Low-Earth orbit does not provide many second chances, but it does provide audits. Boeing’s CST-100 Starliner is on a collision course with one of them: a cargo-only flight that will transform a routine crew rotation mission into a tough engineering judgment on the spacecraft’s propulsion system.
The catalyst for this was the 2024 Crew Flight Test of Starliner, where what was supposed to be an eight-day visit to the International Space Station ended up being a docked stay that lasted months due to a series of problems with the spacecraft’s thrusters and helium leaks. The spacecraft eventually made it back to Earth without any crew on board, which caused the ISS to deal with an unexpected “extra” crew member, an issue that is more important as the station approaches its retirement in 2030.
NASA and Boeing later refined the short-term mission: Starliner-1 is now planned as an uncrewed cargo transport and in-orbit validation mission no earlier than April 2026, pending test and certification activities. The contract terms changed with this new mission, cutting Boeing’s definitized missions from six to four, with two remaining as options, and lowering the contract value by $768 million to $3.732 billion. As Steve Stich explained, “This modification allows NASA and Boeing to focus on safely certifying the system in 2026, execute Starliner’s first crew rotation when ready, and align our ongoing flight planning for future Starliner missions based on station’s operational needs through 2030.”
Starliner’s propulsion system is not exotic, but it is packed with opportunities for small problems to escalate to the mission level. There are 28 reaction control system thrusters for precision pointing and translation and 20 larger OMAC thrusters for orbital maneuvers, as well as launch abort engines that are not relevant to orbital operations. The crew capsule that must return to Earth has its own 12 RCS thrusters for attitude control during entry. Unlike the SpaceX Dragon, Starliner jettisons its service module prior to re-entry, so the failure-prone plumbing and valves are not available for inspection after landing.
During the 2024 docking approach, five of the service module’s RCS thrusters had failed over time, with some able to be persuaded to return after resets. Subsequent analysis identified a critical issue to be with Teflon poppet seals in thruster valves that had overheated, warped, and restricted propellant flow, particularly when firing repeatedly in direct sunlight. Subsequent ground-based effort has focused more on replicating this rather than disputing it: One campaign ran profiles from launch through docking with over 1,000 pulses and other aggressive profiles to drive higher thermal conditions, with Boeing’s Dan Niedermaier explaining, “We decided to run additional profiles with longer and more frequent pulses to see if we could more closely simulate the higher thermal conditions the thrusters experienced in-flight.”
Then there were the helium leaks one pre-launch and four that became known in orbit that affected the pressurization system that kept the propellant flowing. Enough helium was thought to be available for a return, but “enough” is not the same as “safe with margin” when combined with a thruster configuration that could partially fall out of family mid-maneuver. It was the crew’s ride home that became the unacceptable risk, not the docking.
These propulsion problems also sit atop the older concern of verification gaps that have repeatedly allowed issues to arise late. Following the 2019 Orbital Flight Test software issues, the NASA Aerospace Safety Advisory Panel expressed concern for “process escapes,” and the program’s history includes issues such as corroded oxidizer valves, parachute link strength issues, and flammability issues in wiring harness tape. The cargo flight, therefore, is more than just a box delivery; it is a test to prove that the solutions are not only credible but also repeatable under flight-like conditions.
If Starliner-1 proves out new “doghouse” thruster assemblies and helium plumbing designs, NASA regains the dissimilar redundancy it has sought since the start of commercial crew operations at a point when the ISS flight manifest is constrained and NASA’s schedule opportunities are shrinking. Otherwise, the next decade on station will continue to cohere around Dragon and Soyuz, and the future of Starliner will be less a second system than a perpetual orbital-level engineering lab.
