A report in The Washington Post has claimed that NASA and SpaceX are locking horns over how the latter plans to send astronauts into space.

Thanks to significant funding from NASA to the tune of billions of dollars, SpaceX hopes to start sending astronauts into space by 2019. They will travel in a modified version of the company’s Dragon capsule, launched aboard their Falcon 9 rocket, called the Crew Dragon.

But NASA is concerned that SpaceX will fuel its rockets with astronauts already on board, known as load-and-go. And Elon Musk’s company does not look like it’s willing to budge.

In a letter released by NASA in November 2017, former astronaut Thomas Stafford laid out some of these concerns from the International Space Station Advisory Committee.

“There is a unanimous, and strong, feeling by the committee that scheduling the crew to be on board the Dragon spacecraft prior to loading oxidizer into the rocket is contrary to booster safety criteria that has been in place for over 50 years, both in this country and internationally,” he wrote.

That letter was actually written way back in December 2016. In other words, SpaceX has known about NASA’s concerns for almost a year and a half, and has done little about it.

So what’s the issue? It stems from how SpaceX fuels its rockets. In order to maximize the amount of fuel it can get on its rockets, it cools it to an extremely low temperature. This makes it denser, so more can be packed into the rocket.

“To SpaceX, the approach is another example of how it is breaking the mold,” Christan Davenport wrote for the Washington Post. “But to others it is an unnecessary risk.”

The risk is that fueling the rocket can be a somewhat dangerous process. This was highlighted in September 2016 when a SpaceX Falcon 9 rocket exploded while it was being fueled for an engine test.

It’s not clear what the solution to this problem will be in the near-future. Currently, SpaceX plans to complete a test flight of its Crew Dragon towards the end of this year. But some reports have suggested both SpaceX and Boeing, which is also developing a spacecraft under contract by NASA called Starliner, will not be ready to fly until next year at the earliest.

Whether SpaceX will have conformed to NASA’s concerns by then, or whether the agency will have to become less risk averse, will be interesting to see.