Nine months after valve issues caused its previous launch attempt to be scrubbed, Boeing’s Starliner spacecraft has launched on its Orbital Flight Test 2 mission atop a United Launch Alliance Atlas V rocket Thursday. OFT-2 departed Space Launch Complex 41 at Cape Canaveral at 6:54:36 p.m. EDT (22:54 UTC), bound for the International Space Station (ISS).

Orbital Flight Test 2, or OFT-2, is a repeat of Starliner’s 2019 OFT mission, now also known retrospectively as OFT-1. An uncrewed test of the Starliner spacecraft, developed by Boeing for NASA’s Commercial Crew Program (CCP), the original OFT had been intended as the last step before NASA entrusted astronauts to the capsule on a Crewed Flight Test (CFT).

Due to the original OFT’s failure to complete a number of key objectives, including rendezvous and docking with the International Space Station, the OFT-2 mission was added ahead of the first crewed flight.

An investigation following the first OFT mission identified three serious issues. The most significant was 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 unable to reach the space station, even after the burn was eventually commanded manually.

Starliner had also suffered intermittent space-to-ground communication issues affecting command and control throughout the OFT mission. A software issue with the service module separation sequence was also found during the mission, which could have resulted in a loss of vehicle had it not been identified and fixed before Starliner returned to Earth.

The reflight, OFT-2, had originally been planned to launch last year, with the spacecraft first rolling out to Space Launch Complex 41 (SLC-41) atop Atlas V AV-082 in preparation for a liftoff at the end of July. This was delayed to August 3rd, in part due to issues at the space station after the arrival of the Nauka module a few days before Starliner’s planned launch.

The August 2021 launch attempt was scrubbed after indications that 13 valves in the service module’s propulsion system were in an incorrect configuration. Subsequent work to resolve the issue at the launch pad, and after rollback to the Vertical Integration Facility (VIF) was unsuccessful – with four valves remaining stuck after troubleshooting in the VIF – and the spacecraft needed to be destacked and returned to Boeing’s factory.

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. Ultimately, Boeing decided to swap out the service module for one that had been due to fly the CFT mission. The new service module incorporates a nitrogen purge system to help mitigate the problem, with spacecraft fueling and pre-launch procedures also amended.

Starliner is one of two spacecraft developed to carry astronauts to and from the International Space Station under NASA’s Commercial Crew Program. The OFT and CFT missions conclude the development and demonstration phase of this program, paving the way for future crew rotation flights to the station. SpaceX has already completed these milestones with their Crew Dragon spacecraft, the other vehicle built for this program, which is currently undertaking its fourth long-duration crew mission to the ISS.

NASA’s Commercial Crew vehicles: Starliner (left) and Dragon (credit: Mack Crawford for NSF/L2)

Commercial Crew was instigated in 2011 to provide NASA astronauts with access to the space station through American vehicles, while leaving NASA free to concentrate its internal human spaceflight efforts on the exploration program now known as Artemis.

Also known as Crew Space Transportation 100 (CST-100), Starliner consists of a reusable capsule that can theoretically seat seven astronauts – although it will typically fly with up to four – and an expendable service module (SM). The SM houses mission support equipment, including thrusters, propellant, and solar panels, and it is jettisoned at the end of the mission to burn up in the atmosphere. Starliner is designed to spend up to 220 days in space docked to a space station and can operate for up to 60 hours in free flight.

OFT-2 marks the first flight of Starliner spacecraft number 2, which has not yet been named. Spacecraft number 3, now named Calypso, carried out the original OFT mission and is now expected to fly again for the CFT when Starliner carries astronauts for the first time.

To launch Starliner, Boeing has partnered with United Launch Alliance (ULA). The spacecraft rode atop a two-stage Atlas V N22 rocket, carrying it most of the way to orbit. Starliner then completed orbital insertion using its own propulsion systems before beginning a day-long chase to reach the International Space Station.

Atlas V’s N22 configuration, which is used only for Starliner missions, consists of a first stage Common Core Booster (CCB), with two solid rocket motors and a dual-engine Centaur (DEC) upper stage. The Starliner spacecraft is mounted atop the Centaur. The rocket does not have a payload fairing, however an aeroskirt is attached to the launch vehicle adaptor at the aft end of Starliner’s service module to help protect the upper stage from aerodynamic loads during ascent.

