A day into its OFT-2 mission, Boeing’s CST-100 Starliner spacecraft arrived at the International Space Station for the first time. Starliner’s docking, which marks a major milestone in Boeing’s demonstration flight for NASA’s Commercial Crew Program, occurred at 8:28 PM EDT on Friday (00:28 UTC on Saturday).
The Orbital Test Flight 2 (OFT-2) mission is the second uncrewed test flight of the Boeing CST-100 Starliner capsule, which was necessary due to the failure of the original OFT mission to complete its objectives in 2019. Starliner successfully lifted off from Cape Canaveral on Thursday, atop a United Launch Alliance (ULA) Atlas V rocket. This began a 24-hour chase that ended in its docking with the International Space Station on Friday.
Starliner docking
Having made an initial phasing burn an hour and 15 minutes after launch, Starliner began to fine-tune its approach to the International Space Station (ISS) with the first in a series of burns to adjust its altitude and change its orbital plane for rendezvous with the station. The second Height Adjust/Plane Change (NHPC) burn was made, with a Coelliptic/Plane Change burn after that.
Starliner at the launch pad before Friday’s liftoff (Credit: Thomas Burghardt for NSF)
Testing of the spacecraft’s systems and docking procedures were still ongoing, with a space-to-space commanding test carried out shortly after the second NHPC burn. After the coelliptic burn, Starliner performed another test to demonstrate the capabilities of its Vision-Based Electro-Optical Sensor Assembly (VESTA), which will be used to identify and track the ISS, determining the distance to docking during the final approach to the station.
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Next, the Terminal Phase Rendezvous Initiation (TPI) maneuver was conducted, putting Starliner on a direct course to intercept the ISS. Further VESTA demonstrations took place before Starliner performed its Inbound Flyaround Initialization Maneuver 1 (IF1) burn. This was the first of two maneuvers aligning the spacecraft with its docking port, the International Docking Adapter – Forward (IDA-F), attached to Pressurized Mating Adaptor 2 (PMA-2) on the forward side of the Harmony module.
Then, the second Inbound Flyaround maneuver was performed. This was followed by the Approach Corridor Initiation (ACI) burn, bringing Starliner closer to the ISS. After the ACI, astronauts aboard the space station commanded Starliner to hold at 255 meters distance in a planned test of the crew’s ability to send instructions to the spacecraft.
After holding for five minutes, Starliner resumed its approach and entered the 200-meter “keep out sphere” surrounding the ISS.
Shortly after it was cleared to enter the keep-out sphere, Starliner automatically retreated to the 200-meter mark in another planned demonstration of docking safety procedures. Starliner held position, and during this time VESTA was once again tested.
Starliner docks with the ISS (Credit: Mack Crawford for NSF/L2)
After extending this hold to resolve a discrepancy involving navigation overlays, the next stage of Starliner’s approach brought it to the 10-meter hold point, holding there to wait for the next docking window due to the previous delay. During the hold at 10 meters, a final VESTA demonstration was conducted before Starliner performed its Final Approach Initiation burn, beginning the last stage of its approach to the station.
Starliner docked with the station four minutes later: 25 hours and 34 minutes after lifting off from Cape Canaveral to begin its mission.
Starliner arrives at the space station loaded with 230 kilograms (500 pounds) of cargo, and a special passenger in its commander’s seat: back aboard Starliner is Rosie the Rocketeer, an anthropometric test dummy that also flew on the original OFT mission.
During OFT, Rosie carried a suite of sensors to record data that would allow engineers to study the conditions an astronaut would experience aboard the capsule. For OFT-2, however, she will not be equipped with any sensors or collect any flight data, as everything that was needed had been collected during the previous flight. Instead, sensors have been placed on and around the seat pallet to gather additional data through the data collection system that had previously been used in conjunction with Rosie.
Speaking during a pre-launch briefing ahead of a previous OFT-2 launch attempt last year, Boeing Commercial Crew Program manager John Vollmer explained that while Rosie is still aboard Starliner, “she’s essentially ballast”, continuing that “We are taking measurements, we’ve put sensors in the seats and other locations in the vehicle, as we found that we thought that would be more valuable since we’d gotten the data from Rosie on the first flight.”
Rosie the Rocketeer aboard Starliner during the 2019 OFT mission (Credit: NASA)
Starliner’s cargo consists of mostly food for the Expedition 67 crew aboard the ISS, along with some small components to help with future spacewalks. During Starliner’s time docked at the station, the astronauts will unload the capsule and fill it with over 270 kilograms (600 pounds) of cargo for return to Earth. This will mostly consist of nitrogen-oxygen recharge tanks which will be refilled before they are returned to the station at a later date.
