Boeing’s CST-100 Starliner and the crew of the capsule’s inaugural crew flight test (CFT), NASA astronauts Butch Wilmore and Suni Williams, will return to Earth from the International Space Station (ISS) no earlier than June 26, Starliner teams said during a news conference Tuesday.
Wilmore and Williams began their sojourn to the ISS on June 5 with plans for an eight-day stay but will now spend a minimum of three weeks aboard the orbital laboratory.
Partners NASA and Boeing—under contract for six Starliner Commercial Crew missions following the completion of the CFT and the vehicle’s certification—had previously targeted a return date of Saturday, doubling the length of the astronauts' mission. On Tuesday, however, they decided to further delay a return to allow teams to collect data from Starliner’s service module.
The service module, which makes small maneuvers to align the capsule such as when orbiting or docking, is an expendable component that is jettisoned before the spacecraft reenters the atmosphere. NASA and Boeing hope to study it further in order to assess issues that have arisen during the mission. Their findings will inform any modifications required before the capsule can be certified to enter NASA’s rotation.
NASA Commercial Crew program manager Steve Stich, ISS program manager Dana Wiegel, Johnson Space Center flight director Mike Lammers, and Boeing Commercial Crew program manager and vice president Mark Nappi addressed those problems Tuesday.
Starliner is contending with three main issues. The most significant is a series of helium leaks traced to the seals between the service module’s thrusters and manifold, of which crews have identified five. The first leak was discovered during prelaunch preparations, two were uncovered on the way to the ISS, and two more were identified after docking.
Another complication, which teams on Tuesday theorized may be connected to the helium leaks, involves the service module’s aft reaction control system (RCS) thrusters. Five of these engines failed to fire during the final phase of the spacecraft’s rendezvous with the ISS. Each phase of the remainder of the CFT requires a different number and arrangement of RCS thrusters.
The last problem stems from a faulty oxidizer isolation valve, which has been closed for the remainder of the mission. All other valves are functioning normally, NASA said.
According to Wiegel, the ISS can hold Wilmore and Williams indefinitely as NASA and Boeing prepare Starliner for undocking. Nappi said 77 of 87 CFT flight test objectives have already been completed, with the remaining few taking place during the trip back to Earth. He described the astronauts’ extended stay as an “opportunity” and a “privilege” to study Starliner further on orbit, which is considered an ideal test environment.
Over the past week and a half, Starliner teams have been busy gathering flight data on thruster firing, docking, and other maneuvers, as well as performing testing and analysis on the ground.
During a hot fire test over the weekend, one of the five RCS thrusters displayed what Stich described as “a strange signature where we’re getting almost no thrust out of that [engine].” Crews have opted to power down the thruster for the remainder of the mission. Three other thrusters functioned as expected.
On Starliner’s first visit to the ISS in 2022, a similar issue occurred where two RCS thrusters failed. Stich theorized that heat may be causing propellant in the engines’ chambers to vaporize.
The hot fire test also reflected a promising outlook for the helium manifold problem: “I would say every single manifold that we looked at [following testing], we saw the leak rates going down,” said Stich.
Starliner requires about seven hours’ worth of helium to undock from the ISS, perform a deorbit burn, and touch down on Earth, representing the final steps of the CFT. On Tuesday, teams estimated that the spacecraft will have at least 70 hours of margin.
According to Stich, three helium leaks, described as larger than the rest, tend to reach their highest rate when the service module’s RCS thrusters are firing, leading he and NASA to believe there may be a link between the two problems. As noted earlier, two leaks were identified after Starliner docked to the ISS, which Stich said placed an unexpected amount of demand on the thrusters.
Stich added that the two leaks discovered en route to the orbital laboratory stabilized while Wilmore and Williams took their designated sleep, suggesting there could be some correlation between thruster activity and leak rate. He theorized that the seals on the service module’s helium manifolds could be getting worn down due to extreme heat, for example.
NASA and Boeing representatives said Wilmore and Williams remain in good spirits aboard the ISS. The astronauts are using their extended stay to perform additional tests and equipment checkouts, including a demonstration of Starliner’s use as a safe haven in the case of a contingency aboard the space station. Stich likened the scenario to seeking refuge in a storm cellar during a tornado.
Crews throughout the week will continue to conduct evaluations of Starliner on orbit and on the ground, including simulations of the mission’s final phase. If all goes according to plan, Starliner, carrying Wilmore and Williams, will undock from the ISS on June 25 and land at White Sands Space Harbor the following morning at 4:51 a.m. EDT. Additional landing opportunities are available every four days, with the next being June 30.
The CFT is intended to be Starliner’s final test flight before the vehicle is certified by NASA for service missions. On Tuesday, representatives did not dismiss the possibility of a delay in certification to address the spacecraft’s frequent issues. The vehicle’s first operational mission, Starliner-1, is scheduled for 2025.
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