Boeing’s Starliner CFT launches on third attempt

by Sawyer Rosenstein

After years of anticipation, Boeing’s CST-100 Starliner capsule has flown its first crew into space on the 100th flight of an Atlas V launch vehicle. Capping off Starliner’s test campaign, the Crew Flight Test (CFT) has sent Barry Wilmore and Sunita Williams on a trajectory to rendezvous with the International Space Station (ISS) where they will remain for about a week.

Launch was initially scheduled for May 6, but was scrubbed just under two hours before the flight due to an issue with an oxygen relief valve on the Centaur upper stage. Liftoff was then scheduled for June 1 at 12:25 PM EDT (16:25 UTC). On this second attempt, the rocket entered a hold at approximately T-3:50 due to a fault in the ground support equipment that controls the launch vehicle’s countdown. The launch took place on Wednesday, June 5 at 10:52 AM EDT (14:52 UTC).

While CFT was the first crewed flight on Starliner, it also marked the first flight of humans on the United Launch Alliance (ULA) Atlas V rocket, as well as the first crewed flight from Cape Canaveral Space Force Station since Apollo 7 in 1968.

Following this launch, Starliner will join SpaceX’s Crew Dragon in the rotation of vehicles contracted to bring crews to and from the ISS for long-duration stays.

First launch attempt

During the countdown on May 6, teams with ULA noticed a valve on the Atlas V’s Centaur second stage that appeared to continually open and close.

Given the fact it was a crew mission and that the vehicle had been in a fully fueled state, teams decided to stand down from the launch attempt. ULA noted that had this been a non-crew mission, it could have likely been resolved during the countdown. The valve, which vents pressure from the liquid oxygen tank, was rated to at least 200,000 cycles, which engineers believe it had surpassed prior to launch. This led to the vehicle rolling back to the Vertical Integration Facility (VIF) where it was replaced.

Teams also noticed a small helium leak in the service module of the Starliner spacecraft. Originally leaking at 7 pounds per square inch (psi), it reportedly grew to between 50-70 psi from an area smaller than a shirt button and approximately 10 sheets of paper thick according to engineers.

This is located inside of one of the reaction control system thrusters onboard the vehicle that help with in-orbit maneuvers as well as deorbiting. The issue reportedly only affects one of the 28 of such reaction control system (RCS) thrusters onboard the vehicle.

Engineers also noted that helium leaks are not uncommon, having been present on other vehicles including early Crew Dragon flights, and pose no danger to the crew.

The Starliner capsule at SLC-41 prior to a May 6 launch attempt. (Credit: Max Evans for NSF)

However, examining this issue did reveal an additional “design vulnerability.” The manifolds for the thrusters are located inside housings known as a “doghouse.” Four of them are located around the vehicle. It was noted that if two of the doghouses experienced a failure that were not perpendicular to each other, it could remove the ability of the spacecraft to use eight of its orbital maneuvering thrusters to perform a deobrit burn, removing a layer of redundancy.

The other certified deorbit maneuvers both involve firing the vehicle’s orbital maneuvering and attitude control (OMAC) thrusters, which are different from the RCS thrusters.

An additional deorbit method was tested and approved as a backup that would see two deorbit burns, each using four RCS thrusters. The crew, who have been quarantined in Houston, reportedly ran through these scenarios in the simulator between launch attempts.

The two astronauts arrived back in Florida on May 28th, followed by their ride rolling back out to the launch pad the next day. Following a special Delta Flight Readiness Review on the 29th, meant to look at all anomalies discovered between attempts, teams gave their approval for launch.

Second launch attempt

During the attempt on June 1, multiple issues appeared during the countdown, however all were resolved prior to entering the terminal count at T-4 minutes. This includes an issue with a sensor on the fuel topping valves for the cryogenic Centaur upper stage, an inadvertent release of 1,000 psi of nitrogen, and an issue with a fan inside one of the crewmember’s spacesuits.

Moments after entering the terminal count, the clock stopped at T-3:50 after a fault was detected in some of the ground support equipment. At a press conference following the scrub, ULA CEO Tory Bruno explained that out at the pad, there are three sets of redundant computers, each full of racks containing individual cards related to each portion of the countdown.

