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Boeing CST-100 Starliner

Boeing · Seven-Seat Commercial Crew Capsule (ISS) · USA · Digital Age (2010–present)

Boeing CST-100 Starliner — Seven-Seat Commercial Crew Capsule (ISS)
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The Boeing CST-100 Starliner is an American crewed orbital spacecraft built by Boeing under NASA's Commercial Crew Program, joining SpaceX's Crew Dragon as the second crew-rated commercial capsule. Development ran from 2010 to 2024. The first uncrewed orbital flight launched on 20 December 2019 and failed to dock with the ISS. The first crewed flight, NASA's Crew Flight Test, launched on 5 June 2024, and the first crew rotation mission, Starliner-1, is planned for 2025-2026. Boeing has built 3 Starliner capsules.

Starliner is a 4.6 m diameter conical capsule rated for up to 7 crew, though NASA flies a standard 4 to the ISS. Dry mass is 13,000 kg, rising to 25,000 kg at launch with the service module attached. The launch vehicle is ULA's Atlas V N22, with a planned transition to Vulcan Centaur. Unlike the European-supplied service module on Lockheed Martin's Orion, Boeing builds its own; it carries 4 RS-88 and 16 RS-87 hypergolic engines for orbital manoeuvring. Reentry uses an ablative ATK PICA heat shield, and recovery is by parachute-and-airbag landing on dry ground in the western US rather than the water splashdown SpaceX uses. Each capsule is designed for up to 10 reuses; the service module is expended on every flight.

Starliner's troubled history has become a case study in NASA and Boeing programme management. OFT-1 in December 2019 missed the ISS owing to a mission-timer error. OFT-2 was delayed by oxidiser-valve corrosion in August 2021 before flying successfully in May 2022, docking and returning to Earth. CFT launched on 5 June 2024 carrying NASA astronauts Butch Wilmore and Sunita Williams. On arrival the capsule developed helium leaks and lost 5 of its 28 thrusters during approach. After a joint NASA-Boeing investigation, the agency judged the capsule unsafe for crewed return. Wilmore and Williams remained on the ISS — an 8-day mission stretched to 8 months — and came home aboard SpaceX Crew Dragon Crew-9 in February 2025. Starliner itself returned uncrewed in September 2024. NASA cut Boeing's contract option from 6 rotations to 3, leaving Crew Dragon as the primary US crew transport. Starliner-1 is now targeted for late 2025-2026.

For Kids — a shorter, friendlier version

The Boeing CST-100 Starliner is an American space capsule that carries astronauts to and from the International Space Station. It is shaped like a wide cone. Up to seven astronauts can fit inside, although NASA usually flies only four at a time.

Boeing built the Starliner under a contract with NASA. The first flight without crew happened in 2019, but it had trouble docking with the space station. In June 2024, the Starliner finally flew its first crew test mission with two astronauts on board.

The Starliner is about as tall as a two-story house. It is heavier than three small cars combined when fully loaded. The capsule launches on top of a big Atlas V rocket. After its mission, it lands on dry ground in the western United States using parachutes and big airbags.

The Starliner is one of two American spacecraft that can carry astronauts to space, along with the SpaceX Crew Dragon. Boeing has built three Starliner capsules. They can be reused up to ten times each, which helps save money for NASA.

Fun Facts

  • The Starliner can carry up to seven astronauts at a time.
  • Each capsule can be reused up to ten times.
  • The Starliner lands on dry ground using parachutes and big airbags.
  • It is one of two American spacecraft cleared to carry NASA astronauts.
  • The capsule launches on top of an Atlas V rocket.
  • Boeing has built three Starliner capsules so far.

Kids’ Questions

How is the Starliner different from the SpaceX Crew Dragon?

The Starliner lands on dry ground in the western US using airbags, while the Crew Dragon splashes down in the ocean. Boeing built the Starliner and SpaceX built the Crew Dragon. Both can carry up to seven astronauts.

Why did Boeing build the Starliner?

NASA wanted two different American spaceships that could carry astronauts to the space station. Having two helps keep flying even if one has a problem. NASA gave money to both Boeing and SpaceX to build a capsule each.

Variants

Starliner (single design)
3 capsules built. Reusable up to 10 times.

Notable Operators

NASA Commercial Crew Program (2024-present)
Sole operator. ISS crew rotation.

Frequently Asked Questions

Why did NASA leave Wilmore + Williams on ISS?

The 5 June 2024 Crew Flight Test produced two serious anomalies during approach to the ISS: 5 separate helium leaks in the service module's propulsion pressurisation lines, and 5 of 28 reaction-control thrusters failing during docking, forcing a manual override. Through July and August 2024, NASA and Boeing analysis could not rule out a recurrence of the propulsion and thruster faults during the higher-stress return trajectory. NASA judged the risk of a crewed return unacceptable and elected to bring the astronauts back via SpaceX Crew Dragon Crew-9, already scheduled to reach the ISS in September 2024 with 2 empty seats. Wilmore and Williams returned to Earth on 18 March 2025 aboard Crew Dragon — an 8-day mission stretched to 286 days. The decision was a major reputational and financial blow to Boeing, and in October 2024 NASA cut its Commercial Crew contract from 6 to 3 rotation missions.

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