For the second time, Boeing has delayed the first crewed flight of its CST-100 Starliner spacecraft — the vehicle the company is building to transport NASA astronauts to and from the ISS, Aviation Week reports. Originally, the aim was for Starliner to carry astronauts for the first time in 2017, but Boeing announced in May that people wouldn’t fly on the vehicle until 2018. The optimistic goal was for crewed flights to begin in early to mid-2018, but now Boeing is saying that the first operational flights won’t occur until December of that year.

Boeing says that production delays and problems with qualification tests are partly to blame for the timeline slip. The company also found a production flaw in September that forced them to get rid of a main element on one of their spacecrafts — a dome that made up the pressure shell of the crew module. All of these complications combined prompted Boeing to push back the development timeline of Starliner by about six months.

"When we were faced with these issues it was time for us to step back and say: ‘Hey listen, we have to readdress [this] and say what’s real and lay in where we are going forward’," John Mulholland, Boeing’s vice president and program manager for commercial programs in space exploration, told Aviation Week.

That means a lot of big production milestones for the Starliner are also being pushed back. Boeing still needs to do a pad abort test — to demonstrate that the Starliner can carry a crew to safety in case an emergency occurs on the launch pad. That was scheduled for October 2017, but now that test won’t happen until January of 2018. And the first uncrewed flight of the Starliner has now moved from December 2017 to June of 2018. Now, the first crewed test flight of Starliner is on track for August 2018, with the first fully operational flights getting underway in December.

The CST-100 Starliner is Boeing’s contribution to NASA’s Commercial Crew Program, a public-private partnership aimed at getting NASA astronauts back on American-made vehicles again. Currently, NASA relies on Russia’s Soyuz rocket to transport astronauts to and from the International Space Station — a ride share that costs the space agency upwards of $80 million per seat. But in 2014, NASA contracted both Boeing and SpaceX to develop crew capsules that could ferry astronauts to the ISS on US vehicles for much lower costs. For the program, SpaceX has been developing a crewed version of its Dragon cargo capsule, which will launch on top of the company’s Falcon 9 rocket. And when complete, Boeing’s CST-100 Starliner will launch on top of the Atlas V rocket — the premier vehicle of the United Launch Alliance.

When NASA came up with the Commercial Crew program, the agency envisioned the first flights occurring in 2015. But that deadline was pushed back to 2017, and further delays have been suspected. A recent report from NASA’s Office of the Inspector General predicted that the first Commercial Crew flights would slip to 2018 thanks to "funding shortfalls" and "technical challenges" related to the design of the spacecraft.

Despite these expectations, SpaceX has yet to formally announce any delays regarding its Commercial Crew program schedule — though it seems likely the company could experience its own timeline slippage. On September 1st, one of SpaceX’s Falcon 9 rockets exploded on a launch pad at Cape Canaveral, Florida as the vehicle was being prepared for a routine test. The explosion greatly damaged one of SpaceX’s launch pads, and the company has since grounded all of its flights while it investigates the cause of the failure. The exact origin of the explosion hasn’t been pinned down yet, but SpaceX says it had to do with a breach in the rocket’s helium system. The company says it will have a better idea of its timeline once the true cause of the explosion is known.

"We continue to review and analyze data from the anomaly," a representative for SpaceX tells The Verge. "We expect to stay on track with our Commercial Crew milestones with NASA, but we'll better know how our schedule will be impacted once the investigation is complete and we get back to flying."

But as the Commercial Crew program experiences delays, NASA’s dependence on Russian access to space continues to grow. Last year, the space agency was forced to buy six additional Soyuz seats for astronauts to get to the ISS in 2017 — a move that cost $490 million. Now it’s looking even more likely that Soyuz seats will be needed for 2018, too.