Updated 10:45 p.m. Eastern with post-launch briefing comments.
TITUSVILLE, Fla. — Boeing’s CST-100 Starliner spacecraft successfully docked with the International Space Station May 20, a little more than 24 hours after its launch.
The spacecraft docked with the forward docking port on the Harmony module of the station at 8:28 p.m. Eastern. Controllers reported a hard docking securing the spacecraft to the station about 20 minutes later, although hatches separating the spacecraft from the station won’t open until around 11:45 a.m. Eastern May 21.
The docking took place more than an hour later than the original schedule as controllers worked through several minor issues. That included the spacecraft’s docking ring, which needed to be retracted and extended again before the spacecraft could make its final approach.
“To the joint Boeing and NASA team, the crew of Expedition 67 would like to offer our congratulations on this momentous occasion,” NASA astronaut Bob Hines, currently on the station, said after the docking was confirmed. “Today marks a great milestone towards providing additional commercial access to low Earth orbit, sustaining the ISS and enabling NASA’s goal of returning humans to the moon and eventually to Mars.”
“This was a really critical demonstration mission,” Kathy Lueders, NASA associate administrator for space operations, said in a briefing an hour after docking. “Seeing that vehicle docked now to the ISS is just phenomenal.”
Neither NASA nor Boeing provided updates about the status of the Orbital Flight Test (OFT) 2 mission for more than 17 hours after a postlaunch briefing May 19, an unusual silence that raised concerns that there was a problem with the spacecraft. A Boeing spokesperson told SpaceNews that the company would provide an update about the mission “in a bit” but the company did not release that update until more than three hours later.
In that update, Boeing confirmed that the spacecraft was generally in good condition, having conducted several tests as planned. One issue was “off-nominal behavior” of a thermal cooling loop on the spacecraft, but the company said the system was still maintaining stable temperatures.
“The ground team did a great job of managing those loops,” said Steve Stich, NASA commercial crew program manager, at the post-docking briefing. He said some moisture may have gotten into the coolant loops that froze out and clogged a filter, causing a pressure rise in the loop. Controllers were able to manage the temperature of those coolant loops, and there was plenty of margin in the system.
The other issue was the failure of 2 of 12 aft-facing Orbital Maneuvering and Attitude Control (OMAC) thrusters during the spacecraft’s orbit insertion burn shortly after launch. In the statement, Boeing said a drop in chamber pressure likely caused the thrusters to shut down.
Mark Nappi, Boeing vice president and commercial crew program manager, said engineers developed a fault three and identified “three or so” plausible causes, which he did not identify, later suggesting the two thrusters may have failed for different reasons. “We may never know what the real cause of what this is because we don’t get this vehicle back,” he said. The thrusters are located in the service module, which is jettisoned before reentry and burns up in the atmosphere.
Other OMAC thrusters continued to work well, performing several maneuvers as the spacecraft approached the station before smaller reaction control system (RSC) thrusters took over for the final approach. The OMAC thrusters will not be used again until the spacecraft’s deorbit burn at the end of the mission.
In addition to the two OMAC thruster failures, two RCS thrusters also shut down during the approach to the station after suffering a drop in chamber pressure. “I don’t think we know quite yet what happened to those thrusters, but the vehicle has plenty of redundancy,” Stich said, including for undocking and landing.
Starliner is expected to remain at the station until at least May 25. Stich said the earliest undocking opportunity would set up a landing at White Sands Space Harbor in New Mexico at 6:46 p.m. Eastern that day, weather permitting. “We’re not in any hurry to come back. We want to learn as much from this vehicle as we can while it’s on orbit.”