Astronauts who rode Boeing’s Starliner to the International Space Station last month still don’t know when they’ll return to Earth.
Astronauts Butch Wilmore and Suni Williams were in space for 51 days, six weeks longer than originally planned, as engineers on the ground to troubleshoot problems with the Starliner’s propulsion system.
The problems are twofold. The thrusters that control the spacecraft’s reaction time overheated, and some of them stopped working as the spacecraft approached the International Space Station on June 6. A separate, but possibly related, problem is a helium leak in the spacecraft’s propulsion system.
NASA and Boeing managers said Thursday they still plan to bring Wilmore and Williams home aboard the Starliner spacecraft. In the past few weeks, ground teams have completed thruster testing on a test stand in White Sands, New Mexico. This weekend, Boeing and NASA plan to fire the spacecraft’s thrusters into orbit to check their performance during a docking with the space station.
“I think we’re starting to get closer to those final pieces of the justification for the flight to make sure we can get home safely, and that’s our primary focus right now,” Stitch said.
The problems have led to speculation that NASA may decide to return Wilmore and Williams to Earth in SpaceX’s Crew Dragon spacecraft. One Crew Dragon is currently docked at the station, and another with a new crew is scheduled to launch next month. Steve Stich, NASA’s commercial crew program manager, said the agency has considered backup plans to bring the Starliner crew home aboard the SpaceX capsule, but the main focus remains on getting the astronauts home aboard the Starliner.
“Our primary choice is to complete the mission,” Stitch said. “There are many good reasons to complete this mission and bring Butch and Sonny home on Starliner. Starliner is designed as a spacecraft with the crew in the cockpit.”
The Starliner spacecraft launched from Cape Canaveral Space Station in Florida on June 5. Wilmohr and Williams are the first astronauts to fly into space aboard a Boeing commercial crew capsule, and the test flight is intended to pave the way for future operational flights to rotate four-person crews to and from the International Space Station.
Once NASA fully certifies Starliner for operational missions, the agency will have two spacecraft qualified to carry humans to the station. SpaceX’s Crew Dragon has been carrying astronauts since 2020.
Tests, tests and more tests.
NASA has extended the Starliner test flight to conduct tests and analyze data in an effort to gain confidence in the spacecraft’s ability to safely return its crew home and better understand the root causes of the engine overheating and helium leak. These issues are inside the 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 air-cushioned landing.
The most important of these tests was a series of ground-fire tests of the Starliner rocket. The rocket 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 multiple times to replicate the pulse sequences it will see in flight. The test simulated two flight sequences to the space station, and five sequences the rocket will execute during separation and deorbit burn to return to Earth.
“This thruster has seen a lot of pulses, probably more than we would have expected to see in flight, and more aggressive in terms of two ups and five downs,” Stitch said. “What we’ve seen in the thruster is the same type of thrust degradation that we see in orbit. In a number of thrusters (on Starliner), we see a reduction in thrust, which is significant.”
Starliner’s flight computers shut down five of its 28 reaction control system thrusters, made by Aerojet Rocketdyne, during a rendezvous with the International Space Station last month. Four of the five engines were recovered after overheating and losing thrust, but officials declared one unusable.
Propellants tested on Earth have shown similar behavior. Inspections of the White Sands propellants showed a bulge in the Teflon seal in an oxidizer valve, which could restrict the flow of nitrogen tetroxide fuel. The propellants, which each generate about 85 pounds of thrust, consume nitrogen tetroxide, or NTO, and mix it with hydrazine fuel for combustion.
The poppet valve, which is similar to the inflation valve on a tire, is designed to open and close to allow nitrogen tetroxide to flow into the propellant.
“This glove has a Teflon seal on the end of it,” Nabi said. “Because of the heating and the natural vacuum that occurs with the firing of the propellant, this glove deformed and bulged a little.”
Stitch said engineers are evaluating the integrity of the Teflon seal to determine whether it can remain intact during the Starliner spacecraft’s deorbiting. No thrusters are needed while the Starliner is attached to the space station.
“Can this seal survive the rest of the journey? That’s the important part,” Stitch said.
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