Bruno noted that upon entering the terminal count, two of three redundant launch sequencers instantly enter flight configuration, whereas a third took an additional six seconds. These sequencers are in charge of time-sensitive parts of the launch including disconnecting umbilical lines and commanding the release of the bolts that keep the rocket on the pad.

While it did eventually come up to full speed, Bruno said the launch sequencer read this as a problem and out of an abundance of caution auto-triggered the scrub.

Despite the fact that all three cards perform the same task, adding redundancy, Bruno noted that all three of these cards need to be fully powered on entering the terminal count given their importance to keeping track of systems prior to the launch.

Once the rocket was detanked and the launch pad deemed safe, teams were able to replace the card at the pad and test it, clearing the flight for its next attempt on June 5.

Previous flight tests

In September 2014, NASA announced that SpaceX and Boeing would each receive a contract as part of the Commercial Crew Transportation Capability contracts. The plan was to get two independent spacecraft up and running simultaneously as a form of redundancy in the event something were to happen to one of the vehicles, leaving it out of commission. The first uncrewed test flight of Crew Dragon occurred in March 2019 with the Demo-1 mission, which docked to the ISS without crew and returned safely.

Demo-2 saw the return of US crew launch capability launching on May 30, 2020, safely bringing Bob Behnken and Doug Hurley to the ISS and back.

The Starliner OFT-1 Atlas V prior to first stage cutoff on Dec 20, 2019. (Credit Joseph Navin for NSF)

Simultaneously, Boeing was working on their test program for Starliner.

The first flight test, originally named Orbital Flight Test and later renamed Orbital Flight Test 1 (OFT-1), lifted off on Dec. 20, 2019. Nicknamed Calypso, the spacecraft launched successfully, however, it encountered a serious problem while trying to enter its planned orbit prior to an ISS rendezvous and docking.

A later investigation found that the mission elapsed timer had polled incorrectly, resulting in the spacecraft not executing its orbit insertion burn at the planned time. This left it in a safe orbit, but unable to complete its primary objective of docking to the Station, even after the burn was eventually commanded manually.

In addition, a software issue with the service module separation sequence was found during the mission, which could have resulted in a loss of vehicle had it not been identified and fixed before Starliner’s return to flight. The capsule landed safely at the White Sands Missile Range in New Mexico after a shorter-than-planned mission.

Teams process the OFT-1 Starliner vehicle after landing. (Credit: NASA)

This resulted in the need for an unplanned second uncrewed flight test, Orbital Flight Test 2 (OFT-2), which faced its own issues.

Originally scheduled to launch in August 2021, the attempt was scrubbed once indicators on the spacecraft noted 13 valves in the service module’s propulsion system were in an incorrect configuration.

While they attempted to fix the issues while still attached to the Atlas V rocket inside ULA’s Vertical Integration Facility (VIF), four valves remained stuck. That resulted in the destacking of Starliner, which was then sent back to Boeing’s facility for repairs.

The root cause of the valve issue was determined to be corrosion from nitric acid, which had formed as a result of water vapor in the air reacting with the dinitrogen tetroxide that Starliner uses as an oxidizer. This led to the replacement of the service module, using one originally planned for the CFT mission since it contained a nitrogen purge system meant to help mitigate the problem along with other processing and fueling procedure changes.

Starliner lifts off on the uncrewed OFT-2 mission from Cape Canaveral Space Force Station. (Credit: Stephen Marr for NSF)

OFT-2 successfully launched on May 19, 2022, docking to the ISS just one day later. After just under six days in space, the capsule successfully landed in New Mexico on its airbags, paving the way for CFT.

The crew

The lineup for who would fly on the first crew flight test of Starliner has changed over the years. The original lineup was to include astronauts Nicole Mann and Eric Boe. Mann eventually flew aboard SpaceX’s Crew Dragon on the Crew-5 mission in October 2022. Boe was found unable to fly due to a medical condition. He is currently the chief of the Vehicle Integration Test Office at NASA’s Johnson Space Center, working closely with both Crew Dragon and Starliner crewmembers.

