NASA and Boeing plan for what might be the last thruster tests on the CST-100 Starliner spaceplane, as the spacecraft just seeks its postponed re-entry to Earth returning NASA astronauts Butch Wilmore and Suni Williams, who are still up in space and anxiously awaiting their return.
In a July 29 briefing, NASA and Boeing managers revealed that the undocking of Starliner is now planned only after a decision is verified by those companies’ teams, who will be continuing to analyze the vehicle’s performance earlier in the mission. That long mission duration accounts for why the spacecraft was to remain on the station for just eight days from its arrival on June 6. NASA reports that an extension of the mission will address other issues with the spacecraft’s Reaction Control System thrusters, which performed poorly during the station rendezvous.
Steve Stich, NASA’s commercial crew program manager, explained that higher temperatures in the thruster housings might have caused the oxidizer to vaporize, reducing thrust. Ground tests at NASA’s White Sands Test Facility confirmed similar thrust degradation, attributed to a bulge in a Teflon seal on the oxidizer line. Engineers are now working to ensure the seal remains intact for the remainder of the mission.
These next tests involve the firing of all but one of these RCS thrusters with Starliner still attached to the ISS, in a double-verify performance, before undocking. Boeing Vice President and the Manager of the Commercial Crew Program Mark Nappi made sure to note that such tests reinforced steps to ensure the spacecraft is ready before tight-loops closure.
Besides the thruster tests, the spacecraft’s helium manifolds will be inspected by engineers for changes in leak rates. Previous inspections of one retired Starliner service module in storage at White Sands showed seal degradation in helium systems likely from nitrogen tetroxide propellant exposure, which may have contributed to the helium leaks detected during the mission.
NASA will do one more round of reviews, including one at the agency level, to approve the return of Starliner. Stich said that the review could take place as soon as late next week. A change of that mission profile will call for no manual maneuvering after undocking, to minimize the loads on thrusters.