NASA held first briefing on results of ground tests of Boeing Starliner maneuvering engines. Five of them shut down while trying to board the spacecraft with the ISS on June 5. To ensure the spacecraft's safe return to Earth, NASA engineers began reanalyzing the engine situation during the bench tests. As it turned out, the maneuvering engines overheated more than the calculated values.
This is not the first time that the Starliner maneuvering engines have suddenly stopped working. This happened during the spacecraft's previous unmanned flight to the ISS, as well as when it attempted to dock with the station during the first manned test mission. The ground crew later released four of them, ensuring the ship's safe docking a day later than planned.
New ground tests have shown that the service module's orbit correction engines are operating at higher temperatures than expected. This is probably due to the tight confines of the service compartment and the heating from the sun's rays. During ground tests, engineers artificially supplied the engines with this excess heat to reproduce the conditions that led to their failure.
In general, the thrusters are hardly involved in the process of bringing the ship out of orbit. To achieve this, the Starliner uses more powerful stern thrusters. But at the stage of undocking and maneuvers near the ISS, the ship still relies on the operation of corrective engines, of which there are 28.
NASA is also investigating the problem of a leaky link anomaly in the helium transport system. This is an unexpected problem for the agency, the reasons for which are not yet clear. Engineers believe this effect can be caused by clogged connections or excess grease on the seals. Experts do not rule out that in the future the design of the links will have to be replaced by another one.
In general, NASA is inclined to believe that the Boeing Starliner could be returned to Earth in late July. In mid-August, the next shift of astronauts will fly to the ISS with SpaceX's Dragon Crew, and the docking slot occupied by the Boeing craft will have to be vacated. Alternatively, as a last resort, the agency will first return the previous shift to Earth, which is not normally practiced (crews hand off shifts aboard the station), making room for the incoming ship.
The Starliner crew, “stuck” of the ISS, said a few words about the ship. According to Barry Wilmore and Sunita Williams, the Starliner is a decent ship, but they wouldn't recommend it for scheduled flights.