WASHINGTON — NASA and Boeing will fire thrusters on the CST-100 Starliner spacecraft docked to the International Space Station this weekend in what may be a final test before approving the spacecraft’s delayed return to Earth.
At a July 25 briefing, NASA and Boeing managers said they have not set a date for the undocking of the Starliner from the station, returning NASA astronauts Butch Wilmore and Suni Williams back to Earth to conclude the Crew Flight Test (CFT) mission. The spacecraft has been at the station since June 6 for what was originally billed as a stay of as short as eight days.
The focus of testing has been on the spacecraft’s reaction control system (RCS) thrusters, several of which were shut down by the spacecraft’s computer during Starliner’s approach to the station. NASA and Boeing recently concluded testing of an RCS thruster on the ground at NASA’s White Sands Test Facility to try and replicate the degradation in the thruster’s performance.
Steve Stich, NASA commercial crew program manager, said the working hypothesis was that higher temperatures in the “doghouses” that host the thrusters may have caused oxidizer to vaporize and reduce the thrust produced. He said engineers ran two “uphill” profiles of thruster firings as Starliner approaches the station and five “downhill” profiles as it departs the station to return to Earth.
“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. We’re seeing reduced thrust,” he said. Analysis of the thruster showed a bulge in a Teflon seal on the oxidizer line, reducing the flow of oxidizer and perhaps explaining the reduced thrust. He said additional work is planned to confirm that the seal can remain intact for the rest of the flight.
Managers also agreed to fire the RCS thrusters on Starliner while still docked to the station. All but one of the thrusters will fire a series of pulses to confirm its performance. The excluded thruster is one that showed very low thrust while on approach to the station.
“We’re going to fire all those thrusters for a number of pulses just to make sure before we undock that whole system performs the way we expected and the way it did last time we checked it,” Stich said.
“With all these test results, we determined that we could run one more set of tests while we’re docked, and is really icing on the cake for us,” said Mark Nappi, Boeing vice president and commercial crew program manager. He added that Boeing will probably do such thruster tests on future Starliner missions to the station, stating that he was unaware that other visiting vehicles perform similar thruster tests prior to their departures.
Engineers will also check the spacecraft’s helium manifolds for any changes in leak rates during the thruster tests. Stich and Nappi said that examination of a Starliner service module, fueled but stored for three years at White Sands, shows signs of degradation of seals in helium systems, likely caused by exposure to nitrogen tetroxide propellant. That could explain the problems with the seals that caused the helium leaks.
Only after the upcoming thruster tests will NASA embark on a final series of reviews, including at the agency level, to confirm that Starliner is safe to return home and set a date. “I think we’re starting to close in on those final pieces of the flight rationale to make sure that we can come home safely,” Stich said, with that agency-level review planned for as soon as late next week.
He added one change in the Starliner mission profile for its return to Earth is to drop plans to perform a set of manual maneuvers after undocking. “We realize now that some of the manual maneuvering put some extra stress on the thrusters, and so we’re not going to do that.”
While NASA has not set a date for Starliner’s return, Stich said they are looking at options through Aug. 19 or 20 for its undocking. That would free up a docking port for the launch of Crew-9 mission on a SpaceX Crew Dragon, allowing for a “direct” handover, or overlap, with Crew-8 before it departs. He said that handover needs to be done by about Sept. 11 to avoid conflicting with a separate Soyuz spacecraft handover.
NASA, he added, continues to look at contingency options that could involve the use of Crew Dragon to bring back Williams and Wilmore from the station rather than Starliner, but that currently he expects them to return on Starliner. “There are a lot of good reasons to complete the mission and bring Butch and Suni home on Starliner,” he said. “Our prime intent is to return with Butch and Suni.”
“I’m very confident we have a good vehicle to bring the crew back with,” Nappi added.