Atlas and Starliner at the launch pad ahead of last year’s launch attempt (credit: ULA)

The rocket that performed Thursday’s launch had tail number AV-085. For its 2021 launch attempt, OFT-2 was originally stacked atop AV-082, but after the delays to Starliner’s mission, the rocket was taken apart. The CCB flew with a single-engine Centaur as part of AV-096, which launched NASA’s Lucy mission last October, while AV-082’s dual-engine Centaur is now part of the AV-085 vehicle that has been tasked with deploying OFT-2.

A new CCB was delivered to Cape Canaveral in April using NASA’s Pegasus barge. Pegasus – which has previously been used to deliver Space Shuttle External Tanks and more recently the Space Launch System (SLS) core stage for the Artemis 1 mission – was used for this delivery since ULA’s RocketShip transport vessel was undergoing maintenance in drydock. The usual backup delivery method, an Antonov An-124 aircraft, was also unavailable due to the war in Ukraine.

Like Starliner’s service module, this booster had originally been slated for CFT. It also carried over the AV-085 tail number that had been assigned to launch that mission.

With RocketShip in dry dock and Antonov aircraft unavailable (for obvious reasons), ULA used the Pegasus barge to bring OFT-2’s first stage to Cape Canaveral. https://t.co/vIZnDMiwz2

— Thomas Burghardt (@TGMetsFan98) April 13, 2022

Atlas V’s East Coast launch facility is Space Launch Complex 41 (SLC-41) at the Cape Canaveral Space Force Station. SLC-41 consists of a single launch pad, with rockets assembled atop a mobile launch platform in the nearby Vertical Integration Facility (VIF) before being moved into position for launch. SLC-41 was originally built for the Titan family of rockets, which it served until 1999 when it began conversion for Atlas. It will also be used by ULA’s forthcoming Vulcan rocket.

The CCB was the first part of AV-085 to arrive at the VIF, with the Launch Vehicle on Stand (LVOS) milestone – marking the start of assembly with the booster being raised into position on the mobile launch platform – taking place on April 20th. Centaur was stacked atop it six days later, before Starliner arrived on May 4th.

The mission’s Flight Readiness Review was concluded on May 11th, confirming that NASA and Boeing were happy to proceed with the launch. This was followed by a Launch Readiness Review, completed successfully on Tuesday, which cleared the way for Atlas to roll out to the launch pad. First motion occurred at 10:14 a.m. EDT on Wednesday, with Atlas and its mobile launch platform secured in position at the launch pad 58 minutes later.

Atlas V and Starliner arrive at SLC-41 ahead of Thursday’s launch (credit: Stephen Marr for NSF)

Starliner’s launch marked the 150th mission for United Launch Alliance, which was formed in 2006 as a partnership between Boeing and Lockheed Martin to operate the companies’ Atlas and Delta rockets and to market them to the US Government. The company operates Atlas V and Delta IV vehicles, having also operated the earlier Delta II rocket until its retirement in 2018. Its next-generation rocket, Vulcan, is currently due to make its maiden flight towards the end of the year.

Atlas V was originally developed by Lockheed Martin in response to the US Air Force’s Evolved Expendable Launch Vehicle (EELV) program – now the Space Force’s National Security Space Launch (NSSL) program. It first flew in August 2002, with Thursday’s mission being its 93rd flight.

The rocket has a nearly-perfect success record, having never experienced a loss of mission. It has completed 91 missions successfully prior to OFT-2, with only one partial failure. This occurred during Atlas V’s tenth flight back in 2007, when a pair of National Reconnaissance Office satellites were placed into a slightly off-target orbit due to a propellant leak. Despite this, the satellites were able to correct their orbit and appear to have performed a nominal mission. This is also the only blip on ULA’s otherwise perfect success record; its Delta rockets having not experienced any anomalies since their transfer from Boeing.

Thursday’s launch began with ignition of the RD-180 engine at the base of AV-085’s Common Core Booster, which occurred at the T-2.7-second mark in the countdown. The RD-180, developed by Russia’s NPO Energomash, burns RP-1 kerosene propellant and liquid oxygen, powering the Atlas booster as it climbs through the atmosphere. As the countdown passes T-0, two Aerojet Rocketdyne AJ-60A solid rocket motors (SRMs) strapped to the first stage ignited, providing an additional kick during the early stages of flight.

Liftoff occurred at T+1.1 seconds, as the thrust from the RD-180 and solid rocket motors exceeds the weight of the fully-fueled vehicle. Atlas climbed away from its launch pad, initiating a pitch and yaw maneuver at the T+6-second mark to place itself onto a northeasterly track as Starliner sets course for the ISS. The rocket flew over the Atlantic Ocean on an azimuth of 50.4 degrees.

About one minute into the flight, Atlas experienced Max-Q, the area of maximum dynamic pressure, and the greatest loads due to aerodynamic stresses. Four and a half seconds later, its speed passed Mach 1, the speed of sound.

The SRMs burned out about 90 seconds after liftoff. Although they were no longer be contributing thrust, their spent cases remained attached to the first stage until the T+2-minute, 19.7-second mark in order to ensure the air density is low enough that they can separate cleanly, and they do not collide with the aft end of the CCB as they fall away.

Booster Engine Cutoff (BECO) marked the end of first stage flight, with the CCB shutting down its RD-180 engine. This took place four minutes and 34.9 seconds after launch, with stage separation about six seconds later. During a brief coast after staging, Centaur’s engines began their prestart sequence and the ascent cover that protected Starliner’s docking port was jettisoned.

Centaur’s Main Engine Start 1 (MES-1) came about 10 seconds after stage separation, with the two RL10 engines igniting to perform their only planned burn. These engines burn cryogenic propellant: liquid hydrogen and liquid oxygen.

Atlas V at stage separation (credit: Mack Crawford for NSF/L2)

For Thursday’s mission the RL10A-4-2 version of these engines were used. This is the version of the engine that was used for all Atlas V launches prior to 2014, however it has since been phased out for single-engine Centaur missions in favor of the RL10C-1. This has a wider nozzle than the RL10A variants and so cannot be used in dual-engine configurations, requiring a return to the previous model of engine for this flight.

Centaur was originally developed in the 1960s, with the dual-engine configuration standard for most of its history as it went through several generations of changes and enhancements. The single-engine Centaur was introduced in 2000 on the short-lived Atlas III, and became standard on Atlas V, whose only launches with dual-engine Centaurs have been Starliner OFT, and now OFT-2. Future Starliner missions will also use this configuration.

During the OFT-2 launch, Centaur fired its engines just once, with the burn lasting seven minutes and five-and-a-half seconds. About twenty seconds after ignition, the aeroskirt that protects the stage from aerodynamic forces separated as this was no longer needed outside of the atmosphere.

Centaur’s burn left Starliner in a 72.9-by-181.5-kilometer (39.4 x 98.0 nautical mile, 45.3 x 112.8 mile) orbit inclined at 51.6 degrees, with spacecraft separation occurring three minutes after main engine cutoff (MECO).

The perigee, or lowest point, of Starliner’s planned deployment orbit lies within the atmosphere, meaning that it is not a stable orbit, and the spacecraft would re-enter before completing a single revolution if it was to remain on this track. A critical milestone in the mission will therefore be the orbit insertion burn, using the spacecraft’s Orbital Maneuvering and Attitude Control (OMAC) thrusters. This began at 31 minutes mission elapsed time and raised the spacecraft into a more stable orbit.

Starliner docks with the ISS (credit: Mack Crawford for NSF/L2)

Starliner will take just over a day to reach the International Space Station. In addition to the maneuvers that are needed to keep it on track for rendezvous, the spacecraft will undergo a series of demonstrations – including an abort burn, an RCS control and attitude hold test, space-to-space commanding to ensure the crew of the ISS can send instructions to the spacecraft, and checkouts of the Vision-based Electro-optical Sensor Tracking Assembly (VESTA) sensors that will guide its automatic docking with the station.

Starliner is currently expected to dock with the ISS at 7:10:24 p.m. EDT (23:10:24 UTC) on Friday – or 24 hours, 15 minutes, and 39 seconds mission elapsed time. It will dock at the forward port of the Harmony module, via Pressurized Mating Adaptor 2 (PMA-2) and International Docking Adaptor-Forward (IDA-F). Starliner will stay at the ISS for about four days before undocking to begin its return to Earth.

Following a deorbit burn, the capsule and service module will separate. The service module will burn up on reentry, while Starliner’s capsule will land at the White Sands Space Harbor in New Mexico. If the OFT-2 mission is completed successfully, Starliner’s next mission will be the Crewed Flight Test (CFT) when it will carry astronauts into orbit for the first time.

Following its use in the OFT-2 mission, Starliner capsule number 2 is expected to be refurbished and is currently expected to fly again on the first operational crew mission, which will follow the successful completion of the CFT.

(Lead image: Starliner launches atop its Atlas V rocket at SLC-41 — credit: Stephen Marr for NSF)