WASHINGTON — Preparations for the first crewed flight of Boeing’s CST-100 Starliner vehicle to the International Space Station remain on schedule for a launch in mid to late April, company and NASA officials said Feb. 17.

The Crew Flight Test (CFT) mission will send NASA astronauts Butch Wilmore and Suni Williams to the station on an eight-day mission, launching from Cape Canaveral and landing at White Sands, New Mexico. The mission, following a successful uncrewed flight to the ISS last May, is intended to be the final major test of the vehicle before NASA certifies it for use on ISS crew rotation missions.

In a call with reporters, Steve Stich, NASA commercial crew program manager, said the NASA and Boeing teams have completed about 80% of the work needed for the mission. He described the next major milestone as a decision in early March to fuel the Starliner spacecraft, which Boeing wants to do within 60 days of launch.

That 60-day window is one measure of several Boeing took to reduce the risk that fuel would react with ambient moisture and corrode valves in Starliner’s propulsion system, which delayed an attempted August 2021 test flight. “We are much more confident today with the mitigation that we put in place with the purge system and the sealing of the connectors,” said Mark Nappi, vice president and program manager for Starliner at Boeing. He called that 60-day window a “guideline” and has asked engineers to look at the possibility of going longer.

Those efforts, also used for last year’s OFT-2 uncrewed test flight, are intended to be short-term measures. “It worked really nicely” on that earlier mission, he said, with an “enhanced” version of that approach developed for CFT. A long-term fix, in the form of a redesign of the propulsion system, is in process. That could be used on the first post-certification, or operational, Starliner mission, and no later than the second, he said.

NASA and Boeing are also wrapping up resolutions to issues with Starliner encountered during OFT-2. According to Nappi, the only outstanding major issue is the failure of Orbital Maneuvering and Attitude Control (OMAC) thrusters in the Starliner service module during that mission. Since the service module is jettisoned for reentry, engineers have had to narrow down potential root causes from telemetry and tests rather than inspection of the hardware.

“When we got down close to closing that fault tree, we recognized with NASA that we wanted to do a little bit of additional testing,” he said. Those tests took place earlier in the month and Boeing is now working with NASA to review the results, which he projected should be done by early March.

One challenge for the mission is a busy schedule of missions using the two docking ports that can support commercial crew vehicles. SpaceX’s Crew-6 mission is scheduled to launch Feb. 26, arriving at the ISS a day later. The Crew-5 Crew Dragon vehicle would undock around March 5, Stich, said. That docking port will then be used by a cargo Dragon mission, SpaceX CRS-27, that will arrive around March 12 and depart a month later, freeing it up for the Starliner CFT mission.

“There’s a swim lane that we’re going to watch very carefully relative to the traffic” at the station, Stich said. That is complicated by the fact that Starliner, for this mission, can only use the forward docking port. “We’ll have to watch weather delays and things like that to see where we end up relative to having a window to go launch CFT.”

A further complication is that United Launch Alliance, whose Atlas 5 will launch Starliner, is also preparing for the inaugural launch of its Vulcan Centaur rocket around the same time. “We’ve been in very close contact with them,” Nappi said. “We still feel that the mid April to late April time slot is good for us. It balances with the ULA priorities.” He added there would need to be further discussions with ULA if the CFT launch slipped beyond early May.

NASA officials on the call also addressed risk perception issues about the Starliner mission. During a Feb. 9 meeting of the Aerospace Safety Advisory Panel, Patricia Sanders, chair of the panel, noted that NASA’s commercial crew program had different views of risks associated with the mission than the ISS and flight operations programs. She compared it to similar debates during development of Crew Dragon about “load-and-go” fueling of the Falcon 9 rocket and its use of composite overwrapped pressure vessels.

“Given that there are differing views among the NASA communities about crew risks for the Boeing vehicle, we are very interested in learning more about the overall process for adjudicating those risks,” she said. That included, she said, what risks would be accepted for the CFT mission versus future post-certification missions.

Stich noted that the commercial crew and ISS programs have different risk systems. For the commercial crew program, the risk focus is on launch and landing, while at the station the spacecraft is quiescent. The ISS program, though, focuses on risks associated with the spacecraft while at the station, like the reliability of its batteries.

“We work lockstep with the [commercial crew program] office and their requirements are very comparable to our requirements,” said Jeff Arend, manager of the ISS program’s systems engineering and integration office, adding that he was looking forward to the Starliner mission. “We’re very excited to see them show up.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...