New details have emerged about warnings issued by a National Aeronautics and Space Administration advisory panel regarding potential fueling hazards on SpaceX’s future manned rockets.

The panel’s safety concerns, which were reported on earlier by The Wall Street Journal, focus on possible dangers stemming from plans by entrepreneur Elon Musk’s Space Exploration Technologies Corp. to fuel rockets while astronauts are strapped into capsules loaded on board.

The supercooled fuel used by SpaceX’s rockets to provide greater thrust requires launches to occur roughly within half an hour of the start of fueling, which wouldn’t give crews enough time to get into position before liftoff.

No other large manned rocket developed over the years by any country, or in operation today, relies on fueling operations with astronauts already in place, according to veteran U.S. and European industry officials.

On Friday, NASA released a Dec. 9, 2015, letter in which the agency’s International Space Station Advisory Committee initially raised the issue and requested detailed answers from senior agency officials. NASA has promised to respond to the outside group of experts in December.