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"We are concerned that there may be insufficient pre-cooling of the tank and plumbing with (SpaceX's) current planned oxidizer fill scenario," former astronaut Thomas Stafford and members of the International Space Station Advisory Committee wrote in a December 2015 letter to NASA.
The letter was released on Friday.
"Without re-circulation there may be stratification of oxidizer temperature that will cause a variation in the input conditions to the oxidizer pump," the letter said.
NASA's space shuttles, for example, used a re-circulation system and pressure lines to help keep liquid oxygen temperatures consistent, Mike Leinbach, former shuttle launch director, told Seeker.
"That's key," said Leinbach.
NASA also has a bit of history with liquid oxygen behaving badly with a composite fuel tank, which was developed as part of the experimental X-33 spaceship in the 1990s.
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"The composite tank experienced cracks when fueled in development tests. Composite was used to save weight. Never could overcome it technically, and contributed a lot to the program demise," Leinbach wrote in an email.