The astronauts who rode Boeing's Starliner spacecraft to the International Space Station last month 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 on the groundwork through problems with Starliner's propulsion system.
The problems are twofold. The spacecraft's reaction control thrusters overheated, and some of them shut off as Starliner approached the space station June 6. A separate, although perhaps related, problem involves helium leaks in the craft's propulsion system.
On Thursday, NASA and Boeing managers said they still plan to bring Wilmore and Williams home on the Starliner spacecraft. In the last few weeks, ground teams completed testing of a thruster on a test stand at White Sands, New Mexico. This weekend, Boeing and NASA plan to fire the spacecraft's thrusters in orbit to check their performance while docked at the space station.
“I think we’re starting to close in on those final pieces of flight rationale to make sure that we can come home safely, and that’s our primary focus right now," Stich said.
The problems have led to speculation that NASA might decide to return Wilmore and Williams to Earth in a SpaceX Crew Dragon spacecraft. There's one Crew Dragon currently docked at the station, and another one is slated to launch with a fresh crew next month. Steve Stich, manager of NASA's commercial crew program, said the agency has looked at backup plans to bring the Starliner crew home on a SpaceX capsule, but the main focus is still to have the astronauts fly home aboard Starliner.
“Our prime 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 was designed, as a spacecraft, to have the crew in the cockpit."
Starliner launched from Cape Canaveral Space Force Station in Florida on June 5. Wilmore and Williams are the first astronauts to fly into space on Boeing's commercial crew capsule, and this test flight is intended to pave the way for future operational flights to rotate crews of four to and from the International Space Station.
Once NASA fully certifies Starliner for operational missions, the agency will have two human-rated spaceships for flights to the station. SpaceX's Crew Dragon has been flying astronauts since 2020.
Tests, tests, and more tests
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 the overheating thrusters and helium leaks. These problems are inside Starliner's service module, which is jettisoned to burn up in the atmosphere during reentry, while the reusable crew module, with the astronauts inside, parachutes to an airbag-cushioned landing.
The most important of these tests was a series of test-firings of a Starliner thruster on the ground. This thruster was taken from a set of hardware slated to fly on a future Starlink mission, and engineers put it through a stress test, firing it numerous times to replicate the sequence of pulses it would see in flight. The testing simulated two sequences of flying up to the space station, and five sequences the thruster would execute during undocking and a deorbit burn for return to Earth.
"This thruster has seen quite a bit of pulses, maybe even more than what we would anticipate we would see during a flight, and more aggressive in terms of two uphills and five downhills," Stich said. “What we did see in the thruster is the same kind of thrust degradation that we're seeing on orbit. In a number of the thrusters (on Starliner), we're seeing reduced thrust, which is important.”
Starliner's flight computer shut off five of the spacecraft's 28 reaction control system thrusters, produced by Aerojet Rocketdyne, during the rendezvous with the space station last month. Four of the five thrusters were recovered after overheating and losing thrust, but officials have declared one of the thrusters unusable.
The thruster tested on the ground showed similar behavior. Inspections of the thruster at White Sands showed bulging in a Teflon seal in an oxidizer valve, which could restrict the flow of nitrogen tetroxide propellant. The thrusters, each generating about 85 pounds of thrust, consume the nitrogen tetroxide, or NTO, oxidizer and mix it with hydrazine fuel for combustion.
A poppet valve, similar to an inflation valve on a tire, is designed to open and close to allow nitrogen tetroxide to flow into the thruster.
"That poppet has a Teflon seal at the end of it," Nappi said. "Through the heating and natural vacuum that occurs with the thruster firing, that poppet seal was deformed and actually bulged out a little bit."
Stich said engineers are evaluating the integrity of the Teflon seal to determine if it could remain intact through the undocking and deorbit burn of the Starliner spacecraft. The thrusters aren't needed while Starliner is attached to the space station.
"Could that particular seal survive the rest of the flight? That's the important part," Stich said.