The risk of violent bubble collapse in densified liquid oxygen

The new-generation reusable rockets, such as Falcon 9 of SpaceX, use densified liquid oxygen (LOX) to increase the payload and prolong the mission duration. The densification is accomplished by supercooling the cryogenic propellant to temperatures near the triple point. Despite the enormous benefits afforded by cryogenic propellant densification, we highlight in this Letter a potential hazard associated with densified LOX: severe cavitation damage caused by violent bubble collapse. We show that with the unique physical properties of LOX under extensive subcooling, the attenuated thermal effects impose less constraint on the dynamics of cavitation bubbles therein, and violent collapse can materialize. The emitted shock pressure is estimated across the parameter space of ambient temperature and pressure. The result indicates a peak impact pressure as high as hundreds of megapascals. Considering the important repercussions for the safety of rocket engines, this study is intended to serve as a wake-up call that more attention should be paid to this issue.