Also flying aboard Starliner are commemorative items, including 14 banners from historically Black colleges and universities, Silver Snoopy pins, and commemorative Rosie the Riveter pins to honor the 19 million American women who contributed to the United States’ aerospace industry during the Second World War.
Orbital Flight Test
Starliner’s arrival at the space station completes a major objective that was left unfulfilled during the original OFT mission, now known retrospectively as OFT-1.
OFT-1 was successfully launched aboard an Atlas V rocket on December 20, 2019, deploying onto a suborbital trajectory as planned, with Starliner’s propulsion system expected to perform the final insertion into the spacecraft’s initial parking orbit. The spacecraft’s Mission Elapsed Timer, however, had incorrectly polled the wrong time from Atlas V and was therefore offset by eleven hours from the actual mission time.
As a result, the spacecraft failed to perform its orbital insertion burn, and the spacecraft switching communications between two TDRS satellites meant that controllers on the ground were delayed in their efforts to command the spacecraft to begin this burn manually.
Because #Starliner believed it was in an orbital insertion burn (or that the burn was complete), the dead bands were reduced and the spacecraft burned more fuel than anticipated to maintain precise control. This precluded @Space_Station rendezvous.
— Jim Bridenstine (@JimBridenstine) December 20, 2019
This delay resulted in the capsule being placed into an abnormal orbit and burning an excessive amount of fuel. Although mission controllers were able to fix the issue with its timer, the spacecraft had burned too much fuel to perform its planned set of phasing burns to rendezvous and dock with the International Space Station, and NASA and Boeing teams decided to call off the attempt.
Despite OFT-1 not reaching its nominal initial orbit, teams were eventually able to place Starliner into a stable orbit and modified the flight plan to allow for a two-day mission rather than the planned eight flight days, completing as many objectives as could be achieved without the ISS rendezvous.
The mission concluded with a successful landing at the White Sands Space Harbor, in New Mexico, on 22 December, however, a review post-flight uncovered three serious issues during the flight. As well as the Mission Elapsed Timer fault, a problem with the service module separation sequence that was detected and resolved in-flight was considered to have been a potential loss-of-vehicle scenario, and communication problems persisted throughout the mission.
Starliner after returning to Earth following the OFT mission (credit: NASA/Bill Ingalls)
With these issues, and major objectives still incomplete, NASA and Boeing determined that an additional uncrewed test flight would be required before Starliner could carry astronauts, which is the purpose of the OFT-2 mission.
With the need for the additional flight determined, Starliner’s second Orbital Flight Test was initially scheduled for July of 2021. This slipped a few days into August due to several issues, including an internet service provider (ISP) outage that prevented the rocket from rolling out to the launch pad on schedule, and checkouts on the ISS following the arrival of the Nauka module, which had thruster problems shortly after docking.
With these issues resolved, Starliner rolled back out to the launch pad with liftoff set for August 3, 2021. During pre-launch preparations for the flight, engineers from Boeing noticed that 13 valves on Starliner’s service module were stuck in incorrect configurations. After a 24-hour delay, crews were unable to recycle the propulsion system to force the valves back into their correct positions, and the decision was made to roll Starliner back to the VIF for further analysis and corrective action, and later to destack the spacecraft for further inspections.
Atlas V lifts off on the OFT-2 mission (Credit: Stephen Marr for NSF)
After tracing the cause of the valve issues to a buildup of nitric acid from the reaction of water in the atmosphere with dinitrogen tetroxide, the oxidizer used by Starliner’s thrusters, Boeing implemented measures to mitigate this problem. The service module was swapped out for one that had been under construction for the next Starliner mission, the Crewed Flight Test (CFT) and preparations for launch resumed.
Starliner lifted off aboard Atlas V AV-085 from Space Launch Complex 41 (SLC-41) at the Cape Canaveral Space Force Station at 6:54 p.m. Eastern (22:54 UTC) on Thursday. AV-085 launched in the Atlas V N22 configuration, only the second Atlas V ever to fly without a payload fairing and with a dual-engine Centaur.
During the OFT-2 mission, Starliner is expected to complete a four-day stay at the International Space Station before it undocks for the return to Earth. If the test flight is deemed a success – and Friday’s docking takes a major step towards this – the next mission will be the Crewed Flight Test (CFT) with the capsule carrying astronauts for the first time.
(Lead image: Starliner docking with the International Space Station — Credit: NASA)
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