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Boeing’s Starliner aircraft will soon conduct a “hot fire” test in orbit as NASA and the company continue to investigate the cause of thruster failures and helium leaks in space.
Starliner will test-fire its 28-thruster response control system (RCS) on Saturday (July 27) or Sunday (July 28) at the International Space Station (ISS), NASA and Boeing announced at a press conference today (July 25). However, only 27 of the RCS thrusters will be used, as one was previously deemed unusable for the return flight home.
Commands will be given from the ground, but NASA astronauts Butch Wilmore and Suni Williams, who are flying in Starliner’s current Crew Flight Test (CFT) mission, will likely participate, as they are former U.S. Navy test pilots.
“We’re going to fire all of these thrusters a certain number of times, making sure that the entire system is performing as we expect and as our last checkout before we leave,” Steve Stich, NASA’s Commercial Crew Program Manager, told reporters during a live-streamed press conference.
“We’ll also have a chance to look at the helium system,” he added. “It’s been six weeks since we last checked the helium system; that was June 15th. So we’ll pressurize manifold by manifold and then hot fire the thrusters and then we’ll have a chance to look at the helium leak rates and verify that the system is stable.”
CFT was initially projected to last about 10 days after launching on June 5, but it faced thruster problems and helium leaks before docked safely with the ISS the next day, so the mission was extended indefinitely while engineers fixed the problem.
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Following ground tests conducted at the White Sands Test Facility in New Mexico in early July, engineers have a better understanding of what happens during the challenging docking.
For example, an Aerojet Rocketdyne test MR-104J thruster similar to those on the Starliner that was in storage for three years before the recent ground run suffered “severe degradation” in the valve system from nitrogen dioxide vapor; thruster flow was restricted by a bulging seal, Stich said. Engineers also found trapped Teflon in a downstream filter, indicating a seal erosion due to “bubbling” under high temperatures.
Stich noted that the behavior of the Starliner RCS thrusters in orbit matches what is observed on the ground. He told Space.com at the press conference that the thruster type was “inherited” from other spaceflight programs at Aerojet (an L3Harris company) but modified for the Starliner’s service module, which houses the RCS thrusters and propulsion system.
“I think what we’re starting to understand much better now on this mission – and we couldn’t have understood this just by staying on the ground and doing more analysis and testing – is how the guidance, navigation and control system have integrated effects. [and] “The flight control system gives the thrusters the command to fire during the rendezvous,” Stich said.
A “doghouse” around each RCS thruster assembly insulates the thrusters and protects them from the cold temperatures of space. (There are four doghouses total on the service module.) What the CFT showed was that a series of RCS thruster pulses in a short period of time, in addition to firings with the orbital maneuvering and attitude control system (OMAC), caused the RCS thruster temperatures to increase more than expected.
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Stich said the problem could be solved not by a design change, but by changing how the thrusters are used in flight (firing them less frequently, for example). He stressed that discussions are ongoing at the most senior agency levels; NASA is planning an agency-level review of the CFT as early as next week, and the agency’s Aviation Safety Advisory Panel will be invited to participate.
Boeing Commercial Crew Program Manager Mark Nappi added that Aerojet Rocketdyne has been working closely with Boeing throughout the mission. Discussions are ongoing on how to address the issue. For example, new seals could be installed. Boeing could ask future crews to fly a different profile to the ISS for the thrusters, or “put different thermal protection inside the doghouse,” Nappi said today.
Test missions like the CFT often encounter unexpected situations, and Starliner has always been rated to depart the ISS in an emergency. CFT astronauts Wilmore and Williams participated in orbital troubleshooting in June and were then reassigned to ISS maintenance duties, now 50 days (and counting).
NASA also extended the CFT’s initial spaceflight to more than 45 days, saying battery performance (which initially reported the limit) was performing better in orbit than expected. Stich said in today’s briefing that the mission is now officially rated for 90 days, or until Sept. 3 (though a landing time has not yet been finalized.)
Understanding the root causes of thruster and helium issues in the RCS, which controls maneuvers in orbit, is important for future Starliner missions. Following the CFT, the Starliner-1 mission, with at least three astronauts, is expected to spend six months on the ISS and begin as early as 2025.
Starliner is one of two private U.S. spacecraft that have sent astronauts to the ISS, along with SpaceX’s Crew Dragon. The Crew Dragon, which has been carrying crew since May 2020, is based on SpaceX’s cargo Dragon, which has been in space several times since 2012. A new spacecraft, Starliner has faced development challenges after its first spaceflight.
Starliner’s first ISS launch in December 2019, without astronauts, never reached its target due to computer issues that kept the spacecraft in the wrong orbit. A follow-up uncrewed mission met its target, but that was in May 2022 after the COVID-19 pandemic hit and dozens of fixes were implemented. (The 2022 docking also had thruster issues that NASA thought it had addressed for the CFT.)
The CFT was postponed to 2023 after flammable tape was found in Starliner’s cables and engineers discovered the parachutes could carry less payload than expected. For months, the mission appeared on track for a May 6 launch date. But hours before liftoff, astronauts were ejected from the spacecraft after a vibrating valve was discovered on the United Launch Alliance (ULA) Atlas V rocket.
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Starliner remained grounded for a month after ULA replaced the valve and discovered a small helium leak in an RCS thruster. Extensive testing and modeling on the ground found no expected problems with the CFT, but analysis did reveal a weakness: Under certain conditions, if enough RCS thrusters in adjacent kennels failed, a reentry option could be compromised. Following engineering and simulator studies, NASA and Boeing approved a new reentry mode that would require firing fewer RCS thrusters at the same time.
The CFT launched on June 5, but after encountering another late countdown error due to a ground equipment failure affecting the Atlas V. NASA and Boeing emphasized that, despite experiencing issues with docking, the Starliner’s stay in space was uneventful and the spacecraft performed well. Also, as former U.S. Navy test pilots, both Williams and Wilmore are accustomed to long-duration deployments and development schedules.
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Starliner and SpaceX received billions of dollars to carry astronauts under a contract with NASA’s Commercial Crew Program in 2014. Following a test astronaut flight that launched in May 2020, SpaceX has sent 11 crew members to the ISS so far: eight for NASA and three shorter-term missions for Axiom Space.
SpaceX missions have been paused for now due to an issue with the second stage of the Falcon 9 rocket during a Starlink satellite launch earlier this month. The Falcon 9 completed a static burn last night, but the Federal Aviation Administration (FAA) has yet to respond to SpaceX’s request to re-fly the missions on a public safety basis; SpaceX maintains that there was no risk to the public during the launch, only the rocket’s second stage (in space) was affected.
SpaceX is currently expected to launch two ISS missions in August, subject to FAA approval and NASA’s independent review: Northrop Grumman’s Cygnus cargo spacecraft and Crew-9 with four astronauts aboard the Crew Dragon.