Spacecraft Commander Barry “Butch” Wilmore is making his third spaceflight. Selected as an astronaut in 2000, Wilmore has more than 8,000 flight hours in tactical jet aircraft. A graduate of the US Naval Test Pilot School, he completed 663 carrier landings, flying 21 combat missions during Operation Desert Storm.

Wilmore’s first flight was aboard Space Shuttle Atlantis on the STS-129 mission in 2009. The ten-day mission to the ISS delivered two Express Logistics Carrier racks plus around 30,000 pounds of spare parts to help with Station orientation.

Barry “Butch” Wilmore and Sunita “Suni” Williams arrive at the Kennedy Space Center ahead of the Boeing CFT mission. (Credit: Max Evans for NSF)

His second flight saw him spending 167 days aboard the Station as a member of the Expedition 41/42 crew, launching on Soyuz TMA-14M in September 2014 alongside two cosmonauts. Over the course of his missions, Wilmore also logged more than 25 hours of spacewalking time during four EVAs.

Also getting her start in the Navy, Sunita “Suni” Williams flew H-46 helicopters and saw overseas deployments to the Mediterranean, Red Sea, and the Persian Gulf during Operation Desert Shield. She later joined the US Naval Test Pilot School in 1993.

Five years later, Williams was selected as a NASA astronaut. Her career with NASA actually began in Russia working with Roscosmos to support Expedition 1, the first long-duration mission aboard the ISS. Williams then conducted science underwater during NEEMO2, taking place in an underwater science laboratory, for five days.

Her first spaceflight assignment was as a member of Expedition 14/15. Originally launching aboard Discovery on the STS-116 mission in 2006, she remained for 192 days. During the stay, she completed four spacewalks. She landed back on Earth aboard Atlantis on STS-117 in June 2007.

Williams returned to the ISS for Expedition 32/33, launching aboard Soyuz TMA-05M in July 2012. Besides taking over as ISS Commander for Expedition 33, she also became the first person to complete a triathlon in space.  In place of the swimming portion, Williams used a resistive exercise device to do weightlifting and resistance exercises that approximate swimming in microgravity. The station already has a treadmill and recumbent bicycle to complete the full triathlon.

During her four-month stay, she completed an additional three spacewalks, bringing her total EVA time to 50 hours and 40 minutes, a record at the time.

Williams landed in November 2012, marking the end of a 127-day mission. She currently has a total of 321 days combined in space.

The spacecraft

The Crew Space Transport 100 (CST-100) capsule flying this mission is named Calypso. Selected by astronaut Suni Williams, it was named after Jacques Cousteau’s oceanography vessel, RV Calypso, and so far, is the only Starliner capsule to receive a name.

Boeing’s Starliner capsule named Calypso is moved to SLC-41 for the Boeing CFT flight. (Credit: Max Evans for NSF)

Altogether, the vehicle consists of a capsule and a service module. The capsule measures 4.5 meters (15 feet) in diameter, which is slightly larger than an Apollo capsule yet smaller than the Orion capsule which NASA is using as part of the Artemis program. The capsule can carry a maximum of seven people into orbit. While this flight only has a crew of two, the plan is to launch operational missions with a crew of four, with an optional fifth seat. Starliner is capable of staying in orbit, according to Boeing, for seven months.

Combined with the service module, Starliner stands 5 meters (16.5 feet) tall. The service module, which remains attached to the capsule until just prior to reentry, contains four launch abort engines designed by Aerojet Rocketdyne. Those are meant to pull the spacecraft away in the event of an on-pad or in-flight problem.

The service module is also equipped with 28 reaction control system (RCS) engines for on-orbit maneuvering. These are in addition to 20 orbital maneuvering and attitude control (OMAC) engines used to maneuver and separate the capsule. The solar panels for Starliner are also located on the aft side of the service module.

The capsule is additionally equipped with 12 RCS thrusters of its own.

When it comes time for reentry, the base heat shield, made of Boeing Lightweight Ablator, takes on the heat of reentry. The heat shield is a honeycomb-like structure that is hand-filled with the ablator. In areas that receive less heat, the capsule is covered with a mix of thermal blankets and heat shield tiles, both of which have Space Shuttle heritage.

Once through the thicker part of the atmosphere, the heat shields are jettisoned, the parachutes deploy, and the airbags inflate to cushion the desert landing.

The launch vehicle

The Starliner vehicle and crew launched aboard ULA’s workhorse Atlas V rocket. Although Atlas V itself has not launched crew, early Atlas rockets flew the first American astronauts into orbit. Originally beginning development as a long-range ballistic missile launch vehicle for the Army Air Force in 1946, it was later adapted to fly Mercury missions. That included John Glenn’s Friendship 7 in 1962, marking the first time an American astronaut orbited the Earth. Atlas also saw later use in the launching of Agena target vehicles for the Gemini program.

Starliner OFT-1 mission launches on Atlas V. (Credit: Mike Deep for NSF)

While still Atlas by name, the vehicle launching Starliner has advanced quite dramatically. This particular Atlas, the 100th Atlas V, flew in what is known as the N22 configuration. The Atlas V has its standard RD-180 engine, which contains two thrust chambers, helping to lift the vehicle off the pad. Atop the vehicle was the capsule, which is not enclosed inside of a payload fairing, as denoted by the “N.” Two strap-on solid rocket boosters, manufactured by Aerojet Rocketdyne, provide an additional 1.55 meganewtons (348,500 pounds) of thrust. This is marked by the first “2” in the configuration.

The second “2” represents the number of second-stage RL-10A-4-2 engines. The pair power the 12.6-meter-long (41.5 foot) Centaur upper stage. The Centaur is a cryogenic vehicle, fueled by liquid hydrogen, different than the kerosene-based rocket propellant 1 (RP-1) used in the first stage.

Included for the Starliner flights is the Centaur Forward Adapter (CFA) as well as the vehicle’s Emergency Detection System (EDS). The CFA provides electrical interfaces with the spacecraft among other tasks. The EDS monitors for any indication that a failure is imminent. As a result, this is also responsible for the jettison of the ascent cover and initiates Starliner spacecraft separation.

The launch pad is also equipped with an Emergency Egress System (EES). Situated on level 12 of the Crew Access Tower (CAT), the same level where the crew enters Starliner, it consists of zipline-style wires and a harness for crew and pad team members to use in case of an emergency.

Each harness includes handles that allow the person to control their speed, including coming to a smooth stop at the end. In the event the crewmember forgets to brake, 30 feet of springs help slow the person to touch down in the landing zone. The system is capable of supporting up to 20 personnel.

Launch Profile

Ambient temperature RP-1 fuel was loaded onto the launch vehicle two days before scheduled liftoff. Loading of cryogenic propellants for the vehicle, including liquid oxygen for both stages and liquid hydrogen for the Centaur , begins six hours before the scheduled liftoff time and takes one hour and 55 minutes to complete. Just one minute after the vehicle is loaded with propellant, at L-4 hours 4 minutes, the countdown enters a hold. While called the T-4 minute hold, it stops the T- clock for four hours. During that time, the L- clock continuously counts down to liftoff.

About three hours before launch, the crew departed the Neil A. Armstrong Operations and Checkout Facility, before being driven to SLC-41. The two astronauts rode an elevator up to level 12 of the CAT. The astronauts then walked across the Crew Access Arm (CAA) and through the white room to be strapped inside Calypso.

Hatch closure took place around one hour and 25 minutes before launch, followed ten minutes later by prelaunch cabin leak checks. The cabin pressurization was completed before the L-1 hour mark.

The nominal flight profile for the Boeing CFT launch aboard Atlas V N22. (Credit: ULA)

At L-20 minutes, the launch conductor starts the terminal count briefing, followed two minutes later by the poll for Starliner to enter its terminal count. At L-15 minutes, CST-100 switched to internal power.

The CAA retracts at L-10 minutes leaving one final go/no-go poll. At L-8 minutes, the Atlas V team conducts its readiness poll to proceed with the terminal count. Assuming all teams are go, Starliner is configured for the terminal count at L-4 minutes 45 seconds.

At four minutes to launch, the T- clock resumes, meaning that the T- and L- clocks are synchronized.

At T-1 minute, Starliner is officially configured for launch.

At T-2.7 seconds, the RD-180 engine ignites, followed by the ignition of the SRBs, with liftoff expected at T+1.1 seconds.

