly into space in Boeing’s commercial crew capsule. The test flight is intended to pave the way for future operational flights in which crews of four rotate to and from the International Space Station.
Once NASA fully certifies Starliner for operational missions, the agency will have two human-rated spacecraft for flights to the station. SpaceX's Crew Dragon has been flying astronauts since 2020.
Testing, testing and more testing
NASA has extended the duration of the Starliner test flight to conduct tests and analyze data in an effort to gain confidence in the spacecraft's ability to safely bring its crew home and to better understand the root causes of overheating thrusters and helium leaks. These problems are in the Starliner's service module, which is being jettisoned to burn up in the atmosphere during reentry, while the reusable crew module, carrying the astronauts, parachutes to a landing with airbag cushions.
Chief among these tests was a series of test firings of a Starliner thruster on the ground. This thruster was from a set of hardware planned to fly on a future Starlink mission, and engineers stress-tested it, firing it multiple times to replicate the series of pulses it would see during flight. The test simulated two sequences of flying to the space station, and five sequences that the thruster would perform during undocking and a deorbit burn before returning to Earth.
“This thruster has seen quite a bit of pulsing, maybe even more than we would expect in flight, and more aggressive in terms of two hills up and five hills down,” Stich said. “What we saw in the thruster is the same kind of thrust reduction that we see in space. In a number of the thrusters (on Starliner) we see reduced thrust, which is significant.”
Starliner's flight computer disabled five of the spacecraft's 28 reaction control system thrusters, manufactured by Aerojet Rocketdyne, during its rendezvous with the space station last month. Four of the five thrusters were recovered after overheating and losing thrust, but officials said one of the thrusters was inoperable.
The thruster tested on the ground exhibited similar behavior. Inspections of the thruster at White Sands revealed a bulge in a Teflon seal in an oxidizer valve, which could restrict the flow of nitrogen tetroxide propellant. The thrusters, which generate about 85 pounds of thrust each, consume the nitrogen tetroxide oxidizer, or NTO, and mix it with hydrazine fuel for combustion.
A valve with a flap mechanism, similar to the valve on a tire, is designed to open and close to allow nitrogen tetroxide to flow into the booster.
“That poppet has a Teflon seal on the end,” Nappi said. “With the heat and the natural vacuum created by firing the thruster, that poppet seal became distorted and actually bulged out a little bit.”
Stich said engineers are evaluating the integrity of the Teflon seal to determine if it can remain intact during the Starliner spacecraft's undocking and deorbit burn. The thrusters are not needed while Starliner is attached to the space station.
“Can that particular seal survive the rest of the flight? That's the main thing,” Stich said.
