Astronauts who traveled aboard Boeing’s Starliner spacecraft to the International Space Station still don’t know when they will return to Earth.
Astronauts Butch Wilmore and Suni Williams, who were engineers on the base due to problems with the Starliner’s propulsion system, stayed in space six weeks, 51 days longer than originally planned.
The problems are twofold. As Starliner approached the space station on June 6, the spacecraft’s reaction control thrusters overheated, and some of them shut down. A separate, though perhaps related, craft involved helium leaks in the propulsion system.
On Thursday, NASA and Boeing managers said they still plan to bring Wilmore and Williams home aboard the Starliner spacecraft. Over the past few weeks, field teams have completed testing of a thruster at a test stand in White Sands, New Mexico. This weekend, Boeing and NASA plan to fire the spacecraft’s thrusters into orbit to test their performance while docked at the space station.
“I think we’re starting to close the final pieces of flight logic to make sure we can come home safely, and that’s our primary focus right now,” Stich said.
These problems have led to speculation that NASA may decide to return Wilmore and Williams to Earth aboard the SpaceX Crew Dragon spacecraft. There is currently one Crew Dragon on the station, and another is due to launch next month with a new crew. Steve Stich, manager of NASA’s Commercial Crew program, said the agency has looked at backup plans to bring the Starliner crew home aboard the SpaceX capsule, but the main focus is to fly the astronauts home on the Starliner.
“Our primary desire is to get the job done,” Stich said. “There are many reasons to complete this mission and bring Butch and Suni home on the Starliner. The Starliner is a spaceship designed to have a crew in the cockpit.”
Starliner was launched from the Cape Canaveral Space Force Station in Florida on June 5. Wilmore and Williams were the first astronauts to go into space aboard Boeing’s commercial crew capsule, and the test flight was intended to pave the way for future operational flights with four-man crews. Go to the International Space Station.
Once NASA fully certifies the Starliner for operational missions, the agency will have two manned shuttles to the airport. 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 to gain confidence in the spacecraft’s ability to bring its crew home safely and to better understand the root causes of overheating thrusters and helium leaks. These issues are contained within the Starliner’s service module, which is prevented from burning up in the atmosphere during re-entry, while the reusable crew module parachutes to an airbag-cushioned landing, with the astronauts inside.
The most important of these tests was the continuous test firing of a Starliner thruster on the ground. The thruster was taken from a set of hardware planned to fly on the future Starlink mission, and engineers put it through a stress test, firing it several times to mimic the sequence of pulses seen in flight. The experiment simulated two scenarios of flying up to the space station, and five scenarios of propulsion during propulsion and a detonation burn to return to Earth.
“This thruster has seen quite a few pulses, maybe more than we would expect during a flight, and is very aggressive in terms of two upstrokes and five downstrokes,” Stich said. “What we saw in the thruster is the same kind of thrust decay that we see in orbit. In many thrusters (on Starliner), we see reduced thrust, which is important.”
During a rendezvous with the space station last month, Starliner’s flight computer shut down five of the spacecraft’s 28 reaction control system thrusters, made by Aerojet Rockettine. Four of the five thrusters were recovered after overheating and losing thrust, but officials have declared one thruster unusable.
The thrust tested on the ground showed similar behavior. Inspections of the thruster at White Sands showed a bulge in the Teflon seal on an oxidizer valve, which would restrict the flow of nitrogen tetroxide propellant. The thrusters, each producing about 85 pounds of thrust, ingest nitrogen tetroxide, or NTO, oxygen and mix it with hydrazine fuel to burn it.
A poppet valve, similar to the inflation valve in a tire, 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,” Nubby said. “Through the heating and the natural vacuum caused by the thruster firing, that poppet seal was ruptured and actually popped out a little bit.”
Engineers assessed the integrity of the Teflon seal through the Starliner spacecraft’s undocking and orbital burn to see if it could remain intact, Stich said. Thrusters are not needed when the Starliner is docked with the space station.
“Can that particular seal survive on other parts of the plane? That’s the important part,” Stich said.