The vehicle then began its pitch/yaw maneuver to place it on a trajectory to rendezvous with the ISS. The vehicle is expected to reach max Q at T+1 minute 1 second, passing the speed of sound four seconds later.

At two minutes 20 seconds into flight, the two SRBs jettisoned, leaving the RD-180 to fire on its own for another two minutes and eight seconds.

Then comes a few events in rapid succession. Booster engine cutoff (BECO) occurs four minutes 28 seconds after launch. Six seconds later, the Atlas booster separates from the Centaur upper stage. The ascent cover is jettisoned six seconds after that, followed by Centaur’s first main engine start (MES-1) at T+4 minutes 44 seconds.

20 seconds after that, the aeroskirt is jettisoned. The RL-10A engines fired for the remainder of the ascent, reaching main engine cutoff (MECO) at T+11 minutes 52 seconds.

The Centaur with Starliner attached coasted for approximately three minutes, with Calypso and her crew set free from the launch vehicle at T+14 minutes 52 seconds.

The vehicle was then released on a suborbital trajectory of 157 by 62 kilometers (98 by 39 miles) inclined 51.6 degrees to the equator. The capsule performed its orbital insertion burn at 31 minutes into the flight.

Rendezvous and docking

Once in a stable orbit and on course for the ISS, Starliner began its rendezvous procedure. In a test unique to the CFT, astronauts tested out the manual flight control system on the way to the ISS, in addition to the automated systems already flown by the capsule on the two previous Orbital Flight Test missions.

Approximately two days after launch, the crew closed in on the station, performing two height adjustment and plane change maneuvers followed by a coelliptic and plane change burn. One more burn placed the spacecraft inside the ISS approach ellipsoid, an imaginary 4 by 2 by 2-kilometer (2.5 by 1.2 by 1.2-mile) ellipsoid, centered at the ISS center of mass.

Starliner had to pause 200 meters away from the station before entering what is known as the “keep out sphere.” This imaginary sphere officially placed the spacecraft inside the area controlled by Station flight controllers on the ground.

Starliner prior to ISS docking on the OFT-2 flight. (Credit: Bob Hines/NASA)

The capsule continued toward the International Docking Adapter located on the forward-facing port of the Harmony module. The port was previously occupied by SpaceX’s Crew Dragon Endeavour, which delivered the Crew-8 astronauts to the ISS following a launch on March 3, 2024. That vehicle was moved on May 2 to Harmony’s space-facing zenith port to make room for Starliner.

Calypso missed the first docking window after troubleshooting issues with reaction control thrusters on the service module. It stopped 10 meters from the docking target awaiting one final go to proceed. Once given, the capsule autonomously docked to the station, with initial contact confirmed at 12:34 PM CDT. This marked the first time a crewed Starliner capsule was attached to the ISS.

Undocking and landing

Once ready and cleared to depart the ISS, Wilmore and Williams will enter the capsule and begin the undocking process. The capsule will perform a fly-around maneuver, getting unique views of the ISS, before exiting the approach ellipsoid to begin preparations for deorbit.

While positioned over the Pacific Ocean, the service module will conduct the deorbit burn, slowing the vehicle down enough to interface with Earth’s atmosphere and begin re-entry. Following the burn, the service module will detach and burn up.

Drogue chutes deploy from a Starliner test article during a parachute reliability test. (Credit: Boeing)

The crew module’s descent through the atmosphere will see the exterior reach temperatures of 1,650 degrees Celsius (3,000 degrees Fahrenheit). Once the vehicle reaches nine kilometers (30,000 feet) above the ground, Starliner will jettison the forward heat shield which protects the parachutes during reentry.

First to deploy are two drogue chutes to begin slowing the spacecraft down. Once those are jettisoned, the three main orange and white parachutes will deploy and inflate, dramatically slowing the vehicle down.

Once over its landing target in White Sands, New Mexico, airbags will inflate 0.9 kilometers (3,000 feet) above the ground, allowing them to cushion the crew for a softer landing once they touch the ground.

If all goes well, the first operational mission to the ISS, Starliner-1, will launch on a six-month mission no earlier than 2025.

(Lead image: The Atlas V with Starliner rolls out to the launch pad ahead of the CFT mission. Credit: Sawyer R for NSF)

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