The CST-100 Starliner spacecraft, which Boeing recently planned to conduct a test flight, was delayed due to unexpected values in a valve included in its propulsion system just before launch.
On August 13, Boeing dismounted the spacecraft, which was continuing maintenance work on the Atlas V rocket, and conducted detailed problem analysis again at C3PF, the Boeing commercial cargo handling facility at the Kennedy Space Center. The National Aeronautics and Space Administration (NASA) said the issue has been addressed and will begin flying when ready.
The problem found with the Starliner is that the 13 valves that make up the propulsion system close unexpectedly just before launch. The Atlas V rocket was moving from the launch pad to the vertical integration building for rocket assembly to enable spacecraft inspection.
Boeing said on Aug. 12 that nine of its 13 valves could be returned to an open state using electrical and thermal technology, but the other four valves remained closed and the job was not completed. Boeing explained that the cause of the problem was that NTO, which is mainly used as a rocket oxidizer, penetrated into the Teflon material sealing the valve, which reacted with the moisture on the opposite side of the valve, corroded the valve and stuck it. He was able to open the valves somehow, so nine valves were able to work properly, but the remaining four were difficult for rockets, so he decided that there was no other way than to cancel C3PF.
Boeing did not encounter any problems in a pre-test conducted in May, but during launch, moisture ingress and corrosion of the valve may have been caused by exposure to high humidity during the assembly of the spacecraft or on the launch pad. The day before the scheduled test launch date, there was also a problem that thunderstorm had an effect on the sensor value.
For whatever reason, dismounting a spacecraft from a rocket means a monthly planning delay. Related information can be found here.