A panel of expert advisors to NASA have expressed concern about SpaceX’s plans to fuel its Falcon 9 rockets with astronauts on board, The Wall Street Journal reports. At a meeting on Monday, members of NASA’s Space Station Advisory Committee argued that fueling is a "hazardous operation" that shouldn’t be performed anywhere near people. The committee said it has had these concerns before, but their anxieties have increased after one of SpaceX’s Falcon 9 rockets exploded on the launchpad during routine fueling procedures.

Fueling is a "hazardous operation"

The most vocal about these propellant risks was Air Force Lt. General Thomas Stafford, who chairs the advisory committee. It’s the second time Stafford has called into question SpaceX’s fueling practices. In 2015, he sent a letter to NASA headquarters in which he argued that fueling a vehicle with people onboard goes against decades of human spaceflight procedures, according to The Wall Street Journal. Typically, people have boarded rockets after the vehicles have been fueled, in order to minimize accidents that may spring up during the loading process.

In a statement, SpaceX maintained that it has "designed a reliable fueling and launch process that minimizes" the risks posed to people. Additionally, a NASA safety review board approved a report in June about the hazards posed by the Falcon 9’s fueling, SpaceX says.

But the company also notes that its fueling process isn’t exactly set in stone either. In a recent update, SpaceX said it still doesn’t know the exact cause of the September 1st explosion, but the company did manage to recreate a failure with the Falcon 9 during fuel loading tests in Texas. Now the company is working on improving its fueling processes as it tries to return to flight, and "corrective action" may be taken depending on what SpaceX learns from its investigation. "As needed, any additional controls will be put in place to ensure crew safety, from the moment the astronauts reach the pad, through fueling, launch, and spaceflight, and until they are brought safely home," the company said in a statement.

"Any additional controls will be put in place to ensure crew safety."

Currently, SpaceX fuels its rockets around 30 minutes before they launch, and the plan for future crewed flights is to have astronauts board the Falcon 9 before propellant is loaded into the vehicle. It’s a much different procedure than NASA used for the Space Shuttle: fueling occurred hours before takeoff, and astronauts didn’t board until after the bulk of fueling was complete.

The reason SpaceX has such a short turn around time has to do with the propellant the company uses. SpaceX uses a super cool propellant that’s much colder than what was used on the Space Shuttle. These cryogenic temperatures increase the density of propellant that can be used in the rocket, giving the vehicle much more power. But it also means that the company doesn’t have a whole lot of time between fueling the vehicle and takeoff, since the propellant runs the risk of warming up too much.

Additionally, SpaceX has something the Space Shuttle didn’t have: a nearly instantaneous crew abort system. The Crew Dragon — the company’s vehicle that will carry astronauts to space — is embedded with small engines in its hull that ignite if something goes wrong on the launchpad or during launch. These engines can carry the Crew Dragon up and away from a malfunctioning rocket and get its passengers to safety. SpaceX successfully tested the abort system last year though it has never been used with people onboard. CEO Elon Musk claims that Crew Dragon would have been able to save any passengers had they been onboard the rocket that exploded in September.

The concerns from NASA’s Space Station Advisory Committee come as SpaceX is getting ready to ferry NASA astronauts to and from the International Space Station as part of the agency’s Commercial Crew Program in late 2017 or 2018.