Boeing CST-100 Starliner spacecraft sits atop a ULA Atlas V rocket in July 2021.Boeing/John GrantBoeing is hoping to launch its Starliner crew capsule for a second time in an attempt to dock with the International Space Station. Boeing's first try way back in December 2019 failed to reach the correct orbit but gave it valuable data. The company seemed ready to try again, but its launch attempt was scrubbed Tuesday -- the second delay in less than a week.Engineers "detected unexpected valve position indications in the propulsion system" during a health check of the spacecraft after Monday's electrical storms in the region, Boeing said Tuesday. It's uncertain if the storms were responsible for the technical issue.The company and NASA considered Wednesday as a possible target for a new launch time, but the valve issue continues to haunt the mission. "Engineering teams have ruled out a number of potential causes, including software, but additional time is needed to complete the assessment," NASA said Tuesday night. There is no new launch date at this time. The mission was originally scheduled to take off Friday, but that was delayed due to an issue Thursday with a Russian ISS module firing its thrusters shortly after docking with the station. That knocked the space station around and forced teams to evaluate the station's status."The International Space Station team will use the time to continue working checkouts of the newly arrived Roscosmos Nauka multipurpose laboratory module (MLM) and to ensure the station will be ready for Starliner's arrival," NASA said in a July 29 statement.NASA will livestream the launch when it eventually happens.When Starliner does finally launch, it will lift off on a United Launch Alliance (ULA) Atlas V rocket. The capsule will be packed with around 400 pounds of crew supplies and cargo. If all goes well, it'll dock with the space station about 24 hours later. Docking will also be covered live by NASA TV.Software defects and a communications link problem led to a premature end to the original Boeing test flight in 2019, though the CST-100 Starliner capsule landed safely back on Earth. The upcoming Orbital Flight Test-2 (OFT-2) mission is a chance for Boeing to thoroughly vet its hardware and software before a crew of three American astronauts would fly on Starliner.Both Boeing and SpaceX are part of NASA's Commercial Crew Program, which is all about sending astronauts to the ISS from American soil. SpaceX has now delivered 10 astronauts to the ISS, and Boeing would like to catch up. But first, it'll need to show that its Starliner can safely reach the ISS and return to Earth.Starliner will spend between five and 10 days at the ISS before bringing research samples back to Earth. Boeing will aim to bring the spacecraft back for a parachute landing in the desert of New Mexico."OFT-2 will provide valuable data that will help NASA certify Boeing's crew transportation system to carry astronauts to and from the space station," NASA said in a statement July 22 after concluding a flight readiness review.The mission is a key step for NASA's plans to run regular crewed launches from the US, ending its reliance on Russian Soyuz spacecraft. Boeing is also looking ahead at its first crewed mission, Boe-CFT, which it had been hoping to launch within the next six months. The delays with OFT-2 could mean a longer wait before people fly on Starliner.Follow CNET's 2021 Space Calendar to stay up to date with all the latest space news this year. You can even add it to your own Google Calendar.