The crew of Boeing’s Starliner prepares for Wednesday’s launch

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The final attempt at the inaugural launch of Boeing’s Starliner crewed spacecraft is on track for Wednesday after a computer problem halted the countdown just moments before liftoff on Saturday.

The historic mission, called Crew Flight Test, is scheduled to launch at 10:52 a.m. ET from the Cape Canaveral Space Force Station in Florida. The event will be streamed live on NASA’s website, beginning at 6:45 a.m. ET.

Veteran NASA astronauts Butch Wilmore and Suni Williams will be the first crew to ride on the Starliner, launched into orbit by an Atlas V rocket built by United Launch Alliance, a joint venture between Boeing and Lockheed Martin.

The long-awaited flight was 3 minutes and 50 seconds from liftoff Saturday afternoon, when an automatic hold was activated by the ground’s launch sequencer, or the rocket’s launch computer. The teams safely extracted the astronauts from the capsule, and Williams and Wilmore returned to the crew quarters while the rocket was being fueled.

Joe Raedle/Getty Images

Boeing’s Starliner spacecraft is aboard a United Launch Alliance Atlas V rocket at the Cape Canaveral Space Force Station in Florida on May 31.

United Launch Alliance technicians and engineers assessed ground support equipment over the weekend, examining three large computers housed inside a shelter at the base of the launch pad. Each computer is the same, providing triple redundancy to ensure the safe launch of manned missions.

«Imagine a large rack that is a large computer where the functions of the computer as a controller are separated separately on individual cards or printed-wire boards,» Tory Bruno, president and CEO of United Launch Alliance, said during a Saturday press conference. «They’re all standalone, but together, it’s an integrated controller.»

Cards inside the computer are responsible for various key systems that must occur before launch, such as loosening the screws on the base of the rocket so it can lift off after ignition.

During the last four minutes before launch, all three computers must communicate with each other and come to an agreement. But during Saturday’s countdown, a card on one of the computers responded six seconds slower than the other two computers, indicating something was wrong and triggering an automatic hold, according to Bruno.

Over the weekend, engineers evaluated the computers, their power supply and the network communication between the computers. The team isolated the problem to a single ground power supply inside one of the computers, which supplies power to computer cards responsible for key countdown events — including the rocket’s upper stage refueling valves, according to an update shared by NASA.

The Starliner teams reported no signs of physical damage to the computers, which were removed and replaced with spares. Meanwhile, mission experts continue to analyze the failed power unit to better understand what went wrong.

Other computers and their cards have also been evaluated and are all working as expected, according to the ULA team.

The Starliner mission management team reviewed the computer replacement troubleshooting steps that were taken and agreed that Starliner is «on track» for Wednesday’s launch, according to an update from NASA.

«I really appreciate all the work by the NASA, Boeing and ULA teams over the past week,» said Steve Stich, manager of NASA’s Commercial Crew Program, in a statement. “In particular, the ULA team worked very hard to quickly learn more about these issues, inform our NASA and Boeing teams, and protect them for this next attempt. We will continue to go step by step.»

Mission teams successfully resolved several other issues that arose earlier in the countdown to Saturday’s launch attempt. Those problems included the loss of data from the ground valves responsible for refilling liquid oxygen and hydrogen into the second or upper stage of the Atlas V rocket before liftoff.

Both liquid oxygen and hydrogen, which are used to fuel the rocket, are boiled off while the rocket is on the platform before launch, so that replenishment lasts until liftoff. After assessing the problem, the mission teams switched to a redundant valve data system and were able to continue the process.

Weather conditions are 90% favorable for Wednesday morning’s launch, with the only concern being cumulus clouds, according to the US Space Force’s 45th Weather Squadron. If the launch doesn’t happen Wednesday, there’s another chance Thursday at 10:29 a.m. ET, according to NASA.

If Starliner successfully launches, astronauts will spend just over 24 hours traveling to the International Space Station.

Joe Skipper/Reuters

Veteran NASA astronauts Butch Wilmore (left) and Suni Williams are seen Saturday before the second Starliner launch attempt.

After docking, Williams and Wilmore plan to spend eight days living in the orbiting laboratory, joining the seven astronauts and cosmonauts already aboard. The two will continue to test the functionality of the Starliner spacecraft while docked at the space station and then return home in the same capsule. He is expected to parachute to land at one of several designated locations throughout the southwestern United States.

Meanwhile, the two astronauts remain in quarantine to protect their health before launch, and practice procedures and simulations ahead of the historic flight, according to NASA